May 3, 2018
Installation Errors Part 2 – Shims and Shim Handling
By Tom Shelton
The difference between luck and success is this; Luck is when you achieve a desired result despite mistakes made along the way. Success is when you follow a known process to achieve the expected result.
When attempting a precision shaft alignment, precise practices must be followed. One thing for certain in precision alignment is the need to eliminate soft foot. One of the most misunderstood aspects of this is proper shimming. I hear all the time how dirty shims are reused, or that the removed stacks are placed directly back under the feet where they came from. This is called BAD PRACTICE! I will bullet point and explain best practices regarding shimming of machinery.
- Use new shims.
Crusty, dirty, oily, bent, or otherwise corrupted shims are no longer precision tools. They have the potential of causing soft foot. If you want to chase an alignment, reusing shims is a sure way to get there. If you want fast accurate alignments, toss the old shims in the recycle bin and start fresh. All compressibility must be removed from under the feet. Unless you have a chemical process that eats stainless steel, stainless pre-cut shims are the best bet due to the compression resistance and durability of stainless steel.
- Use the proper size.
Pre-cut shims come in various sizes from 1 ½” to 8”. These are typical industrial slotted shims. Other variations are available. What matters most is to use the shim that is properly sized to the hold down bolt. As an example, a ½” bolt would require that an “A” shim, which is 2”x2”, be used. The slot size is 9/16” which allows room around the thread, but not a gap large enough to allow for foot distortion when the bolt is tightened. Manufacturer tables are available on the web.
- Measure your shims.
Any shim over .025″ thick should be measured for accurate thickness. I have seen variations of up to 8 mils between actual and stated thickness. Use quality brand shims. Most quality brands will have a stated tolerance and that “No Measuring Required”. Measure them anyway, its best to know for sure.
- Consolidate shims.
Minimize the number of shims under the feet of a machine by consolidation. Too many shims can lead to some strange problems such as machine sliding on the stacks or false soft foot caused by a spring action in the stack. Also, too many shims can allow the ingress of dirt and debris. When we handle or insert the shims there is a strong likely hood of sliding foreign debris in under the foot. Less shims, less potential for dirt. Mathematically any number can be achieved with 3 shims. A good rule of thumb is to use no more than 5 shims under the foot. If it takes more than five .125” thick shims than a machined spacer plate is needed or extra thick shims (.250” thick) which are offered by some shim manufacturers.
- Step shim vs. Single partial shim.
When an angular soft foot is discovered, there are many thoughts on how to correct it. A single half shim is the best answer. Contact on each side of the hold down bolt is all that is needed. Step shimming (multiple cut shims) has the potential to create problems if not done well. On large equipment with severe angular soft foot stepping maybe needed. There are some mold-able plastic shims on the market, I am not convinced of their precision properties. I would love to be convinced otherwise, but no one has been able to give me proof so far that they work as claimed.
- GOOD MATH SKILLS REQUIRED!
I always recommend that one person take care of counting and placing shims. In the noise and confusion of the industrial environment, things can easily mixed up. Since shimming is critical to the process, individual control is a best practice.
- Shim the stationary machine.
I believe that all feet should have some amount of shim under them. This allows for vertical correction of every foot. The possibility of being base bound is minimized depending how thick of a shim is used under the stationary feet.
These are the basics of shimming. The VibrAlign Blog has pages dedicated to proper shim technique and application. Here is a list of related blog posts which you can click on to view: