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Stopping stress crack propagation


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#1 NRoshier

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Posted 04 August 2020 - 02:51

The old car I'm restoring with my son uses a swing axle rear end. Suspension wise it's not the best, but the engineering in one area is an issue: The wheel hubs are located on a taper and with a key in a slot. The issue I want to revise is that the key is staked in place with sharp triangular stakes either side. These are crack propagation points are there have been a number of axle failures and wheel losses as a result. Obviously we will crack test them, however would it be a viable fix to grind the stake out and then shot peen the whole tapered area?

https://i.ibb.co/F7s...e1-spitfire.jpg



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#2 Greg Locock

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Posted 04 August 2020 - 08:33

That's nasty. I suppose it is so that the key doesn't come out when you pull the hub off, and doesn't slide inboard and jam everything when reassembling. If there are no cracks you could drill or EDM 1.5mm holes at the tip of the triangle, or even take out the whole of the triangle. That increases the radius of the crack and so stops it propagating. Then machine the key out to a proper Pratt and Whitney key shape, ie round ends.



#3 NRoshier

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Posted 04 August 2020 - 13:27

Thanks for the reply and ideas. It is curious to note that key failures and axle failures were known for some time but the part continued to be produced that way. I discovered today that the axles can be bought new - they are still made in the UK. I'm trying to confirm that I can buy them without the staking done ... yep reports are that some new shafts had the staking done as per the original parts! The shafts are not that expensive, but the shipping is not great! There is also a needle roller bearing running directly on the shaft, just before the taper.



#4 Allan Lupton

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Posted 04 August 2020 - 14:39

Some 35 years before the Herald/Spitfire, the taper-and-key hub/halfshaft fitting was used on the Austin Seven - and Austin quite sensibly used a Woodruff key as per this picture:
BO18threadphoto.jpg
 
One would have assumed the Herald hub arrangements would have been similar and if not could become so when rebuilding.


Edited by Allan Lupton, 04 August 2020 - 14:50.


#5 NRoshier

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Posted 04 August 2020 - 22:10

I agree that shaft is better. I have thought about getting some made locally - I even have access to a lather big enough, however I'm not racing this. Racing modifications include using forged MOPAR axles from a Dart, which are considerably thicker and stronger. This means considerably more modifications to other parts, which I'd like to avoid. Really this car will be used for touring, posing and the odd motorkhana. Still the ramifications of a breakage are significant as per this video:



#6 Greg Locock

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Posted 05 August 2020 - 00:40

Wheels falling off are rarely a good look. In this application (tapered shaft) I am slighly leary of woodruffs, as the key can rotate during assembly and lock up. Of course a hefty dose of black loctite will weld the key in place.

 

Now, the real question is, why bother with a key at all? A lathe relies on tapers doing what they do,. This obsession with keys has always baffled me. They should be used for alignment during the assembly process abut not see any loads in a properly designed joint.



#7 NRoshier

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Posted 05 August 2020 - 03:53

Probably eased the need for close fit in production or maybe it just always been done that way?



#8 Allan Lupton

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Posted 05 August 2020 - 08:39

Now, the real question is, why bother with a key at all? A lathe relies on tapers doing what they do,. This obsession with keys has always baffled me. They should be used for alignment during the assembly process abut not see any loads in a properly designed joint.

In many applications with unidirectional loading that's right.

In this case it would be tempting to go keyless and use LH/RH threads on the securing nuts to make them self-tightening. However there is the over-run case to consider (and also the reverse gear case) rotationally and the bending case due to cornering can break the taper if it's not clamped firmly.
 



#9 rachael

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Posted 15 August 2020 - 17:41

Suspect the key is just there so if you don't tighten the wheel nut the wheel can't rotate and undo the nut. Grinding out the offending notch and shot-peening would be a huge improvement - avoid EDM as it tends to leave micro cracks in the surface which are worse in fatigue than the original stakes.

 

Rachael.



#10 Greg Locock

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Posted 16 August 2020 - 01:01

I didn't know that, thanks