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MG Midget and Sprite Technical - New shafts ready to go

Ready for when I pull my diff to replace thrust washers, some freshly "growlerized" half shafts.

Fully MPI crack tested (all OK) then machined in their hardened condition using CBN (carbon boron nitride) inserts. Four inches at the narrower diameter then tapered at a 1 degree angle back to the full diameter.

The machinist (my uncle) said it was interesting the difference between the two shafts. One is an original shaft, the other a repro. I forget which way round it was (damn... I was drinking!) but one was super hard on the outside, but (relatively) soft in the middle and the other was hard through the whole machined thickness (about 2.5mm).

I will ask next time I speak to him.

Malcolm


Malcolm Le Chevalier

Close up of the ends.


Malcolm Le Chevalier

edit "cubic boron nitride" not carbon. Sorry, fingers moving faster than brain is thinking.
Malcolm Le Chevalier

Are you going to have them shot peened?
David Billington

Good question... Don't know. I am going to get in touch with a few companies, see if they want to do it and at what price.

If I can't find someone to do it, then in my mind the altered profile with no surface treatment should still be better than an original shaft that has been treated using whatever method they use (nitriding, induction hardening, peening or whatever) as it addresses the route cause of failure. It is known that half shafts break even when they have a surface treatment, therefore we know failures isn't due to the type/quality/lack of surface treatment, but due to another issue, i.e. the poor geometry of the splines.

But getting them peened should be the ultimate in wizz-bang cool!

Or watch as I don't and they snap at my first full bore race away from the lights! :-D

Cheers,
Malc.
Malcolm Le Chevalier

Malcolm,

The design of the SAE splines is one of the main issues but that is what we have to deal with unless you want to go for the FL fine splines and matching sun gears. I have always thought the standard shafts are shot peened due to the finish on the shaft and with regards the fatigue life of the shaft it can be very beneficial in extending the shaft life, your machining would have removed that where the original surface was removed. In defence of the standard halfshaft offering from an engineering point of view and functional life it was likely OK and failed halfshafts have likely covered far more than the design life of the car or modern more grippy tyres led to higher loads leading to failure. Other makers adopted better involute splines which could be rolled rather than machined so maybe a cost saving and stronger by design but maybe BMC didn't see the expense as being worthwhile for this product.
David Billington

Indeed. Totally agree on all that.

Yes old shafts appear to be shot peened. Newer shafts, I don't know. They have a smooth blackened finish, some sort of hardening that I can only assume does not have the same beneficial compressive stressed skin as a peened shaft.

My uncle, and I am sure a lot of others on here, ran a midget for 20 odd years and never broke a shaft. Agreed they are being run way beyond their fatigue design life, but there is no way of measuring how much of said life has been used.

So what to do? 1. Find NOS shafts - no chance. 2. Buy reproduction parts - are they any good or just the usual half baked rubbish? 3. Convert to fine spline or EN24 or something - £££. 4. Try to modify and improve.

So far I am about £45 out of pocket (not including the resale cost of my old shafts when I am done). So if I do/can get them peened at a reasonable cost, I should have a cost effective and long term solution.

All good fun :-)
Malcolm
Malcolm Le Chevalier

David, very true, though regarding "it was likely OK and failed halfshafts have likely covered far more than the design life of the car" this may have been true for the original A30/moggy application (though even they were know to break halfshafts) but when it came to a sports car application and the vigorous use many were put to when young I'm not so sure - particularly in the case of the pre-1275 cars with their lower spec material.

Certainly some owners have had good runs without halfshaft breakage, but those competing in them knew to be wary - so much so that some took a spare to any event. And MG/BMC actively promoted this sort of use (and good on them for doing so) so it's a relevant factor in design life.

Interestingly, they addressed it in the B series during production of the MGA by going to fine rolled spines, so the engineers were aware of it. Another example of accountants having the last say? We did at least get an improvement in material spec.

