• Load-sensing hydraulic systems with a fixed-displacement pump reduce excess pump output pressure compare to systems where there is only a pressure relief valve set to the maximum system pressure.
• The pump provides pressure according to the load plus an insignificant threshold over this.
• This feature is realized by using feedback from actuators through the signal lines to an unloading valve.
• Shuttle valves located in the signal lines serve to pass the highest load pressure to the unloading valve.
• The unloading valve is closed by the signal pressure, the valve's spring adds a threshold.
• The signal pressure appears when at least one of the directional control valves is in a position to direct the flow to the actuator.
• Flow creates differential pressure on a restriction in a channel. The value of this pressure depends on the velocity or volume flow.
• The differential pressure or Δp is the difference between the pump output pressure and the highest load pressure.
• The pump always provides its maximum flow.
• Unused flow is dumped through the unloading valve when the differential pressure reaches the value corresponding to its spring force.
• If there is no signal pressure, the pump provides standby pressure.
• The standby pressure level is usually 15 - 35 bar and must be measured provided closed-center directional control valves are in the neutral position.
• Standby pressure is also determined by the unloading valve spring action, but 5 - 7 bar higher than the differential pressure level.
• A pressure relief valve (prv) ensures protection if the unloading valve is closed by the pressure that exceeds the maximum allowed pump output pressure.

Standby pressure

Initiate. When the directional control valves are in the neutral position there is standby pressure in the system. This pressure is mainly determined by the spring force of the unloading valve (1). The pump is discharged through this valve as the preset spring pressure is overcome. Power loss in this mode is less, than in conventional closed center systems without load feedback and the unloading valve. Losses are determined by the value of unused flow at the corresponding system pressure.

Motor1 / 125 bar

Initiate. If directional control valve 1 (dcv1) is opened and directional control valve 2 (dcv2) is closed, the load pressure passes from motor 1 through the signal line to the common shuttle valve (shv) and moves it to the right side position. Then the load pressure impacts on the unloading valve (1) from the spring side, providing additional force along with the spring action to close the valve. Now this valve closes according to the pressure level from the actuator plus the spring force action value as a threshold. If the flow demand from Motor 1 is less than the pump output flow then excess flow will be dumped again as a new pressure setting of the unloading valve is overcome. Power losses are determined by the value of unused flow at the corresponding system pressure.

Motor2 / 150 bar

Initiate. If motor 2 is activated and the load of this motor is more than the load of motor 1 then the shuttle valve will be moved to the left side position to pass the load signal from motor 2 to the unloading valve (1). Now the valve closes according to the pressure level from motor 2 plus the spring force as a threshold. If the total demand of flow is corresponding to the maximum pump output flow then there is no flow through the unloading valve. Flow is compleately utilized. Power loss is minimal, but the less loaded actuator starts to move faster. To resolve this issue usually pressure compensators are implemented. The operation of these elements will be reviewed later in the discussion of load-sensing hydraulic systems with a variable displacement pump.

Motor1 / blocking

Initiate. When the maximal setting of the pressure is exceeded, for instance if motor 1 is blocked then the pressure relief valve (prv) will open to drain excessive working fluid into the tank. Power losses are determined by the value of unused flow at the relief valve setting pressure. Cancel animation.