Damage Equivalent Test Methodologies as Design Elements ... - MDPI

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Damage Equivalent Test Methodologies as Design Elements for Journal Bearing Systems Florian Summer *, Philipp Bergmann

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and Florian Grün

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Chair of Mechanical Engineering, Montanuniversität Leoben, 8700 Leoben, Austria; [email protected] (P.B.); [email protected] (F.G.) * Correspondence: [email protected]; Tel.: +43-3842-402-1403 Received: 31 October 2017; Accepted: 8 December 2017; Published: 14 December 2017

Abstract: The current paper addresses the field of experimental research of journal bearing systems. In this regard, the challenges are dealt with concerning simultaneous testing with a close correlation to the industrial application and with a high resolution of tribological processes. Concerning this aspect, two damage equivalent laboratory test methodologies for journal slide bearing systems are presented, and their ability to visualize certain performance parameters of bearing systems are emphasized (for instance friction performance, (start stop) wear processes, and seizure events). The results clearly emphasize that the applied methodologies provide accurate findings regarding specific effects of selective parameters/changes on the performance of bearing systems, such as polymer overlays may result in improved mixed friction sliding conditions if designed properly, and they provide superior start stop wear resistance; the use of specific corrosion inhibitors can successfully prevent tribo-corrosion on bronze bearings; a decrease of oil viscosity increases solid friction share but decreases fluid friction; lubricant anti-wear additives are able to improve seizure resistance and sliding properties of bearing systems depending on formulation harmonization; and novel bearing material coatings, e.g., sputtered SnCu, can significantly improve emergency running capabilities. Keywords: journal bearings; damage equivalent testing; seizure; start stop wear; simulation

1. Introduction In various applications of technical systems, there is a need to hold, guide, and support linear moving or rotating parts by means of slide bearing components. In particular, slide bearings, also referred to as journal bearings or plain bearings, are used in different types of internal combustion engines under various operating conditions [1,2]. Hence, journal bearings are used in manifold application areas, which are, time and again, additionally subjected to profound and rapid changes to affect an improvement in the performance; e.g., re-design of combustion engines to limit exhaust gas emissions. As a result, on the one hand, product development times of bearing systems including bearing material, shaft material, and the lubricant in between need to be short, whereas, on the other hand, simultaneously a large scope of solutions need to be provided. Basically, decisive performance parameters of journal slide bearing systems are related to their reliability and efficiency for all lubrication conditions [2,3]. Reliability of journal bearings for a sufficient life time covers several mechanical and tribological requirements depending on the lubrication regimes of the journal bearings. With regard to tribological demands, especially tribological system robustness (e.g., high resistance against seizure events, wear resistance during start-stop and resistivity against disturbance variables such as dirt particles) represents a main criterion. The efficiency of a journal bearing is rated by means of the resulting frictional losses, viz. the coefficient of friction (COF, µ). The development and design process of slide bearing systems according to the previously described demand is conventionally based on either testing or simulation, or rather a combination

Lubricants 2017, 5, 47; doi:10.3390/lubricants5040047

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Lubricants 2017, 5, 47

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The development and design process of slide bearing systems according to the previously

thereof. The latter has not yet progressed so far in order to fully elucidate the complex processes described demand is conventionally based on either testing or simulation, or rather a combination during thereof. boundary sliding. In in this regard, material of bearing materials, The and lattermixed has notfriction yet progressed so far order to fully elucidatedesign the complex processes shaft materials, and lubricants studies phenomena rely on experiments. during boundary and mixed and friction sliding.on In tribological this regard, material design still of bearing materials, shaft materials, and lubricants andofstudies on tribological still rely experiments. Basically, experimental investigations tribological systemsphenomena can be carried outon according to various Basically,described experimental of tribologicalofsystems canbearing be carried outcategories according tocan various test categories ininvestigations [4]. The classification journal test be defined test categories described in [4]. The classification of journal bearing test categories can be defined accordingly by splitting investigations into those with full scale systems and those testing reduced accordingly by splitting investigations into those with full scale systems and those testing reduced systems.systems. A visual depiction of the following explanatory notes is seen in Figure 1. A visual depiction of the following explanatory notes is seen in Figure 1.

Figure 1. Journal bearing test possibilities.

Figure 1. Journal bearing test possibilities. Full scale tests provide a final performance evaluation of the whole system including the performance during application. Forperformance instance, in [5],evaluation field examinations of high speed diesel engines Full scale tests provide a final of the whole system including the are presented. The results of these activities highlight the main tribological bearing damages performance during application. For instance, in [5], field examinations of high speed diesel engines occurring in the field. The damages are based on several categories of wear (adhesive, abrasive, are presented. The results of these activities highlight the main tribological bearing damages occurring corrosive, etc.) based on root causes; e.g., oil film breakdown due to overload, vibrations or in the field. The damages based on several categories wearor(adhesive, abrasive, etc.) insufficient oil supplyare in case of adhesive wear or seizure of events; oil contamination withcorrosive, foreign based on root causes; film breakdown due to overload, oroften insufficient particles or wear e.g., debrisoil considering abrasive wear. However, full vibrations scale tests are incapableoil of supply elucidating occurring tribological contact nor particles the corresponding in case of adhesiveindividual wear or processes seizure events; or in oilthe contamination withitself foreign or wear debris friction losses. In this regard, investigations of selective parameters or directed optimization considering abrasive wear. However, full scale tests are often incapable of elucidating individual processes are hardly possible. This limits usability of full scale tests for screening applications and processes occurring in the tribological contact itself nor the corresponding friction losses. In this in-depth studies of single effects. Furthermore, reality proves that every application is somehow regard, different. investigations of selective parameters or directed optimization processes are hardly possible. Therefore, full scale tests are inefficient in terms of costs and time. This conflicts with This limits usability of full scaleand tests for changes screening in-depth studies of single shortened product cycles rapid of applications the applicationand requirements. Considering this, effects. Furthermore, reality that application is somehow different. Therefore, fullefficient scale tests are laboratory testproves systems onevery a reduced scale provide valuable advantages towards investigations of costs bearing systems. inefficient in terms of and time. This conflicts with shortened product cycles and rapid changes In literature, several concepts of subscale for journal bearing systems can be scale found.provide of the application requirements. Considering this,systems laboratory test systems on a reduced Bearing test rigs, representing subassembly systems loaded by hydropulsating or electromechanical valuable advantages towards efficient investigations of bearing systems. actuators, are widely used in combination with customized test procedures [1,6,7]. These test stands In are literature, several of subscale systems journal bearing and systems can be found. successfully usedconcepts in predicting hydrodynamic loadfor bearing capabilities wear-fatigue Bearingphenomena test rigs, representing subassembly systems loaded by hydropulsating or electromechanical [8]. However, several studies point out the limited measurement and control accuracy of theseare rigswidely for certain investigation topics owing to complex rig assemblies [9,10]. actuators, used in combination with customized test procedures [1,6,7]. These test Hence, besides bearing test rigs, reduced systems on tribotesters, also referred to as tribometer stands are successfully used in predicting hydrodynamic load bearing capabilities and wear-fatigue test rigs, have emerged aiming for high visualization power of tribo-processes and manifold phenomena [8]. However, several studies point out the limited measurement and control accuracy of measurement parameters. In this regard, tribological model scale set ups including micro abrasion these rigs for certain investigation topics owing to complex rig assemblies [9,10]. testers [10], scratch tests [11], or pin-on-disc systems [12] are compromisingly carried out to gain a Hence, bearing test the rigs, reduced systems on tribotesters, referred to as ittribometer test betterbesides understanding about basic tribological phenomena of slidingalso surfaces. However, is also rigs, have emerged aiming for high visualization power of tribo-processes and manifold measurement emphasized in literature that various basic model test configurations, such as pin-on-plate, fail to reproduce damage mechanisms and tribological of journal bearing applications, such scratch parameters. In this regard, tribological model scale functionality set ups including micro abrasion testers [10], as shown in [13]. Hence, damage equivalency is regarded as a mandatory aspect when using a tests [11], or pin-on-disc systems [12] are compromisingly carried out to gain a better understanding model test configuration for bearing research in order to transfer results from model tests onto about the basic tribological phenomena of sliding surfaces. However, it is also emphasized in journal bearing application environments [14]. More seldom, tribological test configurations for literature that various basic model test configurations, such as pin-on-plate, fail to reproduce damage mechanisms and tribological functionality of journal bearing applications, such as shown in [13]. Hence, damage equivalency is regarded as a mandatory aspect when using a model test configuration for bearing research in order to transfer results from model tests onto journal bearing application environments [14]. More seldom, tribological test configurations for bearing systems are reported in literature, which can be referred to as being closer to the component. They characteristically use


