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 a hydraulic power unit?
What is hydraulic pump load sensing control?
Load sensing control is so called because the load-induced pressure downstream of the directional control valve (DCV) is sensed and hydraulic pump flow adjusted to maintain a constant pressure drop (and therefore flow) across the DCV.
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 DCV 20% of its stroke. The winch drum turns at five RPM. For clarity, imagine that the DCV's spool-adjusted metering notch 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 (DCV) 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 hydraulic pump flow so that pressure upstream of the orifice increases by a corresponding amount. This keeps the pressure drop across the orifice (DCV) constant, which keeps flow constant and in this case, winch speed constant.
Because the hydraulic 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 hydraulic pump power limiting control?
A constant power or power limiting control operates by reducing the displacement, and therefore flow, from the hydraulic 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 hydraulic pump pressure limiting control?
Pressure limiting control limits the maximum operating pressure of the hydraulic 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.
Hydraulic pump control problem
A new hydraulic pump was ordered for the project from a leading manufacturer. When the hydraulic power unit was commissioned, the power limiting control was not functioning.
When advised of the problem, the manufacturer maintained that the pump 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.
Plug-in hydraulic pump controls
The power limiting control on this particular hydraulic 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 hydraulic pump.
If you examine the two hydraulic pump 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 pump fitted to the hydraulic power unit and it did not have this orifice fitted. I advised the manufacturer and requested that they dispatch one 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 an orifice, fitted it to the pump and handed the hydraulic power unit 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 hydraulic pump discussed above - but only its load sensing and pressure limiting controls. Had the functionality of the power limiting control been tested, the pump 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.
Editor's note: for more information on variable pump controls and how they work, read pages 152 to 170 of: Industrial Hydraulic Control
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