Fundamental Study on Preload Loss in Tapered Roller Bearings

Tribology Online, 11, 2 (2016) 326-332. ISSN 1881-2198 DOI 10.2474/trol.11.326

Article

Fundamental Study on Preload Loss in Tapered Roller Bearings Tomoya Hotta1)*, Shoji Noguchi2) and Tohru Kanada3) 1)

Dept. of Mech. Eng., Graduate School of Science and Engineering, Tokyo Univ. of Science 2641 Yamazaki, Noda, Chiba 278-8510, Japan 2) Dept. of Mech. Eng., College of Science and Engineering, Tokyo Univ. of Science 2641 Yamazaki, Noda, Chiba 278-8510, Japan 3) Div. of Mech. Eng., College of Science and Engineering, Kanto Gakuin Univ. 1-50-1 Mutsuura-higashi, Kanaza-ku, Yokohama, Kanagawa 236-8501, Japan * Corresponding author: [email protected] ( Manuscript received 30 July 2015; accepted 26 February 2016; published 30 April 2016 ) ( Presented at the International Tribology Conference Tokyo 2015, 16-20 September, 2015 )

Tapered rolling bearings are applied to support differential gears in vehicles. Though it is categorized as radial bearing, axial load is required as preload to support the radial load while preventing the inner ring and the outer ring from separating. However, when axial load is applied to the tapered rolling bearing, starting friction becomes larger. This is due to the increase in sliding friction between roller end face and large flange face of the inner ring. In case of position-preload, the bearing width reduces due to the wear and the amount of preload dwindles. This phenomenon is called ‘preload loss’ but nowadays there is still no report regarding this phenomenon. This study explained the relationship among taper, bearing width and preload loss by observing the change of preload while rotating the tapered rolling bearing. The taper rolling bearing was installed using position-preload. Furthermore, we also propose a bearing mounting method which can reduce the preliminary preload loss based on the previous researches and viewpoints. Keywords: tapered roller bearing, preload loss, friction coefficient, wear, bearing width

1. Introduction Tapered roller bearings are used in differential gears in most vehicles and wheel shafts of 500 and 700 Series of Shinkansen bullet trains [1,2]. It is categorized as radial bearing. The outer and inner rings can be separated. Certain axial load is required to support radial load. There are some cases in which the axial load works as an external load. Generally, certain amount of loads are preliminarily worked in axial direction when the bearing is assembled. This axial load is called “preload” and its magnitude is determined to some extent by empirical rule based on the magnitude of radial load. To reduce the cost, position-preload technique is applied by using a spacer to fix the position of the bearing. Figure 1 shows contact between the large flange face of inner ring and the end face of tapered roller. Then, when a thick lubricating oil film is not formed at the contacting part, the position of the surface will change (decrease compared with the default position) due to the wear [3]. Furthermore, in raceway and rolling contact surfaces, certain plastic deformation arises in the extent of surface roughness. Consequently, the position of Copyright © 2016 Japanese Society of Tribologists

surface intends to decrease [3]. In this way, the bearing width of tapered roller bearing decreases with rotation due to the wear and plastic deformation. In case of position-preload, the axial load is applied by screw with

Fig. 1

Contacting condition in tapered roller bearing 326

Fundamental Study on Preload Loss in Tapered Roller Bearings

fixing the position. Therefore, after assembly of bearing, if the bearing width decreases, the magnitude of preload becomes smaller than the default magnitude. This phenomenon is called “preload loss” [4]. Preload loss influences greatly the reduction of rigidity, increase of sound and vibration, and unstable rotation in tapered rolling bearing. Although preload loss is a well-known phenomenon, there is still no quantitative research which has been done on the preload loss. Thus, in this research, the relationship between the bearing width and the preload loss was studied using three types of tapered roller bearing from 30306 series. The assemble method to reduced preload loss is also experimentally clarified. The results were discussed. 2. Relationship between bearing internal specifications and change of bearing width As shown in Figure 1, each elements of a tapered roller bearing has an inclined contacting angle. As shown in Fig. 2, when wear and plastic deformation occur, the surface height of the outer raceway decreases by te compared with the initial default position and decrease of bearing width becomes Te which is larger than te. Table 1 represents the relationship between the surface position and the bearing width. Based on Fig. 2 and Table 1, we can say that at the raceway and rolling contact surfaces, the decrease in bearing width is greater than the decrease in surface height. However, at the large flange face and the roller end face, the decrease in bearing width is smaller than the decrease in surface height.