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parts of bearing half shells in a block-on-ring testing configuration and are employed for bearing material characterization [15,16]. Their advantage is the ability to perform tests independent of a journal bearing manufacturer, as bearing half shells from series production can be used. However, Lubricants 2017, 5, 47 3 of 16 these test set-ups provide a limited transferability of the results obtained on a model level as being more ofbearing a linesystems contactaredue to large differences the the closer blocktoand ring. To achieve reported in literature, whichin can be curvature referred to asofbeing the component. transferability from subscaleuse tests to of real life bearing systems, similar contact areand needed in They characteristically parts bearing half shells in a block-on-ring testingconditions configuration employed for bearing material characterization [15,16]. Their advantage is the ability to perform the firstare place followed by motion equality. tests independent a journal bearing manufacturer, as bearing half shells from series production The current paperofpresents successfully established damage equivalent test set ups under can be used. However, these test set-ups provide a limited transferability of the results obtained on a laboratory conditions using ring-on-disc (RoD) model test configuration and a bearing segment model level as being more of a line contact due to large differences in the curvature of the block and (BS) system. In particular, the abilities of the herein proposed testbearing methodologies for characterization ring. To achieve transferability from subscale tests to real life systems, similar contact of friction efficiency and tribological damage mechanisms of bearing systems are demonstrated with conditions are needed in the first place followed by motion equality. current papercompilation presents successfully establishedtribometric damage equivalent test each set ups under the aid of a The comprehensive of representative results for system. laboratory conditions using ring-on-disc (RoD) model test configuration and a bearing segment (BS) system. In particular, the abilities of the herein proposed test methodologies for characterization of 2. Test Methodologies Employed friction efficiency and tribological damage mechanisms of bearing systems are demonstrated with the aid of a comprehensive compilation of representative tribometric results for each system. 2.1. Approach

The the presented test methodologies implemented on tribometer test rigs follows the 2. use Test of Methodologies Employed approach given in Figure 2. A valuable experimental test for tribological purposes needs to trigger 2.1. Approach similar contact processes to the real application. Furthermore, testing has to consider the whole system The use of the presented test methodologies implemented on tribometer test rigs follows the performance which derives from the interactions of the topographical and chemical characteristics of approach given in Figure 2. A valuable experimental test for tribological purposes needs to trigger the surfaces, physical and to chemical the lubricant, and similarthe contact processes the real properties application. of Furthermore, testing hasthe to overall considerconditions the whole under which sliding takes place, for e.g., speed, load, temperature, environmental conditions, or contact system performance which derives from the interactions of the topographical and chemical characteristics of bearing the surfaces, the physical chemical propertiesto oftest the lubricant, and the overall situation. Concerning systems, it is ofand utmost importance on a reduced scale to provide conditions under which sliding takes bearings. place, for Therefore, e.g., speed,application load, temperature, sliding and damage equivalency to engine related environmental testing on a reduced or contact situation. Concerning bearing systems, it is of utmost importance to test on a scale is conditions, a mandatory aspect for optimization of journal slide bearings [6]. The herein proposed test Cite>[ HYPERLINK ing and damage equivalency to engine bearings. Therefore, methodologies combine high visualization power of tribological processes under laboratory conditions, application related testing on a reduced scale is a mandatory aspect for optimization of journal slide high efficiency, and excellent cost effectiveness as well as tribological equivalency of contact bearings [6]. The herein proposed test methodologies combine high visualization power situations of and damage processes to under real life journal bearing high systems. Hence, these test are tribological processes laboratory conditions, efficiency, and excellent cost configurations effectiveness as tribological equivalency contact processes realthe lifelaboratory journal referredastowell as damage equivalent. Costofand timesituations efficient and testsdamage are carried out to with test bearingaccompanied systems. Hence,with theseprecise test configurations are of referred to as damage equivalent. Cost and configuration measurement the tribological performance. This provides time efficient tests are carried out with the laboratory test configuration accompanied with precise comprehensive and in-depth studies of bearing systems. Subsequently, knowledge obtained with these measurement of the tribological performance. This provides comprehensive and in-depth studies of test rigsbearing can besystems. transferred to application, keeping in mind the restrictions Subsequently, knowledge obtained with these test rigs canby be reducing transferredthe to degree of realism. application, keeping in mind the restrictions by reducing the degree of realism.

Figure 2. Experimental approach. COF = coefficient of friction.

Figure 2. Experimental approach. COF = coefficient of friction. 2.2. Damage Equivalent Model Test Method—Ring-on-Disc

2.2. DamageAEquivalent Model Test Method—Ring-on-Disc schematic depiction of the applied set up and the area of testing are shown in Figure 3a. In a ring-on-discdepiction (RoD) test of configuration, moving disc/ring and a fixed A schematic the appliedcontact set upbetween and theaarea of testing are shown indisc/ring Figure 3a. In a counterpart in an axial direction takes place. The herein presented set up uses plain contact partners ring-on-disc (RoD) test configuration, contact between a moving disc/ring and a fixed disc/ring immersed in oil for bearing investigations and, therefore, is able to mimic contact situations and counterpart in an axial direction takes place. The herein presented set upfriction. uses plain contact damage processes of real scale bearing systems in boundary and mixed Hence, seizurepartners immersed in oil for bearing investigations and, therefore, is able to mimic contact situations and events especially or wear processes as well as mixed friction performance of bearing systems can be damage processes of real scale bearing systems in boundary and mixed friction. Hence, seizure events especially or wear processes as well as mixed friction performance of bearing systems can be addressed


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with this set up. The disc specimens, which are made of the bearing material, are realized with four notches splitting the area of contact. This results in easy oil entrainment into the contact. The test set up differentiates from many other model set ups in terms of reproduction of contact situations of real scale bearings owing to the following considerations:

• • •

Large area contact situation and prevention of line contact situations potentially occurring due to misalignment and bearing of the load through the edges. Ensured oil entrainment into the contact realized by oil notches. Rotary sliding condition.

For instance, the difference to the pin-on-plate set up has been emphasized in [13], for e.g., the difference in lubrication type, area of contact or depiction of friction, and temperature characteristics. The ring-on-disc configuration is implemented within a tribometer test rig from Phoenix Tribology Ltd. (Kingsclere, UK). Tribological properties and phenomena can be tested and recorded comprehensively due to a broad range of set values (loading, speed, and heating) and measured values. Owing to a low level of friction power, the tribo-processes and failure mechanisms are decelerated and can be studied in detail. Figure 3b shows a photo of the TE92 test rig with the ring-on-disc configuration mounted as well as several measurement sensors applied. Additionally, a computer-aided design (CAD) drawing of the RoD test cell is depicted, giving a closer look into the set up assembly. The loading of the test cell is realized with the aid of an air bellow. Two thermocouples facilitate the measuring of the temperature level of the ring specimen (4.5 mm below the contacting surface) as well as a contact near temperature (1 mm below the contacting surface) within the shaft specimen closely below the area of contact. The temperature level is settled by using the ring specimen temperature, whereas the system can be heated by two heating elements beneath the oil bath. The coefficient of friction is deduced by a load cell (considering the distance ratio of the load cell and the contact from the axis of symmetry), which is mounted on the left of the test cell and comes in contact with the pivot mounted oil bath under loaded conditions. The motor and the test cell are isolated from each other, which enables the measurement of the electrical resistance, also referred to as contact potential (CP), of the contact between ring and disc according to the method of [17]. Additionally, qualitative depiction of wear processes is possible by using a capacitive displacement sensor responding the relative wear height between ring and disc. Furthermore, an acoustic emission technique using a Piezotron Sensor Type 8152B from Kistler (Winterthur, Switzerland) with a sampling frequency of 850 kHz has been successfully integrated [18]. The range of set parameters and measured quantities, which can be recorded with a frequency up to 10 Hz, are listed below:

• •

Set values: normal load (0–10 kN), sliding speed (0–3000 rpm), temperature level (20–200 ◦ C) Measured values: friction torque (0–15 Nm), contact temperature (0–250 ◦ C), specimen temperature (0–250 ◦ C), contact resistance (0–51 mV), wear height (0–500 µm), acoustic emission (50 kHz–400 kHz)

Furthermore, a modified ring-on-disc set up using thrust washer counterparts has already been successfully designed. The narrowing gap in the case of thrust bearing specimens facilitates a hydrodynamic effect and enables test conditions closer to a bearing operation. Details as well as selective results of the thrust washer ring-on-disc set up are presented in [19]. Considering cost aspects of the specimen, manufacturing the system with plain surfaces is done more frequently and represents a sort of a standard version.


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Figure 3. Ring-on-disc and test test area/skills; area/skills; Figure 3. Ring-on-disc experimental experimental methodology methodology (plain (plain contact): contact): (a) (a) Set Set up up and (b) Implementation of the set up on a tribometer test rig TE92. CP = electrical resistance (b) Implementation of the set up on a tribometer test rig TE92. CP = electrical resistance measurement. measurement.

