Load Sensing Hydraulic Pumps Explained: How They Work and How to Set Margin Pressure

A load sensing pump reads the pressure on the load side of the control valve through a small signal line and strokes itself to hold a fixed pressure difference, the margin, across that valve, so it delivers only the flow the valve opening asks for, at a pressure just above what the load needs. You set the margin (also called delta-p or the differential) with one gauge on the pump outlet and one on the load sense port, at part flow and below the maximum pressure setting, by turning the load sense screw until outlet minus signal pressure equals the factory figure: for example 14 to 22 bar on a Rexroth A10VSO DFR, 160 psi plus or minus 20 on most Vickers PVB and PVQ load sense pumps, and 10 to 35 bar on a Danfoss Series 45 H frame.

Standby is what the pump holds when every valve is in neutral and the signal line is vented: margin plus a little. The rest of this page explains why, what the numbers do, and what goes wrong.

Load sensing vs pressure compensated vs fixed pump: what is the difference?

Eaton's training booklet on its medium-duty load sensing pumps lays out the three systems this way, and its numbers make the point better than any definition:

System	What the pump does	Where the energy goes
Fixed pump, open center valve	Full flow all the time. Pressure is whatever the load demands; a relief valve caps it.	Any flow not used goes over the valve or relief at load pressure. Eaton's example: 20 gpm pump, 5 gpm used at 2,000 psi, wastes 17.5 hp. At the relief (3,000 psi, 20 gpm) it wastes 35 hp.
Pressure compensated pump, closed center valve	Variable flow, but it always tries to hold the compensator setting. With nothing moving it sits at full compensator pressure.	Throttling losses whenever the load needs less than the compensator setting.
Load sensing pump, closed center LS valve	Variable flow and variable pressure: just the flow the valve opening calls for, at load pressure plus the margin. With nothing moving it drops to low pressure standby.	Eaton's same 5 gpm at 2,000 psi job runs at 2,200 psi and takes 6.4 hp. At the end of stroke the pump goes to high pressure standby, losing only internal leakage.

The arithmetic behind those figures is the standard one: hydraulic horsepower = psi x gpm / 1714. Run it on your own machine with our hydraulic calculator.

Most load sense pumps are also pressure compensated. The compensator (Rexroth calls it the pressure controller, Vickers and Eaton the pressure limiter or high pressure spool, Danfoss the PC spool) overrides the load sense function and caps maximum pressure. So a load sense pump has two settings, and both matter.

What is a load sensing hydraulic system, and how does the pump work?

The control on the pump has two spools. Eaton's medium-duty version is a good one to picture:

1. At rest the flow spool's spring (200 psi on Eaton's pump) holds it over, the stroking piston drains to tank, and the swashplate goes to full angle, ready to pump.
2. Valves in neutral. Flow dead-ends at the closed center valve and outlet pressure climbs. At 200 psi it beats the spring, the spool ports oil to the stroking piston, and the pump backs off to near zero stroke. That is low pressure standby. The signal chamber behind the spool is vented through a small orifice in the valve to tank.
3. Operator moves a spool. The valve connects load pressure into the signal line. That pressure plus the spring pushes the flow spool back, the stroking piston drains, and the pump comes on stroke.
4. Steady flow. The pump settles wherever outlet pressure equals signal pressure plus spring. In Eaton's example the outlet reads 800 psi and the load 600 psi: exactly the 200 psi spring. Open the spool further and the pump strokes up to keep that 200 psi drop, so flow follows spool position, not load.
5. Cylinder bottoms out. Flow stops, signal equals outlet, and pressure rises to the pressure limiter (3,000 psi in Eaton's example), which destrokes the pump into high pressure standby until the load moves or the spool is centered.

With several valve sections, each one has a sensor check so the pump always responds to the section calling for the highest pressure. Rexroth describes its DFR control the same way: the pump flow is set by the pressure difference across an adjustable orifice, such as a directional valve, in the line to the actuator, and the pressure control overrides the flow control.

What is margin pressure, and what is standby pressure?

• Margin (delta-p, differential, LS setting): the pressure drop the pump holds across the metering valve while it is supplying flow. It is set by the load sense spring.
• Low pressure standby: what the outlet reads with all valves in neutral and the signal vented. Rexroth says it lies about 1 to 2 bar (15 to 30 psi) above the margin setting on an A10VSO DFR. Vickers sets a maximum for it in its service test.
• High pressure standby (cutoff): what the outlet reads when an actuator is stalled, set by the compensator or pressure limiter.

