Variable displacement pump with flow control?

[diyfarmer]

Hello all,

My boss got a good deal on some hydraulic parts so I am building a power unit to use on whatever application he dreams up. It consists of a 40 hp Deutz engine with a gearbox on the back that has two outputs. One output drives a gear pump which is simple enough. The other output is driving a variable displacement piston pump. Currently I have the output of the pump going through a pressure compensated adjustable flow control and then an open center directional control valve. When testing with an unloaded cylinder, the gauge shows basically zero until the cylinder reaches then end of its travel. This is what I would expect with an open center valve. Now for the questions:

1. Would there be any benefit or detriment to installing a closed center valve? I don't know if a variable displacement pump is better off running at full flow, no pressure or no flow, full pressure.
2. If we installed a closed center valve, would the flow control cause any weird behavior with the pump?

I talked to someone at Eaton, but without knowing the full application, they couldn't offer much advice. With a generic power unit, there is no one specific application.

Thanks in advance for the help, it is greatly appreciated.

 

[hydtools]

What is 'the gauge' measuring and where in the circuit?

Is the flow control a two-port or three-port valve?

Do you want a constant flow or constant pressure circuit for the cylinder?

Ted

 

[diyfarmer]

Sorry, I realize now that I wasn't very clear regarding the gauge. It is measuring the output pressure from the pump, before the flow control. The flow control valve has three ports: in, controlled flow, and excess. With the flow control, we are hoping to obtain constant flow.

 

[hydtools]

Is the variable displacement pump pressure compensated? Or manually adjusted?
I will assume the pump is pressure compensated. With that flow control, the pump will always be at full stroke and the valve will be bypassing the excess flow with constant flow out the controlled port. You should use a two-port pressure-compensated flow control which will cause the pump pressure to increase, destroke the pump and reduce flow to the flow control setting. The pump will increase stroke or decrease stroke in response to various flow control settings.
The three-port flow control valve should really be used with a fixed displacement pump.

Ted

 

[diyfarmer]

Thanks Ted, that makes sense. The pump is pressure compensated. I suppose plugging the excess flow port of the flow control does not make it a two port flow control?

How does all this relate to using an open center versus closed center control valve?

 

[kcj]

I would go to the closed center valve.

From past experience I know that some (or maybe many) PC piston pumps need to have a minimum of a couple hundred psi load to keep the piston shoes against the swashplate as they go past TDC on the compression cycle. Running at full flow and no pressure destroyed them in hours.

In this case the priority flow control might hold the load pressure up high enough to solve the slipper issue, but as ted says, the pump would be on load and consuming hp, converting it to heat across the bypass port of the flow control

 

[hydtools]

With the three-port flow control use the open center directional valve. If you use a closed center valve you increase flow out the excess port, pump does not go to zero flow.

With the two-port flow control use either. A closed center valve will cause the pump to go to zero flow, standby pressure. The open center valve will just bypass the flow control ouput at line and component loss pressure. The pump will operate fine with either setup. Which you use may depend more on how frequently you cycle the system. If demand for pump flow is infrequent, then closed center reduces standby power and flow is available when demanded. If demand for pump flow is frequent, then open center reduces frequent pump stroking cycles and flow is always available.

Yes, you can block the excess port to get a two-port flow control.

Ted