• In load-sensing hydraulic systems (LS-system) with a variable-displacement pump, flow and pressure are automatically set according to the request from an actuator. It makes the system energy efficient.
• This feature is realized by using feedback from actuators through signal lines to a flow control valve (1).
• Shuttle valves located in the signal lines serve to pass the highest load pressure to the flow control valve.
• The flow control 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 increases output flow until the differential pressure is equal to the spring force of the flow control valve, causing the valve to shift its position to restrict the flow.
• The pump always tends to provide the required output flow value in order to obtain the differential pressure equal to the spring force.
• Since the differential pressure is constant, the flow to the actuator also remains constant regardless of the load.
• 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 flow control valve spring action, but 5 - 7 bar higher than the differential pressure level.
• When the maximum set system pressure level is exceeded, a pressure limiting control valve (2) or "cutoff" valve will reduce pump displacement.
• A pressure relief valve (prv) ensures protection if the pressure limiting control valve is out of order or a pressure spike has happened in the system.

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 flow control valve (1). The pump displacement decreases to the minimal volume from the initial maximal value. Minimal flow in the system is needed for cooling, supplying standby pressure, compensation of internal leakages and lubrication purposes. The swash plate in standby mode should not reach a neutral position. This can be realized either with a mechanical restrictor or with an orifice inside the control piston. The orifice opens a way for oil to flow out of the control piston when the swash plate moves closer to the neutral position. Opening the way for oil causes a pressure drop. The pump displacement stops decreasing. Power loss in this mode is minimal, because flow and pressure are too low.

Motor1 / 125 bar

Initiate. If the directional control valve of motor 1 (dcv1) is opened and the directional control valve of motor 2 (dcv2) is closed, then load pressure will be passed through the signal line from motor 1 to the common shuttle valve (3) and move the valve to the right side position. Then load pressure impacts on the flow control valve (1) from the spring side and reinforces the spring action to shift this valve to the left side position. Now the channel towards the control piston is closed. The pump output pressure increases according to the load and impacts on the bias piston. The pump output flow increases until the value of differential pressure Δp does not overcome the spring force action of the flow control valve (1). When the spring force is exceeded, the flow control valve (1) is shifted to the right side position against the spring action and opens the channel towards the control piston. Now pressure impacts on this piston. The output flow decreases, the flow control valve (1) is shifted to the left side position and closes the channel towards the control piston. Thus, the pump provides pressure according to the request (load) of motor 1 plus the spring force of the flow control valve. The pump output flow depends on the load, the spring force setting of the flow control valve (1) and the orifice size of the directional control valve. Internal leakages can cause insignificant fluctuation of the pump swash plate for their compensation. Power loss is determined by the value of differential pressure - Δp.

Motor2 / 150 bar

Initiate. If motor 2 is activated - the directional control valve is opened - and the load of this motor is more than motor 1's load then the shuttle valve (3) will be moved to the left side position and open the signal line to provide the load signal from motor 2 to the flow control valve (1). The pump also increases the output flow according to this request. The pressure compensator (pc1) installed on the inlet of the directional control valve (1) automatically restricts the delivery orifice to prevent an increase of flow through motor 1 due to an increase of the pump output flow according to the request from the more loaded actuator (motor 2). Power losses arise from the differential pressure value - Δp plus the difference between the load pressure of motor 2 and the load pressure of motor 1.

Motor1 / additional load 50 bar

Initiate. Load increasing on motor 1 can lead to a situation in which the load pressure signal from this motor becomes higher than that from motor 2. The shuttle valve (3) then moves to the right side position and opens the signal line to provide the load signal from motor 1 to the flow control valve (1). Then the load pressure impacts on the flow control valve (1) from the spring side and reinforces the spring action to shift this valve to the left side position. The channel towards the control piston will be closed. Rising pump output pressure impacts on the bias piston to increase the pump displacement. The pump output flow will increase slightly to restore the differential pressure according to the spring force of the flow control valve (1). Therefore, the differential pressure will be a constant value and the flow through the directional control valve remains the same. The pressure compensator (pc2) installed on the inlet of the directional control valve (2) automatically restricts the delivery orifice to prevent a change of flow on motor 2 due to an increase of pump pressure according to the request from the more loaded actuator (motor 1). Power losses arise from the differential pressure value - Δ p plus the difference between the load pressure of motor 1 and the load pressure of motor 2, since motor 1 is more loaded compared to motor 2. To minimize power losses it is important to minimize the load difference between actuators.

Motor2 / disengage

Initiate. When motor 2 is disengaging, the output flow of the pump becomes more than necessary. It leads to increasing pressure upstream of the compensators or on the left side of the flow control valve (1). So, it is shifted to the right side position against the spring action and opens the channel towards the control piston. Now pressure impacts on this piston and decreases the output flow. The output flow decreases until the balance between the differential pressure value - Δ p and the spring force of the flow control valve is restored.

Motor1 / blocking

Initiate. When the maximal setting of the pressure is exceeded, for instance if the motor 1 is blocked then the pressure limiting control valve (2) or "cutoff" valve will be opened and pass pressure to the control piston. The pump displacement will start to decrease, preventing damages in the system. The pressure relief valve or shock valve will be opened by a pressure spike to drain excessive hydraulic fluid if the pressure limiting control valve (2) can not provide a sharp decrease of the output flow. The crack pressure for the pressure relief valve should be set 20-25 bar higher than for the pressure limiting control valve (2). Power loss depends on the setting of the "сutoff" valve and the value of internal leakages. Cancel animation.