fluid power
jerry carlin, Eaton Hydraulics Operations
fluid power
Everything you
wanted to know…
…about valves but were too afraid to ask. Take heart with
our pulsating guide to controlling the pressure and flow of the
lifeblood of every materials handling machine
W
hile today’s hydraulic
systems are highly
sophisticated, they are
based on relatively simple
devices operating on a few
easily understood principles.
In its simplest form, a hydraulic system
consists of a pump driven by an external
power source that uses pistons, vanes or
gears to pressurise a fluid and move it
through the system in paths determined
by valves to power cylinders, motors,
and other devices that transform the
flow of pressurised fluid into motion to
do useful work.
In lift-trucks and ground support
equipment, that useful work typically
includes manipulating a mast or other
load-handling device, steering the
vehicle and, in many cases, providing
the energy to propel it as well. Each of
these functions, in turn, depends on
precise control of the flow of hydraulic
fluid within the system, and that
control is provided almost entirely
by the valves.
While the operating
principles of pumps, motors
and cylinders are
intuitively understood,
valves tend to be more
mysterious. In reality
however they are no
more difficult to
understand than
other system
components.
Valves may
be broadly
categorised by
function.
They either
control flow
rate,
pressure,
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direction, or some combination of these
attributes. Flow- and pressure-control
valves are generally based on metering
the fluid through an orifice or other
restriction at an adjustable rate. A
simple needle valve is a good example of
this technology, although in today’s
systems the control capabilities are
much more sophisticated.
Directional-control valves are used to
control the path the fluid takes through
the system. Raising and lowering the
mast or forks of a lift-truck, for example,
is controlled by a directional valve, as
are virtually all other motions produced
by the system. Even steering-control
units are basically rotary directionalcontrol valves.
At the most fundamental level, there
are two typical forms of directional
valves – those that control the flow of
the fluid by moving a spool within a
bore to selectively connect different
ports, and those that control the flow
with a poppet or poppets. Directional
valves are also distinguished by the way
in which the control element is actuated,
with the most common options being
manual, hydraulic and electric.
Spool-type valves
In a spool-type valve, fluid is routed
from the external ports through the
valve body to cavities or openings in
the wall of a central bore. The simplest
configuration is a two-port valve
having only an inlet and an outlet. A
cylindrical, linearly movable spool
within the bore controls flow from the
inlet to the outlet.
The simplest spool has a groove, or
metering notch, machined into its
diameter that is of sufficient length to
allow fluid to flow between the two
openings when it is properly positioned.
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RIGHT ANGLE CHECK
Pilot stage
flow path
also incorporate additional metering
edges to dump the pump flow to tank at
low pressure drop in neutral, and restrict
the pump’s flow path to the reservoir
when the spool is displaced to build
enough pressure to move the cylinder.
The basic operating principle can be
extended almost indefinitely by adding
ports, orifices and grooves to the valve.
And, as we will shortly see, it need not
be limited to a single manual actuator
either. But regardless of how complex
the valve becomes, the basic operation
remains the same.
1,000 psi
valve setting
980 psi pilot stage pressure
1,000 psi system pressure
From pump
To system
Main system
flow
Check valve with
orifice
From top: A simple right-angle
check valve permits flow in
one direction only. This is a
very simple poppet-type valve;
a simple pressure-relief valve
opens when system pressure
overcomes the spring force; a
more complex proportional
pressure-relief valve with
electronic pilot control
from top: A balanced piston-relief valve
responds to a hydraulic pilot signal; a
simple needle valve controls flow through
the cylinder in one direction – note the
check valve under the flow control that
allows unimpeded flow in the opposite
direction; a more complex flow-control
valve provides automatic regulation of
cylinder travel
It is usually biased to one position by a
spring, and displaced to the other
extreme position by a manual lever or
other actuation means.
If the lever is pushed to move the
spool and compress the spring, the
spool edges or metering notches either
open or close the internal openings in
the valve body, depending on the spool
and body design and the initial spool
position. When the lever is released, the
spring forces the spool back into the
initial position.
This type of valve would typically be
used to turn an actuator, such as a
unidirectional hydraulic motor, on and
off. It could also be used to connect or
disconnect a function or portion of the
system to supply pressure.
In the next level of spool valve
complexity, four ports and internal
cavities are employed in the valve body,
in combination with two spool
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positions and at least two grooves and
pairs of metering edges in the spool.
This type of valve is commonly used to
control rotation of a bi-directional
hydraulic motor that does not require
an unpowered position. Such a valve
can also be used to automatically apply
pressure to one side of a cylinder piston,
while opening a drain path back to the
reservoir on the other side.
