Five Minute Facts  

Addressing the Issue of Soft Foot

Mike Olszewski | President, Reliability Concepts

Soft foot refers to distortion occurring at the base of a rotating machinery. This happens when at least one machine foot is not the same height as the others. A common problem on machine frames, soft foot can make it difficult to properly correct shaft misalignment. It can cause machine vibration and reduced motor life if not addressed.


There are many potential causes of soft foot, all of which can present different issues with the machine as it operates.

  • Wrong number of shims: Excessive shims beneath one foot can cause the base to be uneven, which causes distortion. Missing shims can establish an imbalance on the equipment, even as the foot bolts are tightened.

  • Twisted feet: A bent foundation can make the machine unstable, similar to an uneven table that has one leg shorter than the others.

  • Excessive tension on feet: The motor will not run as evenly as it should if bolts warp the equipment’s feet.


Soft foot can cause the shaft’s positioning to change in relation to the stationary machine it’s coupled to. This misalignment of the shafts causes excessive vibration, which creates undue wear in the bearings, rotors, and coupling. Ultimately, the end results can include lack of production from downtime and even machine failure.


  • Check base plates and foundations: Confirm these are leveled to the manufacturer’s guidelines, clean and free from dents.

Ensure shims are clean and flat

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Po Chen
Po Chen
6 years ago

Hi Mike Olszewski,
Thanks for your information.
Could you also share what kind of freq. will be occurred when soft foot?
Thank you so much.

About the Author

Mike Olszewski President, Reliability Concepts

During my many years as a vibration analyst and reliability expert & have witnessed belt driven machines that are “Bad Actors” which are unreliable and have chronic failures.  Misalignment issues like this case are why the B.A.T. Belt Alignment Tool® was invented. 

This blower has a base mounted the motor on the fan pedestal and it was a chronic “Bad Actor” which frequently tore up belts & showed high vibration readings indicating that there were belt problems. When the alignment was checked with a single laser belt alignment tool it did not show a misalignment condition. However, the blower still mysteriously destroyed belts & other system components. Over 8 years it destroyed 28 sets of Belts (84 Belts), 3 Sets of Bearings & 2 Drive Shafts. When the alignment was finally checked with a B.A.T. Belt Alignment Tool® it showed the “TRUE” severe misalignment. It was so far out of alignment that it required .200” shims on the outboard motor feet for the correction. After shims were installed, Vibration was reduced to acceptable levels, efficiency and reliability were restored. The B.A.T. Belt Alignment Tool® showed the true misalignment and made the correction moves obvious, fast and easy the first time.