Original Half Shaft Failures

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jaekl
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Original Half Shaft Failures

Post by jaekl »

I have a related posting in the mechanical section but I've decided to post one here for more visibility.

For the last fifty years I've been the custodian of six active half shafts. Two cars are factory spec 948 Minors and one is a 1275 powered Minor van. One of the 948s has had three RH failures by basically one driver over the years. Two failures were the typical breakage at the spline but on the recent one the hub flange sheared off and the spline looks very serviceable. We've all heard stories of this being a common problem but I really don't think it is unless there is something wrong with the axle. I acquire this particular car with about 45,000 miles and the first failure was about 12000 miles later then another in 16000 and this last on with another 14000 miles. Being a 3/4 floating axle design, the half shaft also performs the duty of holding the wheel up right, so side load may be creating some fatigue loading. This could explain higher mileage but does not explain why I've experienced low mileage failure on the same car, same side. If anything this vehicle is driven more conservatively than the other two.

This is not a request to hear about the virtues of hardened half shafts. I'm trying to identify the cause of premature failure. I'm requesting anyone who has experienced a half shaft failure in a factory tuned BMC vehicle with this axle design to reply with the details such as which side, which side of the road you drive on and roughly the mileage. Of course any pertinent comments.
Thank you.
ManyMinors
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Re: Original Half Shaft Failures

Post by ManyMinors »

Were your replacement halfshafts secondhand? If they were, do you know whether the replacements were taken from the same side? ie, had they been in use on the RH side of the donor vehicle? My understanding is that they should always be fitted to the same side once used - otherwise premature failure can occour.

I too have driven Minors for 50 years or so. I might have been particularly fortunate but have never suffered a halfshaft failure despite driving a Minor very regularly. One car, which I owned for 11 years, was fitted with a 1340cc engine and along with regular road use competed in sprints and autotests. Standard back axle fitted with various differentials but always standard halfshafts. Never a failure.
I know they DO sometimes break though. My wife broke one while out doing the shopping locally in our completely standard 1098cc Minor!

A friend has owned his Minor 1000 since new and driven it extensively - covering well over 300,000miles. Apart from regular oil changes and a new clutch, no replacement transmission components have ever been required and the gearbox and back axle remain completely original!
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Re: Original Half Shaft Failures

Post by Admin »

I've only ever had two failures in well over 100,000 miles in Minors, one pulling away slowly at a junction a couple of years back. And the other you can read about in next Minor Matters!
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jaekl
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Re: Original Half Shaft Failures

Post by jaekl »

The previous replacement were most likely used since I just grabbed what I had around, probably used, but I don’t recall if they had any wear marks and which side they were from. This current replacement was used and the wear indicates it was from the other side. The recent failure of the flange shear can be dismissed as an outlier because it was not BMC but Borg Warner

The responses I’ve received from four boards confirm among the diehard, long term owners that factory cars should last well over 100,000 and with spirited driving. Nothing to create a perception that half shafts are a problem. However, that doesn’t mean it didn’t happen for those who experienced early failure enough to always carry a spare most likely moved on to something other than a Minor and theirs is no longer with us.

As for the cause everything points to the impact of the differential hitting the face of the spline to the point of deformation which causes stress concentration and then a crack. Surely, we can accept that the last vigorous start or spin in loose gravel wasn’t the only straw that broke it. Wear of the spline is considered normal but it has an unacceptable point but I don’t know how that was established. On the other hand if the shafts and the differential are not parallel the entire face will not be engaged so a localized loading will result and thus cause an earlier failure. Oddly, the Borg Warner one still had a black finish on the spline faces indicating there had not been any wear yet. It will be interesting to see how this recent replacement survives being installed on the wrong side. It’s possible now that it is using a fresh surface it may survive quite well. I’ll have to locate the previous two stubs and see what the splines look like. I do remember a very granular surface and then a shear at the center. My earlier replacements may have been high mileage ones or at least unknow history. The lack of wear on the Borg Warner shaft is very encouraging.

There were comments of the design of the shaft in that it would be better to neck down past the spline. That is done mostly to reduce weight and save material. The design diameter is the root of the spline. The necking down does not reduce the stress concentration. Our torsion bars are necked down because they are met to twist and the twist is to be in the length, not at the splines. Whereas the half shafts are not to twist and being larger than the root of the spline, they don’t, well at least not much. If this was the only reason not to switch sides, it wouldn’t be a problem. Reversing torsion bars will not necessarily cause failure, but it can result in the need to retrim them a few times. Whenever something is loaded past its yield point, residual stresses remain on the outer elements such that next time it’s loaded it goes from a negative stress to a positive but less than its yield point. If the loading is reversed, this residual stress gives the outer elements a head start in their trip to the yield point which now comes much earlier and the torsion bar will have a new set point. Unless there was a crack somewhere, it should not fail.
Thanks for the feedback.
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Re: Original Half Shaft Failures

Post by philthehill »

