Replacing bronze bush in an aluminum alloy casting

Hello all, I have a casting which has 2 bronze flanged bushes installed in the lugs on the casting, these bushes then support a hollow steel throttle control rod.

The rod only turns half a revolution back and forth but due to vibration etc the inner bore of the bushes have worn and need replacing. what I would appreciate advice on is: The bushes are a tight fit! should I heat the casting before attempting to tap out the bushes with a drift of some sort?

Then, I am struggling to find a commercially available replacement bush and so will have the replacements machined, question is, how do I ensure the replacements are a tight fit? presumably they will need to be machined slightly larger than the aperture? and the casting heated before inserting the bush. If my assumption is correct how many thou bigger should the O/D of the bush be machined? or is there another way of ensuring a proper fit?

Thanks in advance of any advice.

Terry

Hi,

Aluminum expands so much that you can easily heat it in the kitchen, given permission. Outer bearing rings in model airplane engines more or less fall out of the crankcase casting when heated to 150-180 deg Celcius. That is even if very difficult to remove when cold. As for the fit, I would try to make them as large as the ones you remove. They were good enough!

Good luck

Kai

Hi, I'm sorry I assumed this were aluminum castings.

I really do not know if a cast iron casting expands more than a bronze bush when heated.

I'm quite shure that someone on this forum knows, though. What kind is it?

Kai

Could you fit a bush into the bush or is it a critical piece of kit?

How big is the casting? Could it be mounted on a machine?

 picture would probbly help a lot

---------

It is an ali casting skarven

"Replacing bronze bush in an aluminum alloy casting"
 

Edited By Ady1 on 23/10/2012 22:54:35

Ok you have a few issues here and getting the old bush out is one of them .

Alloy castings will need very carefull heating if they are in any way delicate as alluminium will lose a lot of its strength if heated too much .

If possible i would try to grip the flange of the bush and see if it can be turned .

If so try twisting and pulling .

If not then i would try filing a groove through from the inside using a triangle shaped file then try to crush the bush in so it releases- being carefull not to cut into the housing with the file of course !

I would use heat as a last resort as it is difficult to control and alluminium will not show how hot it is until it melts - you can blacken it with the soot from an oxy acetylene torch and this will burn of just before it melts but knowing how much weaker the casting is at that temperature is a lottery and what aboult distortion ?

You could use a large soldering iron to apply some heat on smaller items and this will weaken any loctite that may have been applied to the old bushes .

Another way is to heat up a suitable piece of steel until it a drop of water will skitter across the surface then sit the item on it until it is too hot too touch comfortably as the alluminium will suck up the heat very quickly - obviously if the item is large this will not be practcal .

I take it that you do not have a lathe to make the new bushes yourself ?

Once you have these made- with the bore undersized (.001-0.003 inch ) you will have to push them into the housing and ream them otherwise both bushes will not be aligned to each other and the forces applied to the bushing by the housing will more than likely squash it a little or knock it out of round so the throttle rod will not fit .

As for the press fit it wil depend on what temperatures this item will have to endure as the coefficient of linear expansion of alluminium and its alloys is different to that of bronze but i would suspect .02mm or .001 inch would suffice and you can use loctite etc .

If the bushings are thick enough you could possibly bore/drill them out oversize and push and loctitein a new liner then ream as above - maybe easier .

Ian

A hot air Heat gun will be better than a flame for this, they can go up to 600C. I doubt you will need that much temperature anyway.

Can you not devize a small puller that will apply pressure as you heat the part and then the stress is only on the piece you work on .

You cannot know the size of the hole for the bush until you remove it to obtain or make a new one.

Clive

A heat gun sounds like a good idea, and comblne it with a bearing puller arrangement?

The safest and easiest way to heat it is to put it in the oven and set the thermostate to the temperature you want. After 20 minutes, depending on your oven, you are ready to remove the bushes. I would suggest starting at 100 C. This will normally do the trick, but I do not think the casting will be harmed by 200C if that is neccesary. Most ovens go up til 250C.

(You will have to ask your wife first)

With this method there is virtually no risk of overheating. I have often done this with aluminum castings.

You should also clean it well first. Othervice it will change the taste of your food for years.

This method is also useful for curing high temperature epoxy, which need controlled temperature increasing in steps over long time.

Kai

Wow, thanks all for your informative replies, took them onboard and gave it a go, I was able to grip the flange and try turning and pulling whilst cold but there was no way it was going to shift. So I applied a gentle application of heat from a propane torch and tapped it out with a parallel punch. I'm confident I didn't use to much heat because the bush fell on the floor and I was able to pick it up without being burned!

I think, when I install the new bushes I'll machine them to an interference fit and use loctite, as there was evidence of some kind of adhesive residue on the old bush.

Anyhow thanks again.

Terry: Ideally you need to use one method or the other. Loctite requires a gap to work properly. Using it with an interference fit would just be a waste of expensive liquid.

Regards,

Andrew

Andrew, that's noted, thanks.

Unless I'm missing something in the original request, it looks like half a story. What is it part of? Lawnmower? Submarine?

What size are the bushes approximately? More info please.

It's off a de Havilland Gipsy Major aero engine, the machined casting is on the rear of the engine and houses the pressure oil filter. It has bushed lugs at both ends and the throttle control cross shaft passes through, the bushes have worn internally and need replacing, the internal bore is half inch.

Would make a hell of a lawnmower

Who knows what the casting is made of then; probably somebody's aluminium saucepan, with the odd park railing thrown in for good measure! We had an engine failure on our Tiger Moth which was caused by the valve rockers falling off one cylinder when the cylinder head casting suffered a progressive fatigue failure.

Regards,

Andrew

Is this casting on an engine that is going to go back into an aircraft and fly? Is so don't do anything before seeking professional advise since the casting contains part of the lubrication system.

If not, do what people who rebuild old motorcycle engines do. Clean the casting thoroughly (boiling in soap, not washing up liquid or biological stuff, and water removes an awful lot of oil from a casting) heat in an oven until spittle dropped onto the surface fizzes and boils and then gently drift the bushes out. You could crack test the casting afterwards but you might get a shock and find many cracks, hopefully small. Remember if you Loctite the new bushes in, to remove them the casting may have to be heated to a much higher temperature (Loctite is said to be good to 200C at least).

JA

Hi terry lee 2, Thank you for your reply. I enjoyed the lawnmower touch. Sounds very interesting. Best of luck with your project. It could have been a submarine, but we won't go there.

Regards, Bear

I' d try boiling water to heat the casting, My book is a bit older than the Gipsy Major, its the 120hp D.H.Gipsy3 but I think the timing gear cover if not the same, is very similar. Sorry can't find the repair sceme for that shaft. Is your engine 130hp, or 145hp, ie Tigermoth, or Chipmunk. Ian S C