Thread: Problems with defenders steering sector shaft

Originally Posted by rick130

Dougal, do you reckon it's a machining/transition issue or just too small a section and that point is obviously the weak point ?

I couldn't say without inspecting both and doing a lot of work.

But generally with fatigue if the stress is low enough (i.e. the load applied is low enough) then you can last for millions of cycles without any problems. The higher the stress gets, the more critical every little transition becomes. Any transition that's a bit rough becomes a great place for a crack to start.

I think inspection/testing is the way to sleep at night. For example if the parts that break have 15 years and 300,000km service, then an inspection of the replacements at 7 years and 150,000km more would make a lot of sense.
Overload testing doesn't shorten the fatigue life, it actually increases it. Because it over-stretches and stress-relieves the material around the crack which takes a lot of the usual smaller cycles to overcome.

Just like bike spokes. If you don't stress-relieve them by stretching then they can break heads off within a couple of years.

I've only seen one broken one in my career,but I wonder how many series 1,2 and 3 Landrovers have come to grief over the decades due to the relay shaft breaking in half ? These are significantly smaller in section than Defender/Disco sector shafts, although being unpowered and with the trackrod high up at front they would only be subject to attack from shocks due to the crap leaf suspension as opposed to impacts from the linkage striking offroad obstacles and input torque from the hydraulic power assist.
Wagoo.

Edit. BTW,As with all tech discussions I'm happy to be proven wrong,I'm also here to learn. but IMO the theory that heavy duty trackrods contribute to sector shaft failure is not correct. By way of explaination lets imagine the trackrod and tierod ends are strong enough not to bend or break when the trackrod strikes a rock or stump.
What would happen? Well if the object is solid enough the vehicle would come to a sudden stop with both front wheels remaining parallell.Strike the obstacle with a standard trackrod and what happens? The trackrod bends easily and quickly wrenches the left and right hand swivel housing in opposite directions. Guess whats attached to the front of the left swivel housing. Yes the drag link, which is also attached at its other end to the sector shaft via the Pitman arm. So this sudden and barely resisted shock loading from the left swivel being wrenched to left lock applies a massive and sudden torque force to the sector shaft. A torque force that a stronger trackrod would have reduced