Dan Patterson 
Flanders Electric Motor Service, Inc. 
Evansville, Indiana 
Technical Services Committee Member 

Ensure A Quality Product 
In the previous article referenced above, I covered methods and criteria for testing motors in the service center. Service centers make every effort to ship a quality rebuild. On occasion, the test-run data may have suspicious characteristics. Even though the motor meets the vibration standards, further investigation is warranted. The motor may exhibit a noise, rumble, or exces­sive bearing temperatures. Spectral data might contain harmonic families, or wave-form data contains impacting. In this circumstance, I will make the statement: “It will never be as close to a motor repair service center as it is now.” Accurate test-run docu­mentation can prove invaluable as a comparison tool when judg­ing the performance of a motor once it is installed. After all, you did ship a quality product, didn’t you? 

Induced And Introduced Vibrations 
So just when you believe you have this motor repair gig under control, the phone rings. “Hello, this is Mr. Big at (insert your largest account here) and we just installed the motor you repaired and it is shaking badly. Did you guys balance it when you had it in the shop?” Does this sound familiar? Here are some common causes of why vibration exists on a quality rebuild. 

Insufficient or absence of coupling lubrication
In the old days, it was necessary to see the coupling components when using the face and rim method. After completing the alignment, the next step was to hand-pack the coupling with lubricant. With the advancement of laser alignment, couplings are as­sembled prior to the alignment process then lubricated at a later time (hopefully). A coupling with no lubricant will cause a vibration with high 1-times turning speed and multiples thereof. See Figure 1. Cou­plings will also generate heat which can be confirmed by a non-contact temperature probe or thermal camera. 

Improper belt tension
Belt tension is a necessary step to insure proper transmission of torque without overloading the bearings. Belts that have too much tension may result in vibra­tion readings with high accelera­tion values. Belts with too little tension result in sub-synchronous (below turning speed) vibration related to belt RPM and multiples of belt RPM. 

Pulley run-out
An eccentric pul­ley, be it on the motor or driven equipment, will cause turning speed vibration of the shaft to which it is mounted. Vibration amplitudes will be higher in-line with the belt. 

Misalignment
Misaligned or bent shafts will affect the running condition of the motor as well as the driven equipment. Proper alignment is necessary to ensure longevity and smooth running equipment. There are no disad­vantages to having both shafts precision aligned to true target values. Misaligned or bent shafts will generate a high 1-times vibra­tion with multiples. Additionally, axial vibration may be high. 

Incorrect key length
On occa­sion, the repaired motor will be received and shipped without shaft accessories. It is the end user who determines the coupling or pulley that is mounted to the motor shaft during installation. Some customers have spare mo­tors that require the coupling or pulley to be removed from the failed motor and mounted on the spare, resulting in the failed mo­tor arriving at your facility bare shafted. Proper key length is vital to the balance condition of rotors. Improper key length will result in higher than normal 1-times shaft speed vibration. Proper shaft key length is often overlooked when performing repairs. See Figure 2. An improperly sized key will cause an unbalance condition and increase the vibration levels at 1x rotational speed. To ensure you have the proper key length, use the following formula:

Proper key length = Total shaft key slot length + coupling key slot length DIVIDED BY 2.

This will produce a key that will fill the entire coupling slot, to provide proper power trans­mission, and provide enough exposed key to compensate for the removed shaft material of the unused slot length. A properly sized key should fill approxi­mately half of the exposed shaft key slot. 

Soft-foot conditions
When a motor is installed and soft-foot conditions are ignored, frame twist results. Not only will this cause an uneven air gap, but in severe cases it will also result in misalignment of the motor’s bearings. Uncorrected soft-foot conditions may produce vibra­tion at twice-line frequency and excessive acceleration values if the bearings become misaligned. 

Insufficient structure or supports
We are fully aware of adequate founda­tion requirements; we all have one back at the shop. Is the motor mounted on a flimsy base that resembles something that was built using a minimum amount of steel? Is the grout below the sole platecracked or missing? Are the sole plate bolts loose or miss­ing? Are the motor supports mounted to standard floor depth concrete or has the floor been removed and a foundation been provided? Insufficient structural supports may amplify even the smallest of vibration from lack of dampening. Compare vertical vibration levels at various points on the base surrounding the motor, to confirm a base/founda­tion problem. 

System natural frequencies
The mounted motor and base com­bination, when installed, will produce a system natural fre­quency that may differ from your test condition. As with insufficient supports, amplification of certain frequencies will be present. Natu­ral frequencies and critical speeds can be confirmed through impact testing and coast-down data. 

Overloaded conditions
Remem­ber when you first saw this mo­tor? It may have had a bent shaft, broken shaft, blown winding, or thrusted bearing. The problem may not have been the fault of the motor, but rather a condi­tion that existed with the driven equipment. Was the driven equipment repaired while the motor was away? If the motor had a phase unbalance which resulted in a winding failure, was that condition corrected? Don’t fail to look for root causes of motor failure in the first place. Unbalanced current (one cause of which is unbalanced voltage) will cause increased twice-line frequency vibration. 

When diagnosing motor prob­lems, it is always wise to keep an open mind. 

Categories: Testing, Motor testing, Vibration

Tags: Vibration

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