A hydraulic actuator receives pressure
energy and converts it to mechanical force and motion. An
actuator can be linear or rotary. A linear actuator gives force
and motion outputs in a straight line. It is more commonly called
a cylinder but is also referred to as a ram, reciprocating motor,
or linear motor. A rotary actuator produces torque and rotating
motion. It is more commonly called a hydraulic motor or motor.
4-1. Cylinders. A cylinder
is a hydraulic actuator that is constructed of a piston or
plunger that operates in a cylindrical housing by the action of
liquid under pressure. Figure 4-1 shows the basic parts of a
cylinder. A cylinder housing is a tube in which a plunger
(piston) operates. In a ram-type cylinder, a ram actuates a load
directly. In a piston cylinder, a piston rod is connected to a
piston to actuate a load. An end of a cylinder from which a rod
or plunger protrudes is a rod end. The opposite end is a head
end. The hydraulic connections are a head-end port and a rod-end
port (fluid supply).
- a. Single-Acting Cylinder.
This cylinder (Figure 4-1) only has a head-end
port and is operated hydraulically in one
direction. When oil is pumped into a port, it
pushes on a plunger, thus extending it. To return
or retract a cylinder, oil must be released to a
reservoir. A plunger returns either because of
the weight of a load or from some mechanical
force such as a spring. In mobile equipment, flow
to and from a single-acting cylinder is
controlled by a reversing directional valve of a
single-acting type.
- b. Double-Acting Cylinder. This
cylinder (Figure 4-2 must have ports at the head
and rod ends. Pumping oil into the head end moves
a piston to extend a rod while any oil in the rod
end is pushed out and returned to a reservoir. To
retract a rod, flow is reversed. Oil from a pump
goes into a rod end, and a head-end port is
connected to allow return flow. The flow
direction to and from a double-acting cylinder
can be controlled by a double-acting directional
valve or by actuating a control of a reversible
pump.
- c. Differential Cylinder.
In a differential cylinder, the areas where
pressure is applied on a piston are not equal. On
a head end, a full piston area is available for
applying pressure. At a rod end, only an annular
area is available for applying pressure. A rod's
area is not a factor, and what space it does take
up reduces the volume of oil it will hold. Two
general rules about a differential cylinder are
that-
- d. Nondifferential Cylinder.
This cylinder (Figure 4-3) has a piston rod
extending from each end. It has equal thrust and
speed either way, provided that pressure and flow
are unchanged. A nondifferential cylinder is
rarely used on mobile equipment.
- e. Ram-Type Cylinder. A
ram-type cylinder is a cylinder in which a
cross-sectional area of a piston rod is more than
one-half a cross-sectional area of a piston head.
In many cylinders of this type, the rod and
piston heads have equal areas. A ram-type
actuating cylinder is used mainly for push
functions rather than pull.
- Figure 4-1 shows a
single-acting, ram-type cylinder. A single-acting
ram applies force in one direction. This cylinder
is often used in a hydraulic jack. In a
double-acting, ram-type cylinder, both strokes of
a ram are produced by pressurized fluid. Figure 4-2 shows
this cylinder.
- Figure 4-4 shows a telescoping,
ram-type, actuating cylinder, which can be a
single- or double-acting type. In this cylinder,
a series of rams are nested in a telescoping
assembly. Except for the smallest ram, each ram
is hollow and serves as a cylinder housing for
the next smaller ram. A ram assembly is contained
in a main cylinder housing, which also provides
the fluid ports. Although an assembly requires a
small space with all of the rams retracted, a
telescoping action of an assembly provides a
relatively long stroke when the rams are
extended.
- f. Piston-Type Cylinder.
In this cylinder, a cross-sectional area of a
piston head is referred to as a piston-type
cylinder. A piston-type cylinder is used mainly
when the push and pull functions are needed.
- A single-acting, piston-type
cylinder uses fluid pressure to apply force in
one direction. In some designs, the force of
gravity moves a piston in the opposite direction.
However, most cylinders of this type apply force
in both directions. Fluid pressure provides force
in one direction and spring tension provides
force in the opposite direction.
