August 26, 2011
Was Soft Foot the Problem?
By Patrick Lawrence
We all know what soft foot is, correct? Soft foot is when we are sitting at dinner and the table rocks. Our solution: put a matchbook, folded piece of cardboard or sugar pack under the leg that needs to be raised even with the other three. Now let’s eat dinner without our drink spilling.
In machinery alignment the effects of soft foot are much more damaging than spilling your drink…especially if you get free refills. Soft foot will distort and stress the machine’s frame leading to problems. Rotational centerlines on the shafts will not be consistent leading to problems with coupling wear, rotor air gap, bearing/seal fit and clearance issues. Since lasers were introduced, they have made the measuring of misalignment faster and easier than the old dial days and our assumption is that the soft foot measurement and correction process is just as easy. Here is a story that will open your eyes.
We were called into a foundry to align a blower that was a pain-in-the-neck for several years. The blower was just repaired and the vibration analyst concluded there was an alignment problem. The maintenance supervisor was confused and frustrated because he had just hired an outside contractor that used a laser to align the blower. We were brought in to correct the problem because the supervisor thought it was a thermal growth problem. The first step was to measure the as-found alignment condition but the values were very inconsistent. It wasn’t shaping up to be a usual thermal growth study so we decided to start all over. We loosened all the hold down bolts and here is what we found under each foot. It could be assumed that there was a very bad soft foot problem. But after completing our prealignment procedure and checking for soft foot in two different steps, there was no significant soft foot.
When using a laser alignment system, the heads are mounted on the shafts at the 12:00 position, far away from the feet of the motor. When the foot is loosened, the laser will measure the movement of the shaft and calculate the lift at the foot. Looking at the three soft foot conditions, two of the three will be measured incorrectly: angled base and bent foot. The system sees movement but doesn’t know how the foot is lifting. The service company probably saw lift of some value, shimmed accordingly, remeasured and still saw lift then corrected and corrected, hoping for a magical solution. This is quantifying soft foot not qualifying. The best way to correct soft foot is to use a feeler gauge or precut shim to “qualify” if the foot is bent, the base is angled or simply just a short foot.
Our solution was to start with 0.125” under each foot and with all feet loose, try to remove one shim at a time to catch any obvious soft foot. None of the shims could be moved, so all the bolts were tightened in sequence. After the bolts were tightened, we went around loosening one foot at a time and used our precut shims to see if we had any bent foot or angled base issues. No issues were found so we completed the alignment as usual.
Remember, the most accurate and best way to check for soft foot is to use a feeler gauge or precut shims to determine the exact soft foot condition.
- Short Foot – Shim exactly the value measured
- Bent Soft & Angled Base – Use a step shim, trimming the part of the shim where it does not fit under the foot to make all the thumb tabs line up. Alternatively, use a Sof’ Shoe® shim.
By the way, how did the alignment turn out? Here are the before and after vibration readings: note the level of vibration is now 1/7 the pre-alignment level.