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.