What is the cause of half shaft failures?

[jaekl]

I never bought into the transmission of torque causes the shafts to break and that the differential ratio has anything to do with it either since the torque on the shaft is the momentum of the car's weight only. Of course more HP translate to more torque. Some have stated that wheel bearings can be the cause. HOWEVER, my experience indicates it is car specific. I know the long term performance of six half shafts locations. The longest of 113000 miles on a bone stock 948. The shortest is a 28000 mile truck with a 1275 and oversized tires (roughly equivalent to a 4.1:1 or 4.2:1) that has towed other Minors on a used axle. The third is 46000 miles, also a bone stock 948 under my care. Of all those half shafts ONLY the RH one has broke (three times) on the third Minor. The first one was acquired with maybe 40 or 50,000 miles (the original log is around here somewhere). This car had a self destructive clutch that went away when I put in a different gearbox but I don't think the original owner had any broken shafts and none under my care. The 1275 truck came without axles or much anything else so a used axle was acquired with unknown history. It does have a 4.55 diff, but the oversize tires help. Now the subject vehicle. It had 45000 miles on it when I got it. Put it on the road and drove it until 57000 miles when it was refreshed. At some point I installed one sided sealed wheel bearings. Some time later at 61000 miles I experience my first broken half shaft, so I switch back to original bearings and had the second break at 77000. Now I had a third at 91000 miles. Perhaps the original lasted 61000 miles, the second 16000, and the third 14000 miles. All in the same location on the same vehicle. The first two broke at the end of the spline, but this last one sheared at the flange. That sure made replacement easy. Even easier if I was thinking at the time and didn't proceed with the familiar repair steps.

So what is the cause? Power obviously isn't the issue. Could be a bent axle housing or a poorly aligned differential, but the shaft slides into place easily. The shear at the flange may indicate that the bearing may not be running square to the shaft causing flexing, but it's hard to think it could cause the common failure at the spline. It's stated often that half shaft breakage is common, but of those who have experience multiple breaks, what is your experience? What's the opinion in the BMC A Series rear axle community? I probably should consider installing a different axle and differential.

[ampwhu]

In my view, unlucky. I've never broken any in 35 years and I dont know anyone else with a rear wheel classic that has either.

My only guess is lack of oil in the axle.

[philthehill]

The failure of Morris Minor half-shafts can be caused by several factors. Here are a few:-

1. Metal fatigue.

2. Wear in the diff.

3. Wear in the wheel bearings.

4. Shock loading on the half-shaft.

5. The use of S/H half-shafts.

6. Used half-shafts fitted to the wrong side of the car.

Additional power to the diff and rear axle can accelerate the failure of the half-shafts by any of the above.

One of the first things I did to cope with the additional loading placed on the rear axle/transmission was to buy a pair of new BMC half-shafts from Bullmotif and have them heat treated to a guaranteed transmission capability of 175bhp. They easily cope with the 135bhp plus generated by my 1380cc full race engine. Therefore I have no desire to change the axle for another type.

Fitting the anti-tramp bars to my Minor axle stopped the axle tramp which puts repeated shock loading on the half-shaft.

[jaekl]

Ampwhu, I agree, I don't think half shaft are common in factory Minors or even factory tuned 1275s. Phil, I think you are missing the point. I have no extra power and it's the same location on the same car. For the most part the half shafts are not a wear item. If they were then as soon as they were driven out of the factory, they were used shafts. The only problem with acquiring a used shaft is you don't know the history. However, virtually all Minors these days are used with unknown history and many are not having failures. I'm not talking about a different type of replacement axle but one from another Minor. I'm just trying to identify what manufacturing defect other than a defective shaft is causing the failures. Axle tramp is a good point though, especially since it is more prone on the right side. I'd like to hear from any owners of stock Minors who have experience broken half shafts.

[simmitc]

I drove my first Minor in 1978. It was a 1963 car with 948cc engine, history unknown. The left hand half shaft failed when attempting to move off from traffic lights in 1979, indicated around 112,000 miles and 16 years old. Recovered by RAC, we stopped at a scrap yard on the way home and collected a replacement half shaft that I fitted that evening. The scrap yard said that they had a fairly regular trade in Minor half shafts, nearly always the left hand one failing. They attributed that to the camber of roads where the cars were parked outside houses.

Many years later, I regularly drove two 1098cc Travellers, each covering around 9,000 miles per year on both rural and trunk/motorway routes. Both tow trailers several times a year. One was in the NEC November show and broke the left hand half shaft when attempting to pull away from one of the roundabouts on site as we left after the show. Around 185,000 miles and 36 years old. The second broke a left hand half shaft when attempting to pull away at a roundabout in a local town, about 210,000 miles and 45 years old.

Similarities? All left hand, all at the start of the splined section, all late autumn/winter, all when moving off, but nothing statistically significant. I go with metal fatigue. To put it in perspective, 45 years of 18,000 miles a year = an average of one shaft every 15 years or 270,000 miles. The fact that both Travellers failed within two years is irrelevant.

