In industrial maintenance applications, soft foot refers to a condition in which the mounting feet of a motor or driven component are not even, or the mounting surface upon which the mounting feet sit is not even. Left uncorrected, soft foot leads to shaft misalignment and inefficiency that puts excessive stress and wear on shafts and bearings and likely premature equipment failure and downtime.
Soft foot also can create a frustrating situation in which tightening the mounting bolts on the feet actually introduces new strains and misalignment.
Soft foot is often manifested between two diagonally positioned mounting bolts, similar to the way that an uneven chair or table tends to rock in a diagonal direction. There are two kinds of soft foot:
- Parallel soft foot—parallel soft foot occurs when one of the mounting feet sits higher than the other three.
- Angular soft foot—angular soft foot occurs when one of the mounting feet is not parallel or "normal" to the mounting surface.
In both cases, soft foot can be caused either by an irregularity in the machine mounting feet, or in the mounting foundation upon which the feet rest. In either case, any soft foot condition must be discovered and remedied before proper shaft alignment can be achieved.
A quality laser alignment tool, such as the Fluke 830 Laser Shaft Alignment Tool, can typically determine whether or not there is a soft foot problem on a particular rotating machine.