You can get too hung up on Minor half-shaft failure.
It has to be accepted that they do fail - sometimes with no obvious reason.
As new factory half-hafts are unavailable use has to be made of good S/H units unless you want to spend money on upgraded items.
All you can do is make sure that the splines are not worn or twisted. The half-shafts are fitted to the same side of the axle that they were removed from.
Half-shafts with multi-spline inner ends do not seem to suffer the same degree of distortion as those with the coarse Minor inner splines. It could be that the metallurgy is better.
It has to be remembered that most of the S/H Minor half-shafts on offer or are fitted to a Minor are at least 45 years old and have no known history.
Half-shaft life is pot luck and you may have a half-shaft that gives good service and then again you may have one with a poor service life.
So long as you have a good spare for the right side of the axle I would not be too concerned about half-shaft failure.
As regards torsion bars - they should be fitted to the same side as original. Load twisting the opposite way to original can lead to failure of the bar.

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Re: Original Half Shaft Failures

Post by Admin »

I found a pair of new old stock halfshafts on a social media group last winter. They weren't cheap but I figured nobody's making any more so they can only go up in value, so they are currently in the roof void in the shed still in greaseproof paper :-?
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philthehill
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Re: Original Half Shaft Failures

Post by philthehill »

Well done :wink:
Thankfully I have a spare set of toughened half-shafts on the shelf which are correctly marked O/S & N/S. and which also have the original BMC spline protector sleeves.

Bead blasting is a way of getter a bit of extra strength into the half-shafts.

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Re: Original Half Shaft Failures

Post by simmitc »

So long as you have a good spare for the right side of the axle
But what about when the left side breaks? :roll:
Last edited by Admin on Mon Mar 22, 2021 6:27 pm, edited 1 time in total.
Reason: D'ya know, that was so terrible I nearly edited it out :)
philthehill
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Re: Original Half Shaft Failures

Post by philthehill »

You take the diff out and remove the broken stub :wink:

jagnut66
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Re: Original Half Shaft Failures

Post by jagnut66 »

Were your replacement halfshafts secondhand? If they were, do you know whether the replacements were taken from the same side? ie, had they been in use on the RH side of the donor vehicle? My understanding is that they should always be fitted to the same side once used - otherwise premature failure can occour.
I just want to second this important point.
Halfshafts are not 'handed' when new but become 'handed' over time once fitted, so if removing them to keep as spares or from a spare axle you are stripping (for spare parts, as I have done with a couple) it is important that you label them up as Drivers and Passenger side.
Fitting secondhand ones to the opposite side to the one they were originally fitted to has been known to cause failure and it will never happen in a convenient spot.....
Best wishes,
Mike.
1954 Series 2: 4 door: "Sally" -- Back on the ground with (slave) wheels and waiting to be resprayed......
1970 Triumph Herald 1200: "Hetty" -- Driven back from Llangollen in Wales (twice.....)
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Re: Original Half Shaft Failures

Post by irmscher »

You can take your half shafts and have them heat treated yourself providing they are in good overall condition
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Re: Original Half Shaft Failures

Post by MCYorks »

Fortunately I've not had a half shaft fail yet, but the differential gears did disintegrate once. Now diff-lock is great for off-roading, not so good for normal road driving :cry: I do have a couple of spare half shafts of unknown history, so don't know which side they are from or even if they're off the same axle :-?

I can understand torsion bars becoming 'handed', because as a spring under continuous load they will eventually creep or settle. However, a half shaft is only subject to torque loading when the car is being driven and they spend most time stationary. Surely the half shafts must be subject to a significant amount of 'reverse torque' when using engine braking or reversing. You would expect this to prevent them becoming 'handed' to any significant degree. Maybe in real life situations it doesn't work out like that.
My thoughts are that it's the alternating torque that starts fatigue cracks at the splines. The cracks then gradually grow until they reach a critical point and the shaft snaps. You would expect them to break under maximum load e.g. setting off up hill. But it's apparent from various comments that they can break with relatively minimal loading in some situations. As people have pointed out, they usually break in the worst possible place :-?
In theory, if you could detect the fatigue cracks early enough, you could take the shaft out of service before it breaks. I wouldn't fancy routinely inspecting the half shafts every 1000 miles though :lol:

I did wonder if the Austin A30 / A35 and Spridgets, which all have shorter half shafts than the Minor, fare any better on this front. Apparently they also suffer failures, even though I've been told the Spridgets have tougher half shafts.
jaekl
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Re: Original Half Shaft Failures

Post by jaekl »

The half shaft from the splines to the flange are larger diameter than the root of the splines so therefore are not stressed as much. Surely the diameter of the root of the spline was designed with a safety margin to handle the torque, so there is no yielding of the outer surfaces. There is no permanent "twist" to become handed. On the other hand there is wear on the splines. From either the impact or just normal forces to the face from the differential gears or the width of the spline doesn't have enough safety margin for the shear forces. I will be watching the life of this half shaft because the replacement is from the other side. If the failure mechanism is the due to overloading of the face of the spline such that irrupt change in the surface from loaded to unloaded spline face induces a crack, then I may experience long survival. If so it may actually be beneficial to rotate the half shafts. We'll see.
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