July 18, 2011
By Brad Case
The OJT portion of a recent training class was the alignment of a 3600 RPM electric motor driving a Bell & Gossett Fire Service Water Pump.
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.”
[…] Cupped washers […]
[…] would not stay in tolerance when the hold down bolts where tightened. The bolts where checked for cupped washers which where […]
Have yuou experienced a cracked washer. We had increase in vibs on a 480v 100 hp 3600rpm motor & noticed that one of the washers were cracked. It there a spec on the washer hardness and/or guide to bolt size to washer size to motor foot hole? We may send out the washer to determine metal and hardness
Tim, Yes we have seen cracked washers before as well as cupped. Stan just posted a blog article that addresses your questions regarding hardness etc.. Look for it later today.
That is a fairly common problem, the washers being cupped. Invest in hardened washers and use two on each foot bolt. That will help out tremendously.