Soft Foot in rotating electrical equipment such as motors and generators is condition in which all the four feet are not in equal contact with the surface. This leads to a distortion of the frame once the bolts in the feet are tightened. This, in turn, affects the shaft alignment.
Soft foot can also result in vibration when the machine rotates.
Soft foot can be caused by
- manufacturing defects
- improper shim fitment
- presence of foreign material such as dirt or trash
Soft Foot can be minimized by selecting proper shims and ensuring the the surface of the foot does not have any foreign materials.
http://thealignmentblog.com/blog/2010/06/04/soft-foot-what-it-is-and-how-to-minimize-it/