Gearing Failure

[raisinbran]

Hello All,
My background is electrical engineering, so I apologize if I do not understand the proper terminology.
We have had a gearing failure and there is some dispute as to what has caused the failure. The gears are straight spur gears and reduce the speed from 1800 rpm to around 500 rpm in the gearing of interest. The final output speed is 56 rpm. The installed horsepower is 800.

Looking at the gears, they look like the teeth flowed outward from the contact area toward the outer edges of the gear. The gears were in service for approx. 3 months. Bearing damage was minimal.

There are two theories as to what happened:
1) The gears were too soft; or
2) They ran without oil.

I have the failed gearing, but hardness readings at this point would be futile. Any thoughts as to how to pursue this further.

Regards,
Raisinbran

 

[Fabrico]

Either scenario is a possiblility, along with that of the gears being too narrow.

No breakage means strength is there, but presence of lube is critical. Until you have this under control, you will have little control or insight into failures.

 

[CESSNA1]

RAISINBRAN: Can you post some color photos? If you had plastic flow of the metal as you seem to descibe, and not outright breakage, I would suspect the gear material is soft (not hardened), or the gear material is the inocrrect material.

Regards
Dave

 

[ivymike]

if lack of oil caused the observed material displacement (due to friction heating), I would expect to see blueing of of the surrounding area. I suspect lack of hardness.

Why not have the hardness measured and find out?
guess why you might not: it's a nitrided geartrain and only the teeth are hardened

If the gear is carburized all over, then the hardness should be measurable away from the damaged areas.

How about sectioning the gear and looking at the microstructure variation through the tooth cross-section? I would think that you'd be able to see case depth, etc.

Why not find and measure other gears from the same production run? Gears from current production?

 

[zcp]

Get them to a metallurgist / failure analysis company and get a definite answer. The small investment will be well worth the prevention of future problems. You might also learn important information about your machinery or that you weren't supplied with what you bought.

ZCP

http://www.phoenix-engineer.com

 

[TVP]

raisinbran,

zcp is correct-- have the gear(s) analyzed by a competent metallurgical/failure analysis company before you do anything else to them. There are a number of different items to address including the exact type of failure mode, case depth/hardness, surface microstructure, surface condition/roughness, etc. If you need a recommendation for a good lab in a specific area of the USA, I would be happy to provide one. Good luck.

 

[rob768]

Agrred with last posts. After all, If you don't know a cause of failure, there will be no silution. Some photo's surely might help..

 

[dbaird]

Go to:

http://www.tribology-abc.com/sub1.htm

Check out the various discussions on failure/reliability.

David Baird

Sr Controls Designer
EET degree.
Journeyman Electrician.

 

[Philrock]

In addition to the above, check the power/speed rating of the gears vs. applied loads/speeds.