Axle Insert Reinforcement

jaredyates

jaredyates

4-Place Kit Builder
Staff member
Has anyone done the optional modification to insert a piece of tubing into the axle with Loctite? I'm wondering how to best prep the inside of the existing axle to make it the right size. The original notice suggests using a reamer, which I'd rather not have to acquire just for this limited use.
 
I don't know how much cleanup the inside of the axle takes but what about renting a cylinder hone from the local auto parts house?
 
I plan on using a flex hone when I do mine. A lot cheaper than a lathe or a reamer. Since I will be doing it on installation I am seriously considering rosette welding it in, even I though I have no doubt that locktite sleeve retainer would work just fine.
 
I used a large drill. The inboard part of the axle tube was slightly undersized.The drill cleaned it up nice. The outboard section was good. It did not take much time. Be sure to lather up the reinforcement tube well. Once you tap it in it is not coming out.

Mark
 
It seems like the drill would leave a sharp ridge on the inside where it stops, leaving a potential stress concentration point unless it was dressed out.
 
The axle tube goes inboard past the vertical V support tubes. All the bending moment and resultant stress is at the outboard side of the vertical support tubes. The very inboard end of the axle tube is under very low stress.
 
I'm working on a Patrol so I had to look up and read about what you guys were talking about. The patrol has the same 1.5"x .120 axle, but at less gross weight I guess its OK?

Doug
 
From the archives and BearTracks:

I just got this back from Kevin. It looks like the axles were more than up to the task. The failure was self inflicted. Maybe Bob will make a recommendation yet, but I suspect he will be satisfied with these results. The little things DO matter.

"On the axles, bottom line is the one had a working crack for quite some time. The crack propagated
from a stress riser where the weld bead was ground away on the brake caliper attach ring welded to the axle.
A bit of weld was ground near the bolt hole to clear and square up the surface so the bolt / nut would sit square. This created a sharp little notch that actually had just barely cut into the axle.
Given the event the cracked axle broke from overload on the remaining un cracked portion of the axle.

The other axle is free from cracks. The ripping off of the axle from the gear leg was a pure overload failure.
Rob Gaddy had a metallurgist he knows look at it. His thoughts are conveyed above. "

Pat Fagan

I just finished the axle reinforcement that was in the newsletter a couple of years ago. What a pain. I didn't have a 1.255" reamer laying around and I couldn't find one to buy that was long enough (besides it was $60). I bought a brake cylinder hone to clean up inside the axle (only $15). I honed so much the little stones are completely warn down and the reinforcement tube still wouldn't fit in to the proper depth. I shifted to a new strategy; I used one of those thick Sharpie pens to color the end of the reinforcement tube and put register marks on the axle and the reinforcement tube so I could get it in the same orientation each time. I shoved the colored tube in as far as it would go - then pulled it out and filed where the ink was rubbed off. I repeated this operation many, many times - re-inking the tube each time. Finally, the tube made it to the specificied depth. Before I secured the tube with the Locktite goo (I used Locktite 620) I mounted the gear back on the fuse along with the wheels and measured to see if they were parallel and lined up with the fuselage centerline. I didn't want to have to heat the gear after I used the Locktite. (I got 1-degree of toe in and declared that good enough) Back to welding stuff to the fuselage.

Good luck with your projects,

Jim Van Fossen #411

This mod is recommended for four place Bearhawks that are going to be operated extensively at over 2200 lb.
gross. The tube should be Loctited in place with Loctite #277 or #620 per Loctite instructions.
 
Helidesigner said:
The axle tube goes inboard past the vertical V support tubes. All the bending moment and resultant stress is at the outboard side of the vertical support tubes. The very inboard end of the axle tube is under very low stress.
Click to expand...
How do you know that's where most of the stress-concentration occurs?

I also note the weld geometry is different on old vs new kit legs, so the stress risers will vary depending on that.
 
Being a mechanical engineer I have gotten good at discerning high and low stress areas in structures. Without a picture it is hard to describe but the inboard part of the tube has low bending moment inside of the weldments, hence has pretty low stress. If my wheel falls off I'll let you know. : )
 
I would have been very interested to see an FEA if you had one, also being a mechanical engineer :) I would like to understand the applied forces and stress path in more detail, as I am considering whether or not to do that particular mod. To me the stress during a landing isn't simple-case, and my thinking is aggravated by the complex structure of welded hollow-tubes in unusual geometry
 
Thanks for the input everyone. I am most concerned about material left from weld penetration on the inside of the tube. I've seen some reamers on ebay that would fit, but I'm wondering how well the reamer would do at clearing chips if there is much material in there. Aren't the straight flutes of reamers generally intended for very small material removal only? The most appealing option was an adjustable reamer, which would also answer the question of whether a .125" reamer would allow enough clearance for the tube to fit.
 
You can, no problem. I actually bought a thicker set from Rod Smith waaaaay back when I was going to make my own gear. I went a different route. I scratch-built my fuselage but bought the engine mount and landing gear from (then) Avipro. For the simple reason that if the gear ever gets tweaked I can buy a new one without having to fabricate a one-off duplicate. The engine mount was purchased in case me or another might want to put on a different engine.
 
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