- Figure 4-5 shows a
single-acting, spring-loaded, piston-type
cylinder. In this cylinder, a spring is located
on the rod side of a piston. In some
spring-loaded cylinders, a spring is located on a
blank side, and a fluid port is on a rod end of a
cylinder.
- Most piston-type cylinders are
double-acting, which means that fluid under
pressure can be applied to either side of a
piston to provide movement and apply force in a
corresponding direction. Figure 4-6 shows a
double-acting piston-type cylinder. This cylinder
contains one piston and piston-rod assembly and
operates from fluid flow in either direction. The
two fluid ports, one near each end of a cylinder,
alternate as an inlet and an outlet, depending on
the directional-control valve flow direction.
This is an unbalanced cylinder, which means that
there is a difference in the effective working
area on the two sides of a piston. A cylinder is
normally installed so that the head end of a
piston carries the greater load; that is, a
cylinder carries the greater load during a
piston-rod extension stroke.
- Figure 4-6 shows a balanced,
double-acting, piston-type cylinder. The
effective working area on both sides of a piston
is the same, and it exerts the same force in both
directions.
- g. Cushioned Cylinder. To
slow an action and prevent shock at the end of a
piston stroke, some actuating cylinders are
constructed with a cushioning device at either or
both ends of a cylinder. This cushion is usually
a metering device built into a cylinder to
restrict the flow at an outlet port, thereby
slowing down the motion of a piston. Figure 4-7
shows a cushioned actuating cylinder.
- h. Lockout Cylinders. A
lockout cylinder is used to lock a suspension
mechanism of a tracked vehicle when a vehicle
functions as a stable platform. A cylinder also
serves as a shock absorber when a vehicle is
moving. Each lockout cylinder is connected to a
road arm by a control lever. When each road wheel
moves up, a control lever forces the respective
cylinder to compress. Hydraulic fluid is forced
around a piston head through restrictor ports
causing a cylinder to act as a shock absorber.
When hydraulic pressure is applied to an inlet
port on each cylinder's connecting eye, an inner
control-valve piston is forced against a spring
in each cylinder. This action closes the
restrictor ports, blocks the main piston's motion
in each cylinder, and locks the suspension
system.
4-2. Construction and Application.
A cylinder is constructed of a barrel or tube, a piston and rod
(or ram), two end caps, and suitable oil seals. A barrel is
usually seamless steel tubing, or cast, and the interior is
finished very true and smoothly. A steel piston rod is highly
polished and usually hard chrome-plated to resist pitting and
scoring. It is supported in the end cap by a bushing or polished
surface.
- The cylinder's ports are built
into the end caps, which can be screwed on to the
tubes, welded, or attached by tie bolts or bolted
flanges. If the cylinder barrel is cast, the
head-end cap may be integral with it. Mounting
provisions often are made in the end caps,
including flanges for stationary mounting or
clevises for swinging mounts.
- Seals and wipers are installed
in the rod's end cap to keep the rod clean and to
prevent external leakage around the rod. Other
points where seals are used are at the end cap
and joints and between the piston and barrel.
Depending on how the rod is attached to the
piston, a seal may be needed. Internal leakage
should not occur past a piston. It wastes energy
and can stop a load by a hydrostatic lock (oil
trapped behind a piston).
- Figure 4-8 shows
force-and-motion applications of cylinders.
Because fluid power systems have many
requirements, actuating cylinders are available
in different shapes and sizes. A cylinder-type
actuator is versatile and may be the most
trouble-free component of fluid-powered systems.
A cylinder and a mechanical member of a unit to
be actuated must be aligned correctly. Any
misalignment will cause excessive wear of a
piston, a piston rod, and the seals. Also, a
piston rod and an actuating unit must stay
properly adjusted. Clean the exposed ends of the
piston rods to ensure that foreign matter does
not get into the cylinders.
4-3. Maintenance. Hydraulic
cylinders are compact and relatively simple. The key points to
watch are the seals and pivots. The following lists service tips
in maintaining cylinders:
- a. External Leakage. If
a cylinder's end caps are leaking, tighten them.
If the leaks still do not stop, replace the
gasket. If a cylinder leaks around a piston rod,
replace the packing. Make sure that a seal lip
faces toward the pressure oil. If a seal
continues to leak, check paragraphs 4-3e
through i.
