Anyone involved in precision shaft alignment knows that unintentional movement is the biggest contributor to non-repeatable alignment results. Movement can come from several sources: Soft foot, loose bolts, flexible bases, excessive bearing clearances, improperly mounted or loose sensor mounting hardware, etc.
One of the more elusive and least considered sources of movement is cupped washers on the hold down bolts of the moveable machine. I see this issue at almost every “Shaft Alignment Best Practices” Training Class I conduct. My clients are usually knowledgeable about alignment and how to get their job done, however they typically have not considered the condition and QUALITY of the washers under the hold down bolts to be very significant. It is! All too often I see techs and mechanics performing their alignments and never inspecting the washers. This can kill your shaft alignment!
When cupped washers are present, every time the hold down bolt or nut is tightened, it forces the washer to flex down inside the bolt hole of the motor foot bending it into a slight concave or “bowl” shape. At this point the washer is ruined and should be replaced. Whenever you loosen and re-tighten the bolt, the cupped washer will re-center in the motor foot hole pulling the motor back underneath itself again. This may only be a very small amount, not even visible, but when using precision laser alignment equipment, the problem quickly surfaces. Each time the affected bolt(s) are tightened, the alignment numbers will skew and it will appear that you have a soft foot problem. When in fact, the washer is the culprit.
In the photos below, you can clearly see the cupped washers and in this case a slightly worn (cupped) bolt hole which also contributed to the “unintentional movement”.
The cupped washers were pulling our motor 6 mils to the side moving it out of the horizontal offset tolerance as indicated by the Orange Coupling Icon.
We replaced all the washers at each motor foot with 2 thicker washers and the problem was solved. Our alignment came right in at better than the 3600 RPM tolerances of +/- 0.5mil/1” angularity and +/- 2.0 mil offset.
When performing precision laser shaft alignment, part of the pre-alignment steps should be inspection of the hold down bolt washers to eliminate a potential alignment headache. Even if the washers are not cupped it’s a good idea to replace “thin” washers with thicker double stacked washers to prevent the thin flat washers from possibly cupping during the shaft alignment you are about to complete.