pressure inside a spur gear oil pump

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dave brode
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pressure inside a spur gear oil pump

Post by dave brode » Sat Jun 09, 2018 1:24 pm

All,

I'm trying to understand how the oil cooler plumbing works on an old marine engine. Spur gear pump [like chevy]

Picture shows the pickup line hanging on the pump. You can't see the cooler return in that picture. The other picture shows [a poor view] of a pump with a pipe plug below the pressure side in the cover, where the cooler return line connects. [said pump is from an engine w/o cooler, not all were equipped with a cooler]. You can see the bulge in the housing where the passage goes up to the main feed into the block.

Question: As oil is squeezed through each pair of male/female teeth, and is forced to the outlet passage, is there a short period where the pressure would be a slightly negative before the next pair of teeth meet and start to compress the oil?

Reason to understand: The oil cooler plumbing on an old chris-craft takes part of the oil from the main bearing feed, sends it to the cooler, and returns it to the bottom of the pump, on the PRESSURE SIDE! I don't see how much oil would travel.


Thanks for any insight.
Dave
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Re: pressure inside a spur gear oil pump

Post by Tuner » Sat Jun 09, 2018 2:27 pm

The oil does not pass through the center where the teeth mesh, it moves around the perimeter of the housing, opposite where the gears mesh, around the OD of the gears where they do not touch. Each gear moves half the oil. It enters on one side of the mesh point and moves around in the cavities between the gear teeth to exit at the other side of the mesh point.

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Re: pressure inside a spur gear oil pump

Post by dave brode » Sat Jun 09, 2018 4:27 pm

Thank you, Tuner.

If I were to have looked closer, I might have realized that when I saw the direction of rotation [feeling dumb]. Looking at the shortblock picture, the gear toward the top of the picture must turn CW, and the lower one CCW. ** Note that gear toward the top of the picture is larger in diameter than the other.

Here is a picture showing the plumbing. The line coming from behind the coil is the supply. It is connected to the drilling from the pump mouting surface to the main longitudinal drilling. The return going to the block behind the starter goes back to the bottom of the pump.

I understand that the highest pressure in the system would be inside the pump on the pressure side. I can't for the life of me, see how oil travels to the cooler and back to the bottom of the pump's pressure side. Espeically with the pressure drop in the lines and cooler.

Dave
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Re: pressure inside a spur gear oil pump

Post by Tuner » Sat Jun 09, 2018 5:12 pm

You're looking at it bass-ackwards. It can only flow from higher to lower pressure. When the oil is cold the high viscosity makes it take the path of least resistance to the bearings and when it gets hot the lower viscosity allows more flow through the cooler. Unless there is a thermostat or a spring loaded bypass valve in the system, the cooler acts as a shunt and how much oil passes through the cooler is a result of viscosity reduction at higher oil temperature. Are you sure there is not a control valve of some sort? Is the long projection with the adjustment bolt and lock nut just a slide valve pressure regulator, or does it provide flow through an alternate outlet, perhaps to the cooler or not?

You need to be sure the distributor advance curve is as specified in the book for that engine. Some flatheads can be more sensitive to timing than overhead valve engines. Is the carb a Zenith updraft (or two) with an adjustable main jet?

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Re: pressure inside a spur gear oil pump

Post by dave brode » Sat Jun 09, 2018 5:21 pm

The bolt on the pump is to adjusting the pressure only. It dumps to the sump through a hole in the boss visible.

There is no valving of any sort anywhere, except for a spring loaded bypass valve in the cooler body, similar to that in a chevy oil filter mount. I understand what you mean about cold oil not flowing as well as hot. The oil returning from the cooler would be thicker.

Thanks.
Dave

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Re: pressure inside a spur gear oil pump

Post by Tuner » Sat Jun 09, 2018 5:32 pm

That valve is probably a check valve so it doesn't drain when it the engine is shut down, which would result in every time the engine is started the cooler would need to be filled before oil could go to the bearings

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Re: pressure inside a spur gear oil pump

Post by dave brode » Sat Jun 09, 2018 5:50 pm

Tuner wrote:
Sat Jun 09, 2018 5:32 pm
That valve is probably a check valve so it doesn't drain when it the engine is shut down, which would result in every time the engine is started the cooler would need to be filled before oil could go to the bearings
I don't believe so, as the in/out passages are open at all times. The bypass is between the inlet and outlet passages.


The passage from the pump meets the logitudinal main bearing feed before it goes to the outside and to the cooler. The feed to the cooler just takes some of the oil from the main drilling. You could remove the lines and plug the block at both spots and the engine would oil normally.

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Re: pressure inside a spur gear oil pump

Post by pamotorman » Sat Jun 09, 2018 6:08 pm

sounds like it might be a by pass oil cooler like the old 6 cylinder chevy oil filters that only filter some of the oil.

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Re: pressure inside a spur gear oil pump

Post by dave brode » Sun Jun 10, 2018 12:47 am

pamotorman wrote:
Sat Jun 09, 2018 6:08 pm
sounds like it might be a by pass oil cooler like the old 6 cylinder chevy oil filters that only filter some of the oil.
Correct. I'm just having a hard time that the oil moves from the high pressure side back to the high pressure side [of the pump].

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Re: pressure inside a spur gear oil pump

Post by cjperformance » Sun Jun 10, 2018 1:11 am

dave brode wrote:
Sun Jun 10, 2018 12:47 am
pamotorman wrote:
Sat Jun 09, 2018 6:08 pm
sounds like it might be a by pass oil cooler like the old 6 cylinder chevy oil filters that only filter some of the oil.
Correct. I'm just having a hard time that the oil moves from the high pressure side back to the high pressure side [of the pump].
I have never seen that exact setup but a few thoughts from other similar systems that I have seen,,,
Did you have the pump apart for a look? A small difference in the pickup area at either side of the gears OR small placement difference in the fitting V gear location is all thats needed to induce a small flow volume thru the cooler.
Or is one of the fittings in any way connected to the oil pressure bypass?
Craig.

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Re: pressure inside a spur gear oil pump

Post by dave brode » Sun Jun 10, 2018 1:20 am

Craig,

I have the pump apart, but not with me. I will take a picture of the pump early next week.

The hole with the pipe plug shown in the pump picture is where the return connects on engines equipped with a cooler. It is directly under the outlet, which leads to the mounting flange end, in the top of the pump. There is no connection to the pressure bypass.

Dave

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Re: pressure inside a spur gear oil pump

Post by cjperformance » Sun Jun 10, 2018 2:06 am

dave brode wrote:
Sun Jun 10, 2018 1:20 am
Craig,

I have the pump apart, but not with me. I will take a picture of the pump early next week.

The hole with the pipe plug shown in the pump picture is where the return connects on engines equipped with a cooler. It is directly under the outlet, which leads to the mounting flange end, in the top of the pump. There is no connection to the pressure bypass.

Dave
Ok Dave im keen to see the pictures. Some old stuff is very interesting.
It could simply be working on the small pressure differential between output pressure right at the pump compared to the pressure(slightly lower) in the main oil gallery. Thinner the oil gets, more flows thru the bearings, increasing the pressure differential, so more flows thru the cooler , ie coming out of the pump, thru the cooler and flowing into the main gallery due to the pressure differential. I dont know that for sure, just guessing at this stage. Pics will hopefully show more.
Cheers,
Craig.

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