International Journal of Automation and Computing
7(2), May 2010, 160-166 DOI: 10.1007/s11633-010-0160-1
Running Temperature and Mechanical Stability of Grease as Maintenance Parameters of Railway Bearings Jan Lundberg
Aditya Parida
Peter S¨oderholm
Division of Operation and Maintenance Engineering, Lule˚ a University of Technology, Lule˚ a SE-971 87, Sweden
Abstract: Spherical roller bearings in railway car wheels are critical components whose failure may have catastrophic consequences. The present study aims to analyse the mechanical stability of greases and temperature of bearings as indicators for condition-based bearing maintenance. The performed case study includes nine fully-formulated commercial greases examined in the wheel bearings of five ore cars operated in northern Scandinavia. The studied ore cars travelled a distance of about 300 000 km during a period of three years. Small samples of the greases were taken on eight occasions to test their mechanical stability. In addition, the temperatures of the bearings were continuously recorded. After the test period, the wear, electrical damage, and corrosion of the bearings were examined. One of the findings is that the shear stress of the grease at a certain shear velocity (the certain yieldstress (CEY) value) is a good maintenance indicator and is highly dependent on the grease type. The bearing0 s wear, electrical damage and corrosion also depend on the grease type. However, no oxidation of the greases was identified. The paper also outlines a systematic methodology to determine an overall maintenance indicator for railway roller bearings which is based on the field measurements. Keywords:
1
Maintenance, mechanical stability, railway, roller bearings, grease lubrication.
Introduction
Roller bearings are often lubricated with grease and placed in sealed boxes in order to ensure that the grease stays close to the bearings to guarantee proper lubrication. During operation, when the grease is coming into contact with the rollers and rings of the bearing, this will lead to the mechanical degradation of the grease, as the bearing is acting like a mill[1] . The result can thus be weakening of the consistency of the grease, which will entail higher demands on the sealing properties in order to prevent the grease from leaking out of the box and thus the bearing[2] . Since the sealing properties are dependent on viscosity of the sealed lubricant[3] , it can be assumed that, if the consistency is below a critical value, then the grease will flow out of the box, resulting in lubrication starvation and seizure of the bearings[4] . From the maintenance perspective, it is therefore essential to increase knowledge about factors affecting the consistency of the greases. Also, bearing temperature is closely related to the condition of the bearings[4] , which means that knowledge of temperature-related topics is of interest as well. The present study deals with roller bearings in ore wagons in arctic conditions, which is a typical field of application for grease lubrication of bearings in northern Scandinavia. Iron ore has been mined at Kiruna Mine in northern Sweden for over 100 years. The ore is transported by railroad to Narvik in northern Norway for shipping to foreign markets and to Lule˚ a in Sweden for smelting. This operation is run all year round and requires the railway company to operate during periods of very low temperature (−40◦ C) through mountainous terrain. The temperature in the summer is typically around 15◦ C in the daytime. This presents very severe conditions, and bearing failures on the ore wagons have been costly and, in some cases, catastrophic[5] . In most Manuscript received January 10, 2010; revised March 21, 2010
of the failure cases, it has typically been found, by visual inspection, that most of the grease got leaked out of bearing boxes. Measurements of the consistency of the greases have shown that it had decreased significantly, leading to the conclusion that the reason for the failures was lubrication starvation, because of excessively low consistency, in combination with the limited performance of the sealing. The problem with bearing failures started when leaded grease was prohibited for environmental reasons and therefore was replaced with unleaded grease. Grease with lead seems to be capable of lubricating even if starvation occurs[6] . Research concerning maintenance performance measurements (MPM) has attracted the attention of several researchers. A short-term measure of individual parameters has, for instance, been performed by [7–10]. However, they have not studied mechanical stability as a maintenance factor for roller bearings in railway applications. Several authors have studied the role of organisation in the maintenance process[11−15] . However, in order to have control of the maintenance process, it is essential to acquire knowledge about critical aspects of the behaviour of essential machine elements. The present study therefore aims to analyse the mechanical stability and bearing temperature as critical indicators for predicting maintenance efforts. In this study, nine different fully-formulated commercial greases were examined in the wheel bearings of five ore wagons, used for transporting ore commercially by railroad from Kiruna Mine in northern Sweden to Narvik in northern Norway for shipping to foreign markets. The degradation of the consistency of the grease (mechanical stability) and the bearing temperatures were measured several times during the test period. After the end of the test period, the wear of the bearings was examined. Based on the knowledge acquired, maintenance requirements are proposed and a maintenance indicator concerning grease lubrication of bearings in railway applications is suggested.
