Imagine the following scenario: An electric motor with a distance of 48 in. between the front and back feet is found to require shimming of 0.500 in. under the front feet and 0.100 in. under the back feet for proper alignment to the pump. Perhaps this will result in an excellent shaft alignment of the centerlines of rotation between the machines, but then the motor will be severely misaligned with respect to its own base (Fig. 1).
Because the difference in the shim thickness between front and back feet is so great (0.400 in.), the motor will be angled with respect to its own base by about 8.3 mils/in. (400 mils/48 in.). If the feet are 4 in. long axially, a gap of approximately 33 mils would result between the front and back of each individual foot.
Tightening the anchor bolts and forcing the undersurface of the feet to rest evenly on the base would severely distort the feet and the frame of the motor. This is the heel and toe effect.
Results of machine frame strain
This distortion—machine frame strain—would cause significant vibration when the machine is running, since the distortion produces misalignment of the bearing bores and consequently internal shaft deflections. A soft foot check can easily reveal whether machine frame strain exists.
The machine frame strain would greatly increase the load on the bearings, resulting in a much shorter operating life of the bearings. The strain also would result in higher power consumption and loss of efficiency.
The effect of the angle produced by this large difference in shim thickness would cause the axial plane of rest of the motor feet to be shifted backward from the center of the feet, so that when the anchor bolts are loose, the motor would be resting on the back edges of its feet.
The alignment technician may be surprised that, upon taking new readings after performing the specified shimming correction, he or she would still find the machine out of alignment by about 16.6 mils, since the angle between the motor and its base of 8.3 mils/in. would shift the offset of the shaft by that amount over the 2 in. of run from the center of the foot to the back edge.
Several additional alignment corrections might be needed before the alignment came into tolerance unless the dimension to the feet was changed to accurately reflect the actual contact location of the feet. However, the machine frame distortion produced by tightening the anchor bolts may shift the shim plane a bit, altering the effect on the alignment as well. This, coupled with the somewhat unpredictable effects of the strain on machine movement when loosening and tightening anchor bolts, all conspire to make the alignment technician’s worst nightmares come true on this alignment job.
In order to shim the front feet 0.500 in., more than three shims must be used. A maximum of four (and preferably no more than three) precut stainless steel shims under a foot is recommended in order to prevent the “squishy” type of soft foot. Three shims will always allow any shim thickness up to 0.150 in. to be achieved, providing all 13 standard thickness shims are available.
Solutions for this situation
How is this problem solved? The best way is to reposition the stationary machine so the machine to be aligned will not require shimming that would produce the heel and toe effect. This is often impractical because the stationary machine is stationary; it is often impossible to adjust without great time and effort.
Another solution would be to remachine the baseplate, soleplates, or undersides of the feet of the machine to be moved to eliminate the need for the large differential shimming. Again, this is usually not a practical or economical solution. This leaves the only solution—step shimming.
This requires several thinner shims to be inserted offset from one another in step fashion, to fill the tapered gap between the underside of the foot and its support plane as evenly as possible (Fig. 2). This may require more than three shims but, as with everything, for every advantage there is a disadvantage and here the benefits outweigh the negatives.
Step shimming works since the angles involved are sufficiently small to fall under the “swedge” angle for the coefficient of friction of the materials involved, so the shims will tend to remain in place rather than squirt out. Of course, do not forget to trim off the excess part of the shims sticking out from the edge of the machine foot to prevent possible injury.
Although not elegant, this solution is expedient, easy, and economical, and will prevent the distortion problem, allowing machines to run satisfactorily until the next major outage when the time and resources can be scheduled to fix the problem permanently. MT
Information supplied by Alan Luedeking, Ludeca, Inc., 1425 NW 88th Ave., Miami, FL 33172; (305) 591-8935