Parallel Key In mechanical transmission and assembly, the parallel key is a small yet crucial component that determines whether parts operate securely. Installed between the shaft and hub (such as gears, pulleys, and couplings), it transmits torque and prevents relative sliding. If the size is chosen incorrectly or installed improperly, it can lead to loosened parts, reduced mechanical precision, or even shaft damage. --- How to Select Parallel Key Size Selection is mainly ba