When to Use Custom Shims to Correct Soft Foot

Mike Olszewski | President, Reliability Concepts

One of the most common problems with alignment of machinery is a condition known as soft foot.  This is when one or more of the machine feet are not making solid contact with the mounting base, mounting frame, foundation, sole plate, or other mounting surfaces.  The cause of this can be either the machine foot or discontinuities of the base. The presence of soft foot makes it very difficult to successfully align any machine, in either case, the soft foot condition must be resolved.

If uncorrected soft foot causes twisting or warping of the machine frame, resulting in serious damage. For example, in a motor soft foot can cause warping and flex the motor frame which introduces stresses on the bearings, bends the shaft, causes eccentric stator clearances, excessive heating, cracking of the frame or feet, shaft fatigue, elevated vibration levels and other related problems.

Soft Foot - Warping of the Machine - MisalignmentSoft Foot - twisting of the machine frame - bearings - misalignment

Soft foot can be easily measured and corrected using available tools such as dial indicators or a soft foot detection program available on laser shaft alignment systems.  In most cases applying the correct amount of shims to the affected machine foot can solve the issue.  However, in some cases, the soft foot can be extremely difficult to correct.  Some reasons for this are:

  1. An improperly machined, bent, or angular foot.
    1. Tightening of the mounting bolt in this condition will not only distort the machine, but it will also cause the shaft centerline to shift position.
    2. Even more difficult are multiple machine foot issues.
  2. Short foot.
  3. Angled, uneven base plate or mounting surfaces.
  4. Cracked or fatigued mounting plate, deck, or foundation.
  5. Crowned sole or bed plate.
  6. Bent or twisted machine frame.
  7. Bent or twisted mounting base.

Bent Foot - Short Foot - Angled Foot - CBM CONNECT

In these extreme cases the only solution may be to repair or replace the machine and foundation. Or to use a custom specialty shim.  Knowing the types of shims available and when and how to use them is important and valuable knowledge.  A few types are:

  1. Spherical Shims.
    1. Angular soft foot compensation up to 2 degrees.
    2. Can be used in combination with traditional shims.
    3. No shim cutting required.
    4. Easy and Safe solution.

Spherical Shims - Angular - MisalignmentSpherical Shims - Alignment and Balancing - CBM CONNECT

  1. Adjustable Chocks:
    1. Self leveling.
    2. Height adjustable.
    3. Re-usable.
    4. Easy and accurate mounting of all types of rotating equipment to base frames, steel foundations or concrete.
    5. Eliminates soft foot.
    6. Reduces the cost of equipment foundations by design for the first build or through retrofit.

Adjustable Chocks - Alignment and Balancing - Condition MonitoringAngular Soft Foot - Shims - Misalignment - Condition Monitoring

Angular Misalignment - Shims - Condition Monitoring

Adjustable Chocks - Condition Monitoring - CBM CONNECTSide View of Adjustable Chock Mounting


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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.