A reader of our blog posted a comment stating that he could not cut a step shim to correct for angular soft foot, since it was not permitted in API standards. Which, naturally, made me do two things:

• Look at API Standards, because, at least in my 33 years of aligning machinery, I have never been asked to make sure the base was within API Standards.
• Look into alternatives for step-shimming, to keep within API specs.

As I interpret it, API 610 designates a base flatness of .15mm/meter, which in Imperial is 0.00015/inch, or 0.0018″ per foot. Most design engineers would specify about 1/2 of that, to help cover any variables in the process. Pretty darn flat, but doable with good machine tools and practices.

Here’s the dilemma. The base can be machined to meet these specs, but can these specs be checked, and held, during the installation process? The base may be well within these specs when it leaves the factory, however it is then shipped, unloaded at the destination, often by good-intentioned people who do not know that it is a precisely machined part, and who do not handle it as such.

It may be stored on a shelf, or out in a construction site, sometimes for months. It may then be set and grouted, or simply bolted to a concrete floor. Then, piping and other attachments are fitted to the machine. So each of these steps is an opportunity to impart error.

Do you begin to see the variables that can affect flatness?

If you must install industrial machinery to very precise flatness tolerances:

• Confirm that such flatness specifications are really necessary for your application.
• Take great care to assure that all machinery surfaces in question are clean, flat, de-burred, and smooth.
• Utilize a precision tool, such as Fixturlaser’s NXA Ultimate, which can measure flatness very accurately, to make sure the flatness tolerance is held. It makes no sense to adhere to a tolerance if you have no way to measure and confirm the tolerance is being held.


Lastly, if your company specifications for correcting soft foot do not allow for step-shimming:

• Be sure to confirm flatness standards of all machinery you receive, before such machines are installed, and make sure these standards are in the bid package.
• Make sure the feet of the machines, and the base locations to which they go, are clean, de-burred, and smooth.
• Have some method or tool available to check for soft foot, to help in finding and correcting it.
• Look into products such as epoxy grout and laminar shims,which are supposed to compensate for angular soft foot. Custom-ground tapered shims could be used as well.

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