Just thought you folks should be aware that Crusty the RM saloon that has travelled the world without an issues snapped the nearside halfshaft 3 weeks ago as I was leaving my son's house towed car home and fixed with the usual sprinkling of swear words and curses!
The axle was built by the "Guru" Vince Leek about 15 years ago a warranty claim is pending.
18-10-2021, 09:36 PM (This post was last modified: 18-10-2021, 09:59 PM by Bob Culver.)
Thanks all for excellent photos.
I have a collection of axles which have failed due hammering of the key. But the cars were not completely immobilised so such axles survive and owners retained as there seems a prospect of repairing. The number of distorts the statistics!
Illustrated is a different failure due bending, some from several directions, typical of weight bearing rotating axles. From comments here it seems transverse failure is not uncommon. When tested very many axles show cracks at the key but these tend to follow the curve of the keyway and seem to proceed very slowly.
For those not versed in such matters fatigue cracks advance slowly over thousands of miles gaining speed until the very small remaining metal, clearly distinguishable, finally lets go.
Any slight defect encourages a crack. Oddly the hardness indentation seems to cause this, although often fatigue cracks steer around such compressions (which obseravation prompted the practice of shot peining). On the key side, did the crack start at the key edge? Is there any other trigger, such as a turning mark?
If the key is entirely welllinside the taper, and the taper is a tight fit esp at the thick end, the key should have no influence on crack formation. but some axles seem to heve the key too close to the e hub edge.
(And for the newbies. The Seven axle is defined as 3/4 floating. The car weight is not born by the axle but the tilting force, as when cornering, is. Any bend in the axle or wheel buckle does not help. Curiously with stub axles as on hard driven cars and on trailers , failure often occurs from the top due cornering load.)
I had great difficulty separating the taper to test my axles some time ago but I did not check beyond the key. Now feel compelled to do all again!
Although it is difficult to see clearly from the pics, they both look like fatigue cracks, the initiation point can be any defect or notch that causes a local discontinuity in a highly stressed area. The crack growth is, I believe, indicated by the grey areas surrounding the central break. Fatigue cracks will grow until there is insufficient sound material remaining and then failure occurs.
Brittle failure if my memory serves me correct is more crystalline in appearance.
Fatigue life was always defined as 90% initiation and 10% propagation, so once the crack is initiated life of the component is virtually over.
Originating from experience with our Seven beginning with a broken steering arm in 1950 I have taken a long term interest in fatigue. In constant load lab tests only 10% of life remains after initial miniscule crack but the real world with fluctuating loads is very different. From the sheer number of cracked cranks and steering arms these must operate cracked way beyond 10% of total life. Curiously early overload can extend life! So a few bunny hops with new axle and early prising of the car out of parking space may be beneficial!
(With cast iron the life cracked is very considerable. An uncracked ci Bradford crank is a great rarity and probably a replacement)
Are the wheel bearings badly worn thus transfering more bending load onto the half shaft ??
In an ideal world the bearing should take all the load and not allow the halfshaft to bend.
For such applications wide tolerance bearings are often recommended specifically to keep bending loads off the bearing and its mounting. I suspect the axle will significantly resist bending before any bearing contributes. (except perhaps preloaded taper, not used in this application.) I am not even sure if it is prudent to arrange very rigid clamping of the rear bearing in hub.
(With semi floating axles common on many later cars the axle carries all weight and fracture is more interesting. A youth in a "modern" indulging in wild cornering lost a wheel outside my house. He asked if I could help him refit it....! Hope springs eternal)
