Sleeve Gear Bush Clearance

Yep toppy knows something about the non obvious goings on in AMC lower gears. Only takes one over tight primary event to also twist shafts to a bit of candy cane shape out of line, so worth checking or tends to wallow out the bushes too soon again. Cranks may be straightened by just pounding on convex area but not tranny shafts which have to be unwound with series of corrections following guide of dial gauge, finer and finer and finer till good enough of give up. I had Peels shafts Tungsten Oxide dry friction coated and ran thin ATF more for heat flow of thinner oil than any hope of enough oil to surf parts on. Then could get away with staying in 2nd for miles and miles 6000 and above having the time of my life.

toppy said:

It is not as simple as the expansion rate of one type of metal verses another. As the bush runs on the shaft an the oil separating the two is forced out then the load will generate friction this will generate localized heat in the bush causing it to expand independently for any heat related expansion of the rest of the gearbox. This will now start a vicious circle as the expansion reduces clearance as the bush is trapped by the gear so more friction is generated leading to more heat an expansion an so on.
Mick Hemmings says on his gearbox rebuild dvd that he has seen the main shaft snap as a race bike warmed it engine in the paddock in neutral so no oil was being flung around inside the box. My own shaft show signs of localized heat damage from over tight chain by previous owner

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As oil can withstand pressure generated at the big ends and provide adequate lubrication it will cope with the pressure between the gearbox mainshaft and sleeve gear bushes. If oil was forced out between any two surfaces as you say, then there would be no lubrication anywhere in our engines.

I'm quite sure with gearbox in neutral the fixed 1st and 3rd mainshaft gears are rotating as are there layshaft counterparts, therefore oil splash will be present.

Any friction generated between the mainshaft and bushes will cause both components to heat up, including the sleeve gear due to heat transfer.

Thermal expansion of a hole e.g bush, means the hole gets bigger - not smaller. The clue is in the definition of expand.

I think my calculations are inaccurate - but the theory is ok.

Sorry Al but everything in the engine that spins depending on oil wedge separation and cooling is either got oil pressure flow fed to it or drenched in oil violent forced splash coverage like cam lifters, so your logic does not apply to AMC sleeve bushes.

Surfaces of sleeve bushes can heat very high locally which further thins oil to spray out, nay vaporize exlode out by the spaces pinching under loads AND the centrifugal sling outward though rather thin gaps which then won't let any oil back in - till they stop spining much in 4th and cog rotation carries oil up to trickle down to slowly seep back in. Also not that parked oil level is below main shaft so what little did seep in riding will drain out during cool down.

Others may remain insensitive to the dissolving bush clearances in lower 3 gears but I sure am.

hobot said:

Sorry Al but everything in the engine that spins depending on oil wedge separation and cooling is either got oil pressure flow fed to it or drenched in oil violent forced splash coverage like cam lifters, so your logic does not apply to AMC sleeve bushes.

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Therefore any gearbox, TTI for example, with a needle roller bearing in place of the sleeve bushes, is going to seize due to lack of lubrication - I don't think so.

Roller or ball bearing roll over the surface of their inner and out races and so only need a small amount of oil to prevent surface scuffing . Bushes and shell bearing relay on oil to separate the surfaces so oil needs to be feed in constantly. The cam bushes have oil feed holes in the cases and these channel the spray from the crank. The dig ends have oil fed to them by the pump but the sleeve gear has only the oil that can work past the small gaps.
When you stand in neutral the rear wheel and chain are stationary as is the sleeve gear but the main shaft is spinning as the engines turns the clutch so then first main an lay shaft gear spin to but none of the drive dogs are engaged to transfer motion to the sleeve gear which is bad.The small amounts of oil being moved are at other end of shaft an the speed difference between shaft an bush is as high as I gets so heat build up is inevitable . Many modern car gearbox have their own built in oil pumps to aid lubrication for this reason ..the AMC box was designed for half the power output it has to deal with on even a standard Commando so its no wonder its a weak point

I quizzed TTI about this area to be told their needle bearings got enough lube to allow Peel to run them to max w/o worry of melting. You are either baiting me for fun or just reflexly reacting to something ya ain't yet experienced or thot through, the sleeve bushes ain't gonna seize any more than un lubed swing arm pivot but boy howdy can the surfaces turn to metalic oxide ceramic dry as a bone and give troublesome slack. Not that big a deal for normal users but the Manx racers know exactly what I found on my own the hard way. Steve Manning has posted about this on the forum so not exactly heresay. There is nothing to get mean about, I said I'm sorry you are missing a AMC fact of life, so more appropriate reaction would be double check hobot hocum. I bet if ya took a ride and never touched 4th and returned and pulled covers to get guts out you'd see dry surfaces with obvious burnishing and maybe measurable losses. That's what I found when bushes got so slack it threw the belt to melt tangle inside alternator as I was strafing turns in 2nd 100 miles away from home trying to beat deer as dusk set in. If ya ain't discovered this then maybe ain't having as much immature fun as me on Peel.

