The key to a proper fit for keyways

Selecting the proper size keyway is determined by the shaft size. Sometimes you can put a square peg in a round hole.


Did you ever have someone tell you that you cannot fit a square peg into a round hole? This common phrase is misleading because it details the dimensions of neither the peg nor the hole. Clearly if the peg is 1-inch square and the hole is 4 inches in diameter, then the peg will easily fit in the hole, albeit useless. However, if that same peg was 25 mm square with a tolerance 0/-0.01mm on each side and the hole was Ø25mm with a tolerance of +0.01/0mm, it would slide in easily and grab the hole at the corners.

In October’s column, I mentioned the typical tolerance for keyways. Table 1 details the appropriate key slot and key sizes for various metric bores. Please note that the key sizes in parentheses are older sizes that are no longer commonly used. 

As noted above, when selecting a keyway tolerance, there are two common selections in the metric system. The first is Js9. This is a +/- band clearance; the value of the tolerance is equally oversized or undersized. The second is a P9 tolerance. This is an undersized clearance. The advantage of the Js9 tolerance is that the key can be inserted and the gear manipulated without much difficulty. Whereas the P9 tolerance is a press fit tolerance. Once the key is inserted into the keyway, it is not going to move.

For those engineers who wish to put a square peg in a round hole, please consider the following:

A square consists of four angles, each being 90 degrees. Therefore, there are 360 degrees in a square.

There are 360 degrees in a circle. Therefore, a square is a circle?

Table 1: Flat keys and keyways

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Brian Dengel