A water pump bearing is a device used to support the end of the water pump impeller shaft. Fitting securely around the impeller shaft and tightly int o the water pump housing, the water pump bearing is designed to withstand the stress and load placed upon the water pump shaft by the engine's accessory-belt drive system. In many cases, the water pump bearing is charged with performing double duty as a bearing for the rotating impeller shaft as well as a seal to prevent the coolant from leaking out of the water pump housing.