MRO Today - Stamp out soft foot

Stamp out soft foot
If your equipment is feeling a little out of kilter,
you better check the machines feet

by John C. Robertson

What is soft foot? Its not the end result of hot water, a loofah and a good moisturizing cream.

Soft foot is a frequently overlooked machine problem.

It describes a machines foot that deflects a small amount when the hold-down bolt is tightened.

Imagine a person sitting on a three-legged bar stool that has one leg shorter than the other two.

If that person moves his weight slightly, the stool tips in the direction of the short leg. Its very uncomfortable, even dangerous.

The solution is simply to pack sufficient shims under the short leg until it gets back on an even setting.

This situation is also very common with machinery seatings.

Check each hold-down foot of the driver and driven units for a soft foot condition. If a soft foot condition exceeds plus or minus 0.002 inches, make corrections to ensure the unit sits squarely on the bedplate with no stress.

The many faces of soft foot
Soft foot problems come in many different forms, some more obvious than others. These are:

Parallel air gap: One leg is too short or one baseplate-mounting pad is not level with the other three. This is also caused when the shims under one foot are the wrong size.

Bent foot: The bottom of one foot is not parallel with the base.

Squishy foot: Dirt; grease; paints; rust, bent or burred shims; or too many shims are found under the foot. Use feeler gauges to measure the clearance all around the foots three exposed edges.

Induced soft foot: Two soft feet are located on the same side or end of a machine, and the feeler gauge indicates a gap that is parallel or nearly parallel.

Gaps without soft foot: There is a visual gap under the foot before tightening, and there is none after tightening. A laser check might indicate a relatively small soft foot, but feeler gauges will indicate a much larger gap. Installing shims is not the answer to this problem. The most logical thing to do is machine the baseplate or the machine housing.

External forces also cause soft foot. Some of these are:
" overhung machines or attachments;
" belt, gear or chain loads;
" hoses or stressed conduit;
" structural bracing attached to the machine;
" jack bolts left tight against the machine base;
" poorly finished foundations;
" piping that fails to meet zero-cold-spring tolerances at flange connections.

How to check for soft foot
Individually check each machine foot location on both the driver and driven units for the existence of soft foot. If a soft foot movement is greater than plus or minus 0.002 inches, correct it immediately. The checkout procedure is as follows:

1) Visually check all machine feet for breakage, cracks or bent feet. If these defects exist, correct them before making soft foot checks.

2) Replace all shims that are rusted, dirty or burred at the edges with correctly dimensioned, precut stainless steel shims.

3) Using a feeler gauge or taper gauges, determine the gap between the bottom of the foot and the bedplate. Ensure that the gauges touch more than one point. In many cases, machine feet can suffer from a compound bend, which produces angular irregularities and a tapered gap. If you cant straighten the foot, fit tapered-off shims to correct the condition.

4) Always perform a three-point check at all of the three edges of the feet when investigating soft foot conditions. Make this check by placing a magnetic-based dial indicator stand on a stationary location (usually the bedplate) and ensure the indicator dial stem button is in contact with the machine foot surface. The dial indicator face is calibrated in thousandths-of-an-inch increments, and when the stem is pressed toward the dial, the needle rotates in a clockwise, or positive, direction.

A plus sign (+) precedes all measurements taken in that mode. Depress the indicator dial stem onto the shaft surface enough to rotate the needle (pointer) approximately one revolution. Then set it to 0.000 inches. Slacken the hold-down bolts nut and carefully record the deflection of the needle. Tighten the hold-down bolts nut again to the specified torque value. Carry out this procedure at all of the machines feet, and record all readings for comparison and determination of any soft foot conditions.

The machine foot, or feet, exhibiting the largest movement, and the greatest measurement, is in need of shimming to correct the discrepancy. If the measurement was 0.008 inches, insert a shim thickness of 0.004 inches under that foot as an initial start in correcting the soft foot.

Torque-tighten the bolt, set up the dial indicator again and adjust to 0.000 inches before slackening off the bolt. Record the new measurement. Repeat this procedure until the condition disappears. Make a final all-around check before signing off the job to ascertain the accuracy of the correction. Leave all hold-down bolts tight before closing this inspection.

Keep the amount of shims needed for correction of soft foot and alignment to a minimum. If, for example, 0.100 inches was the required depth of shims, install one 0.090-inch shim and maybe one or two other shims to make up the difference. Each successive shim layer is a potential soft foot cause if air or liquid is trapped underneath.

Note: Never accept the face value of dimensioned shims in a purchased set of precut shims. Always measure them with a calibrated micrometer before installation. Discrepancies of plus or minus 5 millimeters (+/- 0.005 inches) are found on some pre-cut shims. This can throw a carefully executed alignment out of specification without the millwright being aware of it.

Soft foot leads to big problems
Soft foot can distort heavy machine casings enough to cause the machines to fail through shaft misalignment, bearing failures, and mechanical and gear wear. The problem often originates from the initial installation. Since the manufacturers representative set it, its assumed that the job was done correctly.

Always check for soft foot before making any shaft alignment adjustments. You wont regret the little extra time spent on this important task.

John C. Robertson is the maintenance reliability specialist for Strategic Work Systems, a consulting firm based in Greenville, S.C. He is also the author of "Proven Tips for Equipment Troubleshooting." To order the $10 handbook, call .

This article appeared in the February/March 2001 issue of
MRO Today magazine. Copyright, 2001.