NSK has developed a new tapered roller bearing for use in electric vehicles (including hybrid, fuel cell and battery electric variants) that offers long life even under severely lean lubrication conditions. The bearings dramatically boost seizure resistance under lean lubrication, in turn improving transmission reliability and efficiency, and enabling the use of smaller lubrication systems and more compact gearbox designs.As public concern for the environment continues to increase, consumers and businesses around the world are demanding better fuel economy from road transport, leading to the rapid development of electric vehicles (EVs). To this end, improving the efficiency of the gearbox and reducing the size and weight of components is essential in extending the maximum driving range of EVs. Therefore, the bearings used in these applications need to be small and offer low energy loss.
Conventional tapered roller bearings can bear relatively large loads for their size, but have rollers held in place at their end by a rib. As the quantity of lubrication is reduced, conventional tapered bearings will seize more frequently due to damage from sliding contact in this area, making it difficult to significantly simplify lubrication systems and reduce the amount of oil used. Accordingly, NSK set about developing a new tapered roller bearing to surmount this hurdle and clear the path to further progress.
Among the design innovations of the new bearing, NSK created a special cage that retains and distributes oil in fine grooves via capillary action. The oil is held in the cage near the roller end and, when lubrication is low, seeps out of the fine grooves to the area most in need.
Lean lubrication conditions in EVs are more common than many might think. After start-up and initial driving (which lubricates components), EVs often shut off the oil pump to conserve power, resulting in a reduced (lean) lubrication supply. There are also challenges to starting EVs at lower temperatures, such as a dramatically increased oil viscosity, making it more difficult to supply the bearings with oil.
NSK’s new tapered roller bearings achieve seven times higher seizure resistance after shutting off the oil pump in comparison with the company’s conventional alternatives. A further benefit is that gearbox bearings lubricated by oil bath systems can be redesigned to use a more efficient oil spray system. Compared with
NSK’s conventional tapered roller bearings, the latest bearings require 95% less lubrication oil to prevent seizure. As a result, a smaller pump can be used, eliminating any churning loss in the process.
The bearings also offer a reduced width, which has been achieved by optimising the cage shape. In a comparison with NSK’s conventional tapered bearings, the latest product is 10% narrower and has 5% less mass.
