Brass piston and cylinder problem

[zakman]

I had some bits and pieces left over from other projects and decided to make a "Frankenstein" engine, no real plans, I would make the pieces fit. I had 2 brass cylinders with 7/16 bores and a crankshaft that sort of matched. I found an old base I could adapt and lots of cut off's so off I went.

It was to be my version of a twin cylinder spool valve engine with everything in brass apart from the base and the 7BA screws for the cylinder. I got to the point yesterday where I had the body of the engine assembled but still to make the spool valves. The bores were reamed out, pistons polished etc and the whole thing turned over just fine.

It was running (by hand only) dry so I added a couple of drops of light oil to the bores, my nice free engine got tighter and tighter ! I took it apart and the bores were now scored along with the pistons and the slides under the pistons. I thought something must have been binding so I cleaned everything up again to the point where the pistons were as loose as I dare to go, assembled it again and it turned perfectly smooth.

I turned it for a while just by hand and all looked good so again I added a little oil, it instantly tightened up and the bores/pistons were scored again !

I dont know why this is happening, is it a "brass on brass problem" ? is the oil causing it to bind and pick up ? I'm only using light machine oil.

Any ideas ?

 

[natalefr]

It is not a good idea to use the same metal to make the cylinder and piston, normally use steel to make the cylinder and cast iron to make the piston, or brass to make the cylinder and duralumin (AU4G) to make the piston

 

[grg12]

Had the same problem with aluminum piston in steel cylinder in low temperature Stirling engine. First try - dry - runs fine, added tiny drop of light oil to "get better compression" - stopped on the spot. During disassembly i felt that something rubs, polished finish on piston was scratched afterwards on one side... It felt like surface tension of oil pulled piston and cylinder together.

 

[zakman]

Hi and thanks, I should have said that this is a small steam engine, or will be , it has a stroke of only 3/4", this is my 4th scratch build and I've used brass in brass once before with no problems though they do get a bit scored eventually. This is different, its digging in to the bores as soon as you add oil, even when turned by hand, its never been turned with steam or air as the valves aren't made yet.

Weird

 

[zakman]

I should have added a couple of photo's so you can see what I'm talking about, but my camera is pretty poor as you can see. You can just about see the scoring on the piston and this after turning it by hand for a very short time, it just gets tighter and tighter once you oil it.

 

[mcostello]

Remove the oil and try Graphite.

 

[Mechanicboy]

Use bronze against brass without problem. Brass against brass, it's bad combination.. Use steam engine oil.

 

[zakman]

Yes bronze would have been better but I dont have any left, I was using up my scrap and offcuts. Anyhow.....problem solved, the piston bore and the guide tube are separate pieces silver soldered together, one of them is out of line, only by a tiny fraction but enough to push the piston to one side, which pushes the other piston with it. I managed to set it up on the lathe and rebore the guide tubes, being a thou oversize wont make any difference, the cylinder stays the same. Its back together and turning nicely even with a little light oil

I made a twin wobbler a while ago with brass cylinder and piston and its done a lot of running on air and steam (with steam oil), when I get time I'll strip out the pistons and see how there doing.

Thanks guys for all the help but it was my mistake

 

[mayhugh1]

Zakman,
It might be that either your piston or cylinder is slightly out of round. When dry there is enough clearance around the piston so it can move unimpeded through cylinder. But when oil is added, the clearance around most of the piston is closed up by an oil film. The film is then penetrated because of the extreme pressure created by the out-of-round area and the result is metal-to-metal contact.
It could also be the alignment of the connecting rod is slightly off and causing the piston to scuff the cylinder. Again, the oil film could be taking up the clearance and highlighting the issue by creating metal-to-metal contact where the pressure rise is too great. - Terry

 

[Blogwitch]

I have shown this chart a fair few times, and I will most probably have to show it again sometime.

Very easy to use, by following the lines down and across will soon show you which materials work well together, or not.

 

[minh-thanh]

Thanks Blogwitch !

 

[Mechanicboy]

See at the Saito steam engine who has brass/bronze as cylinder/piston material (and other toy steam engines has brass/brass cylinder/piston). They lasted well when the cylinder/piston is lubricated with steam engine oil.

 

[zakman]

Thank you for the chart Blockwitch, interesting, maybe this only relates more to larger steam loco's ? I have a couple of Stuart Turner engines, one from the 1920's and all original which still runs perfectly with it's brass and bronze mix, all the Mamods and similar were brass piston/cylinders. Perhaps smaller engines that dont run under load last well without much consideration to materials. Tubalcain seems to use whatever is laying around

I only mess around with small stuff which is probably just as well

 

[abby]

You beat me to it Zakman , Mamod's always used brass cylinders with brass pistons although these had large clearance and run on wet steam , see how they run with a squirt of oil lol!
My own high pressure engines use gun-metal pistons in cast brass cylinders but I do use a viton ring.
Dan.