The variable speed pulley used on a Troy-Bilt riding lawn mower is composed of two pieces, the fixed frame and the sliding center groove plate. The sliding groove plate slides between the two fixed frame grooves. As the plate slides one way, the pulley formed by one side of the fixed frame and one side of the sliding plate will either get larger or smaller. As one side gets smaller, the other side gets larger.
One drive belt goes from the engine to one side of the variable pulley. Another belt goes from the transmission to the other side of the variable pulley. The tension on these belts is equal when the mower is in operation.
The variable speed pulley is mounted on a sliding plate, which moves the pulley toward the engine or toward the transmission. As the pulley moves toward the engine, the transmission belt gets tighter. This forces the sliding plate in the variable pulley to squeeze the engine belt. The harder the engine belt is squeezed, the higher the engine belt will ride on the engine belt side of the variable speed pulley. Since the transmission is being driven by the variable pulley in its lowest setting, the transmission has more power and less speed.
If the variable speed pulley is moved toward the transmission, the engine belt will force the variable plate to squeeze the transmission belt. The engine belt will then move to a lower setting on the variable speed pulley while the transmission side will begin to ride higher. This causes the transmission to turn faster with less power.