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Soft Foot

Soft foot is a common term used for machine frame distortion. The distortion is caused when one or more feet of a machine differ in height from the others. This, in turn, may be due to differences when the machine was manufactured, a squishy footage with oil film etc. between foot and base, a bent foot, or it may be induced by a pipe to which the machine is attached (e.g. a pipe on top of a pump), which prevents the machine from touching all its feet to its base.

Types of Soft Foot

  1. Air Gap or Parallel Soft Foot: The most common explanation of soft foot is parallel or straight soft foot. When the hold-down bolt is loose, the foot simply does not reach the base, leaving a gap between the foot and the base. The bottom of the foot is parallel to the baseplate though. This condition is easiest to detect using either a feeler gauge or dial indicator.
  2. Downward Bent Foot: A common situation is known as a bent foot. Here the foot is touching the base on the outside portion, but the inside of the foot is bent, creating an angle between the base and the bottom of the foot. When the hold-down bolt is tightened, the foot will deflect and distort the machine frame, depending on what portions of the frame will flex.
  3. Upward Bent Soft Foot: The foot may also be bent upward so that the outside edge is not touching the base and the deflection occurs along the outside of the foot. The tightening the hold-down bolt will result in a distortion of the machine's frame as the inside of the foot is drawn down to the base. Either of these situations can cause frame distortion; however, an inconsistent or warped base will have the same result as a bent foot.
  4. Squishy/Spring Foot: A new deck of cards stacks shorter than an old deck, not because the cards are thicker, but because of the buildup of oil and dirt on each card and the bending and creasing of the cards. The same holds true for shims. Grease, dirt, rust, paint, metal filings, and other substances can build up the thickness of a shim. Bending and creasing can cause a stack of shims to give under pressure.
                                           If these factors are multiplied by several shims, the shim pack can actually have a spring effect. The foot will move when the hold-down bolt is tightened or loosened. A soft foot condition exists even though there is no gap because the foot moves when the bolt is tightened.
  5. Stress-Induced Soft Foot: Perhaps the most difficult soft foot condition to detect is caused by forces that are external to the machine. This is referred to as stress- or force-induced soft foot. It can be the result of pipe strain or stresses induced by the electrical connections as well as drastic misalignment. Binding at the coupling can also induce external forces that create a soft foot condition.

Stress-induced forces can be created during any stage of the alignment process; therefore, eliminating this kind of soft foot may require more than one check. Soft foot can be a major problem. If alignment is attempted on a machine with a soft foot condition, inconsistent readings will make the alignment calculation difficult regardless of the alignment method used. 

Throughout the industry, most alignments are performed without checking for soft foot. The main reason for this is incomplete analysis and the results are hours of frustration, compromised alignments, and machines that do not run as smoothly as possible. If soft foot is corrected after alignment is complete, any shim changes may change the alignment. To have smooth-running machines, soft foot must be eliminated on both the drive and driven machines before performing the alignment. 

Understanding the different types of soft foot is essential in examining the various methods of measuring and correcting soft foot deflection causing machine frame distortion. Some methods do not always accurately measure soft foot, and if the craftsperson does not completely understand the proper procedure, a soft foot condition may go undetected.


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