Soft foot is a condition that occurs when rotating machinery is positioned incorrectly ─ specifically, when one or more of the “feet” of a machine’s base, frame, or soleplate are not making the same point contact as the others. Think of it as chair with one of its four legs shorter than the other three.
When this happens in a fully operational machine with the belts tightened, the machine’s frame distorts as it is pulled toward the base.
The result is often serious wear and damage to the machine, which is why alignment tools and services such as those from Fluke Reliability’s PRUFTECHNIK unit are critical in today’s highly competitive industrial environment.
Four types of soft foot exist, of which two are the most common disorders and two others are secondary conditions:
Parallel soft foot occurs when a foot does not reach the base and creates a gap between the foot and the base. This condition is probably the most common and also the easiest to detect. When the bolts are tightened, the machine’s frame distorts as it is pulled towards the baseplate and creates misalignment. It can often be corrected by simply adding shims of the correct thickness.
Angular soft foot is caused by a machine foot touching the base on either the outside or inside portion of the foot, with the other side of the foot bent away ─ creating an angle between the base and the bottom of the foot. Angular soft foot is often more complex to diagnose because, well, it is happening at an angle.
Squishy foot is caused by having too many shims or too much corrosion or debris under the machine foot. This condition is common after loosening bolts for initial checks prior to alignment, as sediment and debris can build up. Such a problem can happen frequently, especially if a machine has been tightened down for a long time and has been running for months or years before the bolts are loosened.
External force is soft foot caused by stress-induced external forces such as pipe strain, electrical connections, or severe misalignment combined with a stiff coupling. This type of soft foot is the most difficult to diagnose and can occur at any point during machine alignment.
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