Malcolm, the metallurgist at the heat treatment company here that does my work is firmly of the view that the once the machining goes through the hardened outer and exposes the soft core the shaft needs treatment to restore the hardness. (And he's a straight-up guy, not in make-work mode!) It's a cheap processs, worth doing.
Paul Walbran

once the machining goes through the hardened outer and exposes the soft core the shaft needs treatment to restore the hardnes


exactly... its just like a tootsi pop sucker...you lick off the candy shell and expose the soft chocolate tootsi filling that sucker is gone before the hoot owl can say 3... 3 licks to get to the center of a tootsi pop

same thing on a hardened shaft with a jucie core

there were a couple of years where the axles were very superior ... danial stepltons book is a good source

luckly ive got great shafts ... 1971 orginals

congrats malc. I bet they will do well for you


prop
prop

Interesting, thanks Paul.
Malcolm Le Chevalier

Shafts are back from the the peeners. Might get round to pulling my diff this weekend. Woop!

Malc.


Malcolm Le Chevalier

Such a nice finish, it's a shame to hide them in the axle!



Malcolm Le Chevalier

Malcolm, you're right, that very nice looking! Are there technical specs for that procedure if you take axles to a machinist to have them growlerized?
Jack Orkin

Just follow the description here:

http://www.chicagolandmgclub.com/techtips/midget/ra_hs_break.html

In simple terms, start the cut one inch from the end, cut down to the root of the splines, do a straight section for ~4", then taper back to original diameter.

My shafts were machined in the hard state using CBN inserts.

They were crack tested pre-machining, but I skipped any heat treatment steps.

Once machined, they were shot peened along the whole section that was machined + the splined area.

Cheers,
Malc.
Malcolm Le Chevalier

The blend-out taper should be over ~6"
Paul Walbran

Malc

"Such a nice finish, it's a shame to hide them in the axle!"

lol

Reminds me of seeing a nice polished conrod going down the road after it escaped

Good luck with the shafts----
William Revit

Quite right Willy, nicely polished rods should indeed be available for viewing, particulary in this way as their shape is much more interesting and worth discussing/bragging about than the common old borning normal variety. After it's only an easily fixed electrical problem ... get another starter to replace the one cust in two during the rod's exit strategy and you are nearly there!
Paul Walbran

Paul
Yep' I've seen plenty of 6 cyl Holdens with alternator problems-
Reminds me of years ago, I had our Chev racer warming up in the pits and it started choofing on one cylinder and lightly rattling -Then out the exhaust came a mangled head off an exhaust valve -It had been hammered through the seat and rattled it's way out
Quite a talking point around the pits, everybody wanted a look at it----no racing that day
William Revit

Ha ha, love the stories. You crazy Aussies and Kiwis, can't you build an engine that lasts? :-)
Malcolm Le Chevalier

This is one of mine that tried to make a run for it...only got half way out, though!


Dave O'Neill 2

Pardon my possibly daft question..

But, is thus basically shortening the spline length to prevent the stress caused by the splines in the diff 'digging' into the splines on the shaft.

If the splines are machined down, the diff splines won't 'dig' in??
Karl Bielby

In simple terms, yes. If you take a look at a normal shaft as pictured, you can see where the splines have been digging into each other (about half way along the splined section of the shaft).


Malcolm Le Chevalier

If a small deformation can happen, it follows that under more and more repeated loading that a bigger deformation can happen. A bigger deformation leads to cracking and cracking leads to failure.

The idea of machining them down is to stop this happening. The splines themselves are plenty strong enough to take the pure rotational loads i.e. you don't ever see a half shaft where the splines have been torn off and you are left with a plain shaft.

Does that make sense? Sorry... I am bad at explaining things at the best of times and I have had a couple of beers!!

Cheers,
Malcolm
Malcolm Le Chevalier

Sure does! They are designed to handle torsional loads. Any weakness in any stressed material is a potential point.

So have you machined the splines to the exact length of the female in the diff? As to prevent the same thing happening in the diff.
Karl Bielby

I'm not sure it's the digging in that's the problem, but excess of torque twisting the halfshaft.

The area indicated by the blue line is supported within the sun gear, but the section indicated by the red line is the smallest diameter and unsupported, so when the halfshaft winds up, this part has a tendency to twist and break, as it is weaker than the rest of the shaft.

I believe that machining the shaft down to the smaller diameter over a greater length stops the twisting from being concentrated in that short section of unsupported spline.

I can't seem to find a photo of a twisted halfshaft, unfortunately.


