Effect of Lubricating Oil Behavior on Friction Torque of Tapered Roller Bearings

TECHNICAL PAPER Effect of Lubricating Oil Behavior on Friction Torque of Tapered Roller Bearings H. CHIBA H. MATSUYAMA K. TODA Low-friction tapered roller bearings were developed to improve the fuel efficiency of vehicles. Bearing friction torque variation was studied for these bearings with various oil flow rates supplied to the bearings controlled by means of interruption rings. As a result, the effects of oil flow rate and oil behavior on the friction torque of these bearings was clarified. Agitating resistance and viscous rolling resistance can be reduced by restricting oil flow into the bearing, and the friction torque of tapered roller bearings with interruption rings was lower than that of angular contact ball bearings. Key Words: tapered roller bearing, friction torque, oil flow, oil behavior, fuel efficiency 1. Introduction Along with recent lifestyle changes typified by increased use of vehicles and propagation of office automation products, energy consumption on a global basis has been increasing year by year. As a result, global warming, one of numerous global environmental problems we face, has become a very serious issue. Because of this, many programs have been initiated throughout the world in order to protect the environment, and automobile-related laws and regulations are becoming increasingly severe 1). For automakers in particular, a key task is improving fuel efficiency in order to reduce carbon dioxide emissions as quickly as possible 2) 4). In view of these social needs, expectations are very high for efficiency in rolling bearings, more than 100 of which generally are used in one vehicle. It was proposed that there be a change from tapered roller bearings to ball bearings for the purpose of reducing the friction torque of bearings 5). However, this change to ball bearings creates the problems of decreased life, lower rigidity, and lower static safety, which would result in the need for large-sized ball bearings. In general, the torque of tapered roller bearings originates from viscous rolling resistance between the rings and rollers, sliding resistance between the rib face of the inner ring and the end faces of rollers, agitating resistance of lubricating oil (hereinafter oil), and sliding resistance between the cage and rollers. So far, efforts to reduce torque have concerned mainly viscous rolling resistance and sliding resistance 6) 8). However, with conventional technology, the torque reduction has reached only around 10% to 20%, which still is insufficient compared with the torque of ball bearings. Therefore, in order to achieve a torque value similar to that of ball bearings, the reduction of agitating resistance, which has not been considered before, must be studied. Focusing on oil behavior within the bearing, which is considered to influence agitating resistance, we studied the possibility of reducing torque by controlling oil flow. Results are shown hereunder. 2. Test Bearings and Method 2. 1 Test Bearings For this test, single row tapered roller bearings with a bore diameter of 34.9mm, outside diameter of 72.2mm, and overall width of 25.4mm, as shown in Fig. 1, were used. The material for the inner and outer rings is case hardened steel with carburized-quenching and tempering-treatment, and that for the rollers is high carbon chrome steel with hardening and tempering treatment. The cage was a pressed type with mild steel. With tapered roller bearings, a common phenomenon regarding oil flow called pumping action 9) occurs and the oil passes through from the front side of inner ring to the backside. In order to change the total oil flow rate into the bearing, steel interruption rings (hereinafter referred to as attachments) to restrict oil flow have been fixed to the front side of the inner ring. 34.9 dia. 25.4 Fig. 1 Test bearing 72.2 dia. Koyo Engineering Journal English Edition No.168E (2005) 23

