Steel on steel - bearing

[Circlotron]

Not quite an engine but the similarities are obvious.
A friend has a cheap, nasty and quite small air compressor that has an aluminum conrod with no bearing shells, just conrod running directly on the journal. When he first got it given to him free it had a huge big end knock. Eventually we found a replacement rod and all was well. The crank journal is about 1/2" diameter and was still smooth. If it quits again I am considering making a steel rod for it. Balanced be damned!

Is there any issue regarding running the steel rod on a steel crank pin provided the surfaces are sufficiently smooth to begin with?

[Dave Koehler]

I will just say that there is a reason you never see such a thing, eh?
Think it through a little more.

[norm]

I say you would want dissimilar materials . Steel on steel would gall in my opinion

[bigfoot584]

Not quite an engine but the similarities are obvious.
A friend has a cheap, nasty and quite small air compressor that has an aluminum conrod with no bearing shells, just conrod running directly on the journal. When he first got it given to him free it had a huge big end knock. Eventually we found a replacement rod and all was well. The crank journal is about 1/2" diameter and was still smooth. If it quits again I am considering making a steel rod for it. Balanced be damned!

Is there any issue regarding running the steel rod on a steel crank pin provided the surfaces are sufficiently smooth to begin with?

It did have bearing shells, both halves of the rod were a shell, your best bet
would be to bore the bad rod out and make or find shells that would fit from aluminum.
Steel on steel NNNNNNNNNNNOOOOOOOOOOOO good.

[Dave Koehler]

Chances are that if the journal is still reasonably round it will achieve it's useful life with the replacement aluminum rod.
I am all for keeping things working but it's a little bitty, throw away compressor when all is said and done.
Choose your battles wisely.

[modok]

Steel would be about the worst choice for a bearing material.
if the rod is split, cut the split to tighten it up.
If it's one piece, sleeve it.

[hoodeng]

A lot of small engines do not have bearing shells and run the rod straight on the crankpin and give excellent service life as others have pointed out ,but a word of warning ,any plain rod on crankpin is oil specific , If the manufacturer recommends a straight 30W oil ,use it ,if the manufacturer would prefer you use a product they provide there is usually a reason for it.I cant think off hand of any manufacturers that want their products famous for regular replacement.

Many years ago i bought a large twin compressor with rod on journal for peanuts , the owner had gone through a couple of sets of rods and was not prepared to go through it again ,,, strip down , inspect , and what i found looked like oil starvation although the sump was to level on the stick . I thought the oil was very thick although clean ,upon inquiring back to the original owner he told me he religiously used " Motor Honey" or a heavy oil stabilizer in anything that had oil in it that he owned , that was the problem , these things may not have pumps and rely on splash to lubricate so the oil needs to be of a relatively low viscosity as specified by the manufacturer,what was happening was the rod oil dipper tongue was cutting a path through the oil and not dispersing it around the crankcase ,also compressors in sporadic use don't warm up so the temperature thinning of the oil can be virtually non existent.

Cheers.

[Truckedup]

So what about iron on iron? I've seen that a lot. And steel on iron as well.

Yeah, Ford N tractors ran the cam directly on the cast iron block..I believe the cam was cast, but not 100 % sure...On some manual transmissions, gears may spin on shafts with no bushings or needle bearings

[Keith Morganstein]

So what about iron on iron? And steel on iron as well.

Steel and iron, iron and iron, steel and steel, definitely work for some things. I haven’t seen it for crankshaft and rod bearings in engines.

[David Vizard]

Just to throw a monkey wrench into the works here I did, when working as an R&D engineer in the aero-space industry, build a self powered miniature gyro-scope that turned over 500,000 rpm, steel on steel and no oil! Just for the heck of it I will let this comment stew awhile just to see if anyone comes up with the 'how' but suffice to say steel on steel is totally doable with the right lube.
DV

[cgarb]

Was it an air bearing by chance?

[modok]

Iron on hard steel shaft is decent, the iron is less prone to scoring/galling as steel

Iron on iron does work, of course, but usually only for low loads, cam bearings ect, machine tools.

If the rod has to be steel, then DLC coat the pin, that does work well.

You can run anything on anything if the surfaces don't touch, machined perfectly, always lubed, and no dirt, but.... how often does that happen? Not in an air compressor. Probably only has half a crankshaft and flexes all over the place.

[Dave Koehler]

Don't forget that cast iron is porous and will hold some oil on it's own.

[Circlotron]

Hypoid ring gear and pinion (where the pinion shaft is lower than the axle half shafts) have plenty of sliding motion and that's steel on steel. They seem to get along okay. Specific oil though.

[Kevin Johnson]

Was it an air bearing by chance?

I think with steel that an inert gas would be used. I used to use these in a dental lab to carve porcelain teeth.

gas_bearing.png