Load-sensing control

Hydraulic load-sensing control is also referred to as a load-pressure feedback system and adapts certain physical variables to the conditions required by the consumer. These are usually the pressure and/or the flow rate of the hydraulic pump used.

In conventional hydraulic systems, the flow rate to the motor is throttled by conventional pressure control or pressure relief valves. The disadvantage is that the hydraulic pump continues to deliver at maximum output even when throttled, which can lead to very high power losses.

With load-sensing control, by contrast, it is ensured that the pump delivers only the flow rate required by all consumers currently in operation.

Load-sensing control in detail

Load-sensing controls are equipped with either fixed-displacement or variable-displacement pumps. Hydraulic load-sensing controls with fixed-displacement pumps are referred to as open-center systems, and those with variable-displacement pumps as closed-center systems.

Pressure compensators ensure that the pump pressure is regulated. If the consumer in the hydraulic system is switched off, the hydraulic pump delivers only enough to actuate the pressure compensator. The excess flow is routed to the tank in an open-center hydraulic system.

In a closed-center system, the pump’s control piston is actuated by the pressure compensator. This reduces the pump’s delivery volume. The pressure is determined by the spring force of the pressure compensator. The pressure differential between the pressure compensator’s control line and the pump delivery pressure remains constant in this hydraulic system.

The pressure is transmitted via the load-sensing line to the pressure compensator when the control valves for the consumers in the system open. The pressure in the load-sensing line acts in the same direction in the hydraulic system as the control spring. This increases the pressure required to actuate the pressure compensator. The delivered volume is increased exactly until the pressure differential set by the user is fully restored. If multiple consumers are connected in a hydraulic system for feedback via the load-sensing line, additional shuttle valves are required, which continuously output the highest pressure occurring in the hydraulic system. Load-sensing control is often used in combination with power limitation or pressure cut-off.

Function of a load-sensing control

Three potential functions of a load-sensing control are presented below:

In the first function, the system’s drive motor and the hydraulic pump are switched off. Due to the spring pressure of the control cylinder, the pump is set to maximum delivery.

In the second function, the pump is driven and the valves are closed. If the pressure in the delivery state is sufficient, the pressure regulator shifts and the control piston is pressurized. As a result, the hydraulic pump destrokes and an equilibrium is established between the pressure required to actuate the pressure regulator used and the pump pressure.

In the third function, the valves are opened and the pump is driven. As a result, the hydraulic fluid flows to the consumer in the hydraulic system. A line leads back to the pressure regulator via the shuttle valves. The valve spring, together with the pressure from the line, presses against the pressure side of the hydraulic pump. If the valve is opened further, the pressure in the line also increases. As the pressure rises, the pressure regulator opens further and further. In this way, the pump is set to a higher delivery rate via the control piston.