Doug wrote:
My understanding of this subject is this:
It's not really oil PRESSURE that lubricates the bearings,
its oil FLOW. The Spit's oil pump (like virtually all of them)
is a positive displacement pump, which means that the flow is
mostly independent of the pressure at a given RPM.
This is partly true - the pump is positive displacement, so the faster the
engine goes the greater the oil flow - up to a point. Once the oil pressure
passes the Oil Pressure Relief Valve critical pressure, the relief valve will
dump some of the flow back to the sump, so that the pressure drops back to the
critical pressure. This is why your oil pressure should rise with speed up
to, say, 2000rpm, and then should stick at that level.
If your oil pressure never reaches the point where the relief valve kicks in,
or even it doesn't kick in until over around 2,500 rpm, then something is
wrong.
Pressure is, in fact, the critical factor in bearing performance, not flow.
Oil is fed through the gallery into the groove in the bearing. When the
crankshaft is pressed against the bearing by the loads on it, the journal (the
bearing bit of the crankshaft) is forced to be slightly eccentric in the
bearing - one one side the gap between the journal and the bearing shell falls
to as little as 1 micron (1/1000 of a millimetre), while on the other side the
gap can be 100x that. You need the full pressure to maintain oil film in the
thin area, and that is not helped if all the oil is pouring out of the
opposite side.
Donald also made a good point that if there is inadequate pressure in the oil
supply, then less oil will be able to reach the crankpin bearings, or the
camshaft bearings.
Never get an engineer onto his specialist subject, you will never know what
sort of lengthy, dull and incomprehensible reply you will get back!
Best wishes,
Richard & Daffy
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