Fig. 2

3. Experimental apparatus and conditions 3.1. Measuring instrument for bearing width of tapered roller bearing The decrease of surface height due to wear and plastic deformation during rotation is thought as the reason of preload loss for tapered roller bearing, with excluding loosening of fixed screw. Hence, it is important to measure precisely the bearing width before and after rotation. Figure 3 shows a schematic view of measuring instrument used to measure bearing width. This instrument was developed based on principle defined in JIS B 1515-2 (ISO 1132-2). The measuring procedure was as follows. Firstly, a block gauge with dimension of 85 mm was placed and displacement sensor output was set to zero. Next, with fixing the inner ring and applying an axial load (120 N for 30306) to the outer ring, the outer ring was rotated several rounds. After this rotation, the deviation of displacement sensor from the zero output position was measured. By conducting this measurement before and after rotation test, the decrease of bearing width could be obtained. The resolution of displacement sensor used was 0.1 μm. In the measurement of precision displacement, the environmental temperature should be controlled. In this experiment, the measurement was conducted in a constant temperature (20 ± 1°C) room at Kosaka Laboratory Ltd. (Misato Factory). 3.2. Rotation test instrument for tapered roller bearing Figure 4 shows a schematic view of rotation test instrument for tapered roller bearing. Position-preload can be applied in this instrument. The axial loading was done by a screw and position-preload was realized by

Decrease of bearing width of outer ring

Table 1 Relationship of decreases between surface position and bearing width Outer raceway Inner raceway Roller surface Cone back face rib & Roller large end face

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Fig. 3

Schematic view of measuring instrument for bearing width Tribology Online, Vol. 11, No. 2 (2016) / 327

Tomoya Hotta, Shoji Noguchi and Tohru Kanada

Table 2

Experimental conditions

Lubricant Rotational speed Testing time First preload Cooling method After rotation Sampling interval

Fig. 4

Schematic view of rotation test instrument for tapered roller bearing

double nuts locking technique. The test bearing and a thrust ball bearing (51312) were used in the construction of a spindle mechanism. An underlying motor was used to rotate the inner ring of the test bearing. Grease was used as lubricant for test bearings. Load cell was installed above the cover of outer ring to measure axial load applied to the test bearing. Thermocouple was inserted into a fine hole at the cover of outer ring to measure the temperature of test bearing. The materials of parts constructing the instrument were steel material whose linear coefficient of expansion is similar to the test bearing. The rotational speed was set to 285 min-1. This rotational speed was lower compared to the rating condition of the test bearing so that the grease applied to the test bearing did not fly apart and constant lubrication could be assured. A rod was fixed to the cover of outer ring. If this rod pushed another load cell, a rotation torque could be measured. However, in this experiment, the torque was not required to study. Therefore, the rod was used as a rotation-locking for the outer ring to make contact to one of the columnar support. Few thermocouples were installed at the columnar support, basal plate and motor to measure the temperature at each point. 3.3. Experimental conditions The tapered roller bearing tested for 30306 series has inner diameter, outer diameter and bearing width of 30 mm, 72 mm and 19 mm respectively. There were three pitch values of tapered surfaces at which the outer ring and the tapered roller contact. These three values were as follows; (i) standard pitch (α ≤ 17°, ending with no code), (ii) middle pitch (α  20°, ending with code C) and (iii) steep pitch (α  28°, ending with code D). These tapered roller bearings, which had three pitch