2.3. Component Close Test Method—Bearing Segment System 2.3. Component Close Test Method—Bearing Segment System This novel test configuration enables component testing of journal bearing systems in combination This novel test configuration enables component testing of journal bearing systems in with precise depictions of the friction characteristics. In this set up, a two-sided lubricated contact combination with precise depictions of the friction characteristics. In this set up, a two-sided between two real life bearing shells, which are reduced to 120◦ , and a rotating shaft specimen takes lubricated contact between two real life bearing shells, which are reduced to 120°, and a rotating place, see Figure 4a. Geometric equivalency to full scale parts facilitates studies concerning friction shaft specimen takes place, see Figure 4a. Geometric equivalency to full scale parts facilitates studies performance for all lubrication regimes and damage processes, such as start stop wear or seizure events, concerning friction performance for all lubrication regimes and damage processes, such as start stop close to component conditions. The currently used shaft diameter is ~48.6 mm. The curvatures of the wear or seizure events, close to component conditions. The currently used shaft diameter is ~48.6 shaft and bearing are adjusted to equal 1.6 per mill clearance in 360◦ journal bearings. Figure 4b shows mm. The curvatures of the shaft and bearing are adjusted to equal 1.6 per mill clearance in 360° a corresponding photograph of the test rig with implemented bearing segment test configuration journal bearings. Figure 4b shows a corresponding photograph of the test rig with implemented in a loaded condition as well as a CAD drawing of the test set up including shaft and bearing shell bearing segment test configuration in a loaded condition as well as a CAD drawing of the test set up specimen holders. The depiction shows several sensors applied, measuring the ongoing sliding and including shaft and bearing shell specimen holders. The depiction shows several sensors applied, tribological processes during testing in detail. This includes two thermocouples, one installed at measuring the ongoing sliding and tribological processes during testing in detail. This includes two the back of a bearing shell and one placed on the bottom of the oil bath, which is used for heating thermocouples, one installed at the back of a bearing shell and one placed on the bottom of the oil control of the set up by four heating rods. Analogously to the RoD set up, friction is deduced by bath, which is used for heating control of the set up by four heating rods. Analogously to the RoD set torque measurement through the load cell located on the left side of the test cell (considering the up, friction is deduced by torque measurement through the load cell located on the left side of the distances from the load cell to the symmetrical axis of the contact) contacting with the pivot mounted test cell (considering the distances from the load cell to the symmetrical axis of the contact) oil bath assembly. This measurement principle enables precise detection of the friction resistances of contacting with the pivot mounted oil bath assembly. This measurement principle enables precise the contacts between bearing shells and shaft and does not record any dead loads or dragging forces detection of the friction resistances of the contacts between bearing shells and shaft and does not from any supporting structure. Normal load is applied by means of an air bellow and transmitted record any dead loads or dragging forces from any supporting structure. Normal load is applied by to the contact by two lever arms. Also for this set up, an electrical contact resistance measurement means of an air bellow and transmitted to the contact by two lever arms. Also for this set up, an method exists along with acoustic emission recording possibilities. Furthermore, a lubricant pressure electrical contact resistance measurement method exists along with acoustic emission recording sensor can be implemented through a drilled hole in one of the bearing shell specimen holders (high possibilities. Furthermore, a lubricant pressure sensor can be implemented through a drilled hole in loaded region) giving additional insight into the contact situation by visualization of the transition one of the bearing shell specimen holders (high loaded region) giving additional insight into the between boundary and mixed friction regime. The range of set and measured values are given below: contact situation by visualization of the transition between boundary and mixed friction regime. The range of values: set and normal measured values given below: • Set load (0–10are kN), sliding speed (0–6000 rpm), temperature level (20–200 ◦ C) ◦ C), oil bath Measured friction torque (0–15 Nm), •• Set values: values: normal load (0–10 kN), sliding speedbearing (0–6000back rpm),temperature temperature(0–250 level (20–200 °C) ◦ temperature (0–250friction C), contact potential (0–51 mV), lubricant pressure (up(0–250 to 300 bar), acoustic • Measured values: torque (0–15 Nm), bearing back temperature °C), oil bath emission (50 (0–250 kHz–400 kHz) temperature °C), contact potential (0–51 mV), lubricant pressure (up to 300 bar), acoustic

emission kHz–400 kHz)can be carried out up to 10 Hz for all previously listed parameters. Standard (50 data acquisition In addition, COF, the sliding the out normal and CP signal canparameters. be measured Standardthe data acquisition can speed, be carried up toloading, 10 Hz for all the previously listed In precisely by acquisition up speed, to 10 kHz. This canloading, be used and for instance visualize quick events, addition, thedata COF, the sliding the normal the CP to signal can be measured such as start and stop events of precisely by data acquisition upantoengine. 10 kHz. This can be used for instance to visualize quick events, such as start and stop events of an engine.


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Figure 4. Bearing test methodology: methodology: (a) (a) Set Set up up and and test test area/skills; area/skills; (b) Figure 4. Bearing segment segment test (b) Implementation Implementation of of the the set up on a tribometer test rig TE92. CP = electrical resistance measurement. set up on a tribometer test rig TE92. CP = electrical resistance measurement.

3. Results 3. Results 3.1. 3.1. Visualization Visualization of of Friction Friction Figure highlights the Figure 55 highlights the visualization visualization possibilities possibilities of of the the presented presented test test methodologies methodologies regarding regarding friction depict results results obtained obtained with with friction performance performance of of journal journal bearing bearing systems, systems, whereby whereby Figure Figure 5a,b 5a,b depict the bearing segment system and Figure 5c,d show results obtained with a RoD set up. As shown in the bearing segment system and Figure 5c,d show results obtained with a RoD set up. As shown in Figure 5a, the bearing segment test set up precisely visualizes differences of the friction performance Figure 5a, the bearing segment test set up precisely visualizes differences of the friction performance induced variation of the bearing bearing material. material. The been recorded recorded during during start start induced by by variation of the The test test data data shown shown have have been 22, 100 °C ◦ stop testing (cycle number 1800) with a constant load of 1.64 N/mm oil bath temperature, stop testing (cycle number 1800) with a constant load of 1.64 N/mm , 100 C oil bath temperature, and speed ramp. ramp. Basically, bearing materials, lubrication regimes are and aa 55 ss speed Basically, for for both both bearing materials, the the states states of of lubrication regimes are exemplified. highlight the boundary friction indicates exemplified. The The plateaus plateaus of of µ µ highlight the boundary friction regime. regime. The The decrease decrease of of µ µ indicates mixed lubrication regimes with significant hydrodynamic friction share. The reach of a minimum mixed lubrication regimes with significant hydrodynamic friction share. The reach of a minimum value value and a re-increase of the coefficient friction coefficient the transition the friction full fluid friction and a re-increase of the friction describe describe the transition to the fulltofluid condition. condition. This is also emphasized by the CP signal, which moves up, thus indicating the separation This is also emphasized by the CP signal, which moves up, thus indicating the separation of the of the metallic previously in contact. use of materials different materials affects, particularly, metallic surfacessurfaces previously in contact. The use The of different affects, particularly, boundary boundary and mixed friction sliding conditions. Material A represents a softer bimetal bearing and mixed friction sliding conditions. Material A represents a softer bimetal bearing (lining material (lining material and steel back) known for good emergency running capabilities. Material B is an and steel back) known for good emergency running capabilities. Material B is an optimized tri-layer, optimized tri-layer, also referred to as trimetal, bearing product (steel back, lining material, and also referred to as trimetal, bearing product (steel back, lining material, and overlay) with a polymer overlay) a polymer coating. It is seenlower that Material friction incompared a boundary coating. Itwith is seen that Material B provides friction inBaprovides boundarylower sliding regime to sliding regime compared to that of Material A, but, overall, the Stribeck curve generated that of Material A, but, overall, the Stribeck curve generated with Material B is more expanded towith the Material is more expanded to be theplausibly right side. The with notedthe effects can properties. be plausiblyMaterial linked with the right side.B The noted effects can linked material B shows material properties. Material B shows low solid friction properties, but has a higher surface low solid friction properties, but has a higher surface roughness, which requires running in to shift the roughness, which requires running inobtained to shift the Stribeck curve to the left. the obtained data Stribeck curve to the left. Hence, the data precisely describes theHence, tribological phenomena precisely tribological phenomena within the the impact contactsusing taking place. Figure 5b viscosities highlights within thedescribes contacts the taking place. Figure 5b highlights various lubricant the impact using various lubricant viscosities on friction characteristics. As seen, the boundary on friction characteristics. As seen, the boundary sliding regime with the plateau of µ is not affected. sliding regime with the plateau of µ is not affected. With the increase of sliding speed and With the increase of sliding speed and transition to mixed friction, the impact is seen. Withtransition lower oil to mixed friction, thefriction impactasiswell seen.asWith lower oil viscosity, boundary friction as well higher as the mixed viscosity, boundary the mixed friction regime are expanded towards speed. friction regime are expanded towards higher speed. During fluid friction, higher lubricant viscosity During fluid friction, higher lubricant viscosity implies higher friction losses. implies friction losses. Thehigher test set ups are also capable of visualizing systematical effects on the friction performance The test set ups are also capable of effectsthe ondifferences the frictionof performance deriving from material optimization. Invisualizing this regard,systematical Figure 5c shows a common deriving from material optimization. In this regard, Figure 5c shows the differences of a common AlSn20+ bimetal bearing and an advanced AlSn25+ bearing alloy with an enhanced microstructure AlSn20+ bearing and advanced AlSn25+ bearing antin enhanced obtainedbimetal with RoD set up. It an is clearly visible that the alloy alloy with with higher content microstructure and improved obtained with RoD set up. It is clearly visible that the alloy with higher tin content and improved structure shows a better friction performance at low speed when the solid friction share prevails. structure shows a better friction performance at low speed when the solid friction share prevails. Furthermore, the effect of additive tribofilms, owing to interactions of the mating surfaces with the engine oil additive system, on the friction performance under mixed friction sliding conditions can be determined, see Figure 5d. In this test procedure, load (3 MPa) and speed (1.4 m/s) are kept


