How does pressure relief valve work in oil pump?

[inline6]

Was verifying that the pressure relief valve in my oil pump was installed correctly today, and after confirming how it is assembled, I really don't understand how it works.  Can someone explain it to me?

 

Was reading this post:

 

http://forums.hybridz.org/topic/98987-l6-oil-pump-differences/

 

What do the posts/nipples on the pistons do?

Edited December 7, 2014 by inline6

[Tony D]

Spring pressure presses the piston against the top of it's bore.

As oil flowbuilds, the pressure is created by friction in oil galleries and orifices (bearing clearances) that restrict that flow. As a result pressure rises.

As pressure rises, it presses on the relief piston face...once this hydraulic pressure total (area of the piston, multiplied by the hydraulic pressure) eventually overcomes the spring pressure.

 

Spring pressure is rated in "pounds per inch" or other weight-vs-deflection standard.

 

So as your pressure increases, the spring allows the piston to deflect, at some point the piston will uncover a relief port and bleed off pressure.

 

This will counteract spring compression when hydraulic pressure drops, and the spring presses back up and closes the port allowing pressure to rise... This balancing act goes on at that preset pressure.

 

You can see by shimming the spring it will increase spring preload and fluid pressure as a result.

 

Curiously so will cutting the spring as it can increase spring rate... The shim then merely becomes a placeholder for the installed height you cut out of the spring.

 

Clear as mud now?

 

Sometimes the 'needle' on the bore end restricts the inlet to the relief piston chamber, or the relief port making for a smoother response at one end or the other of the regulation spectrum.

 

Then...you can add relief port friction or size which can affect the relief capability of the port which could otherwise just blow to the oil pan unrestricted. I have seen arrangements where synthetic oil ran dangerously high pressure cold, and dangerously low when hot due to relief port and passage design. The reaction of viscosity can make a big difference in how the relief port dies it's job. Thick oil may jam the piston to the bottom of it's bore, completely allowing full bypass... And kind of removing it as it's intended use in the system...

Edited December 7, 2014 by Tony D

[inline6]

Thanks Tony.  I follow.  I guess without the relief, if the shaft speed keeps increasing, the oil pressure would become excessive.  

 

I took off the bottom of the oil pump today and saw that that the post on the piston bottoms out on the aluminum casting in such a way as to set the piston top (base of the post) flush with the side of the cavern that oil flows into.  I think it is a "depth" setting thing... so the piston doesn't come out too far.  

 

I've ordered some of the stiffer springs from Don Potter's widow - she sells them on ebay.  Going to play with setting it up to provide 80 psi at high speed since this engine is supposed to get to 7700 RPM some day.  

 

On a not so fun side note, I tried using a dummy shaft I made out of 1/2" aluminum and a drill to pump oil through the engine today... ran it a full drill speed repeatedly.  Ran counter clockwise, as that is the direction the shaft is supposed to turn, and got no oil flowing at all.  

 

Pulled the pump again, and there was no oil in there at all.  And yes, when I pulled it to swap shafts, I did put it on there "dry". Guess I won't be putting it back on without filling it with oil ever again. 

[Tony D]

Lubriplate 630AA on the Gerotor or Vaseline helps the pump suck oil and prime.

 

The pump output is linear with speed, to a point. It could cavitate on the inlet side. in fact, it's not uncommon with thicker oil to physically blow apart the oil filter canister, it won't flow through the relief point, and pressure rises to the point where the filter explodes... happens on VW's all the time with oversized gears. 

 

In fact, on our compressors, the oil pump is gear type. When they have a hung check valve, I have see them alternately lock the bullgear in place from hydraulic lock from the pumps foot-valve...blown foot valves from excessive pressure, etc. . . On a prior manufacturer, the oil pressure was normally 640psi, with restriction orifices to an inner bearing limiting that to 30psi. .. Start that one with the cold oil permissive bypassed it wasn't uncommon to blow the pressure transducer clean apart with pressures over 1000 psi! Always made me leery when guys plumbed those with 125 psi rated WOG BIP... Saw some of those fittings shrapnelize and stick in things as well. Nothing like standing in front of an electrical panel and hearing a muffled "BOOM" feel something like it hits the panel, and then see oil leaking out of all the unsealed panel openings, including around the electroflurorescent display!

 

Generally though, yes, without a relief valve the pressure would be speed dependent. Common practice is to oversize the pump to allow for relatively constant oil pressure at all engine speeds and accommodate bearing wear through the engine's lifetime.

 

Blueprinting the passages will help with flow and pressure. Going external pickup and feed is nice. If you can find the parts anymore!

[inline6]

My engine builder did blueprint the passages - which I was not expecting.  

 

 

I am running the datsun comp oil pan and pick up for now.  I've not understood the external pick-up set up when I have looked at pics.  I'll be sure to check that out again.

Edited December 13, 2014 by inline6