June 9, 2012
By Stan Riddle
Stainless steel shims have for the most part replaced cutting shims from rolled stock. They speed up the alignment process dramatically. They usually assure a more accurate alignment since they come in many different thicknesses. They are clean, flat, and their thickness is marked on each shim. Since they are stainless steel, they are not prone to rust.
Here are some good guidelines for using shims.
- The thinner shims are sharp. Care should be taken when using them.
- Shims should be installed under motor feet until they contact the bolt and then be pulled back slightly. This is so the bolt threads do not bend the shim as bolts are tightened.
- Shims should be clean and flat.
- Use fewer, thicker shims – not a large number of thinner shims. Try to use no more than 4 shims under each foot whenever possible. If the machine needs a large amount of shim, try to use one thick, steel block, and a small number of shims. The block can be drilled, milled, or torched out, but should be ground flat before use.
- Shim stacks should be measured with a micrometer or caliper. Never just go by the numbers etched onto the shim. In most cases, shims 0.020″ and smaller are from ground stock, with a tolerance of +/- 0.0005″. Shims 0.025″ and thicker are made from rolled stock, and can have a tolerance of +/- 0.005″. This can be important when aligning to close tolerances.
- Dirt is a shim–as is rust, paint, sand, and anything else between the foot and the base. All of this should be taken into consideration when measuring shims already in place. Under most circumstances, painted, rusty, or otherwise tarnished shims should be replaced with clean stainless shims. Used shims can always be cleaned and re-used later.
- When measuring shims for replacement, try to remove the entire stack of shims, being careful not to disturb the stack while removing. Measure the entire stack with a micrometer or caliper. The best place to measure is normally in the inside radius of the shim.
- When angular soft foot is present, it is advisable to cut a shim to fill in the void from the angular soft foot. However, only one shim should be cut. Cutting shims in a “stair step” fashion is not advisable, since it is difficult to get each step shim back to the same place.
- When placing shims under machine feet, “sandwich” thinner shims in between thicker shims whenever possible.
- If traditional rolled shim stock must be used, the stock should be cut cleanly, de-burred, and peened to minimize the possible errors which could occur.
- Shims come in various sizes – A, B, C, D, – for a reason. The first consideration when choosing a shim size should be the diameter of the bolt. The shim should fit around the bolt with sufficient clearance to keep the threads from biting into the shim, yet should not have a gap larger than the washer used on the foot bolt. If the gap is too big, the foot may tend to bend while tightening.
COMMON SHIM SIZES
“A”-2×2”, with a 17/32 (0.531)” bolt slot.
“B”-3×3”, with a 25/32 (0.781)” bolt slot.
“C”-4×4”, with a 1-1/4 (1.25)” bolt slot.
“D”-5×5”, with a 1-5/8 (1.625)” bolt slot.
- Shims should be stocked in various sizes and thicknesses, as needed for alignments.
- Some things are not shims:
- Keep your shims in a box, in order! This will prevent wasting time finding the right shim and keep the shims cleaner.
- Lastly, place shims neatly under the feet. It really won’t make your alignments any more accurate, but is a reflection of your work.
Do you have some shim “best practices”? We’d love to hear them!