I suspect that the reason we don't hear of this type of failure with other makes is because they ndon't last as long as Minors and are not driven as regularly as Minors.

I now have hardened shafts in all three Minors, and have not had any failures - although I really need to wait 40 years to confirm that.

[kevin s]

There are a couple of other factors,

Dynamically it is possible to generate 4 or 5 times the engine torque, basically because of the inertias of vehicle, wheels and engine reacting against each other, this typically occurs during a wheel hop or wheel stick slip event so tyres and suspension set up can have a significant effect.

Clutch peak slip load is a limiting factor for aggressive clutch engagements, not sure about a minor but today this is typically something like 1.5 - 2 times the peak engine torque.

Given the examples simmtic gave above It looks like the shafts have simply exceeded their design life (most cars even now are designed to 100,000 - 150,000 miles), perhaps good advice would be to change them at 100,000 miles if you are worried about reliability.

[oliver90owner]

I’ve never experienced a broken half shaft. Distorted some in my Landrover, but not broken. Cortinas and Escorts were severely stressed but I only smashed the gearbox or differential in those. Also broke a few rear springs on all three of those.

But I have gone through quite a few drive shafts on fwd vehicles... but they are through knocking CV joints, not breakages. I’ve mellowed over the years and now drive more sedately.

[les]

I vaguely recall having a broken shaft once, the cause unknown, however I’ve never dropped the clutch suddenly or indeed been rough with my vehicles. I think you’ve either got engineering ‘sympathy’ or not. I know machinery is not a living thing but like things that do live, I feel for it !

[MCYorks]

I've always assumed that half shafts fail due to stress concentration, resulting from the sudden change in cross-section caused by the splines. The splines are quite deep so the cross-sectional area of the shaft is significantly reduced at this point. In my opinion the torsion bars are probably a better design, as the cross-sectional area slowly increases up to the splines and the splines are not as deep. This would spread out the stresses. Personally I've never heard of a torsion bar breaking on a Minor. It's surprising they didn't use the same design for the half shafts, perhaps it was down to costs.

Regarding a half shaft breaking at the flange end. I suppose a worn or misaligned wheel bearing could cause the half shaft to flex and eventually break at this point. Does anyone on here know if the flange is friction welded to the half shaft or produced in one piece?
I've seen several axles over the years with significant corrosion around the bump-stop/mounting pads. Even leading to oil leaks! This could cause the axle tube to flex excessively and put additional stress on the half shaft and flange, leading to breakage.

[Edward1949]

In my view, unlucky. I've never broken any in 35 years and I dont know anyone else with a rear wheel classic that has either.

In my case, in 50 years I've never broken a Minor one. I've broken one in a 1952 Sunbeam Talbot, and one in an 1962 MG Midget. In both cases the cars had been through multiple owners and odometers had been round the clock. Both cars were often bump-started, often in reverse which couldn't have helped. I remember my brother broke one in his 1957 Ford Popular. There aren't many "sudden death" components in cars, most give some warning of impending failure, but the half-shaft doesn't. A similar modern component is the cam-belt. It works perfectly until the day it snaps!
If I were now planning an extended tour abroad in my Minor I'd probably replace both shafts, or at least pack a spare.

[Keith 66]

I have had two break on my truck, seem to remember both nearside, If you look at a secondhand shaft you will often find a wear point on the shaft at the point it exits the diff scocket, often this will appear sharp & the splines themselves may show that they are starting to twist out of line. The sharp edged wear point is a classic stress raising point for fatigue to start & a shaft like this is guaranteed to snap sooner or later.
Perhaps if the splined socket in the diff had more of a radius its internal splines where the shaft goes in it wouldnt wear the sharp step that will lead to failure. Bit of dremel work on the diff sockets on the bench would sort this.

[Chipper]

My 1970 Traveller has suffered three broken halfshafts; two LHS and one RHS. The car was fine up until the point I replaced the original 1098cc engine with a rebuilt 1275 Midget engine at around 104,000 miles; the first halfshaft (LHS) snapped at 113,000 miles as I was pulling fairly sharply out of a junction going across the busy A13 in Essex - had to get it towed by the AA to a garage near my workplace to have it repaired.

Second and third failures were quite random; the second one (LHS) failed at 157,915 miles at which point I also changed the original 4.22 diff for a 3.9:1 Midget one, which I mistakenly thought might reduce the possibility of breakages!

Alas, it was not to be, for the third (RHS) one broke at 160,007 miles, while I was coasting down a long hill in traffic with my brother on board, waiting for level crossing gates to open, and for some reason the engine cut out and wouldn't restart on the starter, so I attempted a rolling jump start (which got the engine going again), went to let out the clutch and heard a metallic twang out back, then realised there was no drive so guessed a halfshaft had snapped.

I now keep a spare pair in the garage, along with a complete spare axle and diff...