Technical Trend of Bearings for Automotive Drive Train

Transcription

1 SURVEY Technical Trend of Bearings for Automotive Drive Train M. KITAMURA Circumstances surrounding the automobile industry require improvement in fuel efficiency and reduction of CO 2 gas emission in view of preventing global warming. Under such conditions, drive train bearings are also required to provide low torque, compactness and lightweight, etc. Here development trends for each of these bearing needs are briefly introduced. Key Words: drive train bearing, environment, fuel efficiency, weight-reduction, compactness. Introduction Automobiles are required to be more and more improved in the fuel efficiency and the reduction in CO 2 emission considering surrounding circumstances for the prevention of global warming. With regard to automotive drive trains, not only technology improvement in mechanical efficiency, and fuel efficiency through weight reduction, etc. but also new technical developments on power trains as hybrid automobiles etc. have been attempted. On the other hand, improvement in engine performance, steering stability and comfort such as amenity are required. It is thus necessary to respond to such needs for higher power and compactness/ lightweight. Figure shows the needs for drive trains and corresponding technical issues for bearings. In this paper, recent technical trends on automotive drive trains are described. bearings. Consequently tapered roller bearings are more compact and are used in many drive train units. However, there is a problem of larger frictional torque when compared with ball bearings due to the sliding contact between the rib and the rollers. Low friction tapered roller bearing (hereinafter LFT bearings) have been developed with improved sliding contact, optimized raceway profile and reduction in both sliding torque and rolling viscosity torque by about 2~4% when compared with conventional bearings. These bearings are now in mass ion. LFT! Aim Sliding resistance between rib and rollers Needs for drive train units Technical issues of bearings Improvement in fuel efficiency Reduction in unit loss Stoppage of idling HV (hybrid vehicle) CVT (continuously variable transmission) Multiple shifts in AT/MT Light weight/compactness Low cost Improvement in comfort Improvement in shift operation Reduction in NVH (noise/vibration/harshness) 2. Technical Issues Lower torque of bearings Lower viscosity of oil, reduction in oil amount Long life bearings at high rotational speed Smaller bearing mounting space Lower cost (pressed-steel bearing) Improvement in mounting stiffness Improvement in seizure resistance Lighter weight, higher efficiency Materials, heat treatment, internal design Reduction in preload loss Fig. Needs for drive train units and technical issues of bearings 2. Lower Torque, Higher Rotational Speed ) Tapered roller bearings Tapered roller bearings can support both radial and axial load within a smaller size when compared with other types of Running torque ratio.5,, Fig. 2 Aims of LFT bearing LFT! Fig. 3 Effect of low torque Rolling resistance Koyo Engineering Journal English Edition No.64E (24) 9

2 To attain further reduction in torque, reduction in the agitation resistance of lubricants and limiting the supplied amount of oil have been focused as targets and new type of bearings with torque reduced by about 2~3% compared with LFT-2 bearings (improved LFT bearings) have been developed through optimization of the contact angle and roller configuration (diameter, length, number). By adopting the new bearings to the rear differentials, ~2% reduction of fuel efficiency in EC mode is expected. Loss of torque, N m % Full complement Retainer type () Optimized internal bearing design in the same mounting space Rotational speed, min Measurement of lost torque Fig. 7 Effect of low torque Torque ratio Fig. 4 Example of optimal design Gear oil SAE From sliding bearings to rolling bearings Many sliding bearings are used in automatic transmissions and reduction in torque is possible by changing these to rolling bearings. Newly developed extremely thin shell type needle roller bearings enable the replacement with rolling bearings in the same mounting space. Moreover, bearings with controlled oil penetration have been developed when they are to be used adjacent to the oil pump. Reliability of oil seals can be secured as before and drastic reduction in torque can be achieved. Baffle ring 3.5 Fig. 5 Low torque effect on developed 2) Needle roller bearings For planetary gears Presently full complement roller type needle roller bearings are mainly used for planetary gears. However, in view of intensified needs for low torque and the tendency for high-speed rotation coming from multiple shifts, needle roller bearings with retainers have been developed. Torque reduction of about 33% compared with full complement roller types has been achieved, not only through the reduction in sliding friction among rollers resulting from the change to retainer type, but also through the optimal crowning applied to the rolling contact surface to reduce the skew force of rollers. Low rolling resistance Skew force (sliding resistance) u44 u38 Fig. 8 Bearing with controlled oil penetration Rotational torque of developed needle roller bearing vs. conventional bearing (bushing) Rotational torque, N m bearing 8: bearing 2: NRB 8: NRB 2: Rotational speed, min Fig. 9 Effect of rolling bearing Full complement needle roller bearing Needle roller bearing with retainer Fig. 6 Needle roller bearing with retainer 2 Koyo Engineering Journal English Edition No.64E (24)

