Soft foot inspection procedure

When aligning machinery, one of the first steps should be to check for a ‘soft foot’ – i.e., the condition when one or more of the motor feet do not share the same plane or the motor base is not flat.

This step is frequently overlooked when a motor is returned to service following repair. The reasoning may be that, “The motor was on this base before, so the shimming should be correct.” But shims are often lost when a motor is removed from service, and there is no guarantee that the original alignment was correct.

Bolting a soft foot firmly to the base can distort the motor frame, causing vibration. The vibration signature will indicate misalignment – in this case internal misalignment of the motor bearings due to frame distortion.

Fortunately, it is easy to check for soft feet. With the motor bolted down, place a dial indicator at one foot with the travel rod vertical and resting on the foot near the hold-down bolt (Figure 4). Loosen the bolt and observe the indicator movement. Record the deflection and retighten the bolt. Repeat this procedure at each foot.

Next, loosen all the bolts and put the appropriate shim under each foot (e.g., movement of 0.005 in. (0.127 mm) calls for a 0.005-in. shim). Tighten all base bolts and repeat the procedure.

Feet should be shimmed to obtain less than 0.002-in. (0.051-mm) movement. For two-pole machines, especially those with a fabricated frame (formerly called a shoebox design), try to shim for zero movement. 

Angular soft feet are best addressed by machining the feet (Figure 5). In some cases, a millwright will use shims of varying thicknesses, or thermal-setting shims made for this purpose.