The Influence of Spin and Lifetime in Design of Traction Drive Continuously Variable Transmissions

Fundamental issues on the TD-CVT (Traction Drive Continuously Variable Transmission) are efficiency, lifetime and variable speed ratio control. One reason of decreasing efficiency on the TD-CVT is the spin motion. Spin causes power loss, reduces available traction forces, and generates traction forces perpendicular to the rolling direction. Thus, spin has an ill effect on traction drives and should be kept as small as possible to have best performance. For optimal performance, it is necessary to find a traction drive configuration that results in a minimum amount of spin. TD-CVTs operate in the high contact-stress. The achievement of a given number of stress repetitions without failures can only be assigned a probability. Most drives are rated against the 90% probability of surviving the number of stress repetitions associated with the rated number of hours of operation at the rated speed and load. A contact fatigue life analysis for TD-CVTs based on modified Lundberg-Palmgren fatigue theory is used to predict lifetime for a compact ball variator TD-CVT for bicycle. The effects of power, size and transmission ratio on TD-CVT lifetime are shown.