If you want to see how the sleeve gear fails Al just put your bike in neutral with the clutch out then hold a fist full of rev on an after a while you will hear a screeching noise from the clutch area. That is the sleeve gear bush heating an it will be getting wrecked

It always seems to me that running this sort of gearbox for any length of time in any gear other than top is really a form of abuse. I'd be surprised if anyone with any mechanical sympathy who's ever taken one apart could really feel happy thrashing it in the lower gears. They feel happier in top. there is a lot less whirling round under load.

I've never had to resize sleeve gear bushes and not noticed them tightening up under interference.

Yes this is another level of constant awareness operating an obsolete motorcycle trying to get some decent mileage between wrenching to ride again. I feel guilty if not smoothly passing up to 4th w/o lugging or really straining and also letting idle in N so pleasantly to ear but so damaging to sleeve bushes. Sorry to hear your poor bushes sound testing tale toppy but that's the way of school of hard ...
Dang it I didn't think about the bushes in N before toppy, you spoils sport, now 2nd reason not to let idle much, ugh.

hobot habits
For paved road use I like 20-21T ratio but for real life Gravel use that make's 2000ish in 4th just too fast to take. A 19T gives about 30-31 mph indicated in cam oil break in surfing zone, which is another thing to feel guilty of if rpms too pleasant for conditions. Resisting letting it idle before shut down is tough on me to do to as well as rev up 2000ish on wet sump cold starts but by golly I hardly see any ferric dust on sump magnet like I did when green around the gills.

Someone should go wayyyy.... overboard to fit a micro-electric gear oil pump directing spray towards top of gearbox to drip back down over gears,bushes and the like. Smart hunh ? Double-dog- dare ya... :shock:

toppy said:

If you want to see how the sleeve gear fails Al just put your bike in neutral with the clutch out then hold a fist full of rev on an after a while you will hear a screeching noise from the clutch area. That is the sleeve gear bush heating an it will be getting wrecked

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Just like when me and just about every other Commando owner are checking the ignition timing at 5,000rpm in neutral? :lol: :lol: :lol: :lol: :lol:

toppy said:

Roller or ball bearing roll over the surface of their inner and out races and so only need a small amount of oil to prevent surface scuffing . Bushes and shell bearing relay on oil to separate the surfaces so oil needs to be feed in constantly. The cam bushes have oil feed holes in the cases and these channel the spray from the crank. The dig ends have oil fed to them by the pump but the sleeve gear has only the oil that can work past the small gaps.
When you stand in neutral the rear wheel and chain are stationary as is the sleeve gear but the main shaft is spinning as the engines turns the clutch so then first main an lay shaft gear spin to but none of the drive dogs are engaged to transfer motion to the sleeve gear which is bad.The small amounts of oil being moved are at other end of shaft an the speed difference between shaft an bush is as high as I gets so heat build up is inevitable . Many modern car gearbox have their own built in oil pumps to aid lubrication for this reason ..the AMC box was designed for half the power output it has to deal with on even a standard Commando so its no wonder its a weak point

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Bearings need about the same amount of oil as bushes, they both depend on oil to separate the running components. The sleeve gear does not turn in neutral, but the mainshaft does, therefore oil splash from the 1st and 3rd gear pairs will supply oil to the bushes. Layshaft does not turn when in neutral, if it did the sleeve gear would be turning. Every time I've had to remove the gearbox mainshaft it's been coated in gearbox oil.
You really are talking nonsense especially regarding; "The small amounts of oil being moved are at other end of shaft an the speed difference between shaft an bush is as high as I gets so heat build up is inevitable ." How do you set the ignition timing when Boyer, for example, recommend revving to 5,000rpm.
Thanks for the revelation; "When you stand in neutral the rear wheel and chain are stationary...." never knew that.

hello All
RE:: toppy's claim that a bush may contract and therefore allowances must be made. lots of debate over this.
to sort this argument out, can someone try this please. I dont have the parts but if I did I would try this and report back. very simple experiment.

assemble a sleeve gear bush into the gear. accuratly measure the cold bore and record the dimension. in the wife's kitchen oven( i am sure the wife wont mind), heat the bush to 100/110 degress centigrade.

remove the assembly from the oven and measure the bore . record the result.

the result is definitive. game over.

if I had the spares I would definitly do this. wont harm anything
best wishes Bradley

B.Rad said:

hello All
RE:: toppy's claim that a bush may contract and therefore allowances must be made. lots of debate over this.
to sort this argument out, can someone try this please. I dont have the parts but if I did I would try this and report back. very simple experiment.

assemble a sleeve gear bush into the gear. accuratly measure the cold bore and record the dimension. in the wife's kitchen oven( i am sure the wife wont mind), heat the bush to 100/110 degress centigrade.

remove the assembly from the oven and measure the bore . record the result.

the result is definitive. game over.

if I had the spares I would definitly do this. wont harm anything
best wishes Bradley

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They are arguing slightly different things and that test would not be definitive for both cases. If the complete assembly is heated relatively slowly and evenly no problem, what would be a problem is oil film collapse and rapid heating of the bush due to excess friction, the gear will also increase in temperature but at a much slower rate. I have had a failure like this albeit in a Landrover gearbox.

