March 4, 2018
The Soft Foot Primer (Part 3) How to REALLY Minimize Soft Foot Every Time
By Stan Riddle
It is both possible, and desirable, to minimize the negative effects of soft foot. While it may be difficult to completely get rid of soft foot, it can typically be reduced to a few mils, if the following techniques are used.
PREPARATION AND CLEANLINESS
- CLEAN! If you remove the motor, wipe the feet clean, before placing it on the base. Wipe the base off as well. Use clean shims, clean tools, and clean hands. If you don’t or can’t remove the motor, at a minimum brush or blow dirt away from the motor feet. All it takes one piece of grit to cause a problem.
- Stone or file flat any holes drilled for hold-down bolts, especially if they are threaded.
- Get rids of burrs, dents, paint chips, slag – anything that may be an unintended “shim”.
- If the motor is removed, make sure the base is flat. Check with a good straight edge, or laser tool that can measure flatness.
- If the motor is not new, look to see if the feet have been hit with a hammer. If so, they could be bent, or at least deformed on the edges. An old school method is to place the motor on a good clean steel table, with each foot sitting on a file of the same thickness. Drag each file back and forth a few times, and flip the motor over. If the file cuts into one side of the foot, and not the other, it is bent or deformed. While we typically don’t recommend machining motor feet, a slight skim cut with a milling machine may be fix the problem, without putting the motor out of NEMA spec height.
- To start with, make sure the thickness of shim values in each plane match. In other words, the thickness of the shims under the inboard feet should be the same. The shims under the outboard feet should match each other as well. This is a good starting point, and will lessen the likelihood of an angled soft foot.
- Replace any carbon steel or brass cut shims with pre-cut stainless steel.
- Any time you add shims, or remove shims, they should be measured with a micrometer or caliper. Don’t just use assume the thickness marked on the shim is correct. When removing shims, I prefer to remove the entire stack of shims under each foot, and mic the thickness of the entire stack, including any dirt or rust. Ideally, you should mic the thickness at the radius of the slot in the shim, since that’s near the center of the foot.
- Minimize the number of shims under each foot. Try to keep it to 4 or less shims per foot by consolidating shims. Use fewer, thicker shims. If you must cut a shim, be sure it is flat, straight, deburred, and clean.
Follow the pre-alignment steps:
- ROUGH IN. Rouging in the alignment gets the shafts close, to minimize any deflection forces from trying to assemble two misaligned shafts.
- CORRECT EXCESSIVE SOFT FOOT. While the hold down bolts are loose, check for a soft foot. This can be as simple as trying to wiggle the shims underneath the feet, to see if they move freely. If they do, use a shim or feeler gauge to determine how much shim needs adding. Do this to all four feet before tightening.
- TIGHTEN HOLD DOWN BOLTS IN A CONTROLLED, REPEATABLE PATTERN. This is done to minimize the effects of any remaining soft foot. It should also be done in at least three passes, to better control any unintended movement, such as pulling too hard on the wrench, causing the machine to move.
- PERFORM A FINAL SOFT FOOT CHECK. Now that all four feet are tight, loosen one foot at time for a final soft foot check. Check at least 2-3 places under each foot, to see if the gap is the same or angular. If you find different gaps, you may need to use a step shim or plastic shim.
Be mindful that a soft foot condition can re-appear whenever a motor is loosened, shims are changed, jacking bolts are left tight, or any other changes are made. Pay close attention during the tightening process. If the numbers on the display change over a few tenths of a mil, or if you notice one hold down bolt requires more rotation to tighten it, a soft foot condition could exist.
By carefully following these steps, you can and will minimize the possibility of a soft foot. The preparation time may take a bit longer, but the time to achieve a precision alignment will be reduced considerably.