Sorting out shaft connections

Motors and gearboxes that generate high torque and acceleration may make shaft keys obsolete. Designers should analyze stresses and backlash when weighing keyed versus keyless shafts. Stresses in the keyway are critical in evaluating the shaft's torque capacity. Opposing forces exerted by the shaft and hub attempt to shear the key. Loads applied by the shaft and hub keyways can compress and permanently deform the key. Manufacturing tolerances on key and keyway contribute to backlash. Knowing the advantages and disadvantages of keyless and keyed shaft connections is vital when connecting gearboxes and motors. Today's demands for speed, precision, and compactness have motor and drive manufacturers making smaller devices that generate high torque, rapid acceleration, and accurate rotary positioning. But these advances also mean backlash, stress distribution, and balance must be addressed in smaller shaft-locking devices. In many cases it takes keyless connections to handle the dynamic loads, rendering shaft keys obsolete. A straightforward way to compare keyed and keyless designs is in terms of torque transmission. Let's look at an example using a 16-mm shaft. Shaft and key are made of 35S20 steel, with key and keyway dimensions sized according to DIN Standard 6885. The following calculations determine the maximum torque that can be transmitted through both keyed and keyless shafts, as well as the maximum transmissible torque for the key. Assuming the coupling does not slip, torque transmitted through the shaft is: The polar moment of inertia is: /2. And π, the yield stress for 35S20 steel, is 380 X 10 This is the maximum torque that can be transmitted through the keyless shaft before plastic deformation begins. Stress levels on the keyway sides are critical in evaluating keyed applications. Assuming the key will not fail before the shaft, the shaft can hold a maximum torque of: , the effective