Sheared the nosewheel attachment bolts

1736796806875.png

I see some necking of the bolt there. The thread peak diameter is visibly smaller than the shank diameter. That tells me that those bolts have been overtorqued, maybe because they were found loose a few times, and every time they were retightened they were stretched a little until they cracked.
 
Dan Thomas said:
View attachment 137153

I see some necking of the bolt there. The thread peak diameter is visibly smaller than the shank diameter. That tells me that those bolts have been overtorqued, maybe because they were found loose a few times, and every time they were retightened they were stretched a little until they cracked.
Click to expand...

Also look a bit bent, though, too, doesn't it? The right side thread spacing is opening at the top compared to the bottom.

Do the bolts go into a steel part or some cast alloy? It would seem difficult to overtorque to the point of plastic deformation going into alloy unless the bolt was already damaged somehow.

If the failure mode is so common that a replacement kit exists, I think it's fundamentally poor design. Shear load across the root of the threads, at the point you get any crack initiation, they'll come loose, so they'll get tightened, put in a higher stress state, and the whole ball of crap gains momentum on down the hill. Shoulder bolt might have worked a lot better if you could get the shear forces out of the threaded portion?

interesting thread about threads. Inspired me to poke around the internet about a few refresher concepts - learned that my fatigue and durability professor passed last spring...
 
bentmettle said:
Do the bolts go into a steel part or some cast alloy? It would seem difficult to overtorque to the point of plastic deformation going into alloy unless the bolt was already damaged somehow.
Click to expand...
Into the top of the steel cylinder. It's the same sort of arrangement used in plenty of other places, with bolts going into threaded holes in castings or forgings or weld assemblies. Another place where bolts get necked and broken is in brake caliper assemblies. Light aircraft brakes (like Clevelands or McCauleys) used 1.4" AN bolts that run through the cylinder casting and thread into the outboard brake lining support. 70 inch-pounds is the spec, but most get Armstronged to 100 or 150 or more every time the linings are replaced, and they get tired of that. They stretch and break.
 
bentmettle said:
If the failure mode is so common that a replacement kit exists, I think it's fundamentally poor design.
Click to expand...
Not always. A lot of "replacement kits" are offered to facilitate repairing items that can be overlooked during the course of normal maintenance or operation. Or don't become issues until after 30 years in operation. I'd be interested to see the root cause behind this "common" failure and see if there is any previous Piper guidance to check these bolts.
 
It is one of the most perfect examples of poor design. Three short bolts (coarse thread?), loaded in single shear, all operating at a minimum radius, blind tapped mating thread into an expensive part rather than a replaceable nut if the female thread is damaged. No key(s) to carry an indefinite load into a critical control, etc.

Going to a high strength fastener isn't going to change much. Just make the fastener slightly more brittle and more able to rub out the tapped hole threads.
 
nrpetersen said:
Going to a high strength fastener isn't going to change much. Just make the fastener slightly more brittle and more able to rub out the tapped hole threads.
Click to expand...

Disagree a little. The endurance limit of the improved strength fasteners will be higher than the originals so the crack propagation may be delayed or eliminated. You don't have to worry about notch fracture toughness as much if you can avoid the notch, right? OR, if the failure is primarily driven by overtorque, those fasteners will be more yield resistant. Would like to see one that had cracking without being necked.

The other examples that Dan had were kind of more of the same single shear. It's quick and easy and as he stated, fails if the fastener gets abused.
 
Does your Comanche have toe brakes (not all do, as I am sure you know); if so, you'd likely have been able to steer with differential braking.
 
nauga said:
I figured no nuts might signify a different event. But 3?!

Nauga,
triplex for redundency
Click to expand...
As they say in the PR Business when replying to a Press Inquiry, “I have nothing for you on that at this time
“.
 
You must log in or register to reply here.
Back
Top