Soft foot: types, checks and fixes
Soft foot means a machine foot does not sit flat and evenly loaded on its base. It is the number one reason shaft alignments will not repeat, and it can also distort a motor or gearbox casing.
Why it matters
When you tighten a soft foot the casing bends to meet the base. That moves the shaft centreline between the loose and tight conditions. It also squeezes bearing clearances, motor air gaps and gear mesh, so it causes vibration and wear even after alignment looks fine.
Where it comes from
- A twisted or warped base, skid or baseplate.
- A bent, damaged or uneven machine foot.
- Too many shims, or the wrong shim thickness under one foot.
- Dirt, rust, paint, grease or old grout under the foot, under shims or between shims.
- Dents, burrs or flaws on the feet or the base.
- A bolt that is pulled sideways, or jacking bolts left tight against the feet.
- A thin or flexible sub-base, such as light structural tube, that can distort when welded or loaded.
The three basic types, plus one
| Type | What you see | Correction |
|---|---|---|
| Short foot (parallel) | The whole foot is lifted and the gap is the same all over | Shim the measured gap |
| Bent foot | The gap is larger on one side or one corner | Step shim: cut one shim part way and keep its tab |
| Angled base | The base or pad slopes, so the gap is wedge-shaped | Step shim to match the wedge |
| Combination | A parallel gap and an angle at the same foot, often from a thin sub-base | Both corrections together, often a re-work of the base is better |
Laser and dial tools see how the shaft moves at the coupling, not what the foot is doing. A bent foot and an angled base can read the same as a short foot, so you may shim and re-measure over and over without success. A feeler gauge or a precut shim tells you the real shape.
How to check and correct it
- Clean the base and the feet down to bare metal. Wire-brush rust, paint and grease. Remove any shims you can see.
- Do a gross check first: with all bolts loose, try a 0.13 mm (0.005 in) feeler or shim at three corners of each foot. Shim any large gap.
- Rough in the alignment (see the procedure chapter), then tighten all bolts in a fixed cross pattern, in three passes: hand tight, about half, then full.
- Loosen one foot at a time and check it again at three corners with a 0.05 mm (0.002 in) feeler. Shim, retighten, then go to the next foot.
- For a bent or angled foot, cut one shim part way to make a step and keep its tab so you can put it back in the same place.
- Repeat until no foot can be loosened and move more than about 0.05 mm (2 mil), then take your alignment readings.
Do not step several shims to create a taper. It looks like more contact, but it is much harder to put back in the same place each time.
Traps that fool the check
- Compressible material. With bolts loose, a gap filled with dirt or paint reads as supported. Tighten the bolt and it crushes, and soft foot is back.
- Hidden shims. Old, small shims without tabs, or with round holes, can spin round the bolt and hide under the foot. Look under every foot on a machine that has been aligned before.
- Base movement. If the machine foot is thicker than the pad or riser under it, the base can rise to meet the foot while the foot appears fixed. Only a feeler shows it.
- A big move can create soft foot. A machine moved a long way sideways can land on a different part of an uneven base. This is why you rough align first.
- A heavy overhung motor on a gearbox can rock the gearbox casing on its feet. Check soft foot again during the final move, not just at the start.
A torque wrench does not fix this. Alignment values that shift as you tighten are caused by soft foot or debris, not by the torque value. Steel shims between steel faces do not squash.
Questions
How do you check for soft foot?
Loosen one foot at a time with the others tight and try a feeler gauge at three corners of that foot. A gap of more than about 0.05 mm (0.002 in) needs correcting.
What is the soft foot tolerance?
A common limit is 0.05 mm (0.002 in) of lift at any foot when it is loosened. Tighter is better, but chasing less than that rarely pays.
Can a laser alignment system find soft foot?
It can show that a foot lifts, but not why. It cannot tell a short foot from a bent foot or an angled base, so confirm with a feeler gauge or a precut shim.
Why does soft foot come back after I corrected it?
Usually because something compressible was under the foot, such as dirt, paint or rust, or because a shim stack of many thin shims compresses as the bolt tightens. Clean to bare metal and use fewer, thicker shims.
General good practice for horizontal machines driven through a coupling. The machine maker's manual, the coupling data and your site standard come first. Lock out and isolate the machine before any alignment work.