Tom, et al-
This is a classic example of a brittle failure as a result of a high
applied torque, and the fact that it started at an oil passage hole is
not surprising, either. I think it would be a good idea to stop here and
explain a couple of things. The "shear" loading we are talking about
here is not what normally comes to mind when we hear that term, i.e., a
load applied perpendicular to the axis of a shaft. Torsion (or torque)
applied to a circular shaft will produce shearing forces on
perpendicular planes of an element in the part. The combined
perpendicular forces produce resultant normal forces on planes at 45 deg
to the planes experiencing the shear, normal forces that are opposite in
sense - in other words, compressive and tensile forces.
Strong but brittle materials (such as might be found in a crankshaft)
are weakest in tension, so if you combine a brittle material, a stress
concentrating feature like a hole, crack or even a sharp-edged ding in
the surface of the material, and a sudden high-value torsion load ("Just
rev it up to 6 grand and drop the hammer, Bob!") the result just may be
a very spectacular failure like the one Tom described ("Ooh, bummer
Bob... ") with a failure plane inclined at 45 deg to the axis of the
shaft. Collateral damage might include valves, pistons and all the other
rotating and reciprocating parts that hit Warp 9 when the crankshaft is
suddenly free to rotate under the influence of a wide open throttle. The
insides of that VG30T would be pretty interesting to see - not pretty,
but interesting...
Gary McCormick
'70 2000
BS-MET, CSUS '81
Senior Engineer
Northrop Grumman Marine Systems
Sunnyvale, CA
************************************************
Thomas Walter wrote:
>
> Richard,
>
> Of general interest, I stopped by my local metal scrap yard
> yesterday. I am always looking for 6061-T6 end pieces.
>
> In the process of scouting the yard, was a VG30T engine
> sitting there. Hmm, the end of the crankshaft failed in
> shear! Wish I had a magnifying glass, as I was curious
> to see the root failure, but it was a one time overload.
> Hmm, wonder if someone tried out a 50 psi turbo!
>
> Oh, of the failure, only reason I could see, with the
> naked eye, was the failure started at the oil passage
> hole to the last bearing. Shear was at a 45 degree
> to the crankshaft plane. Interesting to observe.
>
> When I was at Cal Poly (California Polytechnic - San Luis
> Obispo) I enjoyed playing in the metals lab. Lots of
> fun stuff. Wouldn't mind a hardness tester at home to
> check out those bolts!
>
> HEY FOLKS - The upper bolts are 7/16-20tpi. The AN7 just
> means a 7/16" bolt. I do not recall the length needed,
> but would recommend double nutting the bolts. Yep, had
> a 'clunk, clunk' on the front end... shims had fallen
> out! Loose bolt.
>
> Cheers,
>
> Tom Walter '67 2000
> Austin, TX '68 2000
|