Bulldozer said: (quoted from post at 04:09:43 04/09/13) and genrators and irrigation pump engines at zero oil pressure with light weight crankshafts and two & four cycle outboard engines and motor cycle engines.
gravity:
its the gravity load on the crankshaft.
What keeps the journal off the bearing surface in a small engine that does not have an oil pump (e.g "zero" engine oil pressure) is the same thing that produces the oil film in an engine with an oil pump. Journal bearings operate on the principle of hydrodynamic lubrication. The oil film that separates the journal surface from the bearing surface is produced by the rotation of the journal dragging the oil inside the bearing clearances around the circumference and constricting it at the narrower load point. That constriction produces a localized pressure gradient inside the bearing that is orders of magnitude greater than oil pump pressure and is opposed to the operating load (and gravity) imposed on the journal. The basic parameters affecting the film thickness are crankshaft load, rotational surface speed of the journal, oil viscosity, and running oil clearance.
You can externally pressurize a normal journal bearing but if the crankshaft is not rotating it remains on the surface of the bearing. It simply sinks to the bottom of the evenly pressurized oil clearance area. When the engine is started and journal rotational speed begins to increase a hydrodynamic pressure gradient is formed at the load point and that pressure gradient acts to push the journal surface away from the bearing surface. The journal/bearing lubrication regime transitions from full boundary mode lubrication (metal to metal) when the rotational speed is zero, to a mixed mode (partial metal to meatl contact) as the speed begins to increase and the film is partially formed, and ideally to full hydrodynamic mode (zero metal to metal contact) when the rotational speed becomes sufficient to produce a gradient greater than the load on the journal. Teh film thickness is determiend by how much the gradient exceeds the load on the journal. Oil pump pressure is not involved beyond maintaining an oil supply adequate to replenish oil lost out the sides of the journal during operation. Small engines traditionally relied on splash lubrication for that oil supply.
TOH
This post was edited by TheOldHokie at 08:17:05 04/09/13 7 times.
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