Grease(NS-7) 285 min-1 5 hours 8 kN Natural cooling 1 second

values and same outer shape dimensions, were applied to the experiment for preload loss. The test bearing was rotated for five hours. In the previous study, it is known that a dynamic torque decreases with test time and converges to a constant value [3], and furthermore, the converging time is a few hours in general. Therefore, five hours for the duration could be considered as enough to obtain the result in this experiment. After turning off the motor power, the experimental instrument was left to cool down naturally to its initial temperature. Axial load and temperature were measured for 24 hours from the beginning of rotation test. Table 2 shows the experimental conditions. 4. Experimental results Figure 5 shows some examples of change in axial load during rotation test. Further details are explained in Fig. 5(b). In every test, as the rotation began, the temperature of the bearing increased due to heat generated inside the bearing. Then, variation of temperature could be seen to some extent and converged to a certain temperature. After five hours rotation, the temperature of the bearing decreased gradually until it finally settled at room temperature. The axial load at this time would be lower than the initial axial load. Test bearing was left to cool down for 24 hours until it reached its initial temperature before taking the final axial load value. Preload loss was calculated by taking the difference between the initial axial load and final axial load. We can see the axial load decrease rapidly from the first preload value in Fig. 5. An enlarged representation of this is shown in Fig. 6. After the rotation of the bearing started, it took this time period to make the motion of tapered rollers stable. It can be thought that this phenomenon was caused in this time period [5]. Generally, the position of tapered rollers varied after assembling it. When the rotation starts, the tapered rollers intended to balance itself by keeping relative motion between the rolling contact surface of the inner ring and the large flange face. Then, the position of tapered rollers intended to incline in the same direction. Table 3 shows the decreased amount of axial load right after the rotation starts. The value in Table 3 is the mean of three measurements. From this, we can see that the smaller the pitch value, the greater the decreased amount Tribology Online, Vol. 11, No. 2 (2016) / 328


Table 3 Decreased amount of axial load just after rotation start Bearing number 30306 30306C 30306D

Fig. 5

Examples of change in axial load during rotation test

Fig. 6

Enlarged representation just after rotation start in Fig. 5(b)


Decrease of axial load 1.76 kN 1.08 kN 0.12 kN

of axial load. For example, in the standard pitch bearing, the axial load decreased by over 20%. From this point of view, the decrease of axial load right after rotation start can be determined. In this research, this axial load was assumed as the initial axial load. Then, preload loss was calculated by taking the difference between the initial axial load and axial load after rotation stop as mentioned above. As shown in Fig. 5, there are bearings that their axial load fluctuate as the temperature gets higher. For example, as shown in Fig. 5(b), there was a case where the temperature went over 60°C. In such case, temperature drift of the load cell should be checked on ahead. Just for the record, the limit of usage temperature of the load cell was 80°C. If the temperature drift was considerably large, the axial load could not be measured precisely. Accordingly, the output of the load cell having a dead weight of 50 N was measured in a large constant-temperature oven whose temperature was changed. The results are shown in Fig. 7. The temperature was increased gradually by 20°C from 10°C to 70°C. The fluctuation of the load cell was about 0.03 V, which is equivalent to 0.03 kN in load unit (Full scale; 20 kN = 2 V). Comparing with the fluctuation in Fig. 5, the value 0.03 kN was negligible and there was no effect of temperature drift of the load cell to the experiment. Referring to preload loss due to decrease in bearing width in position-preload method, if the amount of decrease in bearing width was known, preload loss could be determined by using the relationship between axial load and axial displacement. Figure 8 shows the preload loss can be determined by replacing axial load with position-preload, and replacing axial displacement with bearing width. Due to the wear and plastic deformation, point A (initial default point) moved to point B. The

