Kirk Kirkland
Electrical Repair Service Co.
Birmingham, Alabama
Technical Education Committee Member
Editor's Note: This article is similar to a July 2006 Currents article titled "Tips for Test Running Motors With Roller Bearings." These two articles complement and supplement each other.
End users frequently demand that EASA service centers provide an array of test data at the conclusion of the service/repair process. These tests are normally to validate compliance with the customer’s motor repair specifications. It is also a good idea to have your own in-house specifications so you can prove that you’re compliant with EASA motor repair guidelines such as those found in the Recommended Practice for the Repair of Rotating Electrical Apparatus (ANSI/ EASA AR100-2006).
One of the more common tests involves running the motor no-load and providing the motor owner with electrical test information and vibration spectrums covering various frequency bands. No-load run tests are commonly applied to AC induction motors. In many cases, these motor types are designed for a belted-duty application. That means they may have a roller bearing in the drive end of the motor. The most common roller bearings utilized in belted applications are the two-piece NU type that consists of an inner race mounted on the bearing shaft journal and the rollers caged on the outer race.
Clarify run test procedures
For the purpose of this discussion, the roller bearing is located in the drive end of the motor with an anti-friction bearing held in the opposite drive end of the motor. Special consideration needs to be made so that the roller bearing operating life expectancy is not compromised as a result of run testing the motor without a minimal radial load. On the surface, many customers, and perhaps some motor service centers, think nothing of running a roller bearing motor for as long as it takes to glean test data. Some customers even require minimum run times of an hour or more. In addition, it is not uncommon to find customers applying a belted duty motor with a roller bearing in direct drive applications. If these issues arise, then it is worth a call to the customer to clarify the run test procedures and installation application.
This may not sound like a big deal until there is understanding as to what is happening with the roller bearing as it rotates with insufficient radial load. When the roller bearing motor is running with insufficient radial load, the rollers can act as wipers having a squeegee effect as they skid along the races. You will probably also hear significant cage rattling and squealing coming from the roller bearing. This squeegee effect reduces the critical lubricant from between the bearing rollers and races resulting in potential metal-to-metal contact. Bearing manufacturers indicate that even a short run time, in some cases of less than two minutes, with no minimal radial load, may result in significant and immediate damage to the bearing.
Potential damage to motor
Take the additional step of notifying customers who are test running repaired motors upon receipt or prior to re-installation. The customer may be damaging the motor you just so diligently serviced. You can bet that the motor owner will ask penetrating questions when the expensive roller bearing he bought, as part of the last rebuild, failed prematurely and now he has to service the motor again. Customers must be informed that run testing a motor, with a drive end roller bearing, without a radial load, will damage the roller bearing and shorten its expected design life after the motor is re-installed.
Bearing manufacturers designate minimum radial loads applicable to differing sized roller bearings.
The key is to recognize the need for a protocol wherein a motor, with a roller bearing on the drive end, can be run no-load while minimizing the potential of damage to the roller bearing.
If you have had the experience of engineering belted duty motor applications, you can appreciate the complexity of calculations that may be involved. For the sake of simplicity, a motor service center needs a SAFE and PRACTICAL procedure to address the issue of applying a radial load to motors with a roller bearing.
There are various methodologies that a service center can use to apply a radial load, so one needs to investigate which method best suits their specific needs.
Illustrating the point
Figures 1 and 2 illustrate the application of a radial load on a motor equipped with a drive end roller bearing. The system consists of a fabricated heavy duty adjustable base, mounted on a heavy duty base which must be securely bolted to a test table. The radial load device has two heavy-duty pillow block bearings fastened to the adjustable base with a 2.5” (63.5 mm) shaft on which a pulley is mounted. To take advantage of physics and gravity, the “D” dimension of the load device is less than the “D” dimension of the motor so the resulting belt pull is towards the 7 o’clock position. Each pulley is the same diameter and is taper lock mounted. This arrangement allows the load device to take advantage of rotor weight, coupling weight, pull direction and belt tension to apply a radial load on the roller bearing. A belt tension measuring device can be applied to the belt(s) to confirm belt tension. To increase radial loads, one can increase the pulley sizes, a number of pulley grooves and the number of belts.
Again, this device is very basic; however, it can be the foundation of more sophisticated designs by the addition of load cells or pressure gauges to verify actual radial load being applied to the roller bearing. As with any test operation, the area should be safe and secure.
To summarize, it is important to know that motors having a drive end roller bearing require a minimal radial load to perform as designed. Failure to make accommodations for this design can be costly to the service center and the customer.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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