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KiwiTR6

Mystery of Broken Gearbox Main-shaft Solved (or so we think)

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Sorry, but this is a follow-on from my harsh-shifting OD post that has been running for a wee while now.  I though this was worthy of a new post because it's such an odd event.

As I noted in that post, I lost all drive after having had the OD shifting in and out whilst the car was running on my hoist.  The only suggestion that made sense was that the main-shaft had fractured and after removing the GB and OD (for a second time inside a couple of weeks - much quicker this time though!) that's exactly what I found.

I'm no metallurgist (but I'm sure one of you is and will be able to confirm or correct my analysis) but the fracture appears to be relatively old with just a small section (the circle at 12 o'clock) remaining intact up to the time of failure.  I'm presuming that the stepped nature of the original fracture combined with limited longitudinal play in the shaft allowed the gearbox to continue to function for quite a period.

Anyway, I took the GB and OD to my repairer (I shipped it to him last time) and we discussed how this could possibly have happened as he'd never seen one fail in his 40 years of repairing them.

THEN I showed him a weld repair on the back of the gearbox (centre rear of box just in front of the white cloth) and he immediately responded "that's most likely what's caused the failure".  His view, which I have to agree with as I can think of no other feasible reason, is that the gearbox has been distorted by the weld repair causing the shaft to be forced to run out of alignment and eventually fracture at the circlip groove.  Had I not taken the box to him and simply sourced the parts and rebuilt it myself the same failure would have most likely been repeated (scary!!!). 

Fortunately for me, he has plenty of spare gearbox casings and possibly a spare shaft (to be confirmed) to rebuild the box for me.  

Time will tell, but I have my suspicions that this fracture has been the source of the OD's harsh shifting all along......

 

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Sounds a feasible explanation , maybe when you reinstalled the OD you tightened up everything and distorted the existing fracture enough for it to finally break. Circlip grooves are well known shaft stress raisers.

Hope your OD man has enough good parts to repair the whole assembly so that everything turns freely. 

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Hi Gavin,

I am not a metallurgist but spent many hours with the man at BA in doing my NDT job.

That nice round area just off centre suggests that the fracture was cause by torsion (twisting)

Apart from the moment the shaft snaps there isn't much movement between each side of the shaft to cause the surface to get damage.

Any small irregularity on the outside diameter can start the process and it only gets worse.

 

Many people get torsional and tensile fatigue confused. That round area is the give away.  Unlike a failed four pot crank that breaks at the big end journal.

If this was an aero-engine then those shaft (and other parts) would be MagParticle inspected at every strip down.  That crack had been there for some time.

Roger

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Probably caused by dropping it out of overdrive on a trailing throttle too many times, Ive seen ones come back from the States in worse states than that.

Stuart.

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41 minutes ago, stuart said:

Probably caused by dropping it out of overdrive on a trailing throttle too many times, Ive seen ones come back from the States in worse states than that.

Stuart.

That sounds a very good cause. Any shock load is a no no.

 

Roger

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7 hours ago, stuart said:

Probably caused by dropping it out of overdrive on a trailing throttle too many times, Ive seen ones come back from the States in worse states than that.

Stuart.

Sounds like a harsh shifting overdrive (the original complaint) could be a cause of torsional overload.

I have had a couple of A type overdrives tear loose the pressed in brake ring for the unidirectional clutch. I assumed this was due to the A type having a harsher shift than the J type which is fitted to 3 of my 4 Triumphs. The 4th one is my PI estate which suffered the above failure.

Neil

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Thanks Stuart, Roger and Neil for your comments.

Just to be safe we'll go ahead with replacing the gearbox casing anyway.  The old casing can remain with the car for any future owner should they want matching numbers.

With regard to the shock loading from the OD on downshift, I noticed that just prior to the failure while I was running the car on my hoist with the new non-return valve fitted and filled with 75W80 semi-synthetic gear oil that the upshift was the smoothest I'd ever experienced.  There was a slight delay after flicking the switch then engagement with no shock whatsoever.  

However, the down shift was the complete opposite, immediate and harsh!

Is there a bleed down orifice or device that is out of spec and causing this?

Gavin

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Hi Gavin,

when shfting down coming out of OD ensure that the engines revs are kept higher than before flicking the switch.

 

Roger

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After reading this article I'm inclined to stick with the failure due to misalignment theory rather than torsional overload.....

https://www.efficientplantmag.com/2012/07/failure-analysis-of-machine-shafts/

 

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Edited by KiwiTR6
Added images

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