Fig. 7

Experimental results for temperature drift of load cell Tribology Online, Vol. 11, No. 2 (2016) / 329


Fig. 8

Relationship between decrease in bearing width and preload loss

preload loss was then determined by taking the horizontal difference between points A and B. Table 4 shows the amount of decrease in bearing width before and after the experiment measured by instrument in Fig. 3. The relationship between axial load and axial displacement for the bearing tested could be calculated from the measurement of dimensions and angles of each part constructing the bearing [6]. (i) The initial axial load (mentioned earlier) and (ii) the value in which the preload load loss, predicted from the decrease of bearing width by means of Fig. 8, were deducted and plotted on a line showing the relationship between axial load and axial displacement. Figure 9 shows the results. Identical graphic symbol in each type of bearing indicates the same test bearing. The black-filled symbol represents the predicted axial load after preload loss while the unfilled symbol shows the initial axial load. Since the basic static load ratings for the tapered roller bearing and the thrust bearing in Fig. 4 were 263 kN (considerably large), the plastic deformation of the bearing could be neglected. Therefore in this research, it was evaluated that the preload loss is caused only by the decrease of bearing width.

Fig. 9

Relationship between initial (default) preload and predicted value after decrease of bearing width

Table 5 shows the relationship between preload loss from the experiment to determine the decrease in bearing width (as shown in Fig. 9) and actual preload loss. By comparing both of the results, we can see that both results correspond approximately to each other. Therefore, we can quantitatively verify the preload loss for tapered roller bearing is mainly caused only by decrease in bearing width. 5. Verification experiment reducing preload loss and repeatability of change in axial load Considering the above-mentioned experimental results and knowledge on the preload loss of tapered roller bearing, a mounting arrangement of bearing parts to reduce the preload loss is proposed in this research. This method was verified by experiment explained in the next section. 5.1. Provision of reducing preload loss As shown in Fig. 4, the inner ring was fixed to a rotation shaft. As shown in Fig. 6, when the outer ring was mounted directly and axial load was applied, the

Table 4 Decreased amount of bearing width after rotation test


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Table 5 Relationship between preload loss predicted from experimental decrease of bearing width and actual preload loss

axial load decreased right after the rotation start. The reason of this phenomenon is to intend to balance the positions of all the tapered rollers. Therefore, after rotation start of the shaft in slow speed to balance all the tapered rollers, an axial load as the preload was applied to the bearing. In order to reduce the decrease in bearing width due to wear and plastic deformation, it is crucial to reduce the friction among large flange of inner ring, raceway and roller end face. It is also crucial to reduce the plastic deformation caused by contact between the roller, and the raceways of inner and outer rings. The size of plastic deformation, arising from the contact between the roller and the raceway of inner and outer rings, is similar to the surface roughness. However, if the load is unchanged, farther deformation does not arise [3]. Then, axial load which was greater than the expected preload was applied to have such plastic deformation as the first step. Next, test bearing was rotated for a while to balance the position of all tapered rollers. Finally, the axial load was reduced to the expected preload. In this way, the authors thought that the decrease of bearing width due to the plastic deformation, arising from the contact between the roller and the raceways of inner and outer rings, could be constrained. 5.2. Verification experiment By using bearing 30306C series and the instrument shown in Fig. 4, an experiment to verify the reduction of preload loss was conducted. The experimental conditions were same as the earlier experiment. Firstly, axial load of 9 kN (This load is similar to the maximum axial load in bearing rotation.) was applied to the rotating bearing for 10 minutes. Then, axial load was reduced to 7 kN, which was similar to the initial axial load for 30306C series bearing as shown in Fig. 5(b).


