• 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.
• Power control ensures constant power output of a primary drive at constant speed by reducing the pump output flow when the signal pressure exceeds a preset value.
• A pressure compensator located at the inlet on each directional control valve supports constant flow towards the actuator. The compensator controls the flow to the actuator, changing the size of the orifice. If the load of the actuator is not highest in the system, the compensator will reduce the size of the orifice.

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 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. Since a hydraulic system most often requires either high pressure and low flow or low pressure and high flow, the system can be equipped with an engine that does not provide power to obtain maximum flow and maximum pressure at the same time. In this case, torque limiting control or horsepower control is applied to prevent the engine from lugging.
Torque limiting control can be realized in different ways. Here is considered the situation when the pressure relief valve (torque limiting valve - tlv) is installed in the signal line. The cutoff setting of this valve depends on the swashplate position due to their mechanical linkage. Increasing the load on motor 1 increases the swash plate angle (output flow) due to automatic load-sensing control. At a critical point (depending on the spring setting and the swash plate angle), the torque limiting valve reduces the pressure in the signal line. It causes a shift of the flow control valve (1) to decrease the pump displacement. The angle of the swash plate decreases to reduce the output flow. At the same time the mechanical action on the spring of the torque limiting valve changes - the spring is compressed with more force. The pressure in the signal line becomes higher and shifts the flow control valve (1) to the left side position. The channel towards the control piston is closed. The pump output pressure impacts on the bias piston and increases the swash plate angle (output flow). The process starts again. Pump operation in this mode is called saturation.
When the pump is in saturation mode and pre-compensation logic is implemented (pressure compensators are located upstream of the directional control valves) then the actuator with the highest load can lose its speed until complete stop. It happens because the pump can not increase the output flow to restore the difference between the pump output pressure and the pressure of this actuator (setting of the flow control valve (1) or Δp of the pump). In fact, the Δp of the pump may be even less than the setting of the pressure compensator (the Δp of the directional control valve) in the overloaded line. As a result the flow in this line is weak and the compensator is fully opened. At the same time the difference between the pump output pressure and the load of Motor 2 is bigger than the Δp of the pump, therefore the flow mainly goes to the actuator with the lower load - Motor 2. Pressure compensator 2 (pc2) continues support the constant pressure drop across directional control valve 2 (the Δp of valve2). Motor 2's speed has not changed.Cancel animation.