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2
Field tests
The field tests were performed on five ordinary iron ore wagons, each of which was designed for a weight of 100 tonnes (with the cargo included). The average speed was 50 km/h and the total travelled distance was 319 000 km during the test period of 3 years. The test wagons were employed in commercial traffic and the total weight of the whole train in question, with the cargo included, was 5 200 tonnes. Each ore wagon was equipped with four axles and a total of 16 wheel bearings, which means a load of 6.3 tonnes for each bearing (two bearings in each box). One of the test wagons was equipped with a data logger system for continuous measurements of the temperatures of the outer rings of the bearings. Eight different fully formulated commercial greases (A, B, C, D, E, G, H, and I) were used in the rail wagon selected for the temperature measurements. The remaining four wagons were tested with greases A, B, C, D, E, F, G, H, and I. Grease G as a reference grease was used in the bearings before the present project. After a short while it was observed that severe grease leakage from all the boxes with grease F had occurred, which led to the conclusion that grease F should be replaced with grease I. The grease leakage was due to a large decrease in the mechanical stability. The mechanical stability of the greases was measured on eight different occasions during the test period by means of the shear stress criteria, the certain yieldstress (CEY), introduced in [16] and defined in Fig. 1. This method involves shear stress measurements with a conical viscometer at a specified shear velocity of 10 s−1 . In addition, oxidation measurements were performed. At the end of the field test, all the bearings were examined with regard to wear and rust.
thenic mineral oil and Li-12-OH thickener. A more advanced grease type is composed of polyalphaolefines (PAO) and lithium complex thickener. The following rules were set up for the grease samples. The greases must 1) be fully formulated and commercially available; 2) be recommended by the supplier and the project members for the desired application; 3) represent a wide spectrum of thickener and base oil types; 4) be able to show an national lubricating grease institute (NLGI) (abbreviation to be expanded on first appearance, except if it is very well-known) number between 1.5 and 2.5; 5) be permitted for use in accordance with environmental laws. According to the rules described above, the following test greases were chosen (see Table 1). Table 1
Chosen test greases (adapted from [1]) Base oil
Grease
Base oil
Thickener
viscosity at
NLGI
40◦ C (mm2 /s) A
Mineral
Li-12-OH
200
2
B
Mineral
Lithium complex
185
2
C
Mineral
Li-12-OH
100
2.5
D
PAO
Lithium Complex
220
2
E
Ester
Li-12-OH
90
2
F
PAO
Calcium
130
2
G
Mineral
Li-12-OH
200
2
H
Mineral
Li-12-OH
95
2
I
PAO
Lithium complex
460
1.5
2.2
Mechanical stability
The results of the measurements of the mechanical stability are presented in Fig. 2 (for more information concerning mechanical stability, see [17]).
