Innovative Bearing Concepts for the Powertrains of the Future
Georg von Petery
Modern rolling bearings are indispensable when it comes to the refinement of transmissions with regard to friction and power density. New design space conditions and requirements that this key component will be confronted with by developments such as powertrain electrification may result in optimized designs as well as new bearing types and concepts.
This paper deals with a new bearing design that makes it possible to use low-friction locating/non-locating bearing supports even more frequently than before. It also shows that the consistent optimization of known bearing designs can help further reduce friction, decrease design space requirements and also reach the high speed level of electric drives at moderate temperatures. In addition, the paper describes ways in which application limits can be extended through the use of coatings.
This results in new requirements for bearings, including:
• Higher load capacity with even less friction
• Higher speeds: Today, the speed parameter that is the product of the speed and the mean bearing pitch circle is at around 0.2 - 0.5 million mm/min for oil-lubricated transmission bearings and will increase to up to 1.3 million mm/min for grease-lubricated bearings in electric motors.
• Noise reduction
• Greater temperature differences between the bearing rings: The temperature differences between the bearing inner and outer ring increase from 15 - 20 K to up to 70 K due to the more frequent starts and stops as well as the heat transferred from the rotor. This large temperature difference must be taken into consideration when dimensioning the internal clearance and the heat treatment of the bearing rings.
• Protection from electric current and resistance to non-operate currents
• Increased power density
• Grease lubrication: The temperature limit of available high speed greases is too low. Classic high-temperature greases are thermally very stable but they are limited to speed parameters n x dm up to 1 million mm/min. Schaeffler has resolved this conflict of goals between speed parameters and temperature stability by means of a newly developed grease.
• Oil lubrication: In transmission bearings, the use of oils with ever lower viscosity leads to more frequent operation in mixed friction. The probability of surface-induced damage increases. Some modern transmission oils even promote the occurrence of white etching cracks in the bearings. The Durotect B coating developed by Schaeffler counteracts both effects.
Friction-optimized tapered roller bearings and locating/non-locating bearing supports
Many transmission shafts are currently supported by tapered roller bearings. The reason for this lies in the high robustness of this bearing type and in its very good ability to support loads. In addition, tapered roller bearings are characterized by a comparatively small cross-section as well as mostly simple assembly.
Figure 1 Comparison of ultra low friction tapered roller bearings
Figure 2 Adjusted tapered roller bearing supports (left) versus locating/non-locating bearing supports (right)
Deep groove ball bearings are used as locating bearings and cylindrical roller bearings or radially compact drawn cup cylindrical roller bearings are used as non-locating bearings. One advantage here is that the point contact of the deep groove ball bearing in partial-load operation produces less friction than the line contact of the tapered roller bearing. However, deep groove ball bearings have a comparatively low capacity for supporting loads. In addition, they have a certain axial and radial clearance. That is why their suitability for specific applications must be checked on a case-by-case basis.
Angular roller unit (ARU): Innovative locating bearing with high load rating and low friction
Figure 3 Comparison of the dimensions of a deep groove ball bearing and an angular roller unit with the same load rating
Figure 4Figure 4
Figure 4 Comparison of the design principle of a tapered roller bearing and an angular roller unit (ARU) (left) as well as loading conditions for an ARU with preferred direction (right)
The ARU is suitable for all mounting positions that require a higher load rating than a deep groove ball bearing can provide. The height of the axial forces that occur must be checked in detail and the bearing must be mounted in the preferred direction.
Simulation of various bearing concepts
Figure 5 Mounting position of an angular roller unit (ARU) in a six-speed manual transmission on the transmission output shaft
For a comparison of the bearings, four different concepts are analyzed using the output shaft as an example as well as using identical design spaces.
• Adjusted bearing supports with standard tapered roller bearings
• Adjusted bearing supports with friction optimized tapered roller bearings
• Locating/non-locating bearing supports with deep groove ball bearing and cylindrical roller bearing
• Locating/non-locating bearing supports with ARU and cylindrical roller bearing.