Figure 2 shows test bearings with attachments by type. Type A has a structure that covers the inner ring and cage in order to stop the oil from flowing between the inner ring and cage. Type B has a structure that covers the outer ring and cage in order to stop the oil from flowing between the outer ring and cage. Type C has a structure that covers an area from the inner ring to the inside diameter of the outer ring in order to stop the oil from flowing in from both between the inner ring and cage and between the outer ring and cage. The clearance 3 between the cage and attachment for each test bearing was set at 0.9mm. 2. 2 Test Method 2. 2. 1 Measurement of Oil Flow Rate The measuring equipment is shown in Fig. 3. With this equipment, oil flow rate into the bearings with attachments was measured. A sleeve has been fixed to the end face of the large diameter side of the cage in order to separately measure the oil quantity flowing out of the inside space between the inner ring and cage (hereinafter referred to as the inside oil flow rate) and the same flowing out of the outside space between the outer ring and cage (hereinafter referred to as the outside oil flow rate). 75 W-90 gear oil with viscosity of 86.8 mm 2 /s at 40; was used for this test. The temperature of the oil was maintained as 50±1;. Test procedures are described hereafter. First, a test bearing with a sleeve for measuring the oil flow rate was fixed to the measuring equipment. Next, the oil supply was adjusted in order to maintain the height of oil level at the center of the shaft. Then, the inner ring was rotated under no load with rotational speeds of 1 000 min 1, 2 000 min 1, and 3 000 min 1. At this time, as the oil level height came down due to the pumping action, the oil supply level was adjusted again in order to return the oil level to the center of the shaft. After stabilizing the oil level height at each rotational speed, the inside and outside oil flow rates per minutes respectively was measured. Measurements were done three times for each test bearing and the average values were obtained. 2. 2. 2 Observation of Oil Flow Behavior In order to observe oil behavior inside the bearing when oil flow was being controlled by interruption rings, high-speed video analysis equipment (1 000 frames per second) was used. Figure 4 shows a schematic diagram of the oil flow observation equipment. The body of this equipment and the outer ring of the bearing are made of transparent acrylic resin. For easy observation of the oil behavior, kerosene mixed with 0.12 mass% of pigments (diameter 20-180lm) made of natural mica with surface coated with titanium oxide was used for easy tracing. After placing the test bearing on the equipment, oil was supplied to the center level of the shaft, and then the inner ring was rotated under no load with a rotational speed of 250 min 1. At this time, as the oil level height came down due to the pumping action, oil supply was adjusted in order to maintain the oil level height at the center Support bearing a) Normal b) Type A 3 c) Type B d) Type C Fig. 2 Test bearings with attachments Test bearing Sleeve Oil passing through inside Oil passing through outside of the shaft. Under this condition, oil behavior was observed from the outside of the bearing and the backside of the inner ring. 2. 2. 3 Measurement of Friction Torque Figure 5 shows a schematic diagram of the friction torque measuring equipment. Friction torque can be obtained by measuring, with a load cell, the friction moment that works on the outer ring supported by an air bearing. For this test, 85W- 90 gear oil with viscosity of 178 mm 2 /s at 40; was used, and the oil temperature was maintained at 50±1;. Test procedures are described hereafter. First, a test bearing with a thermocouple attached to the outer ring was used, and a transparent acrylic resin loading housing was placed on the 3 3 Oil Support bearing Pulley Fig. 3 Schematic diagram of oil flow rate measuring equipment Outer ring made of acrylic resin Bearing for oil flow observation Body made of acrylic resin Oil Support bearing Pulley Fig. 4 Schematic diagram of oil flow observation equipment 24 Koyo Engineering Journal English Edition No.168E (2005)

Oil 40mm Load Motor equipment. Next, after oil supply was started under axial load of 4 kn, the inner ring was rotated at 1 000 min 1. At this time, the oil supply was adjusted in order to maintain the oil height level in the loading housing at 40mm height from the end surface of the outer ring. Then, rotational speed was increased to 2 000 min 1 and 3 000 min 1. At this time, as the oil level height came down, oil supply was again adjusted to maintain the oil level at a height of 40mm. At each rotational speed, friction torque was measured when the torque and the bearing temperature became stable. And also, in order to investigate the influence of agitating resistance on torque, friction torque with no oil supply was measured. For this purpose, after the torque and the bearing temperature were stabilized with oil supply maintaining an oil level height of 40mm, oil supply was stopped and friction torque was measured immediately after the inside bearing became empty of oil. Measurements were done four times and the average values were obtained. 