Dave O'Neill 2

Malcolm,

Things are a lot more complicated than what you mention. In a decent material like the halfshafts are made of the very fact that they have been plastically deformed (twisted) will strengthen the material and any further deformation will only occur when the shaft is subjected to higher loads. The big problem is fatigue as that is related to load and cycles, ie high load at low cycles will result in fatigue or low load at high cycles can cause fatigue but steel has a limit below which fatigue won't occur. In the halfshaft most failures are going to be fatigue from repeated load cycling which occurs over a long period but won't be enough to twist the shaft but will result in fatigue cracking. The SAE spline design doesn't help this as it concentrates stress at the base of the spine flanks due to their buttress shape. The first time I checked my halfshafts after being informed of the problem they weren't twisted but the radial cracks from the base of the splines was clear to the naked eye but I ran a dye penetrant check just to see what it looked like, they got replace with better ones shortly after and they got welded along the spline to make sure they didn't get used again.
David Billington

Regarding fatigue cracking I have also seen fine cracks on the nose of the final drive pinion, I presume due to the razor edge nature of the way they were finished but the cross pin failure killed the diff rather than failure of the pinion which was still solid if slightly comprimised.
David Billington

Agreed it is a bit more complicated than I make out, that was my attempt at a simplified version avoiding the primer course in fatigue and fracture mechanics!

If you are interested I found this:

http://www.sciencedirect.com/science/article/pii/S2213290216300086

Similar problem on different type of car. Lots of pretty pictures that explain it better than I ever could.

Cheers,
Malcolm
Malcolm Le Chevalier

It's the run-out of the spline when it is simply cut into the shaft and not left proud that really concentrates the stress. This is shown really well by the stress analysis fig 9 in your reference Malcolm.
What intrigues me is that a formula SAE team could make a mistake of having a shaft like that when it was an engineering 101 issue 50 years ago.
Paul Walbran

"You crazy Aussies and Kiwis, can't you build an engine that lasts? :-)"
Our problem is that we are upside down so all the oil goes to the top of the block, leaving the pick-up dry.
As the GM WRC team found when they brought their team of Chevettes here in the 70's for Rally NZ and all retired with "electrical failure" :-)
Paul Walbran

I suspected as much. Stupid gravity!
Malcolm Le Chevalier

Malcolm/ anyone

How prone to failure are the standard EN17 halfshafts with normal road use? I have some I intend to put in the Frog axle (engine will be 1275 standard tune).

Bill
W Bretherton

That depends on how much of a hooligan you are.

I've never managed to break one on a road car.

I have broken a Peter May halfshaft in a race car, but that was the outer end which broke, due to excess play in the bearing.
Dave O'Neill 2

Not a hooligan these days Dave! Back in the day (1972) I remember breaking a half shaft in my then '66 Sprite mk3 (1098) but that was the weaker original shafts.

Bill
W Bretherton

Dunno Bill, I have no real data, but in the past few months there have been two, maybe three people on the BBS here that have broken shafts.

I think it is an increasingly common issue as shafts get older and older.
Malcolm Le Chevalier

Here is a picture of an axle that demonstrates whats been mentioned. Hard to believe that it can twist like that and not break. I'm sure it was close to the breaking point. You can see where part is supported from the sun gear, as Dave mentioned, and part is not. From a local British car mechanic.


Jack Orkin

That's lovely, Jack. You couldn't repeat that no matter how hard you tried.
Martin Washington

I have a gearbox third-motion shaft somewhere which has a similar twist.

I only realised I had a problem when I was changing the diff. I pushed the propshaft forward to clear the front of the diff, but when I tried to pull it back, I found it had jammed on the twisted splines.
Dave O'Neill 2

That one certainly takes the prize, I have seen a number of similar half shafts but none of the twists have made it quite that far. Jack, you caught it just in enough time to save a useful amount of extra work!
Paul Walbran

Sofar my modest collection snapped halfshafts on my then 1500 engine.
The first two(from left) are stock but the last one is a hardened one... but also snapped...


Sofar (knock on wood) no troubles with the fine-spline halfshafts from Frontline with a triple power K-engine


Arie

What was the material spec/origin of the hardened one?
Paul Walbran

No clue Paul, all I know is that it was a Peter May set.
Arie

This thread was discussed between 22/06/2016 and 12/08/2016

MG Midget and Sprite Technical index

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