The effect of ZDDP additive in CVT fluid on increasing friction coefficient between belt elements and pulleys of belt-drive continuously variable transmissions

This paper discusses the effect of the zinc-dialkyldithiophosphate (ZDDP) a dditive on increasing the friction obtained at the sliding interfaces betwe en the elements and the pulleys of a belt-drive continuosly variable transm ission (B-CVT). Rig tests were performed with different types of CVT fluid to verify the ef fect. of the ZDDP additive. The effect of surface roughness on film formati on was also examined. The ZDDP additive increases the friction coefficient at the interfaces due to a film adsorbed on the sliding surface. A quantita tive correlation was seen between the amount of Zn contained in the film an d the friction coefficient as a measure of the B-CVT torque capacity. Under boundary conditions, the friction coefficient between the belt elemen ts and the pulleys decreased as the amount of Zn was reduced due to oil deg radation. Among the elements of the ZDDP additive and Ca phenate detergent, only the Zn concentration showed a correlation with the friction coefficie nt, which is attributed to the fact that the film contained Zn generated fr om the ZDDP additive.