A double-acting hydraulic cylinder
normally requires a three-position spool
valve to provide the ability to hold it
stationary, as well as to control its
direction and speed. This necessitates
at least four ports and multiple spool
grooves and metering edges. The
metering edges control pump inlet
pressure as well as flow into the cylinder
ports and out of the cylinder ports to
the reservoir.
When the spool lever is pushed by
its actuation means, the pressure/drain
circuit is completed, causing cylinder
movement in the ‘up’ direction, and
when the lever is pulled it is completed
and cylinder movement occurs in the
opposite direction. A captured spring
mechanism is normally employed to
return the spool to the centre or neutral
position when the spool actuating force
is removed.
The number of internal metering
edges or orifices in a three-position,
four-port, spool-type directional-control
valve depends on the circuit design:
• In a closed-centre circuit design, the
inlet pressure region of the valve spool
must only block pump flow in neutral
and direct it to the appropriate work
port when the spool is displaced.
• A load-sensing circuit adds the
requirement to drain a signal port in the
neutral position and connect it to load
pressure as the spool is displaced.
• An open-centre circuit allows the use of
an economical fixed-displacement pump,
but it means that the control valve must
Poppet-type valves
In a poppet-type valve, incoming fluid
is routed through an internal opening
which is closed by the poppet
mechanism; typically a spring- and/or
hydraulically-loaded ball or conical
plug. Fluid flow through the valve is
controlled by the opening or closing of
the poppet, which may be accomplished
with a lever or other mechanical device,
hydraulic or pneumatic pressure, a
solenoid, or a combination of methods.
In a normally closed configuration,
the poppet is held against the seat by
spring force and/or pressure. Moving
the poppet through mechanical force,
application of pressure, or venting of
pressure opens the flow path from inlet
to outlet. In a normally open
configuration, this state and sequence
are reversed and the actuation force
closes the poppet.
If the double-acting cylinder circuit
described in the preceding example
were controlled with poppet valves, two
P
Proportional pilot stage
T
DR
Back-up pilot stage
Main stage relief
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below: Spool-type directional valves are
often manually actuated. Note the spring
that returns the spool to the neutral
position when the lever is released
below right: Solenoids are another
common method of actuating spool-type
valves. This is a simple EH control that
can be energised by a pushbutton,
switch, or sensor
T
B
P
A
Coil
Push pin
Armature
would be required for each direction.
Assuming a normally closed valve
configuration, one valve would
pressurise one side of the piston in the
cylinder, while the other would open
the flow path back to the reservoir for
fluid from the opposite side of the
piston. As is the case with spool-type
valves, poppet valves can be configured
to combine both fluid supply and a
return path in a single valve to simplify
circuit design.
Because poppet-type valves use a
metering element having metal-to-metal
contact with the seat in their closed
position, they are capable of much less
internal leakage in the ‘off’ position
than a spool-type valve. This can be
important in situations such as holding
the position of a loaded lift-truck mast,
where any leakage in the valve will
permit the load to drift down, possibly
necessitating the addition of a separate
‘lock’ valve in the circuit.
Internal leakage in a spool valve is
directly related to the precision of the
fit between the spool and the body.
Linear sealing overlap is another factor
affecting internal leakage, but it is of
lesser significance than the clearance
between the outside diameter of the
spool and the internal diameter of the
body. Of course, decreased spool and
bore tolerances and a reduced clearance
may add further manufacturing costs
and affect the valve’s sensitivity to
contaminants.
Spool-type valves can be designed to
provide smooth, meterable operation of
a cylinder or hydraulic motor, possibly
eliminating the need for additional
metering or cushioning devices in the
system. On the other hand, poppet-type
valves may require special means to
achieve fine metering.
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A
In the end, the choice of valve type is
dictated by system requirements and
application parameters. It is definitely
not the case that one type of valve is
better than the other – they are simply
different. Both have advantages for
particular applications.
Valve actuation options
Manual operation of directional valves
is appropriate for systems in which
minimum cost is an important goal,
operator effort and fatigue are not
critical concerns, and closed-loop
control or monitoring systems are not
required. However, even though
functions such as mast raising and
lowering may be initiated with a lever
or cable, the actual control signals are
very often hydraulic, electric or some
combination of the two.
The basic idea of a ‘pilot’ actuation
scheme is to use a small force or remote
signal to control a larger force. In a
spool-type valve, for example, a
relatively low hydraulic pilot pressure
can be applied to one or both ends of
the spool to produce the force necessary
to shift the valve spool. Instead of
directly moving the spool in the
directional valve against spring load,
friction forces and fluid momentum
P
B
T
forces, a small, low-effort operator lever
controls a pressure-reducing valve that
develops and applies the pilot pressure
to the directional valve.
