http://www.elecon.com/gearworld/dat-gw-failure.html
also, I wonder if this guy is still around?
Date: Fri, 14 Feb 1997 10:09:00 +1100
From: "Jones, Andrew AL"
Subject: gears/tore 4 teeth just driving down the road
To: "'Toy4x4 List'"
David:
>>About the loss of strength with lower gears, I know this is true...
>> I tore 4 teeth off my rear pinion gear (4.88s) while
>>just driving down the road. I still can't figure out how I did it.
>>The only reasons I can figure are:
>>1. Improper set-up
>>2. A flaw in the metal
>>3. Stress from use
Norman:
>I would vote for #3. .
> The stress could have happened a long time ago,
>causing a hairline crack in the base of the tooth which
> just took some time to spread
>the rest of the length of the base...
Obviously any of these factors (or others) could have contributed, but I
thinks it's most likely that Norman's ideas are on the right track. I
have done a lot of work with gear manufacture and failure analysis and
fatigue failure is the dominant failure mode in such components. The
nature of fatigue failure is that a crack grows very slowly with each
successive torsional loading (each revolution) from a small surface
stress concentration. This may be a very small surface crack, dent,
nick from manufacture, installation, an overload, wear or anything.
Even without a defect a stress concentration occurs at the root of a
gear tooth and poor set-up (alignment/backlash) can preferentially load
these areas.
Over time (revolutions) the crack grows through the component, reducing
the load bearing area of the tooth/shaft. Commonly the crack will grow
3/4 of the way across the tooth base or shaft diameter before the
remaining metal fails. Final failure generally occurs under loading
that would be 2-3 times less those than necessary to damage the
component in prime condition. Although this final fracture usually
occurs under highish loadings (like jamming into reverse) it can also
occur under "just driving down the road".
Fatigue failure has a very characteristic fracture surface and is
relatively easy to identify, but without seeing the broken teeth your
case sounds fairly typical of this type of failure. This might explain
why failure occurred under normal driving conditions (we are generally
talking about hundreds of thousand revolutions). It also highlights the
importance correct setup and not ignoring minor damage on gears, axles,
shafts and such rotating components subjected to stress in service
(often caused by the "get a bigger hammer" installation technique).
Andrew
B. Eng. (Materials Engineering - Metallurgy - 1st class Hons.)
Masters of Steel Processing.
