Reliability Tips

December 2002

That a perfect .000” alignment can sometimes be the wrong thing to do? Rarely, if ever, do two machines in a machine train generate the same temperatures during operation. These factors can cause perfectly aligned shafts to be misaligned out of tolerance while operating. Let’s take use the following motor driven pump as an example of this fact:

To calculate the amount of expected thermal growth use the following formula:

Thermal Growth = (T Running - T Non-Running) x h x k

• T = Casing Temperature in the Plane Of The Feet
• h = Height from Base plate to Shaft Centerline
• k = Coefficient of Expansion

In this example the motor will experience a rise in the front foot of ~2mils and 1.8mils in the rear foot. The pump will experience a greater amount of growth due to the fact that the fluid cavity will heat up a greater rate. This calculate out to ~6mils in the back and ~5.5mils in the front.

Assuming that the shaft centerlines were a perfect .000” prior to operation, they would be about .002” out during operation. These is right at the edge of acceptable on most machines, and remember, if we were just within the tolerance before operation, we would now be almost .004” out of alignment.

To correct this from happening, we must intentionally lower each machine to the amount of thermal growth expected once zero is achieved. Depending on the method of alignment used these dial indicator readings can be determined without going through the step of aligning to zero and then removing the applicable shims.

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