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Furthermore, the effect of additive tribofilms, owing to interactions of the mating surfaces with 5, 47 7 of 16 theLubricants engine2017, oil additive system, on the friction performance under mixed friction sliding conditions can be determined, see Figure 5d. In this test procedure, load (3 MPa) and speed (1.4 m/s) are kept constant while the heating level has been set as depicted. Oil 1 represents a fully formulated heavy constant while the heating level has been set as depicted. Oil 1 represents a fully formulated heavy duty diesel engine oil, whereas in the case of Oil 2, a surface active additive compound is missing duty diesel engine oil, whereas in the case of Oil 2, a surface active additive compound is missing with with the rest of the formulation being equal. It can be noted that the system with tribofilms forming the rest of the formulation being equal. It can be noted that the system with tribofilms forming based based on the use of Oil 2 provides significantly lower friction losses. Hence, this visualizes the on the use of Oil 2 provides significantly lower friction losses. Hence, this visualizes the possibility to possibility to investigate particular changes of the oil additive system on friction performance of investigate particular changes of the oil additive system on friction performance of bearing systems. bearing systems.

Figure 5. Tribometric test plots focusing on friction performance: (a) Bearing segment test system Figure 5. Tribometric test plots focusing on friction performance: (a) Bearing segment test system showing the effect of various bearing materials; (b) Bearing segment test system showing the effect of showing the effect of various bearing materials; (b) Bearing segment test system showing the effect of different oil viscosities; (c) Friction curves obtained with ring-on-disc (RoD) set up showing different oil viscosities; (c) Friction curves obtained with ring-on-disc (RoD) set up showing differences differences owing to specific material optimization; (d) Mixed friction performance with different oil owing to specific material optimization; (d) Mixed friction performance with different oil additive additive formulation obtained with RoD set up. formulation obtained with RoD set up.

3.2. Assessment of Tribological Damage Mechanisms 3.2. Assessment of Tribological Damage Mechanisms Friction and wear processes are directly connected with each other within tribological systems. Friction andsection wear processes connected withprocesses each other within tribological systems. The following deals withare thedirectly elucidation of damage occurring in journal bearing Thesystems following section deals with the elucidation of damage processes occurring in journal bearing by using the test methodologies herein proposed. systems by using the test methodologies herein proposed. 3.2.1. Evaluation of Wear Processes 3.2.1. Evaluation of Wear Processes Generally, both systems are superbly capable of evaluating wear processes going on in journal Generally, both systems superbly capable of regarding evaluating wear processes ontolerance in journal bearing systems. Figure 6a,bare show the material effect wear resistance andgoing failure duringsystems. mixed Figure friction 6a,b sliding. data effect have regarding been determined with theand RoD set up in a bearing showThe the test material wear resistance failure tolerance temperature test procedure (loadtest and speed constant as shown). 6aup a system using a during mixed friction sliding. The data haveare been determined with In theFigure RoD set in a temperature free bronze is depicted. The plot a running in 6a process at room (RT) testlead procedure (loadlining and speed are constant asshows shown). In Figure a system usingtemperature a lead free bronze and during the initial heating step indicated by the drop of µ. At elevated temperatures of 100 °C,the lining is depicted. The plot shows a running in process at room temperature (RT) and during ◦ stable sliding conditions with low µ, high CP, which indicates the formation of boundary layers and initial heating step indicated by the drop of µ. At elevated temperatures of 100 C, stable sliding protective interface processes and no wear evolution during this period, takes place (the reader may note that during the heating period the wear graph is influenced by the temperature changes).


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conditions with low µ, high CP, which indicates the formation of boundary layers and protective interface that Lubricantsprocesses 2017, 5, 47 and no wear evolution during this period, takes place (the reader may note 8 of 16 during the heating period the wear graph is influenced by the temperature changes). However, during during the ◦heating step to 140 °C, the vulnerable system behaviorShortly is emphasized. Shortlythe theHowever, heating step to 140 C, the vulnerable system behavior is emphasized. after starting after heating starting the second period, the minor changes of temperature in the start of The wearCP second period, theheating minor changes of temperature result in the start result of wear processes. processes. The CP signal drops at first, followed by a minor increase of µ as well as a slight raise in signal drops at first, followed by a minor increase of µ as well as a slight raise in the wear graph. When the wear graph. When the CP reaches its minimum, severe wear processes start, indicated by high the CP reaches its minimum, severe wear processes start, indicated by high wear rates, high µ, and wear rates, high µ, and an increase of friction heat. In contrast, the similar system using a lead free an increase of friction heat. In contrast, the similar system using a lead free bronze lining but coated bronze lining but coated with a polymer overlay is shown in Figure 6b. During the whole test with a polymer overlay is shown in Figure 6b. During the whole test procedure, no significant wear procedure, no significant wear events take place. This is documented by the various measured events take place. This is documented by the various measured parameters and verified by gravimetric parameters and verified by gravimetric wear measurement. The relative wear height is influenced wear measurement. The relative wear height is influenced by the heating situation, which changes the by the heating situation, which changes the capacitive signal. Therefore, a wear graph, which only capacitive signal. Therefore, a wear graph, which only follows the temperature level, indicates that follows the temperature level, indicates that no significant wear processes take place. Furthermore, it nocan significant processes take place.heat Furthermore, canCP besignal seen that no during significant friction heat be seen wear that no significant friction is recorded,itthe is high the whole test, is recorded, the CP signal is high test, example and µ is constant and low after running and µ is constant and low afterduring runningthe in.whole A further of the visualization capability of in. A further example of the visualization capability of sliding and wear processes of the RoD set up sliding and wear processes of the RoD set up is documented by the authors in [20], focusing on the is documented byofthe authors [20], focusing on the functionality of AlSi sliding material. functionality AlSi slidinginmaterial.

Figure 6. Elucidation of wear processes: (a) Mixed friction sliding of lead free bronze lining Figure 6. Elucidation of wear processes: (a) Mixed friction sliding of lead free bronze lining determined determined with RoD set up; (b) Mixed friction sliding of lead free bronze lining coated with a with RoD set up; (b) Mixed friction sliding of lead free bronze lining coated with a polymer overlay polymer overlay determined with RoD set up; (c) Start stop wear evolution of two bearing materials determined with RoD set up; (c) Start stop wear evolution of two bearing materials obtained with obtained with bearing segment (BS) system; (d) Comparison of bearing shells after start stop testing bearing segment (BS) system; (d) Comparison of bearing shells after start stop testing for similar for similar test conditions. test conditions.

Furthermore, wear processes induced by start stop motion can be replicated by the herein Furthermore, wear processes induced motion can be replicated by the (1.64 herein presented test configurations. In Figure 6c, by thestart wearstop evolution during start stop conditions MPa, 120 °Cconfigurations. oil bath temperature, 0–1.25 m/swear in 5 evolution s) obtainedduring with the segment set upMPa, is presented test In Figure 6c, the startbearing stop conditions (1.64 ◦ given. In this regard, two different materials are shown. In the case of the lead trimetal bearing, a 120 C oil bath temperature, 0–1.25 m/s in 5 s) obtained with the bearing segment set up is given. pronounced weardifferent evolution after short of cycles (below settles. bearing, In contrast, it can be In this regard, two materials are number shown. In the case of the20,000) lead trimetal a pronounced highlighted advanced polymer coating instead of the lead coating the start wear evolutionthat afterusing shortan number of cycles (below 20,000) settles. In contrast, it canimproves be highlighted that stop wear resistance crucially. The differences emphasized by the test data are confirmed by surface using an advanced polymer coating instead of the lead coating improves the start stop wear resistance analysis shown in Figure 6c. In this depiction, the conditions of two bearing shells with lead and polymer coating are compared with each other after a similar number of start stop cycles. The wear


