When is a keyway connection preferable for a coupling application?

By Zak Khan

Keyway connections refer to a slot, or “keyway” cut into one part of a coupling while the other portion has a corresponding  “key.” They have some advantages, namely an ability to transmit better torque and reduced slipping. Additionally, misalignment compensation is possible. Manufacturers usually fit the keyways and keys during production.

Applications that benefit from keyway connections include those that require high torque transmission. If especially high torques and high speeds need to be transferred, then two keys and keyways may be used. Keyways can reduce backlash and vibration as well.

There are common standards in use to size and fit keys and keyways. These have their associated tolerances as well. Keys and keyways come in three main types: square parallel keys, rectangular (also called flat) keys and plain tapered keys. Shaft diameters for keyed shafts usually range from 5/16 to 7-in., though manufacturers also offer custom options in some cases. Keys also come with varying standards of fit. These are: commercial, precision and fitted.

Commercial keys and keyways are the most widely used kind. They suit most applications well, and their tolerances usually aim for slight under-sizing when matching keyway and key. They have less stringent requirements and tolerances. Precision keys have stricter requirements. They have tighter tolerances for clearance on the top of the key and their tolerances usually aim for slight oversizing. Fitted keys are the most stringent. This standard requires the use of keys that are oversized, and there are fitted to suit the keyway. The height of the key may also be fitted if required.

Keys are practically useless if improperly fitted. If the fit is off, there is no capacity for better torque transmission. Keys and keyways that fit improperly can cause them to roll, or the coupling can even split.