Crank Pin Lubrication ?

[Hugh Currin]

As some of you know, I'm trying to build a Webster. Slow going but I'm having fun doing it. A question has come up regarding the oiling of the crank pin and large end of the connecting rod. I know many on here are very knowledgeable on all aspects of model engines, so someone must have insight here.

The crank pin is lubricated via an oiler on the end of this crank pin. The crank, with lubrication hole, is shown below (from the Webster plans).

I think we can determine the direction of rotation from this figure showing setting up  the timing.

This shows a position 15 degrees before Bottom Dead Center. Thus the crank is rotating clockwise in this figure. Is this correct?

So, the crank and connecting rod are positioned as below during a power stroke. This seems wrong.

I looked around the Internet some but found little insight. Journal bearings should develop a thin film of oil reducing friction and wear. A journal bearing as below is flooded with oil.

W is the weight carried by the shaft which is rotating clockwise (omega). The friction between the oil and the shaft tends to “pump” the oil into the thin gap between the shaft and journal. This keeps a thin film of oil there rather than having it simply squirt out, hydrodynamic lubrication.

I would think a hole on the bottom of this journal would relieve pressure and bleed off the oil, thus harming the thin film lubrication. Wouldn't it be best to lubricate the journal through a hole at the top, or maybe just right of the top?

If this is true, then lubrication of connecting rod bearings would be best done through holes to the outside.

Since the connecting rod is a compression link, the lubrication hole would always be on the side away from the loading. I suspect this reverses for a pressure feed system. There you want to inject oil into the area of the thin film.

The Webster is different though. It has oil/grease supplied through the crank pin. This hole rotates relative to the connecting rod, so you can't keep it away from the loaded area completely. If the theory above is correct, then the best you can do is keep the hole away during the power stroke.

Unless I'm mistaken, this is opposite of the Webster plans. Or do I have the Webster rotating the wrong way?

So, does this theory sound right? If not, what's the best way to think though the problem?

If it is correct, is the Webster crank pin hole on the wrong side?

Thank you for reading though this rather dry and long post. I look forward to you input. My thinking on this keeps changing, but the above is my current thinking, please correct me if I'm mistaken.

Thanks.

Hugh

[Jasonb]

You would be better off doing as most full size hit and miss engines do and using gravity by having the hole on te conrod big end above the bearing.

Galloway


Domestic


A small flat machined on the wrist pin will allow oil to travel in sideways from the bore, this can be aided by a groove around the piston in line with he pin to scavenge oil.

J

[dieselpilot]

Everything you said is basically correct. This image shows how most RC engines are built. Above a certain speed and load it requires improvement.

For the Webster, I doubt it matters at its load and speed. Jason's idea of adding a reservoir will probably be the biggest improvement.

[Jasonb]

Two differences between the sketch and RC conrods.

1. The RC crancase if full off oil/fuel mix if glow or uses splash lubrication if a 4 stroke. The Webster is open crank so neither apply

2. Most RC cylinders are vertical, Webster is horizontal.

[Admiral_dk]

It's most likely me with a comment to a comment under the build tread that made you stop and think (not necessarily a bad idea) - but I must admit that I kind of forgot how low the load is on the con rod on the Webster when I made that remark. So sorry, but as long as it get's some oil - especially if you do as Jason suggest - there shouldn't be any problems.

2. Most RC cylinders are vertical, Webster is horizontal.

Sorry but that I can't attest to - I had at least two model aeroplane with the cylinder being horizontal and I've seen them "any way round the clock" so to speak. But being "fog lubricated", the orientation isn't that important.

The comment about open and closed crankcase-, is the most important - and I kind of forgot about that too 

[dieselpilot]

The sketch represents oil holes in probably 90% of RC engines. RC cylinders are mounted every which way and it doesn't make any difference, but there is always feed fresh lube.

I forgot there is already an oil cup, so little need to change anything. By putting the oil hole on the low pressure side of the bearing you may find oil is consumed quickly. I've played with a connecting rod on an electric motor shaft and the effects of hydrodynamic pressures are very apparent. It's surprising how hot a 5mm big end gets at 6000 RPM without a load.

I would guess that many a Webster has been built as designed. How many failures are there? The engine usually runs without load for a few minutes at a time.

[Hugh Currin]

Jason: Such an oil reservoir makes sense. The oil hole is at least on the journal rather than the shaft. I see the hole is away from the loaded side. I suspect these are for lower RPM?

The oil holes in the con rod should be OK as long as the hole sees lubricant. It sounds like this would be the case for enclosed cranks, either mist or splash, at least for lower load/rpm.

dieselpilot: It's good to hear my thinking isn't in left field. Once the basics are understood then various options can be evaluated. Even if not of much concern for the Webster, I'm learning a lot. Thanks. And your right, I'd be real surprised if a Webster has ever worn out. Not run, yes. Seized from over heating, yes. Worn out, unlikely.

I did notice the plans call the main journal and piston lubrication "oilers" while the crank pin is called a "grease" cup. Probably crank the cap on a little forcing grease into the journal. So the oil consumption dieselpilot suggest probably isn't a concern.

I think my oil/grease hole in on the low pressure side of the journal. I'm going to leave it there and put the grease cup on as the plans call for. I doubt, for this engine, that we'll ever know if it was the best course of action. But worth the inquiry, I've learned, or been reminded of, a lot.

Thank you all for the input.

Hugh