Load Sensing Pumps

Objectives

  • Explain the purpose of a load sense pump
  • Describe the function of a load sensing pump controller
  • Define the concepts of hunting, standby pressure, and margin pressure

What Is It?

What makes a pump a load sense pump?

A load sense pump has the ability to vary its output flow and the system pressure to just match the demands at the hydraulic actuator.

In effect this means that the pump only works as hard as is needed to do the job, which in turn saves energy and creates less waste heat.

This is known as a load sensing because the resistance of the system (created by the load) is directed back to the pump, where it is sensed.

How Does It Work?

A pump used in a typical load sensing hydraulic circuit will still have a pressure compensator for limiting the maximum system pressure.

In addition, there is a second control device is used to provide fine control over the pump’s displacement and the system pressure during normal working parameters.

Pressure Compensator
Second Control Device
Spoiler... It's the
load sensing compensator...

A load sensing pump has a small control hose connected to its load sensing compensator.

The other end of this hose will connect to a “LS” port on the directional valve bank.

This allows changes in working pressure at the actuator connected to the valve bank to be sensed back at the pump.

What is Going on Inside?

A load sense pump is very similar in function to a pressure compensated pump, with largely the same internal components.

Touch the component names to see the corresponding components highlighted.

The most important thing to remember about the inside of a load sense pump is that it can change its displacement and outlet pressure precisely for what is needed at the actuator at any operational moment.

The Missing Piece

There is another key component needed for any every load sense pump to work that is not part of the pump itself.

That key is a flow control orifice between the pump outlet hose and the point where the load sense signal is detected.

This flow control can be anything from a fixed flow control orifice...

...to a variable position (proportional) directional control valve.


But there must be a pressure differential created across the device.

Pump
Outlet

The reason this flow control is so important is because there must be a constant pressure difference between the pump outlet pressure and the load sense pressure.

This difference will be equal to the spring setting of the load sense compensator.

This is known as the margin pressure.

A Load Sense Pump in Action

Let’s take a look at the load sense pump in action!

The pump starts out simply supplying flow to the motor…

When suddenly resistance is added to the motor!

The motor is at risk of slowing down!

The added resistance causes the hydraulic pressure to rise...

...and the rise is detected back at the pump through the load sense line.

The pressure rise also has the added effect of decreasing the pressure differential across the flow control device.

The increase of the load sense pressure forces the load sense compensator valve to open.

This creates a flow path that allows fluid from the internal control piston to escape into the pump casing and from there out through the case drain line to the tank.

As the fluid leaves the control piston the bias spring pushes the swash plate further on stroke, increasing the pump’s displacement.

This causes the pump’s output pressure to rise and forces the load sense compensator valve to close again.

The pressure increase from the pump also returns the pressure differential across the flow control back to the same value that it was initially.

Equilibrium is quickly returned to the system and the motor continues to spin as if nothing had ever happened.

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