While on the road for VibrAlign ReAligning America, we trainers encounter similar issues at every facility where we conduct our Shaft Alignment Best Practices Training. During our training classes we ask the students what are some of the “pains” that they have regarding the precision shaft alignments that they have attempted in the past. The typical responses are repeatability, movement of the machine while tightening the hold down bolts, etc. These are telltale signs of soft foot. Soft foot is probably the #1 cause of pain when attempting to perform a precision shaft alignment.
Most shaft aligners are familiar with the typical contributors of soft foot such as bent machine feet, bad or rotten machine bases, shim thickness variation, flexible bases. The one that can be easily overlooked is dirt and debris which are compressible. Compressible materials can trick us in to thinking that we have found and corrected a soft foot. As long as we leave all the bolts loose, we may believe soft foot is corrected, however if there is compressible material under the feet or shims, or even between the shims, as soon as you tighten down your hold down bolts and crush the debris you will again have a soft foot.
Obviously dirt and rust are compressible elements. Some overlooked possibilities are dirty or greasy shims, bent or burred shims and especially a thick coat of paint on the base or even on the underside of the machine feet!
You should take the time to wire wheel the base and feet to clean out any dirt, rust, or debris and always use clean shims with no bent edges or fold lines.
Using new shims is always a best practice. Don’t forget to “mic your shims” as part of your process. Shims .050” and thicker can vary in thickness by +/- .005”. This variation could add up rapidly if all the shims in a stack were off spec. Last, to eliminate the reintroduction of dirt, minimize the number of shims under each foot. Most aligners will tell you that anywhere from 3 to 5 is the maximum number of shims per stack. A large stack of shims (as shown in the left photo) can create a “spongy” foot. Consolidate shim stacks under the feet as vertical corrections are made.
7 to 8 shims in this stack. Soft spongy foot!
3 shims in this shim stack, solid footing.
In our training we discuss the importance of correcting soft foot. Correcting soft foot is considered to be essential to achieving a precision shaft alignment in an efficient manner. Cleaning under the feet is essential to properly correct soft foot.
You Rock! I’m a 4th Year Millwright and this site has become my new bible hahaha What are your thoughts on a 24,000 lbs. axial flow pump thats just hanging from the ducting…no base, just a driveshaft connecting the pump to the drive skid? I’m currently installing 12 of them and am convinced they are doomed to fail haha
Thanks… We try. I would love to see some pictures of that set up.