3. Test Results and Discussion Air bearing Load cell Test bearing Fig. 5 Schematic diagram of friction torque measuring equipment 3. 1 Measurement Results of Oil Flow Rate Using several kinds of attachments, the oil flow into a bearing by pumping action has been controlled. Figure 6 shows the relationship between each design of attachment and the oil flow rate. Each inside oil flow rate and outside oil flow rate of the bearing was measured, and the sum of each oil flow rate is the total oil flow rate. As shown in Fig. 6, there was a remarkable reduction in oil flow rate into the bearing for type A and type C. Type A prevents the oil from flowing in from between the inner ring and the cage, and type C also prevents the oil from flowing in both from between the inner ring and the cage, and from between the outer ring and the cage. At the rotational speed of 2 000 min 1, the ratios of the total oil flow rate were 0.25 for type A and 0.17 for type C compared with the normal type with no attachment. On the other hand, the oil flow rate of type B preventing the oil from flowing in from between the outer ring and the cage showed increased results, 1.22 compared to the normal type at the rotational speed of 2 000 min 1. These phenomena show that almost the total quantity of oil taken into the bearing by the pumping action flowed in from between the inner ring and the cage. The Inside oil flow rate Outside oil flow rate Total oil flow rate a) Normal c) Type B reason the total oil flow rate for type B increased compared with the normal type can be assumed to be that the force of flowing in from between the inner ring and the cage increased because the space of the outer ring and the cage was blocked. Also, irrespective of attachment type, when the rotational speed exceeded 1 000 min 1, the inside oil flow rate decreased and the outside oil flow rate increased drastically. This is because the oil flowing through the inside space was moved to the outside space due to the centrifugal action. In other words, the oil flows in from between the inner ring and the cage at the front side and moves within the bearing from the inside space to the outside space due to centrifugal action, and then flows out through the gap between the outer ring and the cage. In view of this, in order to control the oil flow into the bearing, it is considered to be effective to control the gap between the inner ring and the cage. Then, actual oil flow behavior inside the bearing was observed. 3. 2 Observation Result of Oil Flow Behavior In view of the fact that almost the total quantity of oil flows into the bearing through the gap between the inner ring and the cage, oil behavior in the inside space of the bearing was taken into consideration. As the viscosities of the gear oil and kerosene are not the same, the relationship between the rotational speed and the oil flow rate into the bearing was investigated using kerosene. Based on this result, the rotational speed was set at 250 min 1 in order to maintain sufficient oil quantity in the inside space. These observation results are shown in Figs. 7 and 8. Figure 7 is the observation result from outside of the bearing. In the normal type and type B, many bubbles coming out between the rollers and the cage bars of can be observed. This shows that aggressive agitation of oil in the inside space is occurring. The oil taken into the inside space of the bearing through the gap between the front side of the inner ring and the cage moves to the outside space through the gap between the rollers and cage bars due to centrifugal action. At this time, it is assumed that a collision of b) Type A d) Type C Fig. 6 Relationship between rotational speed and oil flow rate Koyo Engineering Journal English Edition No.168E (2005) 25

