Wheel-hub – or “in-wheel” – motors have many potential attractions, including reducing vehicle weights by eliminating the need for engines and power transmission components, as well as increasing cabin space. The drive force to each wheel can be controlled independently (in a similar way to torque vectoring).
But a major challenge to date has been delivering the drive torque needed for acceleration and hill climbing, as well as achieving high speeds. NSK’s prototype motor, which can fit inside a 16-inch wheel, incorporates two separate motors and a transmission consisting of two planetary gears that can deliver both high drive torque and high speeds.
By controlling the speed and torque of the two motors, the transmission system provides smooth gear-changing, even during acceleration. In high gear operation, the two motors revolve in the same direction. When low gearing is needed, they revolve in opposite directions.
NSK has demonstrated the performance of the prototype motor in an experimental vehicle. It is now looking to commercialise specific components, including the wheel hub bearing with an integrated speed reducer, and a one-way clutch. The company is also planning to develop further the motor’s miniature cage and roller bearings, as well as an anti-corrosion bearing, to make them ready for production.