Fan Balancing -- A Case History
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Most office buildings and factories have some sort of ventilation system using motor driven fans. They are usually installed in the basement, or outside, or up on the roof, so when they are in operation circulating the heat, or air conditioning, they cannot be heard in the offices within the building. As they get older, and wear down, the fans become noisier and oftentimes start vibrating the floors, walls, and even telescoping vibration through the building.
If completing the basic equipment maintenance doesn't quiet the operation, then they call me in to "balance" the fan and get rid of the excessive vibration and noise. This past summer, an upstate college called me in to do just that. Their maintenance involved reconditioning the motor, replacing the fan bearings, and even replacing the 8 foot long shaft of the fan. There was still excessive vibration.
When I arrived, I could indeed feel the excessive noise and vibration. I measured almost 6 mils of vibration! After the balancing, the overall vibration levels were reduced to less than .5 mils. Everyone was happy with the "like new" fan operation. However, 4 months later, I was called in to balance the fan again!
This time they had replaced the bearings again, installed new v-belts and sheaves, and tightened all nuts and bolts in the complete fan housing assembly. My vibration measurements confirmed the fan was out of balance, with .10 in/sec velocity in the horizontal plane, and .40 in/sec velocity in the vertical plane. When a fan is balanced, weights are added to the "low" side to allow the circular fan wheel to spin freely with minimal vibration. The weights are permanently attached to keep them from "falling off". How could this fan become unbalanced?
The fan housing is installed on spring isolators. The foot print is about 15' square, and I can't see all of the the springs, so it is possible an isolator failed and the mounting shifted. But I'm not sure how that could cause rotor imbalance. There were no missing weights on the fan wheel from my previous visit. Fans don't just "fall out of balance" for no reason. For this job, I never confirmed a cause for the imbalance.
A re-balance of the fan rotor reduced the vibration from 4.0 mils to less than .3 mils. The unit ran and sounded like new again. Time will tell. I've impressed on them the need for regular PM, so hopefully, this unit will run for years before I need to return. Preventative maintenance is the key. Invest in PM, it will save you money!
Copyright � 2016 Snyder Technical Services, LLC. All Rights Reserved
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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,
Bill Snyder
President - Snyder Technical Services, LLC
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