Wherever there is a surface discontinuity, such as bearing shoulders, snap ring grooves, keyways, shaft threads, or holes, a stress raiser will exist. Shaft damage or corrosion can also create stress raisers. Fatigue cracks and failure will usually occur in these regions. For motors, the two most common places are at the shoulder on the bearing journal or in the coupling keyway region
Although in most cases where an axial load will result first is in a bearing failure, there are numerous examples where the shaft is damaged before the motor is stopped.
Shaft fretting can cause serious damage to the shaft and the mating part. Typical locations are points on the shaft where a “press” or “slip” fit exists. Keyed hubs, bearings, couplings, and shaft sleeves, are examples. Taper fits seem to be an exception to this rule and experience little or no fretting The presence of ferric oxide (rust) between the mating surfaces, which is reddish brown in color, is strong confirmation that fretting did occur. The cause of this condition is some amount of movement between the two mating parts. Fatigue cracking maybe initialed by the presence of fretting. Uncorrected shaft vibration can also worsen this condition. SUMMARY AND CONCLUSIONS
All too often when a motor fails, the major and sometimes only focus is the repair or replacement and gets it “up and running again.” Without diminishing the importance of this goal, time should be spent collecting valuable information that will assist in a root cause analysis.
Preventive maintenance for mechanical parts plays a critical role in maximizing reliability, minimizing repairs and reducing long term costs, by following generally recognized maintenance procedures and specific recommendations for the application of each machine
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