Triumph Repair: rear axle comes out, rear axle shaft, fatigue failure

Question
QUESTION: The rear axle shaft has come out of the U-joint assembly for the second time
now, shearing the retainer pin, on my 1980 1500 Spitfire. The first time, I
rebuilt the trunion and replaced the U-joints on both sides, and torqued the
spring bolts to spec. Since buying the vehicle last fall, I have only put about
600 miles on it. The rear spring, purchased from TRF, was replaced in 2002,
and the previous owner put about 6000 mostly carefree miles on it after the
replacement. What do I need to do to make sure the axle shaft doesn't come
loose again? Thanks for any suggestions!

ANSWER: Hi Earle,
That spline and yoke are a tight fit and the pin is just in there to keep it in place so there should not be any movement in that area. You need to check for any loose play anywhere that can cause a "Hammer" effect on the drive axle. A "Swing" axle design like this does exert some inward pressure on the drive axle but unless there is loose play that can cause a hammering effect, that pin should not get sheared off. If it is the same side each time, that should tell you something. You have not added a large offset alloy wheels have you?.
Howard

---------- FOLLOW-UP ----------

QUESTION: Driver's side (left) both times.  Wheels are stock.  Didn't see any play or
looseness in the suspension when I put things back together the first time,
but, of course, I will look around some more.

I examined the fracture surface on a piece of the pin microscopically and saw
no evidence for ductile or fatigue failure......totally a brittle fracture, which
suggests a very large, one-time stress.

Answer
Earle, if it were mine and you can see that the shear action was inward not outward. An outward shear off would indicate that you were sliding sideways and hit a pot hole or something. A shear off of the pin inward indicates a sharp strike inward and if you look at the lower pivot point of the outer suspension you will see that there is not a lot of leverage on the axle even on a hard bump. So the upper bushing would have to be loose for the wheel and axle to be knocked inward hard enough to shear off the pin.

I would do this, loosen the four bolts that hold the Differential flange to the "U" joint flange so you can insert a feeler gauge or such between the flange and the differential flange to see if a load exists or an excess distance. (with a new pin in) Now place a jack under the outer wheel flange so as to compress the suspension on the failing side of the car. As you raise the jack note the clearance between the "U" joint flange and the Differential flange. This will tell you if an excess load exists on the axle in the travel of the suspension.

This will at least tell you that normal suspension travel is not exerting excess loads on the axle for what ever reasons.

Other work that was done on the rear suspension didn't result in the differential stub axles getting mixed up was it? They look close but are a different lengths.
Let me know,
Howard