Load Sensing Hydraulic Systems

Textbook on ‘Load Sensing Hydraulic Systems in the SI Units

By Joji Parambath

The textbook presents information on the classification, constructional features, working, and essential terms and definitions, sizing, and safety and maintenance aspects of hydraulic accumulators. The book uses the SI system of units.

Textbook on ‘Concepts of Load Sensing Hydraulic Systems in the English Units

By Joji Parambath

Please click on the image links to go to the Amazon Website

The hydraulic system with a load sensing feature provides only the flow and the pressure as required by the connected load. Controlling both the pressure and the flow allows for a significant reduction in losses, improvement of circuit efficiencies, and enhancement of the service life of the system.

A Typical Load Sensing System

The basic load sensing system typically comprises a variable-displacement load sensing pump, a special compensator, and a load sensing directional control valve with proportional flow characteristics.

Load Sensing Variable-displacement Pump

In the variable-displacement pump, the flow can be controlled by varying the angle of its cam plate. This type of control can be achieved with the help of the control piston of the cam plate. The pump along with its compensator senses and responds to the varying flow and pressure requirements of the associated hydraulic system.

1. Variable Displacement pump

Symbolic representation of a variable-displacement pump

Pump Compensator

The compensator, when connected to a hydraulic system, senses the pressure and the flow conditions of the system. It consists of a pressure-flow compensator spool that works against a low-pressure spring {say 20 bar [290 psi] spring} and a high-pressure compensator spool that works against a high-pressure spring {say 300 bar [4350 psi] spring}.

2. Pump Compensator

Schematic and symbolic representations of a pump compensator in a load sensing hydraulic pump

Operation of a Load Sensing System

The load sensing system for the control of a hydraulic actuator consists of a variable-displacement pump, a compensator block, and a load-sensing closed-centre directional control valve. The 20 bar [290 psi] spring of the compensator forces the pressure-flow compensator spool towards the left when there is no pressure in the system. This normal position of the spool provides for the fluid a direct passage from the camplate control piston to flow to the reservoir. As there is no fluid pressure acting on the control piston of the pump, the camplate of the pump is forced to move to its maximum angle position. In this position, the pump is ready to produce the maximum flow.

3. Load Sensing System Initial Position

Schematic and symbolic representations of a simple load sensing system in its initial position

When the pump is switched on, the system will slip into the following modes: (1) low-pressure stand-by mode (when the pump is on and the directional control valve is in its centre position), (2) load-sensing mode (when the directional control valve is actuated and load is moving with the requirement of pressure and flow), and (3) high-pressure stand-by mode during its cycle of operation (when the load comes to a stand-still under pressurized condition).

Authored by JOJI Parambath, Founder/Director, Fluidsys Training Centre, Bangalore


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