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crucially. The differences emphasized by the test data are confirmed by surface analysis shown in Figure 6c. In this depiction, the conditions of two bearing shells with lead and polymer coating are compared with Lubricants 2017,each 5, 47 other after a similar number of start stop cycles. The wear track of9 the of 16 lead coating is clearly visible, whereas the polymer coating appears close to new conditions. Also elsewhere, track of the coating is clearly visible, whereas thewear polymer coatingis appears close to new the superiority of lead polymer overlays concerning start stop resistance documented [21], which conditions. Also elsewhere, the superiority of polymer overlays concerning start stop wear proves the validity of the results herein presented. A detailed study by the authors concerning start resistance is documented [21], which proves the validity of the results herein presented. A detailed stop wear of bearing systems using the herein applied bearing segment system is published in [22]. study by the authors concerning start stop wear of bearing systems using the herein applied bearing Furthermore, the tribo-chemical wear processes can be resolved. Figure 7 depicts the effect of segment system is published in [22]. corrosive wear on lead freetribo-chemical bronze bearing material as well asresolved. the prevention Furthermore, the wear processes can be Figure 7thereof depictsby theusing effectspecific of corrosion inhibitors (CIs). In the case of Oil C, a corrosion layer containing S and Zn forms on the corrosive wear on lead free bronze bearing material as well as the prevention thereof by using bronze material, see Figure 7a. This alsoofvisible the naked eyecontaining as emphasized in forms Figure 7b. specific corrosion inhibitors (CIs).effect In theiscase Oil C, abycorrosion layer S and Zn the bronze material, seeand Figure 7a. conditions, This effect ishighlighted also visible by naked as emphasized in Theseon processes affect sliding wear in the Figure 7ceye showing a temperature Figure 7b. Thesewith processes affectspeed slidingatand highlighted Figure 7c showing a the step test procedure constant 1.4wear m/sconditions, and 3 MPa loading. in The friction curve of temperature step test procedure with constant speed at 1.4 m/s and 3 MPa loading. The friction system using Oil C rises at elevated temperature, thus indicating adhesive processes. In contrast, curve of the system using Oil C rises at elevated temperature, thus indicating adhesive processes. In optimized lubricant design with corrosion inhibiting components (Oil D) results in prevention of this contrast, optimized lubricant design with corrosion inhibiting components (Oil D) results in layer formation, see Figure 7b. The resulting wear performance indicates a stable sliding condition prevention of this layer formation, see Figure 7b. The resulting wear performance indicates a stable pairedsliding with low friction losses. condition paired with low friction losses.

Figure 7. Tribo-corrosive events during mixed friction sliding obtained with RoD set up: (a) Light

Figure 7. Tribo-corrosive events during mixed friction sliding obtained with RoD set up: (a) Light microscopy surface picture of corrosive layer on lead free bronze; (b) Comparison of bronze disc microscopy surface picture of corrosive layer on lead free bronze; (b) Comparison of bronze disc specimen tested with different oil formulations with Oil D containing special corrosion protector specimen tested different oil formulations with Oil D containing special corrosion protector additives; (c) with Tribometric data. additives; (c) Tribometric data. The conclusions and knowledge acquired with testing find their application in the improvement andand development productswith as well as in thetheir numerical life time analysis and The conclusions knowledgeofacquired testing find application in the improvement product design. The cornerstone of numerical wear assessment was laid by Archard and Hirst [23]. and development of products as well as in the numerical life time analysis and product design. The authors proposed the model according to Equation (1), which correlates properties of the mating The cornerstone of numerical wear assessment was laid by Archard and Hirst [23]. The authors surfaces of journal bearing and shaft with the external load P:

proposed the model according to Equation (1), which correlates properties of the mating surfaces of × × journal bearing and shaft with the external load = P: (1) K×s×P

W =s the sliding distance, pm the flow pressure of the softer (1) In this equation, W is the worn volume, pm material, and K a material-related empirical constant describing the probability of the formation of a In this particle. equation,The W istransformation the worn volume, s the sliding distance, pm the flow pressureequation, of the softer wear of Equation (1) into an ordinary differential material, and K a material-related empirical constant describing the probability of the see Equation (2), provides the mathematical base for the numerical wear evaluation, where wformation is the wear height, p the The asperity contact pressure, the sliding(1) speed, is an empirically determined of a wear particle. transformation of vEquation intoand an Cordinary differential equation, parameter(2), thatprovides describesthe themathematical relation between wear and the introduced frictional where energy w is see Equation base forvolume the numerical wear evaluation, during sliding contact [24]. pressure, The stepsvbehind this speed, transformation the division of the wear height, p thesolid asperity contact the sliding and C is include an empirically determined Equation (1) by the nominal contact area and the derivative with respect to time. Consequently, the parameter that describes the relation between wear volume and the introduced frictional energy during normal load P transforms to a pressure which is correlated to the asperity contact pressure to sliding solid contact [24]. The steps behind this transformation include the division of Equation (1) evaluate solely the effects resulting from asperity interactions. The time derivative transforms s into by thev nominal contact area and the derivative with respect to time. Consequently, the normal load while C unites the material intrinsic parameters K/pm.

P transforms to a pressure which is correlated to the asperity contact pressure to evaluate solely the =

×

×

(2)


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effects resulting from asperity interactions. The time derivative transforms s into v while C unites the material intrinsic parameters K/pm . ∂w = C×p×v (2) ∂t In recent years, numerical tools have been developed at the Chair of Mechanical Engineering which allow the investigations of conforming and non-conforming contacts in regard to friction and wear phenomena. The tribological simulations of (thermo-)elastohydrodynamic contacts, for e.g., Lubricants 2017, 5, 47 10 of 16 gear tooth contact and journal bearing, are conducted with COMSOL Multiphysics. By considering Equation (2), wearInand itsyears, effect on the gap’satgeometry can be modelled recent numerical toolslubrication have been developed the Chair of Mechanical Engineering fully, coupled which allow the investigations of conforming and non-conforming contacts in regard to friction and with the equations describing the physics of (T)EHL. For presentation purposes, a numerical scenario wear phenomena. The tribological simulations of (thermo-)elastohydrodynamic contacts, for e.g., derived from conducted tests thebearing, BS was TheCOMSOL modelMultiphysics. of the core parts of BS is depicted gear tooth contact andon journal are chosen. conducted with By considering (2), wear and its effect on the lubrication gap’s canapplied be modelled coupled in Figure 8. In Equation accordance with the real test setting, thegeometry force is onfully, boundary A while the with the equations describing the physics of (T)EHL. For presentation purposes, a numerical Reynolds equation, the equation representing asperity contact pressure according to Greenwood and scenario derived from conducted tests on the BS was chosen. The model of the core parts of BS is depicted in Figure 8. In accordance with the real test setting, the force is applied on boundary A Williamson [25], as well as Equation (2) are solved on boundary B, allowing for the resolution of friction while the Reynolds equation, the equation representing asperity contact pressure according to and wear processes. The contact behavior represented by an analytical function calculated based on Greenwood and Williamson [25], as well as Equation (2) are solved on boundary B, allowing for the the theory by Greenwood and and Williamson is considered unaffected bybyroughness changes due to wear resolution of friction wear processes. The contact behavior represented an analytical function calculated based on the theory by Greenwood and Williamson is considered unaffected by for the entire numerical scenario. The effect of wear is considered in the change of the lubrication roughness changes due to wear for the entire numerical scenario. The effect of wear is considered in gap’s geometry.theInchange interplay with thegap’s initial lubrication gap and thegap structural of the lubrication geometry. In interplay withheight the initialh0lubrication height h0 deformation and the structural deformation u, the wear height determines the lubrication gap’s height. C, and boundaries to u, the wear height determines the lubrication gap’s height. C, D, and E representD,the E represent the boundaries to the surrounding structure. The guiding function of the surrounding the surrounding structure. The guiding function of the surrounding structure by the structure is considered by the application of roller conditions, which restrict deformationisinconsidered the boundary’s normal direction. application of roller conditions, which restrict deformation in the boundary’s normal direction.

Figure 8. ModelFigure and8.boundaries for thefornumerical assessment. Boundary action of external Model and boundaries the numerical wear wear assessment. Boundary A: action of A: external boundary B: application of Reynolds, asperity contact, and wear equations; boundaryboundary C, D, force; boundaryforce; B: application of Reynolds, asperity contact, and wear equations; C, D, and and E: roller conditions representing the guidance of the surrounding structure. E: roller conditions representing the guidance of the surrounding structure. A representative numerical scenario based on conducted tests is depicted in Figure 9. A value of 1.4 × 10−13 [m3/Nm] is chosen for C. The thermal conditions are considered to be constant. For the A representative numerical scenario based on conducted testsframeworks is depicted 9. A value quantitative comparison with conducted tribological tests, the numerical allowsin forFigure the − 13 3 implementation of temperature dependent oil parameters. In the initial state, the system operates in of 1.4 × 10 [m /Nm] is chosen for C. The thermal conditions are considered to be constant. For the fluid lubrication regime under a low external load of Fext = 1500 N, a unit load of 1.65 MPa, and a the quantitativesliding comparison with numerical frameworks allows for speed v of 0.37 m/s,conducted which is kept tribological constant until t =tests, 100 s. Fthe ext is totally carried by the force resulting from the hydrodynamic pressure F hyd . By increasing F ext the system is steadily the implementation of temperature dependent oil parameters. In the initial state, pushed the system operates towards harsher loading conditions. The system counteracts the increasing load by decreasing the in the fluid lubrication regime under a low external load of F = 1500 N, a unit load of 1.65 MPa, ext fluid film thickness, which results in an increased hydrodynamic load carrying capacity. However, and a sliding speed v of 0.37 which is kept constant until t = 100 s. Fext when the fluid film m/s, thickness drops within the range of surface roughness, asperities startistototally interact, carried by the resulting in a rising Fasp. At 40 s, the system turns into the mixed lubrication regime indicated by the force resulting from the hydrodynamic pressure Fhyd . By increasing Fext the system is steadily pushed onset of the asperity contact force Fasp. Simultaneously, the wear volume starts to rise and depicts an towards harsher loading conditions. The increasing loadof by exponential run until the time of 100 system s at whichcounteracts start stop cyclesthe begin. During the times lowdecreasing the speeds, which the hydrodynamic carrying capacity hydrodynamic is insufficient. Consequently, Fasp contributes to fluid film thickness, results load in an increased load carrying capacity. However, help carry the external load. Within this period of time, wear due to solid contact occurs and a rise of when the fluid film thickness drops withinDuring the range of of surface roughness, the wear volume can be observed. the phases sufficient hydrodynamicasperities load carryingstart to interact, capacity due. to slidingturns speed,into the total is carried solely hydrodynamically resulting in a rising Fasp Ata sufficiently 40 s, the high system theload mixed lubrication regime indicated by the while asperity contact is absent and the cumulative measure of the wear volume remains constant.

