Soft foot refers to distortion occurring at the base of a rotating machinery. This happens when at least one machine foot is not the same height as the others. A common problem on machine frames, soft foot can make it difficult to properly correct shaft misalignment. It can cause machine vibration and reduced motor life if not addressed.

Causes

There are many potential causes of soft foot, all of which can present different issues with the machine as it operates.

• Wrong number of shims: Excessive shims beneath one foot can cause the base to be uneven, which causes distortion. Missing shims can establish an imbalance on the equipment, even as the foot bolts are tightened.

• Twisted feet: A bent foundation can make the machine unstable, similar to an uneven table that has one leg shorter than the others.

• Excessive tension on feet: The motor will not run as evenly as it should if bolts warp the equipment’s feet.

Issues

Soft foot can cause the shaft’s positioning to change in relation to the stationary machine it’s coupled to. This misalignment of the shafts causes excessive vibration, which creates undue wear in the bearings, rotors, and coupling. Ultimately, the end results can include lack of production from downtime and even machine failure.

Solutions

• Check base plates and foundations: Confirm these are leveled to the manufacturer’s guidelines, clean and free from dents.

Ensure shims are clean and flat