Categories: Shaft Alignment

How Many Shims?

By on June 13, 2011

In a recent email to VibrAlign, Mike wrote:

“In using your shims what is the most shims you can place under each foot? I have always been taught to use no more than 5 shims under each foot. Is this true?“

Mike, good question!

VibrAlign’s official stance is no more than 4, if possible. However, that’s not always possible. So, here’s a “real world” answer:
• Use the smallest amount of shims possible.
• They should be flat and straight, free from burrs, slag, or dents.
• Shims should be durable enough to offer stable support. In other words, the shim should be able to support the weight of the machine that sits upon it, without its thickness changing over time. Shims should also be able to survive the elements into which they are used, such as rain, chemicals, caustics, etc.
• Shims should be checked with a micrometer or caliper to verify the thickness.
• Always use the thickest shims possible to minimize the number of shims used under a particular machine foot.
• If shims are bent, damaged, rusty, or otherwise deformed, they should not be used.
• When shim stacks are removed to change the thickness of the stack, care should be taken to carefully measure the entire stack, including the dirt, grit, rust, etc. The shim stack should either be cleaned before re-use, or new shims should be used.Inserting Shim for shaft alignment
• When shims are placed under a machine foot, they should be carefully slid under the foot until they make contact with the bolt, and then slid back about ¼”, to make sure it does not contact the threads of the bolt. This way, when the bolt is tightened, it will not bend the radius of the shim, and deform it.
• Shims should be stored in a neat, clean, and orderly fashion.
• When aligning a machine, try to lay shims on a shop towel, or other clean surface, to minimize the risk of adding dirt or other contaminants under the machine foot.
• When aligning electric motors with cooling fins, the fins make excellent places to place the shims during alignment. The shims can be placed in the fins above the foot under which they will be placed, helping to minimize errors in shimming.

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