Specific question about load sensing pumps

[mechowl]

I have searched the forums and been unable to find answers to my questions.

I need help understanding how load sensing pumps work. Please correct any statements that are not true, as well as fill in the details that I am not grasping.

I understand that the load sensing compensation's goal is to save power. Specifically for metering of a dc valve when adjusting speed of actuators?

It does this by sending fluid to reduce the angle of the swash-plate and "destrokes" the pump limiting flow.

I'll use a simple setup as an example to ask questions. Components consist of a...
- Load sensing PUMP
- DC VALVE
- CYLINDER

Lets say that the pressure compensator is set to 3000psi. The load sensing compensator is set to 150psi. The max flow from the pump is 60gpm. The load induced at the cylinder requires 800psi and the speed of choice requires 40gpm.

When you start the pump up with no induced load it will immediately reach the pressure compensator setting of 3000psi but stabilize at its minimum swash angle to reduce flow. Since all components require lubrication the minimum swash angle is slightly above zero running oil throughout the pump and draining to tank through the case drain. I've read that it maintains the load sensing spring setting during idle of 150psi... The only way I can put this together is that for the oil to be able to drain out of the case drain it first has to shift the spool against the LS spring?

Next we will activate the cylinder...

When your metering flow at the dc valve there will be a pressure drop. Since we are metering flow, we need the pump to react and send out ONLY THE REQUIRED amount of fluid. This would mean that it should have to shift the spool to allow the signal to be transferred from the cylinder line to the swash plate piston and destroke the pump to maintain the required flow.

The shifting of the piston is another part I'm not understanding...

If this were true wouldn't the one side of the spool be at 950psi (800 for cylinder, 150 for dc pressure drop) and the other side of the spool to be at system pressure of 950psi as well? That would mean the spool forces would be balanced and the spool would not shift?

I've read that the load sensing spring setting (150psi) is what controls the pressure drop at the dc valve. I would have thought that with different amounts of metering (from a small orifice to a large one) that you would have different pressure drops. How and why does it maintain 150 psi drop?

 

[hydtools]

perhaps this will help:

http://www.hydraulicspneumatics.com/200/FPE/Pumps/Article/True/6403/Pumps

Ted

 

[mechowl]

Thanks Ted. That link has helped. Still confused on the metering part. I noticed in the link it was saying that the pressure drop across the valve stays the same (whether you meter 1% of max flow or 99%). My guess is that when you adjust how much you meter the pump automatically sensing the increase or decrease in flow requirements and adjusts the pump to suit. Correct?

So normally without the ls pump, when you meter it would vent off a relief valve wasting power. The more you meter the higher pump pressure becomes. In a simple circuit with ls pump, dc valve, cylinder, and tank wouldn't one side of the ls spool see pump outlet pressure and the other side sees load+spring setting. With the difference across the valve being the same as the spring setting the spool would be balance and not allow flow to de-stroke the pump. What am I missing?

Thanks!

 

[EdDanzer]

This link may help:

http://www.insidersecretstohydraulics.com/load-sensing.html

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com