a) Normal b) Type A c) Type B d) Type C Fig. 7 Observation of oil flow in bearing from outside view a) Normal b) Type A c) Type B d) Type C Fig. 8 Observation of oil flow in bearing from inner ring back face side view flows occurs because of the large quantity of oil being maintained inside the bearing with the normal type and type B. On the other hand, in case of type A and type C with attachments restricting the oil flow into the bearing, the quantity of oil maintained is small, so the oil moves smoothly from the inside space to the outside space with little collision of flows. It is apparent that the agitating resistance of oil is attributable to such collision of oil flow. Because this collision occurs when the oil moves from the inside to the outside space, it can be assumed that the agitating resistance can be reduced, resulting in lowering the friction torque of the bearing, by making such movement of the oil from the inside to the outside space smoother. Figure 8 shows the observation result from the backside of the inner ring. While the end surfaces of rollers on the normal type and type B cannot be identified, the same end surfaces on type A and type C can be clearly identified. It can be assumed that this is because the oil flow rate is smaller and the flow is smoother on type A and type C compared with the normal type and type B. 3. 3 Result of Torque Measurement It was clarified that smooth flow of the oil inside the bearing can be made possible by reducing the intake of oil quantity by attachments at the front side of the inner ring. Assuming that control of the oil flow has a large influence on lowering the friction torque, actual torque was measured and the relationship with the design of the attachments was investigated. Figure 9 shows the results of measurements. As assumed, it was found that the torque of type A and type C was very small, where the oil flow rate is smaller and the oil flow is smooth. At a speed of 2 000 min 1, the torque reduction ratio was 36% on type A and 40% on type C compared with the normal type. In view of this, it is apparent that bearing torque can be reduced remarkably by controlling the oil intake between the inner ring and the cage. Figure 10 is a summary of the data in Figs. 6 and 9 showing the influences on friction torque of the inside and outside oil flow rates. Reduction of the inside oil flow rate coupled with reduction of the outside oil flow rate results in lowering the friction torque. Further, based on a comparison of regression curves for both, it is clear that reduction of the inside oil flow rate had a greater influence on lowering friction torque than reduction of the outside oil flow rate. This supports our assumption that the collision of oil flows, which occurs when the oil moves from the inner space to the outer space, is the cause of increased agitating resistance leading to the increase in friction torque. 1.6 1.4 1.2 0.8 0.6 0.4 0.2 4 000 Normal Type A Type B Type C Fig. 9 Relationship between rotational speed and friction torque 26 Koyo Engineering Journal English Edition No.168E (2005)

As discussed above, it was clarified that it is possible to decrease the agitating resistance inside the bearing by fixing the attachment to the front side of the inner ring and reducing oil flow intake from the gap between the inner ring and the cage, which ultimately results in lowering the friction torque by 40%. Incidentally, in addition to lubrication, one more important role of oil flow inside the bearing is to cool the bearing by absorbing the heat generated by friction caused by the rolling or sliding contact of each part. In other words, decrease in oil flow rate may lower the cooling capability. Accordingly, bearing temperature changes were also studied using as test bearings the normal type and type C, which showed the lowest friction torque. Figure 11 shows the relationship between the rotational speed and the friction torque under the conditions of sufficient oil supply and without oil supply. The temperature in Fig. 11 shows the temperature of the outer ring at a rotational speed of 2 000 min 1. The difference in friction torque at the time of oil supply and the time of no oil supply can be regarded as the torque generated by the agitating resistance. As can be seen in Fig. 11, at the rotational speed of 2 000 min 1, the outer ring temperature was 55.3; for the normal type, the friction torque was 1.25 N m with oil supply, and it was 0.98 N m without. This difference of 0.27 N m can be attributed to the friction torque caused by agitating resistance. On the other hand, the outer ring temperature was 67.8; on type C, friction torque was 0.71 N m with oil supply, and it was 0.60 N m without. If the only reason for this lower torque on type C was the reduction of the agitating resistance, the friction torque on the normal type without oil supply and type C without oil supply should have been equal. However, actually there occurred a difference of 0.38 