Oversized bushing

Mikey52

Line Up and Wait
Joined
Jul 26, 2023
Messages
578
Display Name

Display name:
MMADD
I need an over sized bushing for a NAS75-3-009. Any one have an idea. Outside diameter needs to go to about 7/16 in.
 
?NAS75-3-009 is a bushing

You need a bushing to fit over a bushing?
Or want to replace it with one of different dimensions?

ASSCo has a variety

Also
 
I need an over sized bushing for a NAS75-3-009. Any one have an idea. Outside diameter needs to go to about 7/16 in.
If this is a specific aircraft requirement, some OEMs offer oversized bushings under non-standard part numbers.
 
It looks similar to a Cessna Shimmy Damper Bushing.

The oversize is quite a jump!
 
If Damper ;suggest you match bushing to aircraft rather than ream to match
Bushing. The hole will be elongated. Match bushing to that measurement. I doubt the 7/16 would legal.
 
If Damper ;suggest you match bushing to aircraft rather than ream to match
Bushing. The hole will be elongated. Match bushing to that measurement. I doubt the 7/16 would legal.

What's the legal way to fix this? I'm not an A&P, so it's a matter of curiosity.

If the hole is elongated, don't you pretty much have to ream it out to some round shape? It's pretty difficult to make oval bushings isn't it?

thanks!
 
A larger bushing OD implies a larger hole in the structure, which implies a weakened structure. I am familiar with that problem on Cessnas, and there is too little metal there to allow major oversizing.

The root problem is dynamic imbalance of the nosewheel. In most shops, if that wheel/tire assembly gets any balancing at all, it's a static balance, not dynamic, and there is a huge difference between the two. Your car's wheels get dynamically balanced when you get new tires, and cars have been getting that since the late 1960s. Airplanes get nothing, or just a static balance which can actually make the dynamic imbalance worse. The shimmy damper only masks the symptoms, and the torque links and everything else gets torn up as that wheel tries to shimmy.

I have made many posts on this subject and will not explain it again. Do a site search on "nosewheel shimmy." I have cured many shimmying nosewheels by simply dynamically balancing them. It saves all the nosegear stuff as well as airframe stuff. Instruments and radios don't appreciate being shaken violently either.
 
I've read about the shimmy and balancing - was curious about the legalities of who gets to decide how oversize to make the hole, not as interested in the shimmy part.
 
What's the legal way to fix this? I'm not an A&P, so it's a matter of curiosity.

If the hole is elongated, don't you pretty much have to ream it out to some round shape? It's pretty difficult to make oval bushings isn't it?

thanks!
A new steering collar ("arm assembly") is needed. #27 here:

1730909138620.png

That #27 is pointing at the hole that is worn. Very little margin around it. I have never seen any legal means of reaming it for a larger bushing.

Expensive repair. This is what comes from accepting nosewheel shimmy. Some people even say "that's what Cessnas do." That's accepting mediocrity, and I would not. I figured out how to do dynamic balancing, and our flight school did not have shimmy problems.
 
It’s possible that newer Cessna manuals may provide allowable wear dimensions. If so , even .010 may be allowable. With current cost of parts
it might be worthwhile to contact a DER.
 
For certain, do not ream or drill the hole any larger for the #27 part above!

Measure and record the minor and major diameters of the oblong hole with a plug gage. Make (from a free machining steel such as 120 ksi Stressproof) a simple ID/OD bushing with the desired ID (.1875 or a little under for an AN3 bolt.) Make the OD equal to the desired major diameter. Force that custom bushing onto a .1875 mandrel, or simple, just retain it with a AN3 bolt into a chuckable tooling stub.

Put that mandrel and bushing assembly in a 4 jaw lathe chuck. Use a dial indicator to offset the mandrel stub/bolt assembly equal to the difference between the major and minor diameters. Paint the concentric bushing with machinist bluing. Skim the OD of the stub until the bluing is gone.

You now will have a thin wall bushing that will fill the egg shaped hole. Hopefully you started with a free machining steel material that has good hard properties and won't wear the original hole any more and it now can be pressed and perhaps Loktited into place. The only difference is the new hole will be located slightly differently, but this is minor in the great scheme of things.

Some fiddling with the bushing OD, installation chamfers, and tolerances will be necessary. Maybe someone would want to make a kit? It would certainly be desirable to have a pressfit hard enough to minimize future wear.
 
Any rebushing oversize is sketchy and does not address the real problem. This is how it stacks up:

1730939549413.png

The upper green line is pointing at the worn bushing hole. That hose is in an aluminum casting, which rotates on the oleo barrel, and that barrel is mounted to the firewall via a couple of brackets. The lower green line is pointing at the steel lug that is welded to the oleo barrel, and it does not move.

The disassembled shimmy damper:

1730939740667.png

The upper green line points to the lugs on the side of the cylinder, and those are the lugs that straddle that aluminum lug with the bushing in it. The lower green line is pointing at the ends of the piston rod, and it connects to the non-moving steel lug on the oleo. So the cylinder itself moves, not the rod. Odd. And that cylinder, being mounted from its side instead of its end, bounces up and down all the time while taxiing, and rocks on that bushing and seriously wears the hole in the aluminum steering collar in an oval AND hourglass fashion. Shimmy just adds to the agony.

The AN3 bolts also wear, and so does the hole in the rod end and steel lug.

The nosewheel is mounted in the fork, which is held by the piston that goes into the oleo barrel. A dynamically-imbalanced nosewheel makes that fork twist left and right, which moves the torque links ("scissors") that are mounted on the fork and on the steering collar. The torque link bushings wear and so do the holes that the bolts mounting them go through.

1730940308214.png

One of the BIG problems is that mechanics don't get those two long bolts tight enough. They pass through aluminum lugs on the fork and steering collar, and through "spacers" that ride in the torque link bushings. Those spacers are made to fit precisely between those lugs, and tightening the bolts properly pulls the lugs in a tiny bit to clamp those spacers so they cannot move. Mechanics leave the bolts a little loose, so now the spacers move and chomp depressions in the lugs and now there's too much slop to get the lugs tightened against even new spacers. Tiny shims have to be made to fill those depressions. The moving spacers bang the bolts around, and they wear their holes in the aluminum castings, wallowing them out. Ugly.

This same phenomenon is also seen in the rudder and elevator hinges, where the same principle is used. A spacer in a bushing, with the bolt securing the spacer between the hinge lugs. The 180 and 185 stabilizer aft mountings also use this principle, and if those bolts are not properly torqued, the spacers move and the bolts wallow out the bolt holes in some VERY expensive structural parts in the tailcone. With the rudder and elevator hinges, it's the hinge lugs that get torn up.

The whole nosewheel shimmy damper design is inadequate to handle a dynamically imbalanced wheel, and that stuff all gets torn up. Replacing it just results in the same losses. Take the bearings out of that nosewheel and take it to a motorcycle shop that has a dynamic balancer. Spending the money in the right place prevents spending it in the wrong places.
 
Last edited:
It’s possible that newer Cessna manuals may provide allowable wear dimensions. If so , even .010 may be allowable. With current cost of parts
it might be worthwhile to contact a DER.
This is what the 172R & S Service manual says:

1730941375460.png

Replace as necessary. No different from the old manuals.
 
Back
Top