Figure 6 compares the calculation results for fifth gear in each bearing variant. The friction losses shown in the top part clearly show that conventional tapered roller bearings that are adjusted against each other have the highest friction. Optimized tapered roller bearings already achieve significantly lower friction torques, but they are outperformed by locating/non-locating bearing supports using deep groove ball bearings and cylindrical roller bearings. The locating/non-locating bearing supports using ARU and cylindrical roller bearings perform similarly well in terms of friction as the solution with deep groove ball bearings.
Figure 6 Simulation of friction (top) and rating life (bottom) for the 5th gear of a manual transmission
For calculated life, tapered roller bearings not specially adjusted to low friction show the best performance. However, they are over dimensioned in many applications and should not be preferred because of their high friction. Optimized tapered roller bearings continue to offer a sufficient rating life, even if it is much shorter than for bearing supports with conventional tapered roller bearings. Generally, the locating/non-locating bearing concepts should be preferred due to their excellent friction characteristics. However, the calculated life is not sufficient for the solution using a deep groove ball bearing in fifth gear. With the ARU as the locating bearing, however, a longer rating life can be achieved than with optimized tapered roller bearings, while also having lower friction. Thus the conversion from adjusted tapered roller bearing supports to locating/non-locating bearing supports without modifications to the design space would only be possible by using an ARU bearing as a locating bearing.
Validation of simulation results
Figure 7 Comparison of the simulated friction torque and the friction torque measured on the test stand for the new ARU bearing design
Figure 8 General design of the test stand used to validate the rating life calculations for the new ARU bearing design
Figure 9 Increase in radial load capacity and reduction of design space requirements and weight through an improved design using an RNU308-E-XL type cylindrical roller bearing as an example
Bearings for automatic transmissions and CVT
Transmissions have a large number of components with rolling bearing supports. The designs used essentially depend on the transmission type and its structural design. In manual transmissions, double-clutch transmissions and CVT, the main bearings tend to be tapered roller bearings, deep groove ball bearings and cylindrical roller bearings or drawn cup cylindrical roller bearings. The constant mesh gears are supported by needle roller and cage assemblies. By contrast, automatic transmissions with a converter have a large number of thrust needle roller bearings, radial needle roller and cage assemblies and planetary gear bearings.
Planetary gear bearings are subjected to extreme acceleration in stepped automatic transmissions as a result of high relative speeds, with some as high as 5,000 times the acceleration of gravity. The resulting forces lead to elastic deformation of the planetary gear bearing cages as well as to increased friction and cage wear caused by an unfavorable contact geometry. The shape optimization of the cage, in particular the cage bars between the rolling elements, has led to a significant improvement in durability. The use of an additional, directed blasting process and thus the application of residual compressive stress can increase cage strength even more. Testing on planetary gear set test stands has confirmed the increase in strength and also showed reduced cage wear. Coating with Schaeffler’s Durotect M can have a positive effect on cage friction, which largely depends on the cage surface, and reduce it by up to 25 %.
Figure 10Figure 10
Figure 10 Increase in robustness against WEC damage with Durotect B
Figure 11 General measures against current damage to bearings
Figure 12 Bearing concept requirements for electric drives
Besides optimized rolling bearings, more and more smaller assemblies have lately found their way into transmission applications. The combined use of components allows the development of integrated bearing solutions that offer advantages with regard to operating efficiency as well as mounting and installation space.
Figure 13 Integration of bearing systems using a bearing cartridge for a hypoid drive as an example
Figure 15 Bearing with integrated resolver
Figure 14 Locating/non-locating bearing supports as a compact unit with a shaft and gear teeth for a hydraulic auxiliary drive in commercial vehicles
Reducing friction while ensuring a high level of robustness remains the primary task of transmission bearings, irrespective of whether they are used in conventional or electrified powertrains. It has only recently become possible to optimize bearings in a very short time by using the OPTIKIT program developed by Schaeffler. Analyses of tapered roller bearings have shown that this can yield increased friction advantages of up to 20 %. Volume production of tapered roller bearings of this so called ultra low friction design recently began for axle drives.
