October 16, 2013

# Balance Your Couplings

By Tom Shelton

Balancing your couplings is not essential to a great precision shaft alignment, though it is an essential part of a job well done. As part of your prealignment steps/checks, the keyways of the driver and driven machine shafts should be set 180 degrees apart from each other. This will help to correct any influence that is being exerted on the shafts by incorrect key lengths or incorrect set screw lengths in the coupling hubs.

The formula for determining the proper key length is; Length of keyway on hub + length of keyway on shaft divided by 2. Then multiply by .95. (hub key length + shaft key length = X, X ÷ 2 = Y, Y x .95 = proper key length)

Example: Motor shaft keyway is 5” long, the coupling hub keyway is 3” long.

5″ + 3″ = 8″, 8″ ÷ 2 = 4″, 4″ x .95 = 3.8”, The key should be 3-51/64” long.

In addition to having the proper key lengths, the coupling set screws should completely fill the full depth of the threaded hole on the hubs.

These practices will help you gain the maximum benefit from your precision shaft alignment by minimizing imbalance at the coupling.

Am I right in thinking that the key length calculated here is just about balance? Not anything to do with the key shear stress and length needed for that issue; assumes that aspect is not a problem.

You are correct.

There was a time when we would cut stepped keys to make sure the keyway was completely and properly filled. I even had a little program that would draw the key to be cut for the machinist (that probably doesn’t surprise some of you).

I am curious about the 95% multiplier…why 95%? Is it to account for a shaft keyway that is sloped at the motor? If it isn’t, then how do you account for a sloped keyway?

That is actually the reason. Thanks.

Do you use the same formula if you have an aluminum couple?

The formula calculates the amount of material that is removed for the key way. This formula is based on steel in both the shaft and hub. You would need to calculate the metal removed and determine which metal you are going to use as a key. Therefore, this is not the correct formula.