Reference material 1 Graphic symbols for fluid power

(1) Hydraulic graphic symbols (JIS, ISO, UTS)

● Basics of graphic symbols ● Pipelines and connections 27 Quick-action
1 Pipeline 15 Main line couplings

(1) Removed state

1 Without
non-return valve
2 Connection point 16 Pilot line
of pipeline and 2 With non-return
passage valve (Self-sealing
coupling)

3 Shaft, lever, rod 17 Drain line (2) Mounted state

1 Without
non-return valve
2 With one-sided
check valve
4 Pump, 18 Connection of (Self-sealing
compressor, pipelines coupling)
motor, pressure 3 With double-sided
source check valve
(Self-sealing
5 Gauge 19 Flexible line coupling)

28 Swivel joint

(1) For one pipeline

6 Non-return valve, 20 Crossing of or
swivel joint pipelines (2) For three pipelines
(coupling)

29 Mechanical joints
7 Link connector 21 Flow directions
part (1) Rotating shaft
Roller (1) Hydraulic flow

(2) Pneumatic flow

8 Valve
22 Flow direction in
valve
(2) Lever, rod

23 Pipelines
connected to tank (3) Joint part

(1) Pipeline which edge

9 Flow direction does not enter fluid
Fluid inlet and (Fluid)
outlet (2) Pipeline which edge
(Air)
enters fluid

10 Flow direction (3) Pipelines connected

to head tank (4) Joint with fixed point

24 Vent line
11 Rotation direction
30 Signal transmission
paths

25 Outlet port (1) Electrical signal

12 Filter
Heat exchanger (1) Closed state
Lubricator
Water trap (2) Opened (connected
state) (2) Other signals

13 Assembling unit
26 Fixed restrictor

(1) Choke

14 Adjustable case (2) Orifice

(Note) Detailed symbol, Mnemonic symbol

Hydraulics Basic Training Material 1

● Pumps and motors ● Cylinders 49 Input system
31 Fixed displacement 38 Single-acting
hydraulic pumps cylinders (1) Input system (basic
symbol)
(A) One directional (A)
flow (1) Without spring (2) Lever system

(B)Two directional (B) (3) Push button system

flow (2) With spring
(4) Pedal system

32 Variable
(3) Ram cylinder 50 Mechanical system
displacement
hydraulic pumps (A)
39 Double-acting (1) Push rod system
cylinders
(B) (2) Spring system
(1) Single rod
(3) Roller system

33 Fixed (4) Pulling roller system

(2) Double rod
displacement
hydraulic motors (A)
40 Cushioned 51 Cylinder systems
cylinder Single-acting type
(B)
(1) Single sided type (1) Without spring

34 Variable (2) With spring

(2) Double sided type
displacement
hydraulic motors (A)
41 Differential (3) Double-acting type
cylinders
(B)
52 Solenoid
operated system
42 Telescopic
35 Fixed displacement (1) Single-coil type
cylinders
hydraulic pump/
(1) Single-acting
motors (2) Double-coil type
(2) Double-acting
(C)Both the pump (C)
and the motor are 53 Hydraulic motor
in the same one 43 Diaphragm type systems
direction cylinders
(1) One-way type

(D)The pump is in (2) Two-way type

(D)
one flow direction, 44 Pressure
and the motor is transducers 54 Electric motor
in the opposite systems
flow direction (1) Similar fluid
(1) One-way type

(E)Both the pump (E) (2) Different fluid (2) Two-way type
and the motor
flow in both 55 Combined systems
directions 45 Air-oil actuator
(1) Sequence operated
36 Variable system
displacement 1 Electro-hydraulic
hydraulic pump/ (C) control
motors 2 Electro-
pneumatic control
● Control methods
(D)
(2) Selection operated
46 Spring type
system
1 Electric or
(E) hydraulic control
(ISO) 2 Electric or
pneumatic control
47 Adjustable spring
type
37 Semi-rotary 56 Support systems
actuators (ISO)
(1) Detent system
48 Pilot types
(A) (B) (2) Lock system
(1) Direct operation type
(3) Over-center system
(2) Indirect operation type (A) (A)
(A)Pressurized
control
(B)Depