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Understanding Loadsensing Control
Brendan Casey
When things go awry with a piece of hydraulic equipment,
the maintenance technician is usually the <rst on the
scene. For the technician’s troubleshooting efforts to be
effective, he or she must understand how the equipment
operates.
One type of hydraulic control system in widespread use,
but not well understood, is load-sensing control.
Load-sensing describes a type of variable pump control
used in open circuits. It is also termed this because the
load-induced pressure downstream of an ori<ce is sensed
and pump Eow is adjusted to maintain a constant pressure
drop (and therefore Eow) across the ori<ce.
The ori<ce is typically a directional control valve with
proportional Eow characteristics, but a needle valve or even
a <xed ori<ce can be employed, depending on the
application.
Power-saving Control
In hydraulic systems subject to wide Euctuations in Eow
and pressure, load-sensing circuits can save substantial
amounts of input power (Figure 1). In systems where all
available Eow (Q) is continuously converted to useful work,
the amount of input power lost to heat is limited to inherent
ineNciencies.
In systems <tted with <xed displacement pumps where
100 percent of available Eow is required only intermittently,
the remaining Eow not required passes over the system
relief valve and is converted to heat.
This situation is compounded if the load-induced pressure
(p) is less than the set relief pressure - resulting in
additional power loss due to pressure drop across the
metering ori<ce (control valve).
A similar situation occurs in systems <tted with pressurecontrolled (pressure-compensated) variable pumps, where
only a portion of available Eow is required at less than
maximum system pressure. Because this type of control
regulates pump Eow at the maximum pressure setting,
power is lost to heat due to the large pressure drop across
the metering ori<ce.
A load-sensing controlled variable pump largely eliminates
these ineNciencies. The power lost to heat is limited to the
relatively small pressure drop across the metering ori<ce,
which is held constant across the system’s operating
pressure range (see bottom of Figure 1).
Figure 1. Flow-pressure-power Diagrams for Fixed,
Variable and Load-sensing Controlled Pumps (Peter
Rohner)
Load Sensing Circuit
Con<guration
A load-sensing circuit typically has a variable displacement
pump, usually axial-piston design, <tted with a load-sensing
controller, and a directional control valve with an integral
load-signal gallery (Figure 2).
Figure 2. Typical Load-sensing Circuit Schematic
The load-signal gallery (LS, shown in red) is connected to
the load-signal port (X) on the pump controller. The loadsignal gallery in the directional control valve connects the A
and B ports of the control valve sections through a series
of shuttle valves. This ensures the actuator with the
highest load pressure is sensed and fed back to the pump
control.
To understand how the load-sensing pump and directional
control valve operate together, consider a winch being
driven through a manually actuated valve. The operator
summons the winch by moving the spool in the directional
valve 20 percent of its stroke. The winch drum turns at <ve
rpm.
For clarity, imagine that the directional valve is now a <xed
ori<ce. Flow across an ori<ce decreases as the pressure
drop decreases. As load on the winch increases, the loadinduced pressure downstream of the ori<ce (directional
valve) increases. This decreases the pressure drop across
the ori<ce, which means Eow across the ori<ce decreases
and the winch slows down.
Constant Pressure Drop
Equals Constant Flow
In a load-sensing circuit, the load-induced pressure
downstream of the ori<ce (directional valve) is fed back to
the pump control via the load-signal gallery in the
directional control valve.
The load-sensing controller responds to the increase in
load pressure by slightly increasing pump displacement
(Eow) so that pressure upstream of the ori<ce increases by
a corresponding amount. This keeps the pressure drop
across the ori<ce (directional valve) constant, which keeps
Eow constant and in this case, winch speed constant.
The value of the pressure drop or delta p maintained
across the ori<ce (directional valve) is typically 10 to 30 bar
(145 to 435 PSI). When all spools are in the center or
neutral position, the load-signal port is vented to tank and
the pump maintains standby pressure equal to or slightly
higher than the load-sensing control’s delta p setting.
High-end load-sensing directional control valves feature a
pressure compensator at the inlet to each valve section.
The section pressure compensator works with the spoolselected ori<ce opening to maintain a constant Eow rate,
independent of the pressure variations caused by the
operation of multiple functions at the same time. This is
sometimes referred to as “sensitive load-sensing”.
Because the variable pump produces the Eow demanded
only by the actuators, load-sensing control is energy
eNcient (fewer losses to heat) - which may result in lower
oil oxidation rates and longer Euid life, and improves
actuator control.
Load-sensing control also provides constant Eow
independent of pump shaft speed variations. If pump drive
speed decreases, the load-sensing controller will increase
displacement (Eow) to maintain the set delta p across the
directional control valve (ori<ce) until displacement is at
maximum.
Load-sensing pump controls typically incorporate a
pressure limiting control, also referred to as a pressure cutoff or pressure compensator. The pressure compensator
limits maximum operating pressure by reducing pump
displacement to zero when the set pressure is reached.
Read more on hydraulic system best practices:
The Seven Most Common Hydraulic Equipment Mistakes
Symptoms of Common Hydraulic Problems and Their Root
Causes
The Negative Effects of Suction Line Filtration
How Do You Know if You're Using the Right Hydraulic Oil?
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About the Author
Brendan Casey
Brendan Casey has more than 20 years experience in the
maintenance, repair and overhaul of mobile and industrial
equipment. For more information on reducing the operating
cost and increasing the... Read More
Machinery Lubrication (3/2006)
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