Fig. 1
2.1
Method for determination of CEY value
Selection of proper grease samples
Because of the financial limitation of the project, the total number of test greases was set to only nine. The greases must be suitable for the railway application concerned, and only a limited risk of bearing failure could be accepted, since the wagons were used in commercial traffic. At the same time the greases were carefully selected such that they can represent a wide spectrum of grease types in order to make it possible to draw useable conclusions. Today, about 1 000 greases are available in the market, and the most common grease types are based on naph-
Fig. 2
Mechanical stability (CEY) from field tests
It is seen that grease A is particularly good concerning mechanical stability, while grease F (with a calcium thickener) shows very weak mechanical stability, and therefore this grease had to be rejected after about 60 000 km. It is well-known that the mechanical stability of calcium greases is good to satisfactory[18] . Generally, it was seen in the field
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International Journal of Automation and Computing 7(2), May 2010
test that grease leakage through the sealing occurs if the CEY decreases below 200 Pa. This means that CEY measurements are suitable as a criterion for consistency and mechanical stability and are thus valid as an indicator of maintenance for this particular application. It is also seen that the reference grease, G, has weak mechanical stability, which was expected due to the leakage problems which initiated the present project. It is interesting to note that both grease A and G are of the same type and viscosity, the only difference being the amount of thickener (grease A: 12 wt%, grease G: 8 wt%). It is therefore natural to assume that increasing the amount of thickener will increase the mechanical stability. As seen in Fig. 2, the main mechanical degradation is present within the first 25 000 km. After that, the consistency is mainly constant for most of the greases during the whole field test. Measurements on different places in the bearing boxes indicate approximately the same consistency and colour, indicating that most of the entire grease volume has migrated around the interior of the box. The migration of greases is defined as the velocity of a mass of grease when it is subjected to vibration and gravitational forces. Vibration excitation has been found to have a widely varying effect on the creep transport of grease, suggesting that this parameter may be important in the selection of greases for specific applications where vibration is present[19] . However, in the present investigation, no significant correlation was found between creeping properties and the results from the field tests. Since, the bearing temperatures were acceptable (see Section 2.3 below), it can be stated that the main reason for the mechanical degradation was mechanical working of the grease in the bearings. The mechanical stability can be expressed as the quotient CEY%, which is defined as the CEY value of the last CEY measurement from the field test divided by the CEY value of new un-destroyed grease (see Table 2). Table 2
2.3
Fig. 3
Bearing temperatures
It is seen that grease I shows particularly high temperatures. The reason for this is its high base oil viscosity[1] . For the same reason, grease E, with the lowest viscosity, shows the lowest temperature. During the wintertime (about −15◦ C) the average bearing temperatures were between 15◦ C and 50◦ C, probably depending on the base oil viscosity. During the summertime (about 20◦ C) the average bearing temperatures were between 30◦ C and 55◦ C. Temperature peaks of about 70◦ C have also been measured, but those temperatures will not have any significant influence on the mechanical stability. During the start sequence of the train, it was also found that the temperature rise is about 0.5–1◦ C/min. The average temperature differences between the bearings and the outdoor air for all the greases, for the whole test period, are presented in Table 3. Table 3
Temperature difference between bearings and outdoor air and absolute bearing temperatures
Grease
Mechanical stability of tested greases
Temperature
Temperature
Winter (◦ C)
Summer (◦ C)
Difference
Absolute
Difference
Absolute
A
49
34
21
41
B
45
30
14
34
C
42
27
18
38
D
45
30
20
40
Grease
CEY %
Travelled distance (km)
E
31
16
13
33
A
80
319 000
F
-
-
-
-
B
65
319 000
G
44
29
17
37
C
63
319 000
H
43
28
15
35
D
56
319 000
I
66
51
29
59
E
52
319 000
F
15
60 000
G
28
235 000
H
30
235 000
I
74
240 000
Bearing temperatures
Temperature measurements were performed on 16 occasions within the field tests. The temperature measurement equipment was capable of storing temperature measurements lasting 17 s each. One example of such measurements performed in the summertime is presented in Fig. 3, where the temperature of the outside air is represented by the curve marked “outdoor”.
Note that the increased temperature difference during the wintertime is due to the increase in the base oil viscosity. It can also be concluded that the energy losses in the bearings are dependent on the temperature difference. The energy losses will increase when the bearing temperature increases.
2.4
Bearing wear
Today it is not possible, for practical reasons, to measure the wear of bearings online, and therefore bearing wear is not considered as a maintenance parameter. (However, since the wear of bearings is of general interest, the results obtained in the present study are presented) you could have given even more solid reason and thus make this consistent with the previous text.
