How to make a silver steel axle

[AdrianRParticipant @adrianr18614]

Hi,

In my wobbler engine is says to use a silver steel axle in either a brass or bronze bush.

I did not have any bronze so used CZ121 brass but the bush seems to be wearing faster than I would expect. In about 1hr it has gone from a good fit to having noticeable (an audible) side play. The surface of the silver steel has a ground finish and I am wondering if it should be polished before use. Or is using brass really a bad idea.

I am using SAE68 oil.

Thanks,

Adrian

[AdrianRParticipant @adrianr18614]

[Tony Pratt 1Participant @tonypratt1]

Silver steel looks nice but is not always round due to poor centreless grinding this could be causing you a problem,also Brass is not known as a bearing material.

Tony

[SillyOldDufferModerator @sillyoldduffer]

Should be OK, any chance of some close-ups?

First reaction, I'm suspicious of the oil, and the ground finish. Do you mean ISO68 rather than SAE68? And silver steel usually is polished, maybe you have something different? Is it heat treated and scaly?

Some oils have unkind to Copper additives – the label should mention it, but rapid wear suggests the bearing is running dry – is that possible?

Dave

[AdrianRParticipant @adrianr18614]

The oil should be OK, it is Morris ISO 68 that I use on my lathe, no nasty yellow metal eating additives.

The silver steel is from M-machine, it was what I was expecting, nice and shiny with a smooth ground finish. Not polished, more of a brushed look.

This is what I am making **LINK**

I drilled the bush at 5.8mm and reamed to 6mm, and it looked pretty good to me. So I dont think it was a bad finish in the bush.

When I run the engine the oil goes black, just like using metal polish. That is why I was wondering if people polish the shaft before fitting, just to remove some of the sharp edges on the ground surface.

Maybe I should stop being tight and by some bronze.

Adrian

[old martParticipant @oldmart]

You might look at sintered bronze for the bush. Giving the axel a polish with some 1200 paper would do no harm as long as it is well cleaned before re assembly.

[David George 1Participant @davidgeorge1]

I have just finnished the same engine yesterday. I used phosphor bronze for the bush and silver steel shaft, no problem. I think you have the wrong brass?  I got my bronze from M machine  metals.

David

Edited By David George 1 on 04/04/2020 19:17:42

[HopperParticipant @hopper]

Posted by AdrianR on 04/04/2020 17:35:51:

… In about 1hr it has gone from a good fit to having noticeable (an audible) side play. …

That sounds bizarre. Even regular brass should not wear that quickly. Unless it is defective material?

What rpm are you running the engine at? They are designed to chuff along at about 100rpm on 5 to 10 psi air pressure, not screaming flat out on 100psi etc.

Few things you could do to make it last longer would include make the flywheel lighter and move it in closer to the bearing to minimise overhang. Drill a small oil hole in the top of the bearing and add a countersink so you can lube the shaft directly with drops of oil. Add an outrigger bearing on the far end of the crankshaft to support it – just an A frame with brass bush in the top. Cut away some of the crankshaft so it is counterbalanced against the weight of the piston.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Hopper on 05/04/2020 03:07:46:

Posted by AdrianR on 04/04/2020 17:35:51:

… In about 1hr it has gone from a good fit to having noticeable (an audible) side play. …

That sounds bizarre. Even regular brass should not wear that quickly…

…

I agree, something is wrong!

I doubt it's the brass, silver-steel or ISO68. That's reasonable combination for a lightly loaded bearing in a model engine. CZ121 may not be the best bearing material, but it's certainly not wrong. Used to make bushes, lightly loaded bearings and hinges.

Time to eliminate possible causes. Given it's a wobbler, overloading or overheating the bearing seem unlikely and this can't be normal wear.

That oil is coming out black suggests the problem isn't an ordinary lubrication failure.

Corrosion is unlikely on a new engine.

More likely, a construction related fault. The suspects include:

• Rough surface on the brass or silver-steel. This may be because the bearing was insufficiently polished on assembly, but I'd suspect contamination. Anything that can embedded in the brass like a steel filing or a speck of grinding material will do a lot of damage.
• The bearing fit is either too tight or too loose. The oil has to develop enough pressure to float the axle on the oil rather than make metal to metal contact. May be impossible if the oil can't penetrate properly because the axle is an over tight fit. And it may not float if the oil escapes due to a very loose fit.
• The axle and bearing aren't running true and the misalignment overloads part of the bearing surface. It might even run dry on the tight spots while oil escapes through the gaps. (Tony mentioned the interesting possibility that the silver-steel isn't round. Never seen it myself, but centreless grinding as used to make silver-steel rods can misfire and produce Reuleaux polygons, which have the interesting property of having constant diameter without being circular. The British 50p coin in an example. Or a faulty grind might cause obvious lobes. If either is suspected, make a new axle.

Does the axle show any sign of binding when the engine is turned over by hand? Misalignment may cause the bearing to run tight and loose over several revolutions. The error could be in the engine frame or due to a bent part, especially the axle.

If misalignment or a mis-fit can't be blamed, then I'd take the bearing and axle apart and have a good look for damage with a strong magnifying glass or loupe. If the axle is scratched, something harder than brass did it – look for contamination in the brass. It may be too late to determine if the original cause was a tight or loose fit because severe wear has occured.

Whatever the original cause, the cure is to either tolerate the problem (it's a model) or to replace the brass &/or axle. When reassembling, make sure everything is clean, including oil-ways that might hide swarf, and true.

Have to admit my own investigations of plain bearing bother haven't always been conclusive. Although damage is often easy to see, the cause may not be obvious. I suspect many a University Research team is kept busy by bearing problems.

Dave

[Author]

Posts