onset of the asperity contact force Fasp . Simultaneously, the wear volume starts to rise and depicts an exponential run until the time of 100 s at which start stop cycles begin. During the times of low speeds, the hydrodynamic load carrying capacity is insufficient. Consequently, Fasp contributes to help carry the external load. Within this period of time, wear due to solid contact occurs and a rise of the wear volume can be observed. During the phases of sufficient hydrodynamic load carrying capacity due to a sufficiently high sliding speed, the total load is carried solely hydrodynamically while asperity contact is absent and the cumulative measure of the wear volume remains constant.


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Figure 9. Quantities of the numerical wear scenario.

Figure 9. Quantities of the numerical wear scenario. Hence, the developed numerical methodology in COMSOL Multiphysics represents a significant and effective tool for the assessment of wear due to solid contact in tribological contacts. Hence, the developed numerical methodology in COMSOL Multiphysics represents a significant A promising tool for the evaluation of wear was developed whereupon the incorporation of suitable and effective tool for the assessment of wear due to solid contact in tribological contacts. A promising test data and the comparison between numerical and test results is the subject of current tool for theinvestigations. evaluation of wear was developed whereupon the incorporation of suitable test data and

the comparison between numerical and test results is the subject of current investigations. 3.2.2. Seizure Performance Screening

3.2.2. Seizure Performance Screening This section elucidates the capability of the presented test methodologies to visualize emergency running properties of journal bearing systems and demonstrates the potentialities of the

This section the capability of the test of methodologies visualize emergency obtainedelucidates experimental results contributing to presented the optimization bearing systems to concerning this running properties of journal bearingproperties systems and andseizure demonstrates the are potentialities of the aspect. In principle, emergency performances tested according to obtained standard test procedures by means of load tests by increasing the normalsystems loading either linearly or in experimental results contributing to the optimization of bearing concerning this aspect. a step wise manner [26]. Hence, the systems are dragged into severe sliding conditions, such as may In principle, emergency properties and seizure performances are tested according to standard test occur during application operation, and the ability to withstand this stressing is detected. procedures byFigure means load teststhebyexperimental increasingresolution the normal loadingprocesses either linearly or in a step wise 10 of demonstrates of tribological during emergency running conditions of the ring-on-disc set up (Figure 10a) and theconditions, bearing segment (Figure manner [26]. Hence, the systems are dragged into severe sliding suchsystem as may occur during 10b). For the given examples, a characteristic trimaterial bearing, composed of a steel back, a lead application operation, and the ability to withstand this stressing is detected. bronze lining, a Ni barrier, and an AlSn sputtered coating, has been tested in combination with Figure 10 demonstrates the experimental resolution of tribological processes during emergency 34CrNiMo6 shaft material and fully formulated heavy duty diesel engine oil. running conditions of systems, the ring-on-disc set of upsliding (Figure 10a)temperature, and the bearing segment system (Figure 10b). For both upon the start at room the coefficient of friction starts to decrease. The decrease of the COF trimaterial can be attributed to running-in of the mating surfaces anda the For the given examples, a characteristic bearing, composed of a steel back, lead bronze of and the viscosity due to heating. coating, The contact resistance raises immediately at the beginning lining, a Nidecrease barrier, an AlSn sputtered has been tested in combination with 34CrNiMo6 in both cases. At the RoD set up, the CP signal stabilizes for the given mixed friction regime due to shaft material and fully formulated heavy duty diesel engine oil. formation of isolating oxide and boundary layers. In the case of the bearing segment system, For both systems, and upon thecontact start of slidingvie at for room temperature, the coefficient friction starts hydrodynamic solid situations domination and, subsequently, the CPofsignal shows more hecticofbehavior (thecan reader note thattotesting seizure performance in thesurfaces case of and the to decrease. Theadecrease the COF be may attributed running-in of the mating bearing segment system requires bearing shells with a smaller area of contact, and owing thisbeginning decrease ofthethe viscosity due to heating. The contact resistance raises immediately attothe aspect and the assembly of the test configuration, the base voltage drop is higher compared with in both cases. At the RoD set up, the CP signal stabilizes for the given mixed friction regime due RoD). At the beginning of the seizure tests, the temperature is heated up to a 100 °C steel specimen to formation of isolating oxide boundary layers. In bearing the case of thesetbearing segment temperature for RoD and and oil bath temperature for the segment up. Further heat system, development can be attributed to friction heat. the further courses of the test plots demonstrate, hydrodynamic and solid contact situations vie forAs domination and, subsequently, the CP signal shows bothbehavior set ups are(the ablereader to highly resolve subsequent processes including scuffing a more hectic may notethethat testing tribological seizure performance in thethe case of the bearing process of the two friction partners. In the case of RoD, µ stays low and constant during the load segment system requires bearing shells with a smaller area of contact, and owing to this aspect and stages up to 13 MPa. This can be traced back to the good sliding properties of the Al based coating. A the assembly of insight the test configuration, the base voltage drop is by higher compared with RoD). At the deeper into the tribological processes going on is provided the wear and CP signal. First it ◦ C steel to beseizure noted that the initial increase of the response of the sensorspecimen is based on temperature the beginning has of the tests, the temperature is heated upcapacitive to a 100wear for RoD and oil bath temperature for the bearing segment set up. Further heat development can be attributed to friction heat. As the further courses of the test plots demonstrate, both set ups are able to highly resolve the subsequent tribological processes including the scuffing process of the two friction partners. In the case of RoD, µ stays low and constant during the load stages up to 13 MPa. This can be traced back to the good sliding properties of the Al based coating. A deeper insight into the tribological processes going on is provided by the wear and CP signal. First it has to be noted that the initial increase of the response of the capacitive wear sensor is based on the external heating, which affects this measurement principle. However, thereafter, under almost constant temperature

Lubricants 2017, 5, 47 Lubricants 2017, 5, 47

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externala heating, which affectsraise this of measurement principle. However, thereafter,minor underwear almost conditions, slight and constant the wear graph is seen, thus indicating events, constant temperature a slightThe andCP constant raise ofex-ante the wear graph seen, thus apparently assigned to theconditions, bearing overlay. signal drops hinting at is imminent failure indicating minor wear events, apparently assigned to the bearing overlay. The CP signal drops events. Shortly after, the wear rate increases. At this point, µ as well as the system temperatures are Lubricants 2017,at 5, imminent 47 12 of ex-ante hinting failure events. Shortly after, the wear rate increases. At this point, µ 16 as still low. Hence, a catastrophic failure has not yet occurred. However, afterwards, scuffing takes place, well as the system temperatures are still low. Hence, a catastrophic failure has not yet occurred. thus raising µ afterwards, and the corresponding friction heat, which requires a stopthereafter, of the movement and the external heating, which affects measurement under However, scuffing takesthis place, thus raisingprinciple. µ and theHowever, corresponding friction heat,almost which constant temperature conditions, a slight and of constant raiseisofprovided the wear graph seen, thus test. requires Similar resolution of movement severe failure bearings the is bearing a stop of the and processes the test. Similar resolution of severeby failure processessegment of indicating minor wear events, apparently assigned to the bearingduring overlay.stable The CP signal conditions. drops system. In contrast to the RoD test, small spikes of µ are detected sliding bearings is provided by the bearing segment system. In contrast to the RoD test, small spikes of µ are ex-ante hinting at imminent failure events. Shortly after, the wear rate increases. At this point, µ as Thesedetected events during exhibitstable the system whenevents dragged from conditions to severe sliding properties conditions. These exhibit thehydrodynamic system properties when dragged well as the system temperatures are still low. Hence, a catastrophic failure has not yet occurred. from hydrodynamic conditions to severe boundary sliding conditions. In many the casesmall of thespikes hard sputter boundary sliding conditions. In the case of the hard sputter overlay, are However, afterwards, scuffing takes place, thus raising µ and the corresponding friction heat, whichnoted. overlay, spikes are noted. and Similar to theSimilar RoD setresolution up, the evolution offailure CP, which Similar torequires themany RoD set up, the movement evolution of CP, slightly decreases several load stepsslightly before a small stop of the the which test. of severe processes of the decreases several load steps before the scuffing, indicates the upcoming of failure events. The higher scuffing, bearings indicates the upcoming of failure events. The input of thespikes set up in is provided by the bearing segment system. In higher contrast energy to the RoD test, small of µresults are input during of the set upsliding resultsconditions. more heat development, by theproperties rise of thewhen bearing back stable These events back exhibitnoted the system dragged moreenergy heatdetected development, noted by theinrise of the bearing temperature, as well as the more rapid temperature, as well as the more rapid scuffing processes, which result in of CP drop and from hydrodynamic conditions severe boundary sliding conditions. In an theoverlap caseincrease. of the hard sputter scuffing processes, which result in antooverlap of CP drop and µ/temperature µ/temperature increase. overlay, many small spikes are noted. Similar to the RoD set up, the evolution of CP, which slightly

decreases several load steps before the scuffing, indicates the upcoming of failure events. The higher energy input of the set up results in more heat development, noted by the rise of the bearing back temperature, as well as the more rapid scuffing processes, which result in an overlap of CP drop and µ/temperature increase.