- b. Internal Leakage.
Leakage past the piston seals inside a cylinder
can cause sluggish movement or settling under
load. Piston leakage can be caused by worn piston
seals or rings or scored cylinder walls. The
latter may be caused by dirt and grit in the oil.
NOTE: When repairing a
cylinder, replace all the seals and packings
before reassembly.
- c. Creeping Cylinder. If
a cylinder creeps when stopped in midstroke,
check for internal leakage (paragraph 4-3b).
Another cause could be a worn control valve.
- d. Sluggish Operation. Air
in a cylinder is the most common cause of
sluggish action. Internal leakage in a cylinder
is another cause. If an action is sluggish when
starting up a system, but speeds up when a system
is warm, check for oil of too high a viscosity
(see the machine's operating manual). If a
cylinder is still sluggish after these checks,
test the whole circuit for worn components.
- e. Loose Mounting.
Pivot points and mounts may be loose. The bolts
or pins may need to be tightened, or they may be
worn out. Too much slop or float in a cylinder's
mountings damages the piston-rod seals.
Periodically check all the cylinders for loose
mountings.
- f. Misalignment. Piston
rods must work in-line at all times. If they are
side-loaded, the piston rods will be galled and
the packings will be damaged, causing leaks.
Eventually, the piston rods may be bent or the
welds broken.
- g. Lack of Lubrication.
If a piston rod has no lubrication, a rod packing
could seize, which would result in an erratic
stroke, especially on single-acting cylinders.
- h. Abrasives on a Piston
Rod. When a piston rod extends, it can pick
up dirt and other material. When it retracts, it
carries the grit into a cylinder, damaging a rod
seal. For this reason, rod wipers are often used
at the rod end of a cylinder to clean the rod as
it retracts. Rubber boots are also used over the
end of a cylinder in some cases. Piston rods
rusting is another problem. When storing
cylinders, always retract the piston rods to
protect them. If you cannot retract them, coat
them with grease.
- i. Burrs on a Piston Rod. Exposed
piston rods can be damaged by impact with hard
objects. If a smooth surface of a rod is marred,
a rod seal may be damaged. Clean the burrs on a
rod immediately, using crocus cloth. Some rods
are chrome-plated to resist wear. Replace the
seals after restoring a rod surface.
- j. Air Vents. Single-acting
cylinders (except ram types) must have an air
vent in the dry side of a cylinder. To prevent
dirt from getting in, use different filter
devices. Most are self-cleaning, but inspect them
periodically to ensure that they operate
properly.
4-4. Hydraulic Motors.
Hydraulic motors convert hydraulic energy into mechanical energy.
In industrial hydraulic circuits, pumps and motors are normally
combined with a proper valving and piping to form a
hydraulic-powered transmission. A pump, which is mechanically
linked to a prime mover, draws fluid from a reservoir and forces
it to a motor. A motor, which is mechanically linked to the
workload, is actuated by this flow so that motion or torque, or
both, are conveyed to the work. Figure 4-9 shows the basic
operations of a hydraulic motor.
- The principal ratings of a motor
are torque, pressure, and displacement. Torque
and pressure ratings indicate how much load a
motor can handle. Displacement indicates how much
flow is required for a specified drive speed and
is expressed in cubic inches per revolutions, the
same as pump displacement. Displacement is the
amount of oil that must be pumped into a motor to
turn it one revolution. Most motors are
fixed-displacement; however,
variable-displacement piston motors are in use,
mainly in hydrostatic drives. The main types of
motors are gear, vane, and piston. They can be
unidirectional or reversible. (Most motors
designed for mobile equipment are reversible.)
- a. Gear-Type Motors. Figure
4-10 shows a gear-type motor. Both gears are
driven gears, but only one is connected to the
output shaft. Operation is essentially the
reverse of that of a gear pump. Flow from the
pump enters chamber A and flows in either
direction around the inside surface of the
casing, forcing the gears to rotate as indicated.
This rotary motion is then available for work at
the output shaft.
- b. Vane-Type Motors.