All this said, the correct clearance is................

Al-otment said:

[ If you ream the bushes to get a sliding fit on the shaft at ambient temp. of 0 degC, then at 40 degC the shaft will expand by approx. 0.0000003". I'd be surprised if this caused a problem even if there was no expansion of the bushes.

If my calculations are wrong I'd welcome anybody putting me right.

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Sorry, I did not mean to get you flamed. Your calculations could be about right, at 40C temp rise there is no appreciable expansion. What I see is that it is always the sleeve gear bushings that are shot in the gearbox. Maybe I have a bent mainshaft, but my motor is not making THAT much power, I have never broken a gear, or let the layshaft bearing fail.

The fit may not be the issue, it could be that scrolling in the bushings is important. I have been fitting 3 normal bushings. How would it work if I ( apologies for hijacking this thread) fit one plain and make a scroll in the inside two ?

Greg

Gidday Cheesy
what this experiment would do is determine whether the bush bore expands or contracts when heated. thats all it does, nothing else.

if at 100 degrees the bush shows bore contraction this would definitivly determine the bush bore contracts. even 1/2 thou would show this. of course the reverse may be true.

You are right in saying sudden temperature increase in the bush expands the bush more than normal, because the bush wants to expand more than the steel.
this is where things get interesting. the bush is trying to expand but physically cant. something has to happen. the bore then contracts.
the land rover incident confirms this.

Toppy was challenged over his claim allowance must be made for contraction. thats all. very simple claim. your land rover incident to me confirms that in certain situations, allowance must be made for contraction, especially if it is a precision bush with minimum clearence.

cam bushes are a good example. they run with only a thou or so clearence with no troubles. this is because the case and bush expand at roughly the same rate.

Where the trouble arises is the DIFFERENCE in expansion rates.

So I go with Toppy and say allowance must be made for contraction where the bronze bush and steel shaft is a precision assembly running with minimum clearence and the risk of failure is to be minimised.
the allowance is necessary in critical situations to minimise the risk of failure caused by unexpected temperature rise . the cause of the unexpected temperature rise does not really matter. enough that it happens. The cause may be lube failure, overspeeding, excessive loading, anything really . what matters is the consequences of that temperature rise. I am only addressing the claim that allowance must be made for contractions in the bore.
Corrections and contradictions welcomed. Bradley

Sooooo, um, 0.002"?

The new GENUINE mainshaft I have here measures 0.8101.

According to the factory manual, the minimum ID of the High gear bush when FITTED is 0.8120. That makes a min clearance of 0.0019.
The maximum ID of the bush fitted is listed at 0.8133, which would make for 0.0032 clearance.

I do not see those as service wear limits, but rather as the normal running range.

This info, from the HD shovel manual, gives a similar view.

Main drive gear bushing on mainshaft (loose) 0.0018-0.0032.

With a service wear limit of 0.0018-0.004.

Same application from 2 different manufacturers, with essentially the same spec.

gjr said:

Al-otment said:

[ If you ream the bushes to get a sliding fit on the shaft at ambient temp. of 0 degC, then at 40 degC the shaft will expand by approx. 0.0000003". I'd be surprised if this caused a problem even if there was no expansion of the bushes.

If my calculations are wrong I'd welcome anybody putting me right.

Click to expand...

Sorry, I did not mean to get you flamed. Your calculations could be about right, at 40C temp rise there is no appreciable expansion. What I see is that it is always the sleeve gear bushings that are shot in the gearbox. Maybe I have a bent mainshaft, but my motor is not making THAT much power, I have never broken a gear, or let the layshaft bearing fail.

The fit may not be the issue, it could be that scrolling in the bushings is important. I have been fitting 3 normal bushings. How would it work if I ( apologies for hijacking this thread) fit one plain and make a scroll in the inside two ?

Greg

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Don't know what you mean by flamed, Greg, but I'm happy discussing points like this, within reason. As has been pointed out, the gearbox was initially designed to cope with possibly 30 BHP + factor of safety. The main weak points I've found on my road going bike is the layshaft bearing, 2nd gear pinions and the sleeve gear bushes. I'm not saying the bushes are ideal, but as I've said before, each time I've removed the mainshaft it's covered in oil - lubrication is not the problem. I think it is impossible for a bush of the dimensions we're talking about to expand by 0.001". Theoretical temp. to achieve this would probably be passed melting point. Contraction, which is what Toppy is claiming, does not happen when bronze or steel are heated - they expand i.e dimensions get bigger not smaller. Have a look at some Coefficient of Expansion tables for materials - figures are given. The only way bronze or steel will contract is when temperature decreases. For any significant change in dimension for the sizes we're talking about it would require a change in temp. of hundreds of degrees C. Starting off with 0.003" clearance as Toppy suggests will just mean changing the bushes sooner.

Best improvement in this area in my opinion would be fitting an outrigger bearing to support the mainshaft.