Oil pump basics - need some s'plaining please!

motorv8N · Sep 16, 2008

There have been a few postings about oil pumps lately and I'm hoping someone could explain a couple of things as I'm going to have to pull my TEA20's pump to investigate low pressure.

1) how do they work?
2) what exactly wears out?
3) what are the issues with rebuild kits (to hear folks discuss it sounds like the rebuild kits are near useless, what with aluminum gears, set screws one has to drill and tap from scratch, etc. Some report reusing their old steel gears. If that's so, what's the point of getting the kit at all?)

All help appreciated!

Jerry/MT · Sep 16, 2008

I'm not sure what kind of oil pump the Standard engine has but they generally all work the same way. The flow is accelerated by a rotor or a gear set that adds velocity head to the oil and that's what causes the pressure rise.(the fluid pressure is = ps + 1/2 rho V^2/2, ps is the local staic pressure , rho is the fluid density and V is the velocity of the fluid.) The flow, G, cu in/sec is proportional to the product of the flow area, A and the velocity V, i.e G =A x V, where V ~ Vt, tip speed of the rotor or the gears. The faster the rotor speed the higher the flow and pressure for a fixed area.

The things that wear are the endplates and the driven gear and driving gears themselves plus their bushings or bearings. They cause the clearance to open up and a lot of oil gets recirculated within the pump raising it's temperature and not contributing to flow through the lube system.

I think the comments you've heard about replacement gears has more to do with the quality of parts on the market today. Most of them are made in the "Land of Almost Right" where quality control is virtually non-existent. As result, rebuild kits don't fit correctly, the parts themselves do not meet specs, etc.

Anonymous-0 · Sep 16, 2008

Gear pumps are not velocity pumps, they are positive displacement pumps. When the gears fit the housing well, there is very little clearance at the tips of the teeth, the mesh of the teeth and the sideplates of the pump. So pockets of oil are transported from input port to output port between the gear teeth and then as the teeth mesh that oil is pushed out the output port. Gear pumps for oil and for hydraulics work the same way. Worn housings and worn gears make for leakage past the gear and low pressure. Probably the only sure fix is a whole new pump assembly with gears, plates, and housing all fitted to the same minimum clearances.

Gerald J.

Jerry/MT · Sep 17, 2008

Respectfully, the gear pumps I am familiar with do not "compress" (i.e. no change in volume like a piston pump or a vane pump) the oil so how do you explain how the pressure rise occurs? The fluid come into the inlet and is accelerated by the gears around the periphery of the housing and exits through the discharge opening. The gears accelerate the flow to a higher velocity than the intake velocity and the dynamic head of the fluid is increased. Bernouli's equation says the total head is equal to the static head plus the dynamic head assuming no change in elevation. That's the way I understand that a gear pump works.
Perhaps you have some other insight than I do. do you mind sharing it?

Anonymous-0 · Sep 17, 2008

The gear teeth transport oil from the inlet to the outlet. The tiny clearances in the gear pump prevent the oil running back inside the pump. Adding oil to a region with restricted flow and volume raises the pressure, it happens the same whether the gear pump is turning at 1 rpm or 3000 rpm. The only reason a gear pump may not develop pressure at slow speed is that the system (including a worn pump) leaks have more volume than the pump can supply at a slow speed. Nothing exotic. Pressure with a gear pump has to be controlled by a spring loaded pressure relief valve. If thats not present the pump will stall its prime mover, or split the plumbing.

Gerald J.

Jerry/MT · Sep 17, 2008

Additionally, "constant displacement" really means "constant displacement at a fixed speed" and strictly speaking even that is not correct because speed characteristics always have some slope(dW/dP<0) due to internal losses and other considerations. Many people confuse "constant displacement" with the flow capacity being independent of speed and that is incorrect.

Jerry/MT · Sep 17, 2008

See my second note above and as a reference, Marks Mechanical Engineer's Handbook, 9th Ed, page 14-14, "Capacity" 2nd paragraph, "The rotary pump is a constant capacity machine for a given speed."
It only stands to reason, just from a simple physical view point.
The fluid trapped between the gear tooth and the casing is a fixed element of fluid that is picked up at the intake and deposited at the outlet. It stands to reason that the more elements that are pickeded up at the inlet and deposited at the outlet per unit time, means the flow rate has to increase with the speed of the gears, N. Therefore capacity is directly proportional to N.

motorv8N · Sep 17, 2008

OK -- I'm getting some good theory here - thanks for that. Now, what's the best way of getting back to optimum performance?

Is replacing the entire pump even an option? Is there NOS around? Are there acceptable aftermarket units? This site doesn't seem to list any. Are there other sources for complete pumps? If I have to go with a rebuild kit, which is the best one?

Sorry for the onslaught of questions, I just want to have a plan in place before dropping that pan.

Jerry/MT · Sep 17, 2008

My advise is to holler out in the forum for some of the guys like Aussie Doc or Bob from Oz who have Ferguson's with Standard engines. Since a form of this engine was used in a British car, there may alternate sources of parts. Rebuilds by reputable firms and NOS are sometimes available. Sparex and Bareco(?)parts suppliers are also sources. The Sparex part No for the TEA oil pumpshaft & rotor is S61996 and it crosses with Ferguson part no. 827 501 M91. I have no Idea about the quality of their replacement parts. the majority of th Ferguson folks on the forum have experience with Continentals and their wimpy pump and that's what I think you're reading. The TEA's engine is a different beast so don't assume that comments about the TE's and the TO's read directly across to the TEA.

Jim in OH · Sep 20, 2008

(quoted from post at 18:12:45 09/16/08) There have been a few postings about oil pumps lately and I'm hoping someone could explain a couple of things as I'm going to have to pull my TEA20's pump to investigate low pressure.

1) how do they work?
[b:322d3194ef]The oil pump is a gear pump which is a positive displacement pump. It pumps a fixed volume for every revolution. Teh pressure developed is more about the restriction at the delivery point than the pump. For example gear pumps are often used for hydraulic pumps and can deliver thousands of psi when restircted.. Here are a couple images..[/b:322d3194ef]

2) what exactly wears out?

[b:322d3194ef]The Flat surfaces on both ends of the gears, the ends of the gear teeth and the bearings that support the gears and the pressure relief valve. [/b:322d3194ef]

3) what are the issues with rebuild kits (to hear folks discuss it sounds like the rebuild kits are near useless, what with aluminum gears, set screws one has to drill and tap from scratch, etc. Some report reusing their old steel gears. If that's so, what's the point of getting the kit at all?)

The aluminum gear is the gear that goes to the crank gear.. inside they are steel.. and the one screw to drill and tap is not critical.. you can do it with a hand drill.. a drill press is better..

All help appreciated!

Oil pump basics - need some s'plaining please!

motorv8N

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Jerry/MT

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Anonymous-0

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Jerry/MT

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Anonymous-0

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Jerry/MT

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Jerry/MT

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motorv8N

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Jerry/MT

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Jim in OH

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