A belt-driven fan is bolted to a pulley on the water pump and turns constantly with the engine. Thus, beltdriven fans always draw air through the radiator from the rear. The power pulley on the crankshaft drives the belt. The fan has several blades made of steel, nylon, or fiberglass attached to a metal hub. Any damage or distortion to the fan will cause it to be out-of-balance, and the fan should be replaced. An out-of-balance fan can cause major problems, including rapid and excessive water pump bearing and seal wear or damage to the radiator if the fan blades hit the radiator.