3 2. 2 Longer life Measures for Contaminated Oil Bearings used with oil lubrication are often lubricated in the contaminated oil with metal chips and burrs produced during machining of parts and metallic abrasive powders generated while the bearings are in use. Bearings lubricated in these conditions cause the flaking initiated at surface, which might be originated at the edge of a dent due to contamination from foreign materials or at peeling due to wear. In the case of conventional bearings, life will be very short i.e. / - or /dozensth compared with bearings lubricated in clean oil. Koyo developed both KE-heat treated (special heat treatment) bearings with higher surface hardness and optimized residual austenite and KE-2 bearings with high case hardening. The newly developed bearings have achieved longer life by about and 5 times respectively compared with that of conventional bearings. These bearings have remarkably contributed to compactness, lighter weight and higher capacity of the unit. Life ratio in contaminated oil Cumulative failure probability, % Measures for Higher Load and Higher Moment Increased engine torque and reduced weight of the unit case not only increase the bearing load but also increase shaft deflection. Consequently, taper roller bearings that have line contact can have a shorter life due to the edge stress. Koyo has developed the crowning (logarithm) profile which levels the pressure distribution at the rolling contact surface based on rigidity analysis (Fig. 2) of the case and the shaft utilizing the latest computer technology. The bearings are now manufactured on a commercial basis as LC (Long Life Crowning) bearings. When misalignment is large, LC bearings KE bearing Cost ratio (HC as ) Fig. Aim of KE-2 bearing HC KE 5 5 bearing Life in hours, h Result of life test in contaminated oil Fig. Effect of longer life can endure more than twice the life compared with conventional bearings. The concept of this crowning profile is also applied to standard bearings. bearing bearing Crowning profile Distribution of contact surface pressure Crowning profile Distribution of contact surface pressure.8 Pmax=2 8 MPa Y.8 Y Pmax=4 53 MPa Z (MM) Z (MM) X (MM) X (MM) X X (MPa) (MPa) Fig. 2 Distribution of contact surface pressure Misalignment and life ratio (calculated from contact stress) 3 bearing Life ratio 2 bearing 3 6 Misalignment angle of shaft Fig. 3 Effect of longer life 7.27 Koyo Engineering Journal English Edition No.64E (24) 2

4 2. 3 Light Weight, Compactness Pressed Steel Bearing With the progress of pressed steel processing and heat treatment technology, clutch release bearings using thin pressed steel plates as bearing races have been developed. While attaining the same life as that of conventional bearings, the pressed steel bearings have achieved the weight reduction of about 5% and the cost reduction of about %. Wave spring washer Inner ring Outer ring Sleeve Fig. 4 Comparison between conventional and pressed steel bearing Plastic Retainer Plastic retainers compatible with various gear oils have been developed with newly developed high performance materials having reinforced fillers. Plastic retainers are widely used in ball, needle roller and cylindrical roller bearings etc. Figure 5 shows an application example of cylindrical roller bearings to manual transmissions for a medium duty truck, through which the weight reduction of about % and cost reduction have been achieved. Tensile strength of retainer, kn Tensile strength of retainer, kn For tapered roller bearing and ball bearing Nylon Dipping hours, h For cylindrical roller, needle roller and ball bearing 66 Nylon Dipping hours, h Fig. 6 Strength of plastic retainer 2. 4 Unitization For the purpose of weight reduction in differential units, light alloy materials such as aluminum alloy and magnesium alloy are increasingly used for the carrier material. As a result, reduction in preload variation, improvement in mounting stiffness, weight reduction, compactness, simplified and consistent assembly by the customer is required. Koyo has developed compact unit bearings. Fig. 5 Example of resin retainer Pinion bearing (u45 u95 68) Shim Parts structure Pinion unit Spacer Shaft, carrier Simple Head BRG structure Tail BRG Spacer Unitization Head BRG (A) Tail BRG Oil seal Height adjusting shim Easy adjustment Fig. 7 Example of differential pinion unit 22 Koyo Engineering Journal English Edition No.64E (24)

5 Moreover, in automatic transmissions, design freedom of bearing support position, securing of mounting stiffness, simplified assembly and stabilization of assembly preload are required with regard to complicated construction due to multi- AT/MT. Figures 8 ) and 9 show application examples of automatic transmission unit bearings. Reference ) Mitsubishi Motor Corporation: F4A4 Manual (23-5). Fig. 8 Application example to automatic transmission u85 3. Conclusion With flange Fig. 9 Unit bearing This paper presented the recent technical issues concerning automotive drive train bearings. It is expected, however, that further evolution and diversification of drive train systems and units will advance from an environmental point of view and higher levels of technology not yet seen will be required for the bearings to be used. It is the strongest desire of Koyo to continue presenting many impressive valuable technologies by advancing bearing technological development utilizing such core technologies as tribology, heat treatment and precision ion engineering from a long-term viewpoint. M. KITAMURA * * Deputy Director, Chubu Technical Center, Bearing Business Operations Headquarters Koyo Engineering Journal English Edition No.64E (24) 23