What are Internal Keyways?

Internal keyways on rings, usually in the form of gears, and external keyways on mating shafts have long been the preferred method of transmitting driven power through mechanical systems. The mating keyways on the shafts and rings are sized up as the components become larger, so as to transmit an appropriate amount of power in keeping with the mechanical strength of the components that are joined.

As in many fields of industry, standards have developed so that machined features, including keyways, can be made cost-efficiently and so that manufacturers can respond in a timely manner without the need to create costly specialized tooling for each keyway job. The American standard for keyways is ANSI-B17.1. And it is available where mechanical industry standards are sold.

What Keyways Do

Now before we delve into the details of ANSI-B17.1, let’s talk more about what keyways are designed to do. A female slot is cut in both the external surface of a shaft and on the internal surface of a ring. The location of these slots determines where the ring will ride on the shaft when keyed together. Into the void on each part is placed a square bar of metal called the “key”. The key is sized so that the key would have to be sheared in half to cause failure of the joint. Since shearing a rectangle of metal is a difficult process, it can be seen how this can make a strong, lasting joint. Increasing the length of the key increases the area of metal that must be sheared to cause the joint to fail, so increasing the key’s length is an easy way to increase joint strength.

Since the strength of the joint comes from the cross-sectional size of the key and not the tightness of its fit, the tolerancing of this feature need not be overdone. B17.1 calls for a total width variance of .003” and a total depth variance of .010”. Off-the-shelf keyway tooling is made to perform to these tolerances and using the sizes tabulated in ANSI-B17.1 helps ensure an economical part for which standard tooling is readily available. Tightening tolerances above these standards usually results in difficulties in installation in the field and not a stronger joint.

How Keyways Are Made

Internal Keyways are made with machines that have a single-point, reciprocating tool that follows the axis of the shaft and mechanically moves in with each stroke to create the keyway depth. Internal keyways can be made by both specialized keyway machines and CNC machines with tooling adapted to apply keyways, but it is the fixed tooling that determines the keyway width despite the fact that the CNC machines are programmable, so the .003 width tolerance should still be adhered to even if a CNC machine is used to apply the internal keyway.

Finally, the machines and processes that make internal keyways work best when the internal keyway runs completely thru the part from end to end. Measurement and chip flushing are vastly improved over a keyway design in which one or both ends of the keyway are closed off, called a “blind” keyway. While it is possible to make blind keyways, it adds considerably to cost and difficulty in manufacture. Securely capturing the key is usually easily accomplished by making the external keyway in the shaft a pocket rather that an open-ended slot. Thus, the key is still captured but the pocket feature is more easily made on the external surface of the shaft, rather than in the confined recesses of the ring part.

Courtesy: Matthew Mawhinney | Gazzam Machine | http://www.gazzammachine.com/