What is soft foot?

Soft foot is a common term used for machine frame distortion. The distortion is caused when one or more foot of a machine differ in height from the others.

Soft foot can be frustrating and bad for the health of rotating machinery and the sanity of the technician performing the alignment. Soft foot needs to be corrected in order to achieve a good alignment. When attempting to correct soft foot it is helpful to identify the type of soft foot condition(s) present in the machines. This will allow you to more intelligently approach correcting the defect and get you moving on your way to the rest of the alignment.

In this series we will cover the types of soft foot conditions likely to be encountered during any given shaft alignment job.

Condition 1: Parallel Air Gap

  • Caused by one machine leg that is too short or a base that is not coplanar at all four mounting points.
  • Produces a “rocking” effect as the leg that is located diagonally from the “short” leg is also affected.
  • To correct, simply add shims to take up the gap and remove any rocking effect.
  • Correction is usually made at only one foot, or in rare cases, at both diagonally opposed feet.

Soft Foot - Parallel Air Gap Figure 1




Watch our Shaft Alignment Know-How: Soft Foot video to learn about the effects and importance of measuring and correcting Soft Foot when performing shaft alignment.

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by Oliver Gibbs CRL