Figure 10. Tribometric seizure test plots: (a) RoD test set up using AlSn trimetal bearing; (b) Bearing

Figure 10. Tribometric seizure test plots: (a) RoD test set up using AlSn trimetal bearing; (b) Bearing segment test set up using AlSn trimetal bearing. segment test set up using AlSn trimetal bearing.

Figure 11 10. depicts bearing andtestshaft conditions after showing scuffed Figure Tribometric seizure plots:surface (a) RoD test set up using AlSnseizure trimetal tests bearing; (b) Bearing conditions. This verifies the previously observed processes going on by the measurement quantities. Figure 11 depicts bearing shaft surface segment test set up usingand AlSn trimetal bearing. conditions after seizure tests showing scuffed The emergency capacity of the bearing overlay and the going protecting additives of the conditions. This verifies the previously observed processes on byanti-wear the measurement quantities. 11 depictsupon bearing and shaft surface load conditions seizure that testssoft showing lubricant Figure are overloaded reaching the seizure limit. after This means phasesscuffed of the The emergency capacity of the bearing overlay and the protecting anti-wear additives of the lubricant conditions. This verifies previously observedlayers processes going on by theare measurement quantities. overlay are consumed, andthe sacrificial anti-wear of the lubricants removed and fail to are overloaded upon reaching the seizure load limit. and Thisthe means that soft phases of the overlay are Thewith emergency of the bearing overlay protectingare anti-wear of 11a. the reform the samecapacity speed. The overlay wears off and the substrates exposed,additives see Figure consumed, and sacrificial anti-wear layers of the lubricants are removed and fail to reform with the overloaded upon reaching the seizure loadcounterpart limit. This means that soft phases of the The lubricant substratesare provide higher affinity towards the steel compared to that of the top samebearing speed. Theare overlay wears and the substrates areofexposed, see Figure 11a. The substrates overlay consumed, andoff sacrificial anti-wear layers the lubricants are removed fail to overlay. Subsequently, scuffing processes take place. These scuffing events resultand in galling reform with the same speed. The overlay wears off and the substrates are exposed, see Figure 11a. provide higher affinity towards the steel counterpart compared to that of the top bearing overlay. marks on the exposed substrate areas and material transfer on the steel shaft material, see Figure 11b. The substrates higher affinity towards thescuffing steel counterpart compared to that ofmarks the topon the Subsequently, scuffingprovide processes take place. These events result in galling bearing overlay. Subsequently, scuffing processes take place. These scuffing events result in galling exposed substrate areas and material transfer on the steel shaft material, see Figure 11b. marks on the exposed substrate areas and material transfer on the steel shaft material, see Figure 11b.

Figure 11. Surface structures after RoD seizure tests: (a) Bearing disc specimen; (b) Corresponding shaft counterpart. Figure 11. Surface structures after RoD seizure tests: (a) Bearing disc specimen; (b) Corresponding

Figure 11.shaft Surface structures after RoD seizure tests: (a) Bearing disc specimen; (b) Corresponding counterpart. shaft counterpart.


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The precise detection of tribological processes and high reproducibility of the test set ups can be precise detection of tribological processes and high reproducibility of theFigure test set12ups can usedThe for high quality screening with low scatter of various journal bearing systems. shows be used for high quality screening with low scatter of various journal bearing systems. Figure 12 seizure performance and emergency running properties of various journal bearing systems shows seizure performance and shaft emergency running properties of various journal bearing systems depending on bearing material, material, and lubricant; therefore, it outlines the possibilities depending on bearing material, shaft material, and lubricant; therefore, it outlines the possibilities for for specific studies in this area of interest by using the herein presented test methodologies (the data specific studies in this area of interest by using the herein presented test methodologies (the data have have been determined in seizure load step test procedures as shown in Figure 10a,b). Figure 12a been determined in of seizure load step test procedures shown inand Figure 10a,b). 12a depicts depicts the ranking several bearing materials, liningasmaterials, coatings to Figure withstand seizure the ranking of several bearing materials, lining materials, and coatings to withstand seizure up of to up to critical load values and verifies the obtained results by comparison with literature ranking critical load the obtained results by comparison with literature ranking bearing bearing rigs.values As forand theverifies lining materials, it can be highlighted that, unsurprisingly, AlSnof materials rigs. As for the lining materials, it can be highlighted that, unsurprisingly, AlSn materials possess possess higher emergency capability compared to that of harder lead free bronze material, which higher emergency compared of harder free bronze material, which matches matches relatively capability results known from to thethat literature. In lead the case of tribological coatings, usually relatively results known from the literature. In the case of tribological coatings, usually applied on applied on lining materials with insufficient tribological properties, the tested polymer overlay lining materials with insufficient tribological properties, the tested polymer overlay results in good results in good performance with the RoD set up. Furthermore, superior performance can be performance with the RoD set up.sputter Furthermore, be achieved with a novel Sn achieved with a novel Sn based coating,superior which isperformance the result ofcan a comprehensive optimization based sputter coating,awhich is thematerial result of with a comprehensive process to design a bearing process to design bearing enhanced optimization emergency properties and sufficient material with enhanced emergency properties and sufficient hydrodynamic load bearing capacity. hydrodynamic load bearing capacity.

Figure 12. Impacts on seizure performances of bearing systems: (a) RoD system—effect of bearing Figure 12. Impacts on seizure performances of bearing systems: (a) RoD system—effect of bearing material compared with data from [7]; (b) BS system—effect of bearing material compared with data material compared with data from [7]; (b) BS system—effect of bearing material compared with data from [2]; (c) RoD system—effect of oil viscosity; (d) RoD system—effect of additive formulation for from [2]; (c) RoD system—effect of oil viscosity; (d) RoD system—effect of additive formulation for various bearing materials; (e) BS system—effect of shaft counterpart. various bearing materials; (e) BS system—effect of shaft counterpart.

With the aid of the bearing segment system, reliable results are obtained. This is also verified by With the aid of the bearing segment system, reliable results are obtained. This is also verified by comparison with rankings from literature (Figure 12b). Highest seizure load resistance, which means comparison with rankings from literature (Figure 12b). Highest seizure load resistance, which means emergency running capability, is reached by using a lead based electroplated bearing. Bearing emergency running capability, is reached by using a lead based electroplated bearing. Bearing products products using Al based sputter coatings or polymer overlays provide lower seizure load limits using Al based sputter coatings or polymer overlays provide lower seizure load limits relative to the relative to the tested electroplated coating. tested electroplated coating. With the applied methodologies providing low measurement scatter, the effect of lubricant With the applied methodologies providing low measurement scatter, the effect of lubricant formulation (viscosity and additive formulation) on the emergency running performance of bearing formulation (viscosity and additive formulation) on the emergency running performance of bearing systems can be visualized. As seen, the robustness of various bearing systems tested is significantly systems can be visualized. As seen, the robustness of various bearing systems tested is significantly decreased using lower oil viscosities, shown in Figure 12c. Regarding additive formulation, for e.g., decreased using lower oil viscosities, shown in Figure 12c. Regarding additive formulation, for e.g., the decrease of Zincdialkyldithiophosphate (ZDDP) content affects the performance of AlSn bearing the decrease of Zincdialkyldithiophosphate (ZDDP) content affects the performance of AlSn bearing material negatively, whereas an optimized polymer coating is less sensitive to the decrease of protective lubricant additive amounts, see Figure 12d.


