Figure 1

When motor vibration problems occur, the magnitude and direction of the vibration can give a good indication of where to look for the cause. When vibration is higher in the vertical plane, one of the first things to examine is the base/foundation of the motor. If the high vertical readings are compounded by indications of an eccentric air gap, such as high axial vibration and a predominant twice-line-frequency vibration, a "soft foot" or twisted frame is often to blame.

 

 Construction Basics

To correct "soft foot" conditions, alignment technicians use prefabricated shims (sized to accept the hold-down bolts) under motor feet. What they may not realize is that a cast iron or steel motor frame isn't as solid as it appears. Because the feet are typically at least 1 in. (25 mm) thick, technicians mistakenly assume distortion isn't possible. As a result, they do not place shims to the greatest benefit. A review of motor frame construction basics can be helpful in determining where to put shims to obtain the best support, and therefore the lowest vibration readings. 

 

Figure 1 shows the typical fabricated-frame construction; the holes through the motor feet are sized for the hold-down bolts. Depending on motor size, there will be at least four bolts (one at each corner of the motor), and sometimes more. Each foot may be the customary weldment at one corner of the motor, or it may run the full length of the frame, in which case there are usually four or more bolts along each side of the motor.