Adding load sense to a plain valve

[__Andy__]

Hi,

I am working on connecting a pair of closed-center proportional control valves in parallel to my tractor. The valves have P, T, A and B connections. The tractor has a closed center pressure-flow compensated system that has a load sense port. The pump has a max output of 25 GPM at 2,500 PSI.

My understanding is that if I leave the valves closed then the pump will continually run at high pressure standby and this could generate excessive heat and damage the pump, as well as consuming more diesel. I am looking at how to take advantage of the load sensing capability on the tractor.

My current plan is to add a tee into the P line between the tractor and the valves. From this tee I will connect a normally-closed solenoid which integrates a check valve. The output of the solenoid will go back to the load sense port. In the tractor there is a 0.020" bleed down orifice for the LS.

The P line will be 3/4" hose and the load sense will be 1/4" hose.

My software will automatically open the LS solenoid one second before any valve control and then automatically close the LS solenoid five seconds after the last valve control, so the operator does not have to do anything manually to make this work.

Is that the correct approach or is there a better way without modifying the valves (which is not going to be possible)?

Thanks! Andy

 

[__Andy__]

I am thinking that my scheme will cause the pump to not see a difference in P and LS pressure. Therefore when I turn on the LS solenoid it will immediately ramp up to maximum pressure. Is that right?

If so would I be better off connecting the solenoid and check valve to A/B? If so, how would I do that considering A and B are bi-directional? Using two check valves and a shuttle valve?

Thanks, Andy

 

[FluidPowerUser]

Hello Andy

Your load sense line needs to go on the A and B line, with shuttle valve between them so that the highest pressure is sent back to tractor to increase the flow.

With no signal back to the pump, via the LS port, it will stay at the standby pressure 250 - 290 PSI.

The pump will try to maintain this pressure as the margin between P and LS. As the spools open up and the pressure drop decreases, the pump will increase flow across the spool to maintain the margin pressure.

 

[__Andy__]

Thanks @FluidPowerUser ! Would the following scheme work?

I would switch off the solenoid the instant the valve movement stops so the A/B lines don't bleed down through the orifice on the LS signal on the tractor.

However if I then want to start movement again I would turn on the solenoid to get the feedback, but in the period between turning on that solenoid and commanding movement, won't the A/B lines bleed down?

Sorry, I am not very good with hydraulic diagrams, learning as I go. Andy

 

[FluidPowerUser]

Yes, that’s basically the right idea.

The check valve isn’t required as there will be no pressure from the LS port.

Is it safe to assume that the P,T and LS are on the back of the tractor?

You don’t need the solenoid valve. If you want to make sure your load sense drains down, you use a compensation flow controller to maintain a constant flow rate <0.2 gpm.

 

[__Andy__]

Thanks for that. The load sense will drain through a 0.02" orifice on the tractor. How can I stop the A/B lines from bleeding down between movements?

Yes, P, T and LS are on the back of the tractor.

Thanks, Andy

 

[FluidPowerUser]

If the load sense signal is draining across an orifice, the flow will vary depending on the pressure differential across the orifice. If you use a compensated flow control valve, you can set the total flow to 0.2 GPM and it will remain at that flow independent of the pressure in A and B.

Using a orifice is OK, but you will get a different pump response as the oil warms up. On a cold day, the pump will stay at high flow for longer as the load sense pressure drains down. On a hot day, you will find that the pump response is a little slower and the load sense pressure takes a fraction longer to build.

As it is, the resolving network of shuttle valves only sends the highest pressure signal to the LS port and while 0.2 GPM is leaking out to tank, it makes the machine response more consistent.

At 1450 PSI, the leakage over the 0.02" orifice will be 0.34 GPM
At 2900 PSI, the leakage over the 0.02" orifice will be 0.50 GPM

The relationship between pressure and flow work in your favour here, the flow rate is pretty consistent. However, the oil density will affect these flow rates too.

You don't want the A and B lines to stay at high pressure between movements. That will cause the pump to stay at high flow against a blocked valve and it will just waste diesel. You want A and B to be leaky, but not too leaky.

 

[__Andy__]

OK thanks. Much appreciated.

 

[akkamaan]

My understanding is that if I leave the valves closed then the pump will continually run at high pressure standby and this could generate excessive heat and damage the pump, as well as consuming more diesel. I am looking at how to take advantage of the load sensing capability on the tractor.

I am not sure the author is focused on "true LS", since he seems to be concerned about the standby pressure and how to unload the pressure-compensated pump

With pressure-compensated pumps, we have 3 different "common" systems.
1. "True" Constant Pressure, CP, system
2. Unloaded Constant Pressure, CPU, system
3. (Unloaded) Load-Sensing, LS. system. This system sets the pump pressure from the load pressure in the actuators. When the system is at rest, the pump unloads in the same way the CPU system is unloaded
In recent years, the distinction between an LS system and a CPU system has been blurred, and parts of the fluid power industry are naming even CPU systems LS systems. The reason for the "blurring" is probably that the marking "LS" appears on both directional valve bodies and pumps, no matter of whether the system is built for "true" LS or CPU. So today, when the less knowledgeable see an LS marking on a component, they take for granted that it is for a "true" LS system.
So, if you are only trying to make the system unload the pump when no actuators are used, then this is how you build it.

So,

 

[__Andy__]

Thanks! I will digest that but it looks good! Andy

 

[akkamaan]

Your second task is to control the solenoid loading valve.
If your posted drawing is correct, your directional valve is controlled with a variable hydraulic pilot pressure, approx 0(5)-15 bar.
That means you will need either
1. A hydraulic pilot joystick valve. (common on excavators. With this joystick, you must manually control the loading valve with a simple separate electric control placed on the joystick, or a more sophisticated electronic system that senses the instant pressure drop on the pump main line and that way signals to the loading valve
Or,
2. An electro-hydraulic system with an electric joystick and an electro-hydraulic variable pilot pressure valve. Common on bigger excavators and cut-to-length logging equipment. With this system, you can take the electric signal from the joystick to an amplifier, and then relay the signal to the loading valve

 

[__Andy__]

Thanks. Yes you are right. I have two pilot valves that are controlled by PWM signals from a custom circuit board. The Bosch Rexroth software I was using to draw the diagram didn't have those valves so I omitted them. Andy

 

[akkamaan]

I have put together a more complete schematic for 1 of your 2 directional valves..
You can take the main pilot pressure supply via a T from the pump pressure line and a pressure-reducing valve. 25 bar should be an appropriate pilot pressure. Make sure the pumps' stand-by pressure is at least more than 25 bar (30 bar is good)

This is the principle of how electro-hydraulic control works