MRO Magazine

Machine Alignment: Understanding Soft Foot

The success of your manufacturing business depends on machine reliability. One of the trickiest issues that can lead to plant disruption and impede workflow is the problem of soft foot.

Soft foot occurs when the underside of one or more machine feet do not make full contact with the base. When tightening the machine down, soft foot leads to stress in the machine frame and can alter the shaft centerline, making an accurate alignment nearly impossible.

There are several types of soft foot. A common one is rocking or parallel soft foot. These result from a foot, which is shorter than the others and parallel to the baseplate. Another type is angular soft foot, which is caused by a machine foot bent at an angle relative to the machine frame. Additionally, there is squishy soft foot. This is a result of damaged shims or contamination by dirt or oil that impairs proper seating of the machine. Lastly, induced soft foot is when an external force, such as pressure from attached piping or other equipment, causes displacement of the machine feet.

Soft foot measurement

A number of techniques may be used to determine soft foot prior to commencing alignment:

  • Using a dial indicator mounted on a magnetic foot: position the indicator above one of the machine feet, zero the indicator, and then loosen off the machine foot. Record any change in the indicator reading. Tighten the machine foot down. Repeat this for all machine feet.
  • Using a set of feeler gauges: loosen one machine foot at a time, measure the gap that appears below the loosened machine foot, and record this. Tighten the machine foot and move to the next foot.
  • Using a laser alignment system: loosen one machine foot at a time. The alignment system records the amount of foot lift at each foot. Tighten the machine foot before proceeding to the next foot.

Having determined the amount of soft foot present, it is possible to adjust the machine according to the soft foot condition diagnosed. Let’s take a look at an example, as the diagram to the left highlights.

This example shows classic soft foot problems with a rock across feet B and D. It is tempting to shim both feet to eliminate the rock but this would be a mistake; the best solution is to shim only one foot 80/100 mm and to recheck all four feet.

Many additional soft foot problems may be present, including bent feet strain imposed by pipe work or ‘squishy’ foot caused by too many shims under the machine feet. Some examples of these are shown in the following sketches.

When eliminating soft foot, it is important to follow these steps:

  • Check all four machine feet: any foot showing over 0.08 mm is to be corrected as appropriate.
  • Examine the largest soft foot (or two largest if having the same value) with feeler gauges to determine the type of soft foot: it never hurts to examine the other feet as well, but concentrate on finding and fixing the largest problem first.
  • Correct the condition diagnosed by shimming only one foot (if any).

Once all feet are within tolerance, it is safe to commence the alignment.

Soft foot conditions can cause shaft deflections (which lead to stress in bearings and seals, increased destructive vibrations and premature wear of equipment), so it is important to solve them from the outset.

Since soft foot elimination can be complex and tricky, PRUFTECHNIK alignment systems have been designed to allow for easy and precise measurement. A soft foot wizard is included with the ROTALIGN touch system, providing diagnostic tools to help correct such problems and PRUFTECHNIK experts are on call to assist with just such issues.

For more, check out the educational ‘Understanding Soft Food’ video.