J. Lundberg et al. / Running Temperature and Mechanical Stability of Grease as Maintenance Parameters of · · ·
The wear of each inner and outer ring of the bearings (in total 80 bearings), due to surface distress, galling and fretting, was studied by experts at a bearing company after the end of the field test by means of microscopy analysis. Since the main factor that was varied in the field test was the type and viscosity of the greases, the differences in wear between the bearings analysed in this study were probably due to the lubricating properties of the greases. A grade system was introduced which was founded on long term experience, in combination with a wear atlas[20] , and based on the amount of wear spots: 1 = a very small amount of wear (almost like a new bearing), 2 = a small amount of wear, 3 = a typical and expected medium range of wear, 4 = severe wear due to bad lubrication, 5 = very severe wear due to bad lubrication (the bearing should be rejected). The average wear of bearings is presented in Table 4. Table 4
Average wear of the bearings in the field tests
Grease
Wear grade number
Travelled distance (km)
A
2.8
319 000
B
4.5
319 000
C
3.2
319 000
D
3.4
319 000
E
4.4
319 000
F
-
60 000
G
4.6
235 000
H
5.5
235 000
I
3.5
240 000
All the grade numbers corresponding to the actual travelled distances for grease G–I were linearly corrected to become numbers corresponding to 319 000 km, which in the particular case of grease H meant a wear grade number greater than 5. It was found that bearings which had passed 300 000 km only showed about 30 % more wear compared with bearings which had run about 70 000 km (using the same grease). This was probably due to the running-in procedure. In order to avoid any possible problems with the wear analysis due to the running-in procedure, only bearings which had travelled a distance of between 200 000 and 319 000 km were included in the present analysis.
2.5
Other parameters
The oxidation of the greases was measured by an Fourier transform infrared spectroscopy (FTIR) spectrometer at the end of the field test. No oxidation was found, probably because the bearing temperature never exceeded 70˚C. The water content was checked by visual inspection at the end of the field test. No water was found. The bearing rust was measured at the end of the field test of each bearing, and large differences were found depending on the grease type. Moreover, no correlation was found between the base oil viscosity, the thickener type and the electrical damage of the bearings. The viscosity was found not to be a critical parameter since, for the optimum greases, it ranges from 100 to 460 cSt at 40◦ C.
3
163
Evaluation of maintenance specification
All of the greases in the field test were evaluated with the help of the so-called requirement tree method[21,22] . The evaluation of the greases then served as a basis for predicting the need for a maintenance effort. The basic principle of this method is that a number of primary requirements are arranged logically and weighted in relation to each other. The weighting value assigned to each requirement is determined by means of comparing the requirements with each other, based on current knowledge in this field of tribology. All the greases under examination receive a grade that shows how well each respective requirement has been fulfilled, after which the weight is multiplied by the grade. When we have worked through the whole requirement tree, each of the greases is given a “merit number”, which is used for ranking it in relation to all the other greases. In addition to the requirements in the tree, there are demands that all the greases should meet. It is proposed that the “merit number” should serve as a maintenance indicator (maintenance number M N ).
3.1
Maintenance parameters
The parameters which in all circumstances must be fulfilled are as follows: 1) Sufficient mechanical stability (CEY > 200 MPa or CEY %> 40 % at 319 000 km) 2) Acceptable average bearing temperature in the summer (maximum temperature = 70◦ ) These demands are based on the possibility of continuously measuring both the mechanical stability and the temperature of the bearings.
3.2
Maintenance requirements
Two maintenance-related requirements, R1 and R2, are identified: High mechanical stability, R1: The mechanical stability must be as high as possible, since this is essential to permit long service intervals with few refills of the grease. It is also very important in order to eliminate the risk of lubrication starvation due to grease leakage between the service intervals. Low running temperature, R2: A low running temperature of the bearing indicates that the friction is low and that the risk of failure is limited. Measuring the bearing temperatures in both the summer and the winter is therefore important. The internal weighting values for summer and winter temperatures are set to “equally important” in the present case, for this particular application. Measurements of the bearing temperatures are related to the need for extra equipment and costs, but this should be compared with the costs of possible failures. The procedure of evaluating the need for maintenance requirements can be described in five steps. The first step, with the aim of calculating the proposed maintenance indicator, is to evaluate the weighting of each requirement by means of comparing the requirements with each other (see Table 5).
