Control size for loader pump

[CMR]

Would a 25 GPM control work ok with a 11 GPM pump? I haven’t been able to find out what size my pump is, but do know it’s either a 11 or 21 GPM. Thanks very much

 

[lennyzx11]

If a 25 gpm valve is used with an 11 gpm pump. It would be more twitchy (sensitive) to control.

Here’s why. Really Long version.

1st given fact(s)
Pump flows 11 or 21 gpm constantly round and round in the circuit.
Tank>pump>valve>return back to tank.
Lever(valve) diverts flow from constant round and round to move a hydraulic cylinder one direction or the other.
Little bit or a lot of the flow(gpm) diverted to cylinder depends on how much the valve is opened.
Simple enough it seems…
Don’t forget that flow (gpm) MUST be moving. All the time. Round and round.

2nd given fact. Valve gpm is max it can flow through it. 11 gpm valve flows ONLY 11 gpm. No more. Ever.
Can flow less 0-11 but never more (quoth the raven) but I digress…

Here’s the scenario you ask about.
The 25 gpm lever handle moves 4 inches say from center (stop) to end (wide open).

Just right
At 2” of lever travel of the handle (1/2 way) the valve is able to flow 12.5 gpm (1/2 of 25 gpm).
4” travel (wide open) = 25 gpm
Life is good.

Valve too big
11 gpm pump with 25 gpm valve.
All the flow available (11 gpm) would go through the valve in less than 2” of lever travel (1 7/8” actually). That valve can flow 12.5 gpm when 1/2 way open (half of its 25 gpm rated flow max).
Only some of the lever travel moderates flow so sensitive to hand movements (twitchy).

Opposite example. Valve too small.

25 gpm pump and 7 gpm valve.
Lever travels 1/2 way. 3.5 gpm is 1/2 of the max 7 gpm the valve can flow.
So 1/2 way of lever travel (1/2 (3.5 gpm)of the 7 gpm allowed )gets you only 1/7th (3.5 of 25 gpm)of your pump flow.
Wide open only flows 7 gpm (7 gpm valve max limit) or a little over 1/4 of the available 25 gpm from pump.
That other 18 gpm (25-7gpm) is bypassing and heating up with the friction of going through the bypass valve instead of through the control valve to do work.
Cylinder moves slow at 1/4 of speed it could be moving even at max. Like waiting on a woman.

This is ignoring some of the other things like rpm of pump, head pressure, etc but you get the idea.

 

[ericm979]

Hydraulics text books recommend valves that have about 25% more flow than the system.

So you'd want 13 or 14 gpm valves.

 

[CMR]

Thanks very much, very helpful info there.

I have two spare pumps both of which are the smaller size that came with the tractor and which worked this loader just fine, and I could use one of those. Again, no telling the GPM but my question now is what would you guess would be a typical GPM on a pump for a bucket that's 5 feet wide?

 

[CMR]

Would a 11 GPM gear pump work a 5' wide bucket be plenty big enough? The two old pumps I have are half the size of the pump I've been using so I'm pretty sure they are no bigger than 11 gpm.
Prince makes a 14 GPM control and a 25 but nothing between

 

[Cargo pants]

try asking your questions on yesterdaystractors case forum. there will be guys who know a lot about your tractor there, maybe even gone thru what you're going thru

 

[CMR]

I'm not sure those smaller pumps still work, is there any way of running a test to find out what the GPM on a pump is? Prince can't find the GPM on the big one I have from the cast numbers on it, they want a stamped number but I can't find one.

 

[MH49]

If you want to go through the effort, you can calculate the displacement of a gear pump through the dimension of the gears. Insanehydaulics.com (I can't make this stuff up ) has a nice online calculator for doing this: Gear Pump Calculator

Taking lennyzx11 comments one step further, if you have a valve that is oversized for the pump flow it will feel sluggish when you are operating it, particularly with a full bucket or any other big load because you will have to shift the spool further to develop enough pressure to lift the load. This point of the spool travel where you reach load pressure is greatly affected by the amount of flow going through the valve but it is more of a control issue as apposed to a negative to the system. However, if the valve is too small, you will have a lot of pressure drop in the system which will equal heat.

This statement from lennyzx11 need a little clarification.

2nd given fact. Valve gpm is max it can flow through it. 11 gpm valve flows ONLY 11 gpm. No more. Ever.
Can flow less 0-11 but never more (quoth the raven) but I digress…

Open center valves can NOT limit flow. In the example above if you have an 11 GPM rated valve and you feed it 20 GPM with a fixed pump, it will try to flow 20 GPM however, the pressure drop across that valve will be insane and probably the majority of the flow will go over the relief valve (again a heat generator.)