fatigue or impact failure 3

[Tmoose]

Several 100 rpm helical gears have broken their teeth in service. The gearbox manufacturer say they feel the failure of the gear in the attached image appears to be from an impact type overload.

Does this look like a one time overload failure?

thanks

Dan T

 

[desertfox]

Hi Tmoose

How long this gears been in service, the first to images are hard to see because they are quite dark, however on the bottom right image there appears to be river lines which might suggest fatigue.

Regards

desertfox

 

[Tmoose]

A "knock" was detected About 1.5 weeks after being rebuilt with brand new low speed gears. Even at 100 rpm that is 1.5 million cycles.

 

[CoryPad]

It is possible that the part suffered from impact loading that caused cracking and fracture.

I think it is more likely that it had cracking and fracture from fatigue, which could have been exacerbated by occasional high stress events coupled with many lower stress cycles.

 

[tbuelna]

Tmoose,

It's hard to tell what caused the fractures based on the pictures. But here's a few things to consider.

First, the cracks all originate in the drive side flank fillets. This is the region of max tensile stress due to tooth bending loads. Spiral bevel or hypoid gears generally have good tooth bending capabilities due to their high face contact ratio. So this is a bit unusual.

Second, since the cracks originate in the fillet, below the working surface of the flank, you can probably rule out scoring or pitting failures.

Third, the mating pinions should be checked. The bending stress in pinion teeth tend to be higher than in the gear teeth, due to the pinion's smaller diameter. This is sometimes compensated for by adjusting addendums, pitch diameters, etc. to help equalize bending stresses between the pinion and gear. But with this type of damage on the gear, the pinion should also show some effects if the cause was torque overload.

Fourth, if there is only damage on the gear, then the cause may be some sort of manufacturing defect. Problems with heat treat, gear tooth profile or index errors, or abusive machining.

Hope that helps.
Terry

 

[gearcutter]

Really bad quality images.
Anyway, this is what I can see.

There is evidence of a ductile failure.
The fracture surface looks smooth & curved.
A brittle/impact fracture would appear rough, flat and faceted.
The fracture surface shows a large fatigue zone and very small ultimate fracture zone.
There appears to be heavy tooth bearing at the toe along with a very narrow load distribution.
There are ratchet marks directly under the loaded zone.
The fracture appears to have originated at the heavy bearing zone.

Any number of things could cause this.
Without more information there is no point in speculating the cause.
The most obvious thing that stands out to me is the evidence of poor contact conditions.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[desertfox]

Hi T.moose

This site might be of help, if you click on the figures it gives photo's of the failure and its charateristic's

http://www.machinerylubrication.com/Read/150/gear-failures

desertfox