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International Journal of Automation and Computing 7(2), May 2010 Table 5
Evaluation of requirement weights R1
R2
Mechanical
Temperature
stability
difference
1 0
Mechanical stability Temperature difference
Sum
Weight
2
3
0.75
1
1
0.25
4
1
Total sum
In Table 5, each weight is established row by row, by means of answering questions like “what is the importance of the mechanical stability compared with the temperature difference?” This means that the rows are compared with the columns and given one of the following weights: 0 = less important, 1 = equally important, and 2 = more important. The next step is to establish a linear grade system for all the requirements (see Table 6). If the grade for a system in general is less than 1, then a maintenance action must be carried out at once. The grades derive from a consideration of the relative scale of the previously determined parameters, and are thus more or less subjective in character. Table 6
Requirement grade system
Grade
Meaning
1
Identical with demand limit
2
Slightly above demand limit
3
Above demand limit
4
Certainly above demand limit
5
Theoretically perfect
The proposed grades range from 1 to 5. The advantage of this narrow range is that, in dealing inadequately known characteristics of the greases, rough evaluations are sufficient and may be the only meaningful approach. The third step is to define the grades from Table 6 with the results from the field tests and the demands (see Table 7). It is suitable to use a linear scale, in order to achieve a proper correlation between the grades and the measured quantities. Note that grade 5 should correspond to the theoretical perfect values. Table 8
The fourth step in the evaluation process is, by means of the grade system in Table 6 and the chart correlating parameter magnitudes with grades in Table 7, to connect the grades with the actual results for all of the greases. In this connection, it is suitable to use linear interpolation in order to achieve accurate values (see Table 8 R1 and R2). It can be seen in Table 8 that grease A shows the best performance concerning mechanical stability (R1 = 3.7) and grease E shows the best performance concerning the temperature difference between the bearing and the outdoor air (R2 = 3.6). Table 7
Chart correlating parameter magnitudes with grades R1
R2
Mechanical
Temperature
Stability (CEY %)
difference (◦ C)
Grade 1
40
70
2
55
52.5
3
70
35
4
85
17.5
5
100
0
The fifth and final step is to multiply the interpolated grades for R1 and R2 in Table 8 by the weight for each of the requirements (see Table 5) and thus obtain the maintenance number for each of the greases as the total sum of the merit numbers (see Table 8). The total maintenance number of the grease can be treated as a measure of the need for maintenance, since a low value will indicate a need for maintenance. However, it is necessary to remember the most important limitations of the method, which are 1) the marked interdependence of the evaluation criteria, 2) the possible incompleteness of the evaluation criteria.
