Coupling Alignment Tolerances vs. Shaft Alignment Tolerances

A coupling alignment tolerance and a shaft alignment tolerance are different things – VERY different things!

• A Coupling Alignment Tolerance is often (but not always) the maximum amount of misalignment the coupling will operate at before premature failure.
• A Shaft Alignment Tolerance is the maximum amount of misalignment the shafts can tolerate before they begin to transfer radial and axial loads to the bearings, seals, gears, and other affected machine components.

Here are a couple of photos I took of failed coupling inserts. These were taken during some of our training classes. In each instance, I was told that the shafts were aligned to the coupling alignment tolerance. On one, the offset misalignment was about 18 mils. On the other, the angular misalignment was about 3 mils per inch.

While the couplings did transfer power for a brief time, the loads from misalignment, along with frequent starts, caused them to fail before their time. It’s not the coupling’s fault. It is a lack of understanding of the differences between coupling misalignment and shaft misalignment. And the coupling, seals, bearings, and productivity pay the price.

In our quest to Realign America we want to start an alignment movement! We need to accept the fact that the allowable misalignment for the coupling, and the allowable misalignment for the shafts, are two different things. And we need to start calling precision alignment “Shaft Alignment”, not “Coupling Alignment”!

Who’s with me?