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Machining marks

Machining marks – Gear failure

Machining marks are artifacts of gear tooth cutting, final profile grinding or edge chamfering. Generally these marks are cosmetic in nature, however they can be serious issues depending on the mark location and depth as well as machine operating conditions. For example, a groove at the root of a gear tooth is a serious condition as that is the location of the maximum root bending stress and the groove acts as a further stress concentrator. Machining marks are an indication that the gear tooth quality is poor and that all the gear manufacturing processes need to be reviewed. Ideally, machining marks should be identified and analysed fully before accepting the gear in order to understand the effect of the marks on the product life.

Also referred to as: Grinding Lines, Polish Marks, Machining Notches, Stripes, Pressure Marks

Causes

Machining marks result from deficiencies in the gear manufacturing process, such as:

  • Dull or broken cutting teeth
  • Insufficient coolant feed
  • Too many chips in the coolant, indicating that the chip seperator is not working properly
  • Incorrect positioning of the coolant nozzle

Detectability

MethodDetection EfficiencyNotes
Visual inspection✓✓✓Deep notches or machining marks on parallel stage gearing will be visible to the naked eye: rotate the gearbox slowly and carefully and examine all visible gearing.
Borescope inspection✓✓✓Deep notches or machining marks on parallel stage gearing will be visible with a borescope: rotate the gearbox slowly and carefully and examine all visible gearing.
Vibration analysis✓✓✓Vibration will detect anomalies on parts in relative motion to each other. Machining marks that repeat on each tooth or roller will not be identified as a specific defect frequency however may result in an overall raised noise floor.
SCADA dataMachining marks generally will not produce signals that can be picked up by the limited sensors in the gearbox.
Oil debris sensor✓✓Machining marks will not generate ferrous material until some other failure mechanism associated with the machining marks such as micropitting or cracking begins.
Oil sample analysisMachining marks will not generate ferrous material until some other failure mechanism associated with the machining marks such as micropitting or cracking begins. Due to the low frequency of sample collection (every 6 months), oil analysis is not a good tool for detecting damage associated with machining marks.

Discussion

Machining marks are an indication of poor gear quality and deep machining marks should be investigated further to determine their effect on the product life. Machining marks may act as stress concentrator and thereby be the initiation site for both micropitting, macropitting and fractures. Machining marks may indicate that the gears were abused during profile grinding and should be checked for grinding burn.

Severity Rating

RankDescriptionDetectionRecommended Action
1N/AN/AN/A
2N/AN/AN/A
3N/AN/AN/A
4N/AN/AN/A
1
2
No image available
3
No image available
4
No image available
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