What margin and standby pressure does the factory specify?

Pump and control	Margin (outlet minus LS)	Standby	Source
Rexroth A10VSO series 31, DFR and DFR1	Standard setting 14 to 22 bar (200 to 320 psi)	About 1 to 2 bar (15 to 30 psi) above the margin, before system influences	Data sheet RE 92711
Vickers PVB5 to PVB29 CVP, PVQ10 to PVQ32 C**V**	160 psi plus or minus 20 at the flow set point	250 psi maximum	Vickers service test procedure, section 46 (1997)
Vickers PVB10 CVP3, PVQ C**V**C versions	350 psi plus or minus 20	550 psi maximum	Same
Vickers PVE19 and PVE21 CVP	160 psi plus or minus 20 at 13 gpm	250 psi maximum	Vickers service test procedure, section 47A (1997)
Vickers PVE19 and PVE21 CVPC	350 psi plus or minus 50	550 psi maximum	Same. Special builds (dash numbers) have their own figures.
Vickers PVQ25 code C21V11B	11 bar load sense differential with a 207 bar compensator	Not stated	PVQ25 service drawing
Danfoss Series 45 H frame, LS and LB controls	10 to 35 bar (145 to 508 psi), set in 1 bar steps by the model code	Pump holds the LS setting in standby	Technical information 11058669; service manual BLN-10237
Eaton medium-duty load sensing pumps	200 psi spring	About 200 psi low pressure standby	Eaton E-PUPI-TM008-E (1992)
Parker PVP, "A" load sense option	Adjustable; Parker's setup sheet gives no single figure	Can be varied with the differential screw	Parker PVP setup sheet and catalog HY28-2662

Two things jump out. First, the Vickers PVQ C21V11B code reads 11 bar, which is the same as the 160 psi the Vickers test procedure uses, so the code and the bench agree. Second, these are not the same number across brands, so a "standard" margin from one pump is not a target for another. Read your model code, and if your pump is a special (Vickers lists dozens of PVE dash numbers with their own margins), the special wins. For Vickers and Rexroth compensator ranges, see Vickers compensator settings and Rexroth A10VSO compensator adjustment, or start from the Vickers and Rexroth brand pages.

What does changing the margin actually do?

The margin is the pressure that pushes oil through the valve's metering notch, so it sets how much flow a given spool opening passes.

• Margin too low: functions run slow and never reach full speed, because the valve cannot pass full flow on that little drop. Danfoss lists a low LS setting as a cause of limited output flow and of instability. Parker says that with the differential too far below its ideal the pump will not reach the maximum flow the valve allows.
• Margin too high: the pump runs more pressure than the job needs, all the time it is moving oil. Parker says it wastes horsepower and makes heat, though it should not make the pump erratic. Our A10VSO compensator page makes the same point for the DFR: raising the differential buys responsiveness at the cost of standby losses and heat.

Put a number on the heat with the formula above: every 100 psi of extra margin at 20 gpm is about 1.2 hp turned straight into hot oil. That is why "turn it up so it's faster" is a bad fix for a slow machine. If the machine got slow over time and nobody touched the screw, the margin is not your problem. A worn pump, a leaking signal or a worn valve is. Start with hydraulic pump not building pressure and a case drain flow test.

How do I set margin (standby) pressure on a load sensing pump?

The order differs a little by maker, and that matters less than keeping one setting out of the way of the other. Danfoss and the Vickers service test set the margin first at part flow, then the maximum pressure deadheaded. Parker's PVP sheet and Rexroth's A10VSO procedure set maximum pressure first. Here is the Danfoss Series 45 sequence, which is the most complete one on our shelves, with the other makers' differences noted:

1. Fit gauges and warm up. Outlet pressure (M2 on the Series 45), a gauge teed into the signal line at port X, and case pressure at the drain port. Bring the oil to normal operating temperature. Vickers runs its load sense tests at 120°F plus or minus 10.
2. Check the relief. Danfoss is blunt about it: the system relief valve must be set above the pump's pressure compensator setting, or the compensator cannot work. Too little gap between them is also on its list of causes of instability.
3. Run a function at part flow. Danfoss asks for about half pump flow with system pressure kept below the compensator setting. The Vickers bench test loads PVB and PVQ pumps to 1,500 psi at a set flow, and the PVE to 2,000 psi at 13 gpm.
4. Set the margin. Loosen the LS lock nut and turn the LS screw until outlet minus signal pressure equals your figure. On the Series 45 H frame clockwise raises it, about 18 bar (260 psi) per turn. Torque the lock nut (7.5 to 10.8 Nm, 5.5 to 8 lb-ft on the Series 45) while holding the screw.
5. Set maximum pressure. Run a function to the end of its stroke so the pump is at maximum pressure and zero flow, and set the compensator. On the Series 45 H frame that is about 57 bar (825 psi) per turn, clockwise to raise. Danfoss references the compensator to case pressure, so subtract case pressure from the outlet reading.
6. Check standby. Center all valves so the signal vents and read the outlet. It should sit a little above the margin (1 to 2 bar above on a Rexroth DFR; no more than 250 psi on a 160 psi Vickers CVP).
7. Cycle it. Vickers cycles the pump five times from minimum to maximum flow and requires stable pressure, no yoke hunting when the load valve is slammed shut, and pressure returning within 50 psi of the setting.

Parker PVP differences. The load sense line goes to the top port of the PVP's compensator only, with Parker's orifice plug (800599) in that port; Parker says to check it is there when converting from another control. To set maximum pressure, Parker tees the signal line in at the pump outlet (or upstream of the valve you are sensing across), blocks the outlet, sets the maximum pressure compensator deadheaded, and sets it 200 psi above any remote pressure controls. Then the signal line goes back downstream of the sensing valve and the flow is set with the system open. See our Parker PVP and PAVC compensator adjustment guide and Parker PVP pumps.

Rexroth A10VSO DFR differences. Set the pressure controller first, then the differential with port X unloaded. On the DFR1 there is no connection from X to tank inside the control, so the valve system must be able to unload the signal line. The full procedure is on our A10VSO compensator page. For setting compensators and reliefs on other pumps, see how to adjust hydraulic pump pressure.

What are the common load sense faults?

What you see	Likely cause	What to check
Pump stays at high pressure with every valve in neutral; loud and hot at idle	The signal line cannot unload, so the pump thinks a load is still there	The vent orifice or unloading path in the valve. On a Rexroth DFR1 the valve system has to do the unloading. A shuttle stuck toward a pressurized section does the same.
Functions slow, never reach full speed	Margin set low, a restricted or leaking signal line, compensator set low, or a worn pump	Read outlet minus LS at part flow against the factory figure. Danfoss lists a low LS setting as limiting flow and a low compensator setting as stopping full stroke.
Pump hunts or pressure surges	Low LS setting, blocked signal line, compensator too close to the relief, or a chattering relief valve	All four are on Danfoss's instability list. Air in the system also gives noisy, erratic control.
Pressure will not rise above a certain point	Compensator set low, or the system relief set below the compensator	Raise the relief first, then check the compensator against the model code.
Standby higher than the book says	Signal not fully vented, case pressure high, or wrong expectation	Remember standby is margin plus a little, not margin. Danfoss references its compensator to case pressure.
Oil runs hot all the time	Margin set high, pump not destroking, or internal wear	Check the margin, then case drain flow. See why is my hydraulic pump overheating.
Signal line weeping or a fitting wet	External leak in the signal circuit	Fix it. A signal leak bleeds the pressure the pump is reading, so it will under-supply the load.

The control spool itself wears and sticks like any other. Danfoss's chart lists sticking control spools, a broken or missing compensator spring, a worn spool, a spool fitted the wrong way round and contamination in the control. If you are chasing a Rexroth, our A10VSO and A10VO troubleshooting guide covers its controls in more detail.

Frequently asked questions

Related reading

• How to adjust hydraulic pump pressure
• Rexroth A10VSO pressure compensator and DFR adjustment
• Vickers pump compensator settings and adjustment
• Rexroth A10VSO and A10VO pump troubleshooting
• Vickers load sense pump catalogs: PVQ (including the PVQ25 service drawing), PVB, PVE
• Parker PVP piston pumps
• Hydraulic pump case drain flow test
• Hydraulic pump start-up procedure
• Hydraulic oil viscosity guide by pump type
• Hydraulic pump questions answered