One of my co-workers recently fielded a call from a customer, asking, “What is the proper way to size shims under motor feet?” In other words, if the foot is 6 by 4 inches square, how big should the shim be?
Excellent question! And one I wasn’t sure how to answer. I have always tried to pick a shim that was at least 50% of the area of the foot, and more whenever possible. But I didn’t have any empirical data to back it up, so I did a little research.
Some shim manufacturers (Lawton and Accushim as example) rate their shim sizes by horsepower or frame size of the motor they support. So if you are using a standard NEMA frame motor, you can look up the right size shim for your application.
But, not every machine being shimmed meets NEMA specs. So, according to The Shaft Alignment Handbook (J. Piotrowski):
“The rule of thumb is provide at least 80% contact between each machined foot and its point of contact on a baseplate, frame, or soleplate” (The Shaft Alignment Handbook, 3rd Edition, p.300).
I have always tried for a minimum of 50% contact, while making sure the shim is equal around the periphery of the bolt. In other words, there should be an equal amount of shim all the way around the bolt, except for the slot.
That being said, one would have to consider not only the physical dimensions of the shim and foot, but also factor in the mass of the machine, and dynamic radial and axial forces imparted to the foot during operation of the machine. Because a shim in essence makes the foot contact area smaller. So, I blew the dust off my old Statics book, and found out that the modulus of elasticity for 304 stainless steel is about 11.2 million psi, with a Rockwell hardness of 80B. Translated, it’s pretty hard, tough, and strong! So, does it really need to be 80%, or even 50% for that matter?
I’d love to hear your thoughts on how big a shim should be, relative to the foot.