Abstract: The invention relates to a variable speed fan drive. There is a need for a variable speed fan drive with fewer seals and bearings, which is operated by system hydraulic pressure, which has fewer parts and which is simpler to assemble. A variable speed fan drive is provided for a vehicle powered by an internal combustion engine. The fan drive has a driver variable sheave unit, a driven variable sheave unit and a belt engaging both the driver and driven variable sheave units. The driver variable sheave unit includes a driver shaft drivingly coupled to an engine output shaft, a first axially fixed half sheave mounted for rotation with the driver shaft, and a first axially movable half sheave member mounted for rotation with the driver shaft and movable axially on the driver shaft. The first axially movable half sheave member includes a belt engaging flange formed integrally with an axially movable piston member mounted for rotation with the driver shaft and movable axially in response to hydraulic pressure.