Primary Clutch
The primary clutch is mounted to the PTO shaft or output shaft of the motor. It responds to two different forces: centrifugal force from the engine spinning the clutch as well as resistance that the belt transfers from the secondary clutch (more on that later).
Here is how it works. (see figure A) Inside the engine clutch (the clutch mounted to the engine is sometimes called the drive clutch) are some weights (15) that pivot on a pin (16) mounted to the movable sheave. The weight is shaped like a half rounded ramp and pushes against a roller mounted to the center section of the clutch called the spider.
The spider cannot move as it is threaded onto the center shaft that connects the two sheaves together. So when the clutch starts spinning, the weights try to swing away from the center of the clutch towards the outer side of the clutch, but they hit the rollers (13) which are mounted to the spider. So in order for the weight to move, something has to give. The force from the weights pushing against the roller forces the movable sheave to move toward the fixed sheave, pushing the belt higher in the clutch until it reaches full shift.
The clutch also has a spring and a clutch cap. The cap is bolted to the movable sheave and has a hole in the middle in order to allow it to slide down the shaft with the movable sheave. Installed in the middle of the clutch between the spider and the cap is a spring that resists the movement of the movable sheave. As the movable sheave starts to shift, the cap compresses the spring resisting the shift. This is necessary in order to prevent the centrifugal force from causing the clutch to start shifting at an idle as well as to control the rate at which the weights shift out.
In other words, if it weren't for the spring, when the engine started, the centrifugal force would just slam the clutch all the way open. The spring also helps the moveable sheave return to the lower ratio as throttle is reduced.
Secondary Clutch
