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Shurafa

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Reply with quote  #1 
Motor Shop Tests
Marhaba!

I'd like to hear from you about your practice in testing motors at your repair shops. How long do you keep the motor run tested during which you collect the readings? Minutes, an hour, four hours, other standards, no standard?

When I write a formal scope of work and I specify a duration, usually it is 2 hours as a minimum or 4 hours provided the readings have "stabilized".

Regards- Ali M. Al-Shurafa
Rang

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Reply with quote  #2 
In the past I've been involved with the vibration testing of overhauled motors prior to delivery to the customer. Tests have been both loaded and unloaded but typically unloaded. We required that temperatures were stable for at least 15 mins prior to testing.

It's a very worthwhile exercise and more acceptance/workshop/factory testing should be done by industry. When testing first commenced we were failing up to 30% of the assets. By changing bearing suppliers, ensuring correct handling and installation this has been brought down to 15%.

There are some downsides of course and the testing standard should be based on good sound data (chicken or the egg?) and without OEM (bearing manufacturer) assistance you can really cause problems for everyone (delaying delivery times, increasing overhaul costs).

Hope this helps.
RRS_Dave

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Reply with quote  #3 
What the motor is sitting on and how it is fastened (or not) will effect the readings as much or more than warming up. We always used the NASA standard of rubber belt (durometer reading I don't remember) and it worked out well. Better than sitting it on the concrete floor. We also anchored them with sliding bolts like used on a mill bed to some rails that were embedded in the floor which I thought was even better for the larger frame sizes. We let sleeve bearing motors run for an hour and checked temp stabilization every 10 minutes or so. When it stopped moving we took vibes. We had to do large motors unloaded, but we had a load to apply to those under 25 HP.

D
fburgos

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Reply with quote  #4 
Dave, can you elaborate more on the rubber belt, is it for anchoring the motor? or put under the motor feet?
RRS_Dave

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Reply with quote  #5 

Sorry for late reply, didn't see that the post had a new question.

The rubber was placed under the motor feet to keep it from bouncing around on concrete (walking). I forget now (It came from the NASA standards) what the durometer reading was, but it changed for speed of motor. On the ones we anchored down to the embedded rails we didn't put anything other than shims (to correct soft foot) under them.
I'll pull up the standard and copy it for here.

D

From "NASA Reliability Centered Maintenance Guide for Facilities and Collateral Equipment"

9.1.1.4 MOTOR ISOLATION

9.1.1.4.1 Method of Motor Isolation for Measuring Vibration

Place the motor on an elastic mounting so proportioned that the up and down natural frequency shall be at least as low as 25 percent of the test speed of the motor. To accomplish this it is required that the elastic mounting be deflected downwards at least by the amounts shown in the Following table due to the weight of the motor.

When a flexible pad is used the compression shall in no case be more than 50 percent of the original thickness of the flexible pad; otherwise the supports may be too stiff.

MOTOR SYNCHRONOUS SPEED (RPM)     ISOLATION PAD COMPRESSION (INCHES)
                  900                                                      1
                1200                                                     9/16
                1800                                                     1/4
                3600                                                     1/16
                7200                                                     1/64
Note: The required deflection is inversely proportional to the speed squared.

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