A gear wheel is rotatably mounted on a drive shaft. The drive shaft has a plurality of longitudinal splines formed thereon. A drive sleeve surrounds the drive shaft and has a plurality of the inner splines formed on its interior surface. The inner splines slidably intermesh with outer splines of the drive shaft such that the drive sleeve can slide up and down on but cannot rotate relative to the drive shaft. A coil spring biases the drive sleeve downwardly. Drive sleeve teeth are formed around the bottom edge of the drive sleeve. Corresponding gear plate teeth are formed on the upper surface of the gear wheel and are adapted to engage the drive sleeve teeth when the drive sleeve is in its downward most position under the influence of the coil spring. In this position, the gear wheel is operatively coupled to the drive shaft.