As most of you know, I get called out to do shaft alignments quite regularly. It is a major percentage of my business. Most are relatively simple and follow a standard SOP. But this past summer I had a semi-regular client who requested an alignment that really tested my skills.

 

He had reported the overall vibration levels had increased and the motor/blower unit was noisy. They had replaced the coupling flex element, and now wanted the "coupling" laser aligned. We had rebuilt this blower back in 2009, laser aligned the shafts when we re-installed it, and it had operated fine for many years. My initial check of the vibration levels confirmed the blower vibration amplitudes were good, but the motor vibration levels were somewhat high(.32 in/sec - .35 in/sec), although not to alarm level.

 

A check of the shaft alignment showed the shaft alignment to be relatively close. It was only out .010" to .015", which would not be enough to effect the vibration levels. Further investigation revealed the motor had a severe soft foot(over .050")!! Soft foot is the #1 cause of excessive vibration in rotating equipment. When I found the shims were not equal under the motor feet, I asked why they would have been moved since I did the last alignment. The customer admitted he had sent the motor out for a bearing change after the blower repair and shaft alignment.

 

When they re-aligned the shafts, but moved the shims to create the soft foot condition. This Baldor, 40 HP, rolled steel type frame ODP motor has been operating with a soft foot for years! Bolting the soft foot, with a .050" space between motor foot and the pedestal, forces the frame to twist and pull the bearing housing, which is in the motor end bells, out of alignment with the motor shaft. In effect, it bends the shaft. The bent shaft, spinning at 3600 rpm, then turns in a cranking motion that the bearings are not designed to accept. Hence, the high vibration.

 

After solving for the soft foot by essentially leveling the motor, the overall vibration levels decreased by over 50%!! (.015"/sec - .018"/sec). The final result: the shafts are aligned, the soft foot eliminated, the overall vibration levels well within manufacturer's recommended limits. But the motor bearing housings, being made of aluminum, have certainly been damaged. Time will tell how badly they were damaged. They may last another year, or even longer. But when the vibration increases to alarm levels, it would be best to replace the complete motor. Replacing motor end bells can be quite costly, especially in a motor with a stressed shaft.

 

At least this customer had the good sense to realize it might be a misalignment problem, and called me in to help. If he continued to let it run with the vibration steadily increasing, he would be replacing the motor much sooner. Preventative maintenance is the key. Invest in PM, it will save you money!

                                                                  

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Maintenance Minute is published each month by Snyder Technical Services, LLC and is based on the work of former Lamson National Service Manager, Bill Snyder.   Should you have any questions, or problems to discuss, please contact me.

Sincerely,