If hydraulics are the lifeblood of
modern systems, electronics are the
brains, and this combination is going to
change the way we do everything.
Controlling the pilot pressure
electronically, with an electrohydraulic
proportional, pressure-reducing valve
for example, opens a whole range of
feedback and control options not
readily available with manual control or
pure hydraulic piloting. Such
‘electrohydraulic’ systems can be
extremely sophisticated, taking full
advantage of electronic sensors and
computer logic to achieve
unprecedented levels of precision and
efficiency. If the directional valve is
relatively small, its valve spool may also
be electrically shifted directly by a
solenoid or linear actuator.
Practical applications
Probably the most universal function on
a typical lift-truck is raising and
lowering a mast with a cylinder. As we
have seen, this can be accomplished
with either a spool-type or poppet-type
directional valve.
The simplest circuits use a manually
actuated spool-type valve to power the
cylinder up, and depend on gravity to
lower it. A slightly more sophisticated
system might incorporate a loadlowering valve to limit mast speed based
on load, as indicated by cylinder pressure.
As mentioned above, a mast control
circuit based on poppet-type valves may
eliminate the need for a separate valve
to control cylinder drift. It may,
however, be slightly more expensive
than the spool-type option.
The fork tilt function on a typical
lift-truck relies on a counterbalance
valve to maintain control of gravity
loads by metering the fluid flow to
maintain positive supply pressure to the
cylinders. This function can be built
into the spool-type valve, or supplied by
an additional metering valve placed in
the circuit between the control valve
and the cylinder, or even installed on
the cylinder itself. Auxiliary functions
such as rotation and fork tilt can also be
controlled with relatively simple circuits
based on spool-type valves.
Pushing or pulling the control
lever connects different ports
inside the spool-type valve
Valve centred
Valve shaft left
Power steering is
another very common hydraulic
application on lift-trucks and GSE.
As steering is safety related, it needs to
have priority over other vehicle
functions such as mast raising. This is
often accomplished with a priority valve
placed in-line between the pump and
the control valve.
In a system having an open-centre
steering unit, the priority valve will
always send a fixed flow to steering, with
the remaining flow available to other
functions. If a load-sensing steering unit
is used, the priority valve senses steering
demand and proportions fluid flow
between the steering circuit and other
vehicle functions, ensuring that the
steering flow requirement is met before
flow can proceed to other functions.
The steering valve operates like a
spool-type valve, except that the spool
rotates rather than moving linearly. This
is mounted inside a sleeve containing
holes or slots that connect pressure and
reservoir to the appropriate steering
ports as the spool is rotated. In a loadsensing unit, as steering demand is
created, a pressure-sensing line carries
the signal to the priority valve which
diverts enough flow to meet the demand.
As noted above, steering valves may
be open-centre or closed-centre types,
with closed-centre load-sensing systems
typically providing better control and
considerably more efficiency. Steering
valves also typically contain an integral
metering pump to provide manually
powered fluid pressure in case of engine
or hydraulic pump failure.
Emerging trends
Valve shaft right
Adding electrohydraulic control
capabilities to a hydraulic system greatly
enhances the performance of both the
system and the entire vehicle. Replacing
lever-actuated directional valves with
electrically operated directional valves
controlled by an electronic joystick, for
example, reduces operator fatigue by
minimising the amount of force
required to initiate and control a
machine function while simultaneously
improving operational precision.
Electronic sensors and controls are
now available onboard the latest valves,
further integrating the electronic and
hydraulic functions. Integrated sensors
can be much more reliable than addons, and onboard electronics offer
closed-loop control and diagnostic
possibilities as well as CANbus capability.
Electrohydraulic valves having
integrated sensors and controls also give
OEMs the ability to tailor system
performance via software. In practical
terms, that means the same valve can
perform differently in different vehicles,
can be adapted to the skill level or
desires of the individual operator, and
can change its control mode in response
to actual vehicle operating conditions.
This ability of one valve to perform
many programmable functions can
reduce costs for both OEMs and end
users by minimising spare parts
inventories and training requirements.
Electrohydraulic systems enable
advanced capabilities such as load
monitoring, closed-loop control and
whole vehicle integration to improve
operating efficiency. With the addition
of sensors, onboard networks and
computer controls, it is now possible to
coordinate operation of the engine and
hydraulic system in real time to optimise
efficiency while minimising emissions.
This capability will be increasingly
important as global emission standards
continue to tighten in the coming years.
It is worth remembering, however,
that no matter how sophisticated these
systems become, they will still be based
on relatively simple devices operating
on well-understood principles. That part
of the equation has not changed, and
will not change, no matter how
sophisticated the end applications of
hydraulics on lift-trucks and ground
support equipment may become. iVT
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