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Small Motor – Soft Foot Frame & Bearing Problem
Technical Associates of Charlotte
I. BACKGROUND
The production personnel of a South Carolina chemical company reported hearing a new noise
emitting from a pump/motor unit. Maintenance was dispatched to investigate the noise and repair
if necessary. The maintenance mechanic and electrician assigned to the job requested vibration
diagnostics to determine the source of the noise and health of the machine.
II. CONCLUSIONS
A. A soft foot on the motor is warping the frame.
B. A bearing outer race defect exists at position 1
1230 West Morehead Street, Suite 400
Charlotte, NC 28208
, P.C. 1
Small Motor- Soft Foot Frame & Bearing Problem
I. BACKGROUND
The production personnel of a South Carolina chemical company reported hearing a new noise
emitting from a pump/motor unit. Maintenance was dispatched to investigate the noise and repair
if necessary. The maintenance mechanic and electrician assigned to the job requested vibration
diagnostics to determine the source of the noise and health of the machine.
II. CONCLUSIONS
A. A soft foot on the motor is warping the frame.
B. A bearing outer race defect exists at position 1.
III. RECOMMENDATIONS
A. While the motor is operating, loosen the foot hold-down bolt on the southwest foot.
The foot will rise due to the warped frame.
B. Temporarily, until scheduled shutdown, insert a shim the thickness of the gap below
the foot then retighten the bolt.
C. At scheduled shutdown, determine between sprung foot (as in Figure 1) and soft foot.
a. Soft foot will require proper shimming; sprung foot may require milling.
b. During scheduled shutdown, replace motor bearings.
c. During scheduled shutdown, inspect the pump base for settling.
IV. DISCUSSION OF RESULTS
Manufactured by Reliance, the subject motor has an operating speed of 3580 rpm, 25 HP, and is
outfitted with NTN6310 rolling element bearings. It is used to power the re-circulation pump for
a 50% caustic soda storage tank. The driven pump is a Durco I horizontal centrifugal pump.
Figure 2 is a sketch of the unit showing measurements positions.
Figure 2 – Sketch of horizontal centrifugal pump & motor
Figure 1 – Illustration of a sprung foot , P.C. 2
A. Warped Motor Frame
An FFT analyzer was employed. The reading taken at position 2V showed a peak at 2X line
frequency (2 Fline). The peak clearly dominated the spectrum with an amplitude of .5 ips (peak
vibration). Remember, “Concern should be given motors whose amplitude at twice the line
frequency (2 X 3600 cpm in the USA) exceeds approximately .100 ips peak for in service
motors… This would apply to motors ranging in size from approximately 50 HP to 1000 HP”1.
Our analyzer showed five times .100 ips on a motor smaller than 50 HP (25 HP). The survey
three weeks prior to the diagnostics testing showed an amplitude of .305 ips or three times .100
ips (the level at which we should give concern). See Figure 3. The amplitude had increased 64%
in just three weeks. A problem was not only present, but also developing.
Such high amplitude at this frequency might lead one to quickly conclude that the motor is
experiencing an electrical problem because many electrical problems are associated with 2 Fline.
However, mechanical problems should also be considered when motors show high amplitudes at
2 Fline.
It is commonly agreed on that mechanical problems can cause high amplitude at 2 Fline. A warped
motor frame for example. The warp can cause an uneven distance radialy between the rotor and
the stator’s magnetic field. At the location where the distance is reduced due to the warp, a strong
pull may occur relative to the location where the distance is increased due to the warp. Both
events (strong pull/weak pull) happen every rotation of the stator’s magnetic field. The stator’s
magnetic field rotates at the line frequency of 3600 cpm (in the USA) X 2 events = 7200 cpm.
The same frequency involved in many electrical problems, yet in this case caused by a
mechanical problem.
When the mechanic began to insert the shim under the southwest foot (per recommendation 1 on
page 1), he noticed a hairline crack propagating from the southwest foot and extending to the
motor frame. The crack measured several inches in length. This discovery added much weight to
the conclusion that the motor frame was warped and that the high amplitude 2 Fline was the
vibration result generated by the warp. A decision was then made to remove the motor, repair the
crack, determine between sprung and soft foot, and replace the bearings. Figure 6 is a post repair
picture of the hairline motor frame crack.
1
Technical Associates’ Concentrated Vibration Signature Analysis and Related Condition Monitoring Techniques (Level II Seminar Book)
Figure 3 – Spectrum showing high amplitude at 2X line frequency
Figure 6 – Frame crack discovered on motor
(Picture taken after weld repair)
The mechanic determined that a soft foot coupled with improper shimming twisted the frame and
caused stress near the foot area. The stressed area was acted on by high amplitude vibration and
resulted in the hairline crack.
B. Bearing Defect Outer race
To determine the source of the noise reported, previously taken velocity and acceleration
envelope measurements were examined. Figure 4 is an acceleration envelope spectrum taken at
position 1H. A band pass filter of 30k – 600k cpm was used to eliminate rotational vibration. The
resulting spectrum clearly displayed multiples of BPFO indicating that a bearing problem existed.
To help determine the severity, the velocity spectrum (also previously) taken at the same position
was considered. A bearing defect of moderate severity will usually display defect frequencies in
the velocity spectrum. Figure 5 is the velocity spectrum, which displays many multiples of BPFO.
A sideband cursor was placed over one of the defect frequencies. Note that the delta of 10962 is
very near the BPFO fundamental defect frequency of 10920. (BPFO 3.07 X 3557 cpm running
speed = the fundamental BPFO frequency of 10920 cpm).
Figure 4 – Spectrum of Acceleration Envelope Measurement position 1H , P.C. 4
Figure 7 – Outer race defect found at position 1
Having so many multiples of BPFO, this spectrum fits the description of a stage 3 bearing failure
as described in the Technical Associates Illustrated Vibration Diagnostic Wall Chart. It also
indicates that the source of the noise reported originates with the bearing defect.
As per recommendation 2 (on page 1), the motor bearings were replaced. To confirm that a
defect actually existed, the outboard (position 1) bearing was cut open and visually inspected.
Figure 7 is a picture of the outer race.
Figure 5 – Spectrum of Velocity Measurement position 1H showing many
multiples of BPFO , P.C. 5
Figure 8 – Waterfall plot displaying measurements taken before and after repair
Figure 9 – Acceleration envelope spectrum taken after motor repair
V. RESULTS OF CORRECTIVE ACTIONS
Motor repairs encompassed the welding of the frame crack, proper shimming/alignment of the
unit, and the replacement of the motor bearings. A significant drop in amplitude at 2 Fline resulted
from proper shimming/alignment, and repair of the crack. Compare the before and after repair
spectra in Figure 8.
The amplitude at 2 Fline dropped from .305 ips to .026 ips (over 91%). In addition, bearing
frequencies disappeared. Figure 9 is the acceleration envelope spectrum taken after repairs.
Note that no bearing defect frequencies are present.
It is understood that the motor continues to run with low to moderate amplitudes of 2 Fline and no
noises have been reported.
<< Before repair amp = .305 ips
<< After repair amp = .026 ips
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