4
Evaluation of the need for maintenance
It could be concluded by means of visual observation that there is a need for maintenance if the mechanical stability, CEY %, is equal to or less than 40 %. There can also eventually be a need for maintenance if the difference
Interpolated grades
R1
R2
Mechanical stability
Temperature difference
Mechanical stability
Temperature difference
Merit number
Merit number
3.7
2.8
2.78
0.70
Maintenance number (M N )
Grease A
3.48
B
2.7
3.2
2.03
0.80
2.83
C
2.5
3.1
1.89
0.78
2.67
D
2.1
3.0
1.58
0.75
2.33
E
1.8
3.6
1.35
0.90
2.25
F
Not passed
-
Not passed
-
-
G
Not passed
3.1
Not passed
0.78
-
H
Not passed
3.2
Not passed
0.80
-
I
3.3
1.9
2.48
0.48
2.96
J. Lundberg et al. / Running Temperature and Mechanical Stability of Grease as Maintenance Parameters of · · ·
between the bearing temperature and the outdoor temperature (temperature difference) is equal to or larger than 70◦ C. This corresponds to grade 1 for both parameters, which gives the maintenance number M N = 0.75 × 1 + 0.25 × 1 = 1 (see Tables 5 and 7). Therefore, it can be concluded for this particular case that there is a need for immediate maintenance if M N = 1, or CEY % is equal to or less than 40 % or temperature difference is equal to or higher than 70◦ C. The performance of the sealing will be critical concerning the demand for mechanical stability, since the design of the sealing will affect its capacity to prevent leakage, and therefore it is assumed that the mechanical stability can be treated as the main maintenance indicator. The technique of maintenance number calculation is suggested in order to present a total estimation of the condition of a bearing-lubricant system, in cases where several aspects have to be weighed together. For instance, for this particular case, assume that the temperature difference of the bearing was measured to be 35◦ C and the CEY % was measured to be 85 %. This gives the maintenance number M N = 0.75×4+0.25×3 = 3.75 (see Tables 5 and 7), which indicates that the bearing probably will work without any problems. The M N number can thus serve as an indicator of the need for maintenance when determining the suitable time for service. Moreover, the method can be appropriate if many parameters are affecting the condition of the bearing, such as load, angular velocity, oxidation of the lubricant, iron content in the lubricant, etc.
5
Conclusions
The mechanical stability of grease is proposed as the main indicator for maintenance of railway roller bearing sealing with reduced performance. If it is possible, temperature measurements are suggested as a complimentary indicator for condition monitoring of bearings. A maintenance number, M N , is proposed in order to present a weighted value of two or more maintenance parameters. Viscosity is not a critical parameter since, for the optimum greases, it ranges from 100 to 460 cSt at 40◦ C. A grease with a calcium thickener was tested, and it showed low mechanical stability. The CEY number is recommended as a measure of mechanical stability; it was found that leakage occurs if the CEY is less than 200 MPa, which corresponds to CEY % = 40%. The average bearing temperatures varied between approximately 15◦ C and 60◦ C, depending on the base oil viscosity of the grease and the outdoor temperature. It was found that increase in base oil viscosity increases the bearing temperature because of friction losses. No oxidation of the greases was reported. The main mechanical degradation for most of the greases took place within the first 25 000 km. After that, the consistency was mainly constant for most of the greases during the whole field test. The main reason for mechanical degradation was the mechanical working of the grease in the bearings. The bearings that had been in operation for more than
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300 000 km only showed on average about 30 per cent more wear compared with the bearings that had run about 70 000 km. This was probably due to a running-in phenomenon. No correlation was found between the base oil viscosity, the thickener type and the electrical damage of the bearings. No correlation was found between the investigated parameters and the rust of the bearing.
Acknowledgments The authors wish to thank LKAB, Banverket and SKF for their support of this project.
References [1] J. Lundberg. Grease lubrication of roller bearings in railway waggons. Part 1: Field tests and systematic evaluation to determine the optimal greases. Industrial Lubrication and Tribology, vol. 52, no. 1, pp. 36–44, 2000. [2] J. Lundberg, S. Berg. Grease-lubrication of roller bearings in railway waggons. Part 2: Laboratory tests and selection of proper test methods. Industrial Lubrication and Tribology, vol. 52, no. 2, pp. 76–86, 2000. [3] H. L. Johannesson. Analysis of Hydraulic Systems, Division of Machine Design (Analys av hydraulsystem, Institutionen f¨ or Maskinteknik), Lule˚ a University of Technology, 1982. (in Swedish) [4] J. Lundberg. Grease for Wheel Bearings of Ore Wagons in Arctic Environment (Projektrapport: fett f¨ or hjullager till malmvagnar i arktisk milj¨ o), Lule˚ a University of Technology, 1997. (in Swedish) [5] B. Bhushan. Principles and Applications of Tribology, John Wiley & Sons, 1999. [6] R. M. Mortier, S. T. Orszulik. Chemistry & Technology of Lubricants, Blackie Academic & Professional, 1994. [7] R. L. Banks, S. C. Wheelwright. Operations Versus Strategy: Trading Tomorrow for Today, Harvard Business Review, pp. 112-120, 1979. [8] R. W. Hayes, D. A. Garvin. Managing as if Tomorrow Mattered, Harvard Business Review, pp. 67–77, 1982. [9] R. S. Kaplan. Measuring manufacturing performance: A new challenge for managerial accounting research. Accounting Review, vol. 58, no. 4, pp. 686–705, 1983. [10] A. Neely. The performance measurement revolution: Why now and what next? International Journal of Operations and Production Management, vol. 19, no. 2, pp. 205–228, 1999. [11] J. P. Liyanage, U. Kumar. Towards a value-based view on operations and maintenance performance management. Journal of Quality in Maintenance Engineering, vol. 9, no. 4, pp. 333–350, 2003. [12] U. S. Bititci, A. S. Carrie, L. McDevitt. Integrated performance measurement systems: A development guide. International Journal of Operation and Production Management, vol. 17, no. 5, pp. 522–534, 1997.
