Variable speed belt drive sheaves are common on combines. At their simplest, a pair of sheaves at one end of a belt moves closer together or farther apart. Another pair of sheaves at the other end of the belt is spring loaded, and as the upper sheaves widen or narrow, the spring(s) in the driven sheaves forces those sheaves to do the exact opposite of the upper sheaves. The arrangement is used in older combines to speed up and slow down ground speed, often used in newer combines to speed up or slow down belt driven feederhouses, and frequently used to speed up or slow down cleaning fans.
The big thing to remember about variable speed belt drive sheaves is that both the upper and lower sets of sheaves sliiiiide closer together or farther apart. And to sliiiide smoothly, they need to be frequently greased. "Frequently" generally means daily, if not more often. If variable speed belt drive sheaves don't get greased frequently--especially the spring-loaded sheaves-- they don't sliiiide easily. If the drive sheaves are forced apart by hydraulics or mechanical linkages when the operator wants to change the speed of that system, and the driven sheaves don't sliiiide easily, they don't grip the belt tightly. That means the belt runs slack. Which means the belt starts to slip, which builds heat into the driven sheaves, dries up what little grease is in the sheaves, and makes it even harder for the driven sheaves to move in response to the drive sheaves. I've seen guys replace three or four drive belts before they figured out the problem was with un-greased driven sheaves hanging up and allowing the belts to slip and self-destruct.
The bottom line is to keep the sheaves in a variable speed belt system well lubricated. Both the drive and driven sheaves. If you're thinking, "I don't think there are grease zerks on THAT sheave..." get out your owner's manual and double check. There's probably a zerk hidden somewhere on any variable speed belt drive sheave, that would really appreciate a couple shots of grease every day.