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Re: 35PSI - Means What

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Posted by TheOldHokie on September 29, 2012 at 10:55:46 from (

In Reply to: 35PSI - Means What posted by CharnerL on September 29, 2012 at 08:01:38:

R Geiger said: (quoted from post at 13:09:43 09/29/12)

The main thing that determines oil pressure is the clearance in the bearings. Too much clearance and I don't care what oil you use there will be no pressure. Granted you need a good pump to start with but the best pump in the world will not create pressure without the resistrictions from the bearing clearance.

Precisely. And I don't care if you have 80 PSI pump oil pressure that oil pressure won't keep the journals clear of the bearings if the clearances are too wide. The journal is held off the bearing surface by a localized pressure differential inside the bearing shell that is orders of magnitude greater than pump oil pressure - well up into the thousands of pounds. That differential comes from the formation of a hydro-dynamic "wedge" of very high pressure oil (red area in the drawing) opposite the load on the crankshaft. It is produced by the journal surface dragging oil around the bearings as it rotates and compressing it in the constricted clearance opposite the load. Too much bearing clearance and that wedge won't form and the journal is forced down onto the bearing surface by the load. It is this hydro-dynamic phenomenon that allows a splash lubed lawn mower engine to maintain a bearing oil film with ZERO pump oil pressure. The three factors that affect the formation of that wedge are:

  • Bearing clearance
  • Oil viscosity
  • Surface speed of the journal
Pump oil pressure has virtually nothing to do with it.


This post was edited by TheOldHokie at 11:29:01 09/29/12.


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