Cracked Tailwheel Fork

[sfarringer]

Cleaning up the airplane in preparation for annual inspection.
Imagine my surprise when I find this crack in the Scott 3200 tailwheel fork!

This tailwheel assembly only has about 300 hours since it was new, so can't blame it on being 68 years old with 5000 hours on it.

I had bought parts to rebuild the original 68 year old tailwheel, but had not assembled it. Guess I'll assemble it tomorrow!

[swixtt]

Wow that is something!!
Can you take some pics of the rebuild? I am changing out my upper thrust plate sometime and I'm curious on the torque of the head bolt.

[Bruce Fenstermacher]

Well there you go. Matches my cracked bracket assembly. Goes to show just about every part of the tailwheel is subject to cracking.

By head bolt I presume you mean what is called the king pin with the nut at the bottom. This nut does not have specific torque. It's adjustment acts against the thrust plates and springs inside to adjust the drag and dampening of the tail wheel. It needs to be at least tight enough there is no slop in the bearings but after that only tight enough the tail wheel, with a proper caster angle, doesn't shimmy.

I'd count the threads on the nut or turns to remove it and replace it the same for a start point.

[swixtt]

I think mine might be a tad too loose. After replacing my tail leaf pack I still have shiminy shimmy. I compared it to a super mountaineer tail wheel on our field.... Almost identical alignment reassuring my thought on the castor angle.
My upper thrust plate pins were all sheared off when I was in there last... I finally have a new one to install!
I'll be a little more critical of the king bolt torque.

[c170b53]

Also check that the internal shoulders of the two casting haven't been rounded off and when torquing the shoulders are aligned. Should be very obvious with a low time tailwheel, maybe not so obvious with a worn assembly.

[DWood]

The interference between the bushing and the casting must be excessive to cause a crack like that. I would bet it isn't the only one out there.

[sfarringer]

The interference between the bushing and the casting must be excessive to cause a crack like that. I would bet it isn't the only one out there.

I guess I would be surprised if a bronze bushing could exert that much force. Maybe the steel bearing cup is a little more likely.
I would bet some issue with the casting though.

[DWood]

You might want to notify your source to let them know as this sure appears to be a manufacturing defect.
Modulus of Elasticity of Bronze is higher than Aluminum

[c170b53]

My dart; could be porosity, could be an inclusion but I'll go with manufacturing defect where the surface finish on one the bores that was drilled into the casting likely was out of spec. A dull bit would tear the surface of a bore rather than cut a smooth surface, the tear in the bore surface would be the initiation point of the crack.
Take a 10x to it and let us know what you find. I think you might have recourse with the supplier based on the parts time in service. A SDR again due to time in service might also be submitted.

[sfarringer]

Unfortunately, there will be no recourse.
It was un-used when I bought it, but it was not new. I bought it from an individual who had purchased it many years ago but had not used it.

At this point I do not have any reason to think it is more sinister than an isolated occurrence.
But it might be interesting to get a look inside the crack and see if there are any clues.

I think I will put it in boiling water and see if the bearing cup will come out, before sawing thru the side opposite the crack.

I had all the parts to reassemble my old Scott 3200, and put it together today, so the old girl is back on three wheels.

[c170b53]

Oops..In light of the new info, I'll say I was on the wrong track....again
Good to hear you had the foresight to have a spare and underway again.

[hilltop170]

It will be hard to determine the cause. The crack goes thru two small holes in the casting which makes sense due to, if nothing more, the reduction in cross section area of the material not to mention possible stress concentration sources. I think we can totally rule out fatigue since the casting does not see classic stress reversals which cause fatigue. The jagged edge of the crack indicates brittle failure of the casting as would be expected.

The crack intersects the bearing outer race, the bushing, spring, and pin holes. As Jim correctly notes, any stress riser in the holes could have initiated the crack but also, if the pin hole or bearing outer race hole was undersized or if the pin was oversize and forced into a hole with a stress riser, it would increase the possibility of a crack initiating. If the pin had a burr on it and was forced into the hole, the burr could create a stress riser as it was pressed in. It would be interesting if the crack face could be exposed without further damage of the fracture surface.

[bagarre]

That pin isn't pressed in with any amount of force. At least, not on mine. it came out quite easy and dropped back in.

What about the possibility of ice? If it was not well greased, water could collect inside and freeze split it. That's really a stretch but I know that can crack a fuel valve.

[sfarringer]

Opened it up.
I guess I don't see anything too remarkable, but see what you think. There certainly is a thin wall adjacent to the bronze bushing, but the bearing race is pretty well supported.

One other bit of information: This tailwheel did see some service that is different from most. It was flown a couple of hours with a tail ski attached, over a year ago. The tail ski attached using the wheel axle bolt, and also clamps onto the fork arms near the top of the arms. I am not convinced this was the cause, but it probably does result in some different forces than normal.

[TFA170]

While I'm no metallurgist, that's a clean break. That is not a crack that propagated from a small flaw. That type of failure usually happens with a single incident, not a fatigue type failure over time.