N m. This difference is presumably due to the reduction of viscous rolling resistance coming from a decrease in oil viscosity on the contact area. In other words, without oil supply the friction torque on type C was lower by 0.65 N m than the normal type, and it is assumed that 0.27 N m of that was due to the reduction of the agitating resistance and 0.38 N m was due to the reduction of viscous rolling resistance caused by the temperature rise. In order to compare the effect of this friction torque reduction with ball bearings, the result of comparison with a single row angular contact ball bearing 10) is shown in Fig. 12. The friction torque of this ball bearing is lower than that of the normal type tapered roller bearing. However, the friction torque on type C, with oil flow controlled, was lower than that of the ball bearing, and it decreased by 25% at 2 000 min 1 and 36% at 3 000 min 1. This result is considered to come from the reduction effects of both viscous rolling resistance and the agitating resistance by controlling oil flow into the bearing. As a result, it was confirmed that it is possible to achieve lower friction torque than that of ball bearings if both agitating resistance and viscous rolling resistance are reduced by controlling the oil flow into the tapered roller bearing. Moreover, with additional consideration from a design point of view, it will be possible to control the excessive temperature rise and achieve both antiseizure performance and lower friction torque. Inside oil flow rate 0 1 Oil flow rate, L/min Outside oil flow rate Fig. 10 Relationship between oil flow rate and friction torque 1.6 1.4 1.2 0.8 0.6 Normal Type C With oil supply (55.3:) (55.3:) (67.8:) (67.8:) 0.27 N m 0.38 N m 0.4 4 000 Without oil supply Fig. 11 Friction torque comparison data under oil supply and non-oil supply conditions 1.6 1.4 1.2 0.8 0.6 Normal (Tapered roller bearing) Type C (Tapered roller bearing) Ball bearing 0.4 4 000 Fig. 12 Friction torque comparison of tapered roller bearing and ball bearing Koyo Engineering Journal English Edition No.168E (2005) 27

4. Conclusions In order to reduce the friction torque of tapered roller bearings, the quantity of oil flowing into the bearing and its influences were studied. As a result, the following conclusions were reached. 1) The oil flows into the bearing from the front side of the inner ring by pumping action, but a higher quantity of oil flows in from between the inner ring and the cage than from between the outer ring and the cage. 2) Inside the bearing, the oil moves from the inside space to the outside space by centrifugal action, and the friction torque increases due to the collision of oil flows. 3) By reducing the quantity of oil flowing into the bearing between the inner ring and the cage, it is possible to suppress such collision of oil flows occurring at the time of oil moving from the inside space to the outside space of the bearing. This reduces the agitating resistance and, as a result, lowers the friction torque. 4) The reduction of oil quantity lowers the oil's cooling capability and causes the bearing temperature to rise. As a result, the oil viscosity becomes lower and viscous rolling resistance is reduced. 5) By controlling oil flow behavior inside the bearing, both agitating resistance and viscous rolling resistance can be reduced, and it is possible to obtain friction torque lower than that of ball bearings. References 1) K. Minato: Journal of Society of Automotive Engineers of Japan, 59, 2 (2005) 4. 2) T. Kotake: Journal of Society of Automotive Engineers of Japan, 58, 3 (2004) 14. 3) H. Hohjo: Journal of Society of Automotive Engineers of Japan, 58, 9 (2004) 4. 4) Y. Ohijiri: Journal of Society of Automotive Engineers of Japan, 59, 2 (2005) 10. 5) D. Spindler and G. V. Petery: SAE Technical Paper, 2003-01-3743 (2003). 6) H. Matsuyama, S. Kamamoto and K.Asano: SAE Technical Paper, no.982029 (1998). 7) R. S. Zhou and M. R. Hoeprich: Trans. ASME, Journal of Tribology, 113, 3 (1991) 590. 8) Y. Asai and H. Ohshima: Koyo Engineering Journal, 143 (1993) 23. 9) T. Omori, J. Okamoto and T. Wakabayashi: Proceedings of Tribology Conference (in Kita-Kyushu, Japan, 1996-10) 429. 10) H. Chiba, H. Matsuyama, K. Kawaguchi and Y. Takahashi: Proceedings of Tribology Conference (in Tottori 2004-11) 533. H. CHIBA * H. MATSUYAMA * K. TODA * * Core Technology Research & Development Department, Research & Development Center 28 Koyo Engineering Journal English Edition No.168E (2005)