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Big end lubrication

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Andrew Moyes 116/08/2019 13:15:54
106 forum posts
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I am in the final stages of making a Stuart 5a and am making drip feed oilers for the main bearings to Myfordboy’s design. The big end is lubricated as per Stuart’s drawings by an oilway drilled at an angle through the web to the centre of one main bearing. I am wondering how the oil finds its way against centrifugal force to the centre of the shaft in the main bearing. Beyond that point centrifugal force will of course fling it out to the main bearing. I can see how it would work in a pressurised system such as a car engine with an oil pump but not with a gravity feed. It seems to be common practice in model engineering eg many ET Westury engines but not full size as far as I'm aware. Does it work in practice and if so, how?

old mart16/08/2019 16:08:22
444 forum posts
42 photos

Surely with that design the oil is fed to the main bearings and some is centrifuged out to the big end. It couldn't work the other way round.

Andrew Moyes 116/08/2019 17:59:18
106 forum posts
15 photos

Apologies if I didn't make myself clear. The oil is fed in at the main bearing but until it has travelled through to the centre of the shaft, against centrifugal force, it won't go any further. Or so it seems to me.

David Jupp16/08/2019 18:42:42
691 forum posts
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Hydrodynamic action in the oil of the main bearing will generate some pressure, presumably enough to push the oil into the hole for part of the revolution.

duncan webster16/08/2019 18:57:47
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Presumably the centrifugal force sending the oil out to the big end is more than that trying to stop it flowing from the periphery of the main shaft, and so it is a sort of centrifugally driven syphon. It might work better if the main shaft drilling went right to the end and oil was dripped in on the centre-line. Even better if there was a restriction to stop the oil flowing out again, a very short bush pressed in

Andrew Moyes 117/08/2019 09:43:58
106 forum posts
15 photos

Duncan, I'm sure you're right regarding siphoning effect when the oil flow is continuous, for example with a force feed system as in a car. But a drip feed won't provide enough oil to maintain a continuous flow. Once it stops, if my theory is correct, it won't restart when the engine is running.

David, I suppose the hydrodynamic theory could be checked on a model by experiment. Remove the connecting rod, wrap some kitchen paper towel around the big end and secure it with tape. Then run the crankshaft with a motor drive and turn on the drip feed lubricator. See if the oil gets through to the towel.

Was this method ever used in full size? On locos the lubricator was mounted on the big end. On many stationary engines, there was a ring beyond the end on the crankshaft and concentric with it, the inside of the ring being a channel. Oil was dripped into the channel and tubed from there to the big end. That clearly avoids the problem. I'm not sure what marine practice was on multi-cylinder engines which, unlike locos, couldn't be stopped to replenish big end lubricators.

David Jupp17/08/2019 10:10:47
691 forum posts
17 photos

Perhaps, but changing how the shaft is driven will probably move where the 'wedge' in the oil film in main bearing is positioned, so the experiment may not replicate reality.

Worst case would be if the maximum pressure generation from the 'wedge' was to coincide with the inlet from drip feeder.

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