Excess Flow with Gear Pump and Manual Proportional Mobile Valve

[HydraulicsGuy]

This weekend I had at the house a Vermeer Stump Grinder SC362. I was looking at the exposed hydraulics while the operator was taking a break. There was a 2-section gear pump. There were 4 functions: 1 steering cyl, 2 pivoting cyls, 1 up-down cyl, and I think 1 hydraulic motor for driving. There were manual control handles sticking out. The rest of the hydraulics was hidden inside an enclosure. I haven't dealt with any mobile applications at work yet, in the little over a year I've been working in hydraulics. Let's say ONLY ONE of the stump grinder's functions is being operated, and let's say it's being operated at less than full pump flow by a partial movement of the directional valve handle. Say the gear pump flow is 10 GPM. Say the function is only demanding 7 GPM by the operator partially moving the valve handle. 3 GPM has to be dumped. I'm guessing there has to be a feature of the hydraulic system that keeps the gear pump from going to relief valve setting as it dumps the excess flow (3 GPM) over the relief valve. I'm guessing that feature is built into the directional valve, where it dumps the excess flow at operating pressure rather than relief valve pressure, like a bypass-type flow control valve. Looking briefly at some directional valve literature, I have seen it implied that that's what happens, but what I saw wasn't clear to me at all. Is that what's happening, or is it something else? If that is the answer, my next question will be to point me to some directional valve literature that clearly explains/indicates this.

 

[hydtools]

Peter gives some information regarding flow factors Cv and K to relate pressure drop and flow in this article:

https://www.hydraulicspneumatics.co...lves/article/21887929/the-valve-flow-constant

Ted

 

[akkamaan]

On mobile power units we sized the pump to allow running the engine above peak torque speed so that increasing load would sag engine speed toward peak torque, not away from it. That prevented overloading and lugging the engine.

Exactly, that's what I meant with...

preferably at an rpm just about where the engine has its peak torque

I had a "typo" and it should have been "just above" to prevent that lagging below peak torque. Let7's say the operating rpm should be between peak torque and peak power depending on the power demand. For fuel efficiency as low RPMs as possible as long as there are enough torque AND power. And if you run your mobile equipment at just above peak torque and occasionally need more power you just press the "gas pedal" (rpm controller) a bit for a few seconds

 

[zeusfaber]

So if I have a 10 GPM gear pump, and I want 7 GPM to the cylinder, what will the pressure be on the pump side of the DCV? Assume no plumbing pressure drops. Say max allowable system pressure set by relief valve is 3000 psi.

Case 1: load pressure = 2500 psi.
Case 2: load pressure = 500 psi.

Repeat for only 2 GPM to the cylinder.

Assuming you were asking about the circuit I posted, and with the compensator cap set to 150 psi then, ignoring piping pressure drops, the compensator controls pump output pressure to:

Case 1: 2800 psi
Case 2: 800 psi

- and this is true whether the load flow is 2 gpm or 7. The cartridge absorbs the excess flow, opening far enough to control the pump pressure so that 150 psi is dropped across the pressure edge of the DCV. Assuming the DCV is symmetrical, the drop across the return edge will be the same so, while the DCV is open, the load pressure is less than 2700 psi and the DCV doesn't open so far that the edges can't sustain 150 psi at 10 gpm, then the pump will always see 'load pressure plus 300 psi' across the full range of load pressures and flows.

 

[Jacc]

Open Center valves are very common in many applications.
A video explaining here:

https://youtu.be/4ip2v22QQ9o

edit: substituted for link to original akkaman video

Some nice info in these datasheets:

http://www.salami.it/en/monoblocks/13-vdm6a.html

https://www.boschrexroth.com/variou...=1584&search_query=64122&search_action=submit

Some other producers of OpenCenter valves worth checking out are:
Walvoil / Hydrocontrol
Bucher
BLB
Parker

 

[akkamaan]

A video explaining here:

https://www.youtube.com/watch?v=zlU9EcilkSc

Hahaha
Thanks Jacc, I made that animation more than 10 years ago...
Here is the original upload...

https://youtu.be/4ip2v22QQ9o

Of course, the video is so good that people are downloading it and re-uploading to get personal creds... That's fine I am happy people like it that much even though it's only covering a very small portion of the system complexity.
I was working on part 2, which was about "closed center constant pressure" but ran out of "juice" and motivation since it is such a commonly used system and much easier to understand. Open Center Constant Flow is a very complex system to understand and explain when you are operating a 6 function knuckle boom crane with 2-6 functions simultaneously