The problem: Grinding burn on gear
A manufacturer of gears believed the root cause of early failure in their products was grind temper. However, the thermal defect was not being detected by Nital etch inspection.
The testing method: Non-destructive Barkhausen noise
The data showed the “OK” gear measured consistently across all flanks while the “NOK” (the suspected defect) gear has elevated BN signal (mp) on nearly all of the right flanks as shown below.
The increased response is an indication of grind temper that was otherwise undetectable since the burn occurred in an initial grind phase where final grinding removed the burn on the surface of the flank, rendering Nital etch ineffective. In this case, BN is detecting the unfavorable subsurface residual stresses that still remained. This trend was shown to occur on all the suspect gears and further testing was accelerated by a quick comparison between the left and right flanks.
Instrument used for this application
GearScan 500 is an automated grinding burn and heat treatment defect detection system. It is designed for quality control needs of small to medium sized gears.
- Application Note: Barkhausen Noise on a Failed Gear
- Application Note: Grinding Burn Detection on Gears
- Barkhausen Noise as an Alternative to Nital Etch for the Detection of Grind Temper on Gears
- Stresstech Bulletin 4: Grinding damages