With its innovative angular roller unit (ARU), Schaeffler has developed a locating bearing that has a 40 % higher load rating at low friction than a deep groove ball bearing with identical design space. Cylindrical roller bearings with narrow lips offer a reduction in bearing width by up to 20 % or, alternatively, an up to 27 % higher load rating with the same bearing width. This allows low-friction locating/non-locating bearing supports, such as those required for modern manual, double clutch and reduction transmissions of electric axles, to be achieved even more frequently than before.
With electric drives, there has been a trend towards increasing speeds, which means up to 30 % higher speed values for bearings. Reliable bearing supports for these high-speed drives can only be achieved with bearings that are characterized by high manufacturing accuracy in combination with specially developed greases, seals and cages. That is why Schaeffler has developed bearings for electric drives that are combined in a modular range with preferred types and adapted specifically to these new requirements. For applications in which electric current may occur, Schaeffler has developed potential solutions for integrating grounding into the bearing; these solutions have already proved to be effective in tests. Due to the shared use of components, such highly integrated bearings offer benefits in terms of efficiency as well as mounting and design space.
With these very specific bearing arrangements that are customized for the relevant application, Schaeffler will continue to contribute to efficient transmissions for conventional and electrified powertrains.
Modern transmissions are unthinkable without high-performance rolling bearings. The bearings are designed for specific applications and with the overall system in mind. Schaeffler has developed its own calculation programs .
Tapered roller bearings are continuously refined by Schaeffler to increase their performance and reduce the friction torque level. Its latest designs combine individual, application-specific designs with the best possible manufacturing standard. These include the optimized design of the contact angle, special raceway profiling, the use of tapered rollers with optimum length and diameter, the right number of rollers for a specific load, narrow tolerances and improved surfaces as well as the use of improved materials and heat treatment processes .
In spite of the progress that has been achieved in the development of tapered roller bearings, changing to locating/non-locating bearing supports can further reduce the friction power because there is no axial preload of the bearing. The time and money invested in the reconfiguration of the transmissions justifies the change in the bearing concept for fundamental revisions of existing transmissions or new designs. Due to such successor transmissions, manufacturers successively change their transmissions to locating/non-locating bearing supports , Figure 2.
The angular roller unit is a new locating bearing that has a higher power density than conventional deep groove ball bearings. This inevitably leads to the question of what optimization potential there is on the non-locating bearing side. Due to their design, cylindrical roller bearings are ideal non-locating bearings because they permit displacement within the bearing, thus preventing displacements between the bearing outer ring and the housing. NU and N type single-row cylindrical roller bearings with a lip-free inner or outer ring as well as NJ type cylindrical roller bearings with two fixed lips on the outer ring and one fixed lip on the inner ring are particularly suitable as non-locating bearings for compensation in the bearing .
Thanks to simulation-based geometry design and corresponding production processes, it has been possible to reduce the friction of thrust needle roller bearings by up to 50 %, depending on the load ratings and speed levels . The new geometry has slightly curved raceways, allowing approximate point contact at low to medium loads and thus reduced differential slip of the needles, as well as the formation of a full line contact at high loads. Initial applications have already been equipped with this bearing design, and volume production has begun. Testing has also shown considerable friction advantages in planet carrier bearing supports due to very compact thrust needle roller bearings.
Due to the largely radial loads as well as based on cost, deep groove ball bearings . are used as bearing supports for electric motors. The bearing supports of electric motors must ensure the reliable guidance of the rotor to the stator, compensate misalignments and allow safe operation with low noise levels. The bearings are adjusted elastically and axially in order to compensate the increased radial internal clearance that is necessary due to the great difference in temperatures of the inner and outer ring. The bearing rings are dimensionally stabilized to prevent ring growth due to high operating temperatures.
The hybridization of transmissions at the P2 position is of particular interest . In P2 hybrid modules, the electric motor is arranged between the transmission and the internal combustion engine. Two clutches and their clever activation makes it possible to drive with just the internal combustion engine, with electricity only or with a combination of both drives . In addition, coasting and recuperation can be implemented. That is why Schaeffler has developed hybrid modules for the P2 position. The optimized bearing supports for electrified transmissions are used in these hybrid modules as well as in Schaeffler’s electric axles .