A variable pitch sheave has a threaded angular faced discs that forms the V-shaped groove. The v-belt rides in the groove, which are movable. When the discs are moved toward each other, the belt rides higher in the groove and the pitch diameter of the sheave is larger, which increases the driven speed. When the discs are moved away from each other, the belt rides lower and the pitch diameter is smaller decreasing the driven speed. In contrast, with a “fixed pitch” drive design you are limited to a specific drive speed, which is a direct result of the “fixed diameter” of the sheaves.

Drive speed variation can be extremely valuable as a system is commissioned. Variable pitch sheaves are a simple method to vary fan speed to meet the required air flow specifications, allowing the system to be compliant with all engineering and regulatory requirements. Using a “fixed pitch” design would require a sheave combination change out to alter the driven fan speed, which is both time consuming and more costly. Using a variable pitch drive design can also help reduce zone air flow without relying on inefficient dampers, which restrict the air flow. Using variable pitch sheaves allows the motor and fan to operate at optimum speed characteristics.