How it works
A pump or motor casing is stiff, but not infinitely so. When a foot sits on an uneven surface or over a gap, tightening its bolt pulls the foot down and bends the frame. Bearing bores go out of line, internal clearances change and the shaft centreline moves. An alignment done on that machine is done on a distorted frame: it shifts each time the bolts are loosened, and vibration and bearing wear follow.
| Type | What it looks like | Typical cause |
|---|---|---|
| Parallel | One foot short, gap even across the foot | Uneven base, missing shim |
| Angular | Gap tapered under the foot | Bent foot, non-flat pad, distorted baseplate |
| Induced | Feet sit flat but the frame is pulled | Pipe strain, conduit, external forces |
| Squishy | Foot keeps moving as torque is applied | Too many thin shims, dirt, rust or paint under the foot |
How it is checked
The check is done with the machine at rest and all hold-down bolts tight. A dial indicator is mounted to read the vertical movement of one foot relative to the baseplate. That bolt alone is loosened and the rise is noted, then the bolt is retightened and the next foot is tested. A feeler gauge at the corners of a foot that moves shows whether the gap is parallel or angular, which decides between a simple shim correction and a stepped shim or machining. Laser alignment systems include soft-foot routines on the same principle.
Soft foot is checked and corrected before alignment, not after. Feet interact, so the largest reading is corrected first and all feet are rechecked. The acceptance limit comes from the alignment specification; API 686 sets one for process-plant machinery.
The link with grouting
Soft foot that returns after careful shimming usually points below the machine: a distorted baseplate, mounting pads that are not flat and coplanar, or grout that no longer supports the baseplate uniformly because of voids, debonding or oil-soaked cementitious grout. Shims cannot correct a base that moves. A dial indicator between baseplate and concrete with the machine running shows whether the grout bond is gone.
The baseplate pads are checked for flatness and coplanarity before grouting, because epoxy grout locks in whatever geometry it is poured under. The pump foundation repair article covers the diagnosis and re-grouting sequence.