The bearing was rotated for 5 hours. Finally, the instrument was naturally cooled-down for 19 hours. Figure 10 shows the relationship between the time and the change in axial load. For preload in this experiment, 7 kN was applied at the rotation start. This was decided based on the decrease in axial load after rotation start as shown in Fig. 6. By comparing both preload loss obtained from Table 5 and Fig. 10, the reduction effect of preload loss due to plastic deformation could be identified. When axial load was reduced to 7 kN, temperature increased to 35.1°C which was higher than the room temperature. Then, preload loss was calculated using the axial load after the bearing temperature was naturally cooled down to 35.1°C. The preload loss shown in Fig. 10 was 0.42 kN. This value was smaller than 1.21 kN (mean value of preload loss of 30306C bearing as shown in Table 5). Therefore, it was clarified that the preload loss due to plastic deformation was caused by the contact between the roller, the raceway of inner ring

Fig. 10 Change in axial load during rotation test in case of taking a large axial load first Tribology Online, Vol. 11, No. 2 (2016) / 331


Fig. 11 Change in axial load temperature (5 iterations)

and

bearing

and outer ring. Moreover, with reference to wear between the large flange face of inner ring and the roller end face, it was confirmed that applying shot peening and SMAP (Shot Machine A.one Polish) treatment could constrain the bearing width of 30306C [7]. 5.3. Repeatability of change in axial load In case of rotating a tapered roller bearing using position-preload method, the change in axial load can be refer in Fig. 5. However, in general, since the bearing rotates for a long time period, it can be supposed that the bearing performance is affected by iterated preload loss. Accordingly, the result for five times iterations without resolving is shown in Fig. 11. After the bearing temperature became elevated first and returned back to the initial temperature, the axial load became smaller than the initial axial load. However, from then on, the axial load was approximately-same whenever the bearing temperature increased according to the bearing rotation and decreased to the initial temperature before the bearing rotation according to the rotation stop. This means, the preload loss, in other words, the decrease in bearing width occurs only at the first experiment (rotation). 6. Conclusions Using tapered roller bearing (30306), axial preload loss was investigated by applying an instrument for position-preload. The summary is as follows. (1) When a tapered roller bearing was rotated with


position-preload method, the preload decreased compared to the initial axial load. The preload loss became greater when the pitch value of the bearing was smaller (bigger contact angle). (2) The main reason of preload loss for tapered roller bearing was a decrease of bearing width. It was confirmed that the preload loss did not occur after two or more iterated experiment. (3) To reduce the preload loss, the following was confirmed by the experiment. Namely, it was better to apply higher axial load than the expected preload and to rotate the bearing for a short time period. After that, the axial load should be reduced to the expected preload. Then, these procedure was available. The authors wish to thank Mr. Motohiro Horikoshi (student of Graduate School of Science and Engineering, Tokyo University of Science) for his support to the experiment. References [1] [2] [3]

[4] [5] [6] [7]

Ooshima, H., “Low Friction Torque Bearings for Differential Pinion,” JTEKT Engineering Journal, No. 1009, 2011, 37-43. Noguchi, T., “Axle Journal Bearings for High Speed Shinkansen,” Nachi Technical Report, 53, 2, 1997, 38-40 (in Japanese). Noguchi, S. and Sawamoto, T., “Surface Observation of Tapered Roller Bearing in Running-in Process (Part 1),” Journal of Japanese Society of Tribologists, 37, 2, 1992, 142-149 (in Japanese). Minebea, Co., Ltd., “Precision Ball Bearing Products,” 7, 2014, 33 (in Japanese). Okamoto, Y. and Tsujimoto, T., “ECO-Top Tapered Roller Bearings,” NTN Technical Review, No. 68, 2000, 34-38 (in Japanese). NSK, Ltd., “Technical Report,” CAT. No. E728g, 2013, 160. Noguchi, S., Hotta, T. and Kanada, T., “Development of Tapered Roller Bearing with Low Torque (Reduction of Sliding Friction between Large Flange and Roller End Faces Machined by Precision Powder Shot Peening),” Proceedings of the Institution of Mechanical Engineers, Part J Journal of Engineering Tribology, 228, 9, 2014, 937-946.

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