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Not necessarily. The old shims were fitted to bring that motor's shaft height level with the machine or to correct a distorted foot; if the new motor's shaft height is different, the same thickness will now create misalignment. The right approach is to note the shims, remove them and set the height of the new motor again based on measurement.
Record during removal
- The number and total thickness of shims under each foot (the four feet may differ)
- Shim material: wood, rubber, cut sheet metal, stainless steel shims
- The shaft height from the old motor's nameplate or as you measured it
If all four feet carry the same thickness, the purpose was most likely height matching. If a single foot has a different shim, the base is distorted and the new motor may need a similar correction.
Predict the height by calculation
On IEC motors, the shaft height is the frame number itself: 100 mm on a 100 frame, 132 mm on a 132 frame. The difference between the old and new motor's shaft heights gives the rough packing required. For example, if the old one is 112 mm and the new one 100 mm, a raise of about 12 mm is needed; fine adjustment is still done with a dial indicator. You can see the shaft height of each model in the technical drawings.
What should a proper shim be like?
- Do not use wood, cardboard or rubber; they compress over time and alignment is lost.
- Large differences should be made up with a single machined plate, and fine adjustment with a small number of precision shims.
- The shim should cover the whole bearing surface of the foot and be of the slotted type that fits around the bolt.
- Do not stack many thin shims under one foot; they act like a spring.
Correcting the base instead of shimming
If the difference is large and the change is permanent, a raising block welded to the base is a sounder solution. To find a model whose shaft height is close to the old motor, have a look at the frame options in the three-phase motor family.