Figure 4-11 shows a vane-type motor. Flow from
the pump enters the inlet, forces the rotor and
vanes to rotate, and passes out through the
outlet. Motor rotation causes the output shaft to
rotate. Since no centrifugal force exists until
the motor begins to rotate, something, usually
springs, must be used to initially hold the vanes
against the casing contour. However, springs
usually are not necessary in vane-type pumps
because a drive shaft initially supplies
centrifugal force to ensure vane-to-casing
contact.
- Vane motors are balanced
hydraulically to prevent a rotor from
side-loading a shaft. A shaft is supported by two
ball bearings. Torque is developed by a pressure
difference as oil from a pump is forced through a
motor. Figure 4-12 shows pressure differential on
a single vane as it passes the inlet port. On the
trailing side open to the inlet port, the vane is
subject to full system pressure. The chamber
leading the vane is subject to the much lower
outlet pressure. The difference in pressure
exerts the force on the vane that is, in effect,
tangential to the rotor. This pressure difference
is effective across vanes 3 and 9 as shown in
Figure 4-13. The other vanes are subject to
essentially equal force on both sides. Each will
develop torque as the rotor turns. Figure 4-13
shows the flow condition for counterclockwise
rotation as viewed from the cover end. The body
port is the inlet, and the cover port is the
outlet. Reverse the flow, and the rotation
becomes clockwise.
- In a vane-type pump, the vanes
are pushed out against a cam ring by centrifugal
force when a pump is started up. A design motor
uses steel-wire rocker arms (Figure 4-14) to push
the vanes against the cam ring. The arms pivot on
pins attached to the rotor. The ends of each arm
support two vanes that are 90 degrees apart. When
the cam ring pushes vane A into its slot, vane B
slides out. The reverse also happens. Amotor's
pressure plate functions the same as a pump's. It
seals the side of a rotor and ring against
internal leakage, and it feeds system pressure
under the vanes to hold them out against a ring.
This is a simple operation in a pump because a
pressure plate is right by a high-pressure port
in the cover.
- c. Piston-Type Motors.
Piston-type motors can be in-line-axis or
bent-axis types.
- (1) In-Line-Axis, Piston-Type
Motors. These motors (Figure 4-15) are almost
identical to the pumps. They are built-in, fixed-
and variable-displacement models in several
sizes. Torque is developed by a pressure drop
through a motor. Pressure exerts a force on the
ends of the pistons, which is translated into
shaft rotation. Shaft rotation of most models can
be reversed anytime by reversing the flow
direction.
- Oil from a pump is forced into
the cylinder bores through a motor's inlet port.
Force on the pistons at this point pushes them
against a swash plate. They can move only by
sliding along a swash plate to a point further
away from a cylinder's barrel, which causes it to
rotate. The barrel is then splined to a shaft so
that it must turn.
- A motor's displacement depends
on the angle of a swash plate (Figure 4-16). At
maximum angle, displacement is at its highest
because the pistons travel at maximum length.
When the angle is reduced, piston travel
shortens, reducing displacement. If flow remains
constant, a motor runs faster, but torque is
decreased. Torque is greatest at maximum
displacement because the component of piston
force parallel to a swash plate is greatest.
- (2) Bent-Axis, Piston-Type
Motors. These motors are almost identical to the
pumps. They are available in fixed- and
variable-displacement models (Figure 4-17), in
several sizes. Variable-displacement motors can
be controlled mechanically or by pressure
compensation. These motors operate similarly to
in-line motors except that piston thrust is
against a drive-shaft flange. A parallel
component of thrust causes a flange to turn.
Torque is maximum at maximum displacement; speed
is at a minimum. This design piston motor is very
heavy and bulky, particularly the
variable-displacement motor. Using these motors
on mobile equipment is limited.
- Although some piston-type motors
are controlled by directional-control valves,
they are often used in combination with
variable-displacement pumps. This pump-motor
combination (hydraulic transmission) is used to
provide a transfer of power between a driving
element, such as an electric motor, and a driven
element. Hydraulic transmissions may be used for
applications such as a speed reducer, variable
speed drive, constant speed or constant torque
drive, and torque converter. Some advantages a
hydraulic transmission has over a mechanical
transmission is that it has-
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