A client recently engaged me to design and build a hydraulic power unit for a specific
application. The unit comprised a diesel engine driving an axial piston pump fitted with
load sensing, power limiting and pressure limiting control.
What is load sensing control?
Load sensing control is so called because the load-induced pressure downstream of the
directional control valve is sensed and pump flow adjusted to maintain a constant
pressure drop (and therefore flow) across the valve.
For example, let's say we have a hydraulic pump driving a winch thru a manual,
directional valve. The operator summons the winch by moving the spool in the
directional valve 20% of its stroke. The winch drum turns at five rpm. For clarity,
imagine that the directional valve is now a fixed orifice. Flow across an orifice
decreases as the pressure drop across it decreases. As load on the winch increases, the
load-induced pressure downstream of the orifice (directional valve) increases. This
decreases the pressure drop across the orifice, which means flow decreases and the
winch slows down.
The load sensing control senses the load-induced pressure downstream of the orifice and
adjusts pump flow so that pressure upstream of the orifice increases by a corresponding
amount. This keeps the pressure drop across the orifice (directional valve) constant,
which keeps flow constant and in this case, winch speed constant.
Because the pump only produces the flow demanded by the actuators, load sensing control is
energy efficient (fewer losses to heat) and as demonstrated in the above example, provides
more precise control.
What is power limiting control?
A constant power or power limiting control operates by reducing the displacement, and
therefore flow, from the pump as pressure increases, so that the power rating of the
prime mover is not exceeded. The advantage of this type of control is that more flow
is available at lower pressures, so that the actuators can operate faster under light
loads. This results in better utilization of the power available from the prime mover.
The power limiting control overrides the load sensing control.
What is pressure limiting control?
Pressure limiting control limits the maximum operating pressure of the pump. Also
referred to as a pressure compensator or pressure cut-off. The pressure limiting
control overrides both the load sensing and power limiting controls.
Pump control problem
Three, brand new pumps were ordered for the project from a leading manufacturer.
When the three hydraulic power units where being commissioned, I found that the power limiting
control on all three pumps was not functioning.
When advised of the problem, the manufacturer maintained that the pumps had been tested
prior to delivery and that the cause of problem therefore must be elsewhere in the circuit.
Possible external causes were quickly checked and eliminated. While waiting for the
manufacturer to respond to the problem, I checked the schematic diagram of the pump's
control and noticed that a vital part was missing. Click here to view the schematic.
Plug-in controls
The power limiting control on this particular pump is a modular, screw-in cartridge
fitted to the standard pump with load sensing and pressure limiting control. The power
limiting cartridge is a relief valve with a link to the swash plate that increases spring
bias as swash angle decreases. This relief valve limits load signal pressure depending on
swash plate position. When the allowable power setting is reached, the relief valve
intervenes to reduce the load pressure signal to the load sensing control. This results
in a decrease in swash angle and therefore flow. The lower the swash angle and therefore
flow, the higher the load signal pressure and therefore operating pressure permissible.
Because power is a product of flow and pressure, this limits the power draw of the pump.
If you examine the two schematic diagrams closely, you will notice that other than the
addition of a power limiting relief cartridge, there is a second difference. An orifice
is shown just below the load sensing signal connection or X port. Without this orifice
to limit the flow from the load sensing line, the power limiting relief valve cannot
effectively limit the load pressure signal. This means that the power limiting control
cannot function.
I checked the pumps fitted to the power units and none of the three had this orifice fitted.
I advised the manufacturer and requested that they dispatch three of these orifices urgently.
I was astonished by the manufacturer's reply - the required part was on back order. To minimize
any further downtime, I manufactured three orifices, fitted them to the pumps and handed the
hydraulic power units over to the customer.
Test for success
Thoroughly testing new or rebuilt hydraulic components prior to dispatch, ensures that
the component will work the way it should and will perform within its design parameters.
It is possible that the manufacturer tested the pumps discussed
above - but only their load sensing and pressure limiting controls. Had the functionality
of the power limiting control been tested, the pumps would not have been dispatched without the
necessary orifice. This would have avoided an embarrassing mistake for the manufacturer and many
hours of downtime for the customer.
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"This book has the potential to save many
organizations lots of m0ney. It should be on the bookshelf of every engineer, supervisor, planner and
technician that deals with hydraulic equipment... it's worth its weight in gold." Find out more
Alexander (Sandy) Dunn
Plant Maintenance Resource Center
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