166 [13] R. S. Kaplan, D. P. Norton. Transforming the balanced scorecard from performance measurement to strategic management, Part 1. Accounting Horizons, vol. 15, no. 1, 2001. [14] M. Kennerly, A. Neely. Measuring performance in a changing business environment. International Journal of Operations and Production Management, vol. 23, no. 2, pp. 213– 229, 2003. [15] J. H. Lingle, W. A. Schierman. From balanced scorecard to strategy gauge: Is measurement worth it? Management Review, vol. 85, no. 3, pp. 56–62, 1996. [16] L. Hamnelid. Amazing grease or finding the right way to consistency. In Proceedings of the 57th Annual NLGI Meeting, Denver, CO, USA, 1990. oglund. A new method for determining [17] J. Lundberg, E. H¨ the mechanical stability of lubricating greases. In Proceedings of the 5th International Tribology Conference (AUSTRIB), Brisbane, Australia, 1998. [18] Lubricating Grease Guide, 4th ed., National Lubricating Grease Institute (NLGI), 1996. [19] J. Lundberg, T. McFadden. Creep transport of grease subjected to low frequency vibrations. Lubrication Science, vol. 9, no. 1, pp. 71–83, 2006. [20] T. E. Tallian. Failure Atlas for Hertz Contact Machine Elements, New York, NY, USA: Mechanical Engineering, 1992. [21] G. Pahl, W. Beitz, K. Wallace. Engineering Design: A Systematic Approach, Berlin, Heidelberg, German: SpringerVerlag, 1996. [22] J. Lundberg, T. McFadden. Low temperature performance rating criteria for lubrication greases. In Proceedings of the ASCE Conference on Cold Regions Engineering, Fairbanks, Alaska, pp. 153–172, 1996.
International Journal of Automation and Computing 7(2), May 2010 Jan Lundberg has been a professor of Machine Elements at Lule˚ a University of Technology, Sweden since 2000. During 1983–2000, his research concerned mainly about engineering design in the field of machine elements in industrial environments. During 2000–2006, his research concerned mainly about industrial design, ergonomic and related problems as cultural aspects of design and modern tools for effective industrial design in industrial environments. Since 2006, his research interests have completely focused on maintenance issues like methods for measuring failure sources, how to do design out maintenance and how to design for easy maintenance, including automation and computing. E-mail:
[email protected] Aditya Parida received the Ph. D. degree in operation and maintenance engineering. He is currently an assistant professor with Lule˚ a University of Technology, Sweden. Besides teaching, he is actively involved in supervising the research students and projects. His research interests include asset and maintenance performance measurement, RCM, computing and eMaintenance. E-mail:
[email protected] (Corresponding author) Peter S¨ oderholm received the Ph. D. degree in the operation and maintenance engineering. Currently, he is an assistant professor with Lule˚ a University of Technology, Sweden, and presently working with Swedish Rail Road Administration, Sweden. His research interests include emaintenance, reliability and no fault found (NFF), and data computing for the aerospace industry. E-mail:
[email protected]
