Oversized bushing
- Thread starter Mikey52
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Let'sgoflying!
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?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
www.aircraftspruce.com
Also
www.mcmaster.com
You need a bushing to fit over a bushing?
Or want to replace it with one of different dimensions?
ASSCo has a variety
Bushings | Aircraft Spruce
Aircraft Spruce is a worldwide distributor of certified and homebuilt aircraft supplies.
Also
McMaster-Carr
McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.
If this is a specific aircraft requirement, some OEMs offer oversized bushings under non-standard part numbers.
nrpetersen
Line Up and Wait
If it is for a Cessna shimmy damper, find someone with a metal lathe and turn down the bushing OD as needed. BTDT
nrpetersen
Line Up and Wait
Measure more closely. It is probably egg shaped. Make the bushing out of good steel.
bentmettle
Pre-Flight
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!
Dan Thomas
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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.
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.
bentmettle
Pre-Flight
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.
Dan Thomas
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A new steering collar ("arm assembly") is needed. #27 here:
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.
nrpetersen
Line Up and Wait
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.
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.
Dan Thomas
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Any rebushing oversize is sketchy and does not address the real problem. This is how it stacks up:
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:
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.
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.
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:
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.
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.
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Dan Thomas
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This is what the 172R & S Service manual says:
Replace as necessary. No different from the old manuals.
nrpetersen
Line Up and Wait
The egg shaped steel bushing above fix was intended to only address wear in the cast aluminum hole (#27 above) by adding a steel sleeve without removing any more cast aluminum than necessary, and to provide a steel wear surface for the shimmy damper body. Our 172M at 2000 hrs TT didn't seem to have worn any steel parts appreciably.
The need for nose wheel dynamic balance is obvious but fleeting, and not readily available in many areas. How long does it typically last? Anybody have a clever way to do this?
The need for nose wheel dynamic balance is obvious but fleeting, and not readily available in many areas. How long does it typically last? Anybody have a clever way to do this?
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I think it was suggested to take the wheel/tire to a motorcycle shop. Most do tire balancing.
Dan Thomas
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That aluminum casting starts with a steel bushing in that hole. It's the bushing itself that eats the hole bigger.
2000 hours on a 172 is nothing. There are 172s out there with way over 20K hours.
Once done it can last the life of the tire. I started doing it by hand. Take the bearings out and clean all the grease off them. Clean the axle and spacers. Put the bearings and axle in dry, no seals, and put some pressure on the spacers to keep the bearing seated. Hold the tire gently against a wire wheel mounted in the shop bench grinder and spin the wheel up some. Doesn't have to be going real fast. You will note your hands moving in various ways. If they're both going up and down together, its mostly a static imbalance. That's rare. More often, one hand will move more than the other, or both will move opposite each other, and that wheel will wobble. That's dynamic imbalance. Now you need stiff foam blocks and stick-on lead balance weights (Aviall has them, but so does any tire shop) and you cut them into size and put them in the wheel, against the inside of the rim, and use the foam blocks to hold them there. Wear safety glasses or a face shield, and spin the wheel again. You'll have to experiment with various locations on both sides of the wheel, and various amounts of weight until that wheel spins as smooth as silk, then you stick the weights on the aluminum wheel after making sure it's really clean. No more shimmy.
Takes time. I built my own mechanical balancer to speed it up. You can buy electronic ones but you're looking at $4k and up.
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