July 18, 2011
By Brad Case
As the hold down bolts were tightened (in a cross torque pattern) the horizontal offset would change to approximately 5 mils at 50% torque then 8 mils at 100%. After 2 attempts the same amount of horizontal movement took place. The students were bewildered. The machine had no soft foot, the coupling backlash had been neutralized and there was no other looseness. Hmm….. what could it be?
The answer…cupped washers under the hold down bolts. The electric motor feet had fairly large bolt holes with soft steel washers under the hold down bolts. As the motor feet were tightened, the cupped washers “centered” the motor feet holes causing the horizontal offset to change and move out of tolerance.
After replacing the hold down bolts and washers the horizontal offset remained spot on as the electric motor feet bolts were tightened.
One student commented “I never would have guessed the washers could be a problem.”