bearing tolerances

[ivan the terrible]

Hi everyone me again
so next question, I had my engine out of it's crate and oiled it up, made and adapter to connect the air line etc ran it for a while all seems ok but there was a small knock I could hear, on close inspection I found some play in the big end's and mains my question is what are the acceptable tolerances in these bearings.
This engine was overhauled by the great northern steam co but to what specification I have no idea. do they still exist?
[Youtube]https://www.youtube.com/watch?v=AL2wcoE ... GD&index=1[/Youtube]

[TahoeSteam]

That's a lovely engine. I love the cast back columns and turned front columns.

I think the clearance figure is something like 0.001" per inch in shaft diameter.. seems awfully tight for an engine that smol though.

[fredrosse]

TahoeSteam is correct here, 0.001 inches bearing clearance per inch of shaft diameter. I have found this specification on Diesel engines from 1 inch to 12 inch crank journal diameter. The wear limit is usually to about 0.0015 per inch diameter, but that is for single acting engines. For a bearing with reversing load directions, (double acting steam engines) I would expect a smaller wear limit, maybe 0.0012 maximum clearance.

The first time I ran the Margaret S engine (1-1/4 inch connecting rod big end) it seized with a clearance of 0.0005 when cold. Evidently a little heating caused this seizure. So much for my unwise tightening up on the bearing clearance. Fortunately no harm was done, since the engine maximum RPM is in the vicinity of 60 RPM. A higher speed could have really torn things up I think. Just some emery on the crankshaft to remove the Aluminum bearing material that had firmly welded itself to the steel shaft, and a paper shim on the connecting rod bearing cap solved the problem. That repair was done while afloat, and I replaced the paper shim with a brass one, running well for the following 10 years.

[ivan the terrible]

thanks for the reply's chaps
I am very used to diesel and petrol engines and did think the tolerances should be similar just wasn't sure, looks like I'm going to have to dig farther in!
so much for a fully rebuilt refurbished engine!!
hopefully the bearings are the standard Stuart Turner item's as they are within visiting distance from me.

[Lopez Mike]

A small knock? Only a small knock? What a lucky dog you are!

Laughing aside, most of the knocks I've chased in small engines haven't been the big end. They've been reversing loads that don't include much rotation.

A rotating journal spreads the lubricant around and at a surprisingly low r.p.m. develops what is referred to as a wedge of lubricant. In fact the shaft and bearing combination acts as a pump. What you see as the oil pressure on an I.C. engine is really two pumps working in opposition, the 'real' oil pump pushing against the bearing acting as a pump. It's startling to hook up a source of oil to an engine that isn't running and find oil just gushing through the stationary bearings. Hardly any pressure showing on the gauge. We do this regularly when preoiling a new I.C. engine.

Of course it might well be a big end problem but I would look carefully at the wrist pin and crosshead clearances. And even the valve gear. I had an elusive knock once that turned out to be an eccentric not snugged down. My Stuart beam engine has a knock that I know to be caused by insufficient cylinder head clearances. Overdid it at trying to minimize the end volumes. Too lazy to fix it. Little engines are harder to get right than big ones.

[Mike Rometer]

Said 'small knock' could disappear when the engine sees heat!

[Kelly Anderson]

True that the knock may disappear when running on steam (it's amazing how much smoother they run on steam than on compressed air!), but they are a lot easier to check while out of the boat. I use "Plastigauge" to check the bearing clearances on my engine. You drop the bottom half of the bearing, put a piece of Plastigauge in lengthwise at the 6:00 o'clock position in the bearing, reassemble the bearing to the proper torque, then disassemble again and measure how wide the plastic has been squished by the clearance in the bearing against the calibrated packaging of the Plastigauge. BTW, that was a standard way to measure bearing clearance on steam ships, but using lead wire instead of plastic.

I agree with the previous posters that .001 per inch of diameter is a good target for crank bearings. On eccentrics though, I would shoot for .002 to .0025 per inch of diameter, due to the high surface speed that they operate. You don't want one seizing up.

Though knocking bearings can be annoying, as the old saw from steam railroading goes regarding knocks in bearings, "It's better to hear them than smell them."

[barts]

I'll echo Lopez Mike's comments - check those wrist pins. The area is small, the load is high, and there's little chance to develop an oil film w/ the limited rotation, so those bearings are often going to be the first to cause problems. This is an excellent place to use needle bearings, by the way - this is exactly the kind of motion seen in a universal (Cardan) joint, and in car and truck applications needle bearings last for many thousands of hours.

- Bart

[fredrosse]

I second Bart's post above. Wrist pins, without needle bearings, generally have much tighter clearances to survive IC engine loads without the advantage of full rotation found on the connecting rod "big end". The big end bearing can have the advantage of full hydrodynamic conditions, with an oil "wedge" that keeps the metal seperated during operation. The wrist pin only has small, and oscillating motion, and also usually must be significantly smaller in diameter, and thus demands tighter clearances to survive. In the IC industry, needle bearings here are cost prohibitive, but on a "one off" custom steam engine, needle bearings are a very wise choice.

[Lopez Mike]

Don't tell anyone but I've been 'experimenting' for forty years with plastic wrist pin bearings on small Stuart engines up to and including the well known #5.

I use Delrin (acetal resin) on a tool steel or S.S. pin (polished surface) with zero to less than zero clearances. It takes sharp cutters and reamers to do good work but the result is extremely long life and complete silence.

I suspect that this would work well on larger engines but haven't tried it.

Mke