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material negatively, whereas an optimized polymer coating is less sensitive to the decrease of protective lubricant additive amounts, see Figure 12d. In addition, the influence of the shaft counterpart design (material/surface structures) can be investigated using the given test methodologies. In Figure 12e, a comparison of the performance of systems using a forged steel counterpart and systems using a cast iron counterpart is depicted. It can be seen that the forged steel shaft provides higher seizure resistance. The differences can be traced back to abrasion damages, induced by metal flaps with burrs around the graphite inclusions in the case of cast iron. However, by means of optimized surface treatment of the cast iron material, this negative effect can be eliminated. Further details can be found in a corresponding in-depth study by the authors published in [27]. 4. Summary The current paper outlines test possibilities of journal bearing systems on damage equivalent laboratory test methodologies on tribometer rigs by using specific research examples. It has been clearly emphasized that the proposed methodologies are able to highly resolve friction phenomena, wear processes, and seizure events of journal bearing systems on a reduced realism scale. In particular, the following effects have been demonstrated:

•

•

•

Impact of bearing material, viscosity level, and additive formulation on friction losses of bearing systems has been shown. In this regard, the difference between an AlSn bimetal bearing and an advanced polymer coated bearing and the difference between various viscosity levels and oils with and without surface active additives have been highlighted. Start stop wear performance of various bearing materials, prevention of mixed friction wear processes by specific bearing material design, as well as bearing wear prediction with the aid of combining simulation and testing have been presented. The possibility for in-depth studies of bearing seizure with secured transferability into real applications showing the impact of the used bearing material and the shaft counterpart, as well as the effect of viscosity level and additive design of engine oils, have been highlighted.

From the obtained results, it can be deduced that there is still immense potential in optimizing bearing systems in terms of higher tribological robustness as well as low friction performance. Acknowledgments: Financial support by the Austrian Federal Government (in particular from Bundesministerium für Verkehr, Innovation und Technologie and Bundesministerium für Wissenschaft, Forschung und Wirtschaft) represented by Österreichische Forschungsförderungsgesellschaft mbH and the Styrian and the Tyrolean Provincial Government, represented by Steirische Wirtschaftsförderungsgesellschaft mbH and Standortagentur Tirol, within the framework of the COMET Funding Programme is gratefully acknowledged. Furthermore, the support of Miba Gleitlager Austria GmbH and Infineum UK Ltd., by providing bearing material products and lubricants, is greatly acknowledged. Author Contributions: All three authors have contributed substantially to the work presented in this paper and made significant contributions concerning the development of the presented test methodologies. Additionally, Florian Summer conceived, designed, and conducted the experiments, wrote the main part of the paper, and analyzed the test data; Philipp Bergmann contributed to writing the paper as well as in proof reading the manuscript and conducted the simulations; Florian Grün consulted regarding the design of the experiments and interpretation of the test data and also proof read the manuscript. Conflicts of Interest: The authors declare no conflict of interest.


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Abbreviations BS CAD CI COF, µ CP HTHS RoD RT TEHL ZDDP C Fasp Fext Fhyd h0 K pm P, pn p s T u v V wear W Wr , w

Bearing segment Computer aided design Corrosion inhibitor Coefficient of friction Contact potential (contact resistance measurement) High-temperature high-shear viscosity Ring-on-disc Room temperature Thermoelastohydrodynamic lubrication Zincdialkyldithiophosphate Empirical constant [J/m3 ] Asperity load [N] External load [N] Hydrodynamic load [N] Initial lubrication gap height [µm] Empiric constant Flow pressure [MPa] External (normal) pressure [MPa] Asperity contact pressure [MPa] Sliding distance [m] Temperature [◦ C] Solid deformation [µm] Sliding speed [m/s] Wear volume [mm3 ] Worn volume [mm3 ] Wear height [µm]

References 1. 2. 3. 4. 5. 6. 7.

8.

9. 10.

Aufischer, R.; Hager, G.; Hamdard, K.; Offenbecher, M. Bearing Technology Combinations for Low Friction Cranktrains. MTZ Ind. 2016, 6, 56–63. [CrossRef] Aufischer, R. Lager in Verbrennungsmotoren. In Handbuch Verbrennungsmotor; Vieweg+Teubner: Wiesbaden, Germany, 2010; Volume 5. Affenzeller, J.; Gläser, H. Lagerung und Schmierung von Verbrennungsmotoren; Springer: Heidelberg, Germany, 1996. Czichos, H.; Habig, K.H. Tribologie–Handbuch: Tribometrie, Tribomaterialien, Tribotechnik, 3rd ed.; Vieweg & Teubner: Wiesbaden, Germany, 2010. Vencl, A.; Rac, A. Diesel engine crankshaft journal bearings failures: Case study. Eng. Fail. Anal. 2014, 44, 217–228. [CrossRef] Forstner, C.; Mairhofer, G. Application oriented bearing testing. In Proceedings of the 25th CIMAC World Congress on Combustion Engine Technology, Vienna, Austria, 21–24 May 2007. Aufischer, R.; Walker, R.; Offenbecher, M.; Hager, G. Modular Bearing Designs to Cope with the New Engine Designs Demanding High Performance, Lead-Free Solutions, and Robustness. J. Eng. Gas Turbines Power 2014, 136, 122505. [CrossRef] Mergen, R.; Gumpoldsberger, G.; Grün, F.; Godor, I.; Langbein, F. Aluminium-base bearings—Performance, limitations, new developments. In Proceedings of the 25th CIMAC World Congress on Combustion Engine Technology, Vienna, Austria, 21–24 May 2007. Sharma, S.C.; Hargreaves, D. A suitable method for journal bearing wear measurement. Ind. Lubr. Tribol. 2014, 66, 15–22. [CrossRef] Farfán-Cabrera, L.I.; Gallardo-Hernández, E.A. Wear evaluation of journal bearings using an adapted micro-scale abrasion tester. Wear 2017, 376, 1841–1848. [CrossRef]


11. 12. 13.

14. 15. 16. 17. 18. 19. 20.

21. 22. 23. 24. 25. 26. 27.

16 of 16

Kagohara, Y.; Takayanagi, S.; Haneda, S.; Fujita, M.; Iwai, Y. Tribological property of plain bearing with low frictional layer. Tribol. Int. 2009, 42, 1800–1806. [CrossRef] Feyzullahoglu, ˘ E.; S¸ akiroglu, ˘ N. The wear of aluminium-based journal bearing materials under lubrication. Mater. Des. (1980–2015) 2010, 31, 2532–2539. [CrossRef] Pondicherry, K.S.; Grün, F.; Gódor, I.; Bertram, R.; Offenbecher, M. Applicability of ring-on-disc and pin-on-plate test methods for Cu–steel and Al–steel systems for large area conformal contacts. Lubr. Sci. 2013, 25, 231–247. [CrossRef] Grün, F. Functionality of Heterogeneous Sliding Materials for Conformal Contacts. Habilitation Thesis, Montanuniversität Leoben, Leoben, Austria, 2012. Gebretsadik, D.W.; Hardell, J.; Prakash, B. Friction and wear characteristics of different Pb-free bearing materials in mixed and boundary lubrication regimes. Wear 2015, 340–341, 63–72. [CrossRef] Gebretsadik, D.W.; Hardell, J.; Prakash, B. Tribological performance of tin-based overlay plated engine bearing materials. Tribol. Int. 2015, 92, 281–289. [CrossRef] Furey, M.J. Metallic Contact and Friction between Sliding Surfaces. ASLE Trans. 1961, 4, 1–11. [CrossRef] Bergmann, P.; Grün, F.; Summer, F.; Godor, I.; Stadler, G. Expansion of the Metrological Visualization Capability by the Implementation of Acoustic Emission Analysis. Adv. Tribol. 2017, 2017, 17. [CrossRef] Grün, F.; Gódor, I.; Gärtner, W.; Eichlseder, W. Tribological performance of thin overlays for journal bearings. Tribol. Int. 2011, 44, 1271–1280. [CrossRef] Grün, F.; Summer, F.; Pondicherry, K.S.; Gódor, I.; Offenbecher, M.; Lainé, E. Tribological functionality of aluminium sliding materials with hard phases under lubricated conditions. Wear 2013, 298–299, 127–134. [CrossRef] Adam, A.; Prefot, M.; Wilhelm, M. Crankshaft bearings for engines with start-stop systems. MTZ Worldw. 2010, 71, 22–25. [CrossRef] Summer, F.; Grün, F.; Offenbecher, M.; Taylor, S.; Lainé, E. Tribology of journal bearings: Start stop operation as life-time factor. Tribol. Schmier. 2017, 64, 44–54. Archard, J.F.; Hirst, W. The wear of metals under unlubricated conditions. Proc. R. Soc. Lond. Ser. A Math. Phys. Sci. 1956, 236, 397–410. [CrossRef] Bergmann, P.; Grün, F.; Gódor, I.; Herbst, K. Methodology Development for Numerical Evaluation of Wear in Tribological Contacts. In Proceedings of the microCAD 2016, Miskolc, Hungary; 2016; Volume 6, pp. 5–14. Greenwood, J.A.; Williamson, J.B.P. Contact of nominally flat surfaces. Proc. R. Soc. Lond. Ser. A 1966, 295, 300–319. [CrossRef] Totten, G.E.; Liang, H. Mechanical Tribology: Materials, Characterization, and Applications; CRC Press: Boca Raton, FL, USA, 2004. Summer, F.; Grün, F.; Schiffer, J.; Gódor, I.; Papadimitriou, I. Tribological study of crankshaft bearing systems: Comparison of forged steel and cast iron counterparts under start–stop operation. Wear 2015, 338–339, 232–241. [CrossRef] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).