The investigation and practical application of vibration theories to detect cup nicks on tapered roller bearings
Date
2013-07-23
Authors
Fernandes, Marlé
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Timken South Africa manufacture and assemble AP tapered roller bearings which are
used on rail journals. These are also assembled in numerous other facilities worldwide.
The bearings are large and cumbersome to handle. On the assembly line there is no
method of detecting the presence of cup (outer ring) nicks during the assembly process. A
nick is a displacement of metal of very small size on the raceways. High spots of metal
also exist. They are most frequently caused during the assembly of bearings. Cup nicks
are known for repeat customer complaints due to rough rotation. The presence of the nick
can induce premature fatigue spalling on the raceways, thus compromising the life of the
bearing. Accelerometers were mounted on nicked and non-nicked cups and were
analysed by completing Fast Fourier Transforms and Power Spectral Densities (PSD)
while the assembly was rotated in a Lateral Machine. When analyzing the results with a
PSD it was found that it was possible to define a baseline for a good defect-free bearing
at different sampling rates. These were then transposed onto the PSD’s of defective
nicked bearings and it was found that the nicked bearings could be distinguished as
having exceeded baseline limits. The frequencies at which this trigger occurred were not
the associated bearing frequencies calculated for the ball pass outer frequencies. The
energy associated with the rollers rotating over the nicked portion of the raceway excites
frequencies with sufficient energy to transpose into the machine running frequency range
as well natural frequencies of the bearing components. Different severity nicks were
detectable as well as roller-spaced and non-roller-spaced nicks. The nicks with high spots
excited the most energy. Tests were performed to show that a cone (inner ring) nick was
also detectable by exceeding the baseline limit. Testing performed showed that the limits
were also applicable when the accelerometer was mounted on a machine component and
not directly on the cup. Recommendations include the implementation of the testing on
the production line to increase the sample size of acceptable bearings for the baseline
definition. The data analysis method can be fully automated to compare measured results
to set limits in a reasonable time frame to not compromise production output. Nicks are
detectable as long as a whole spectrum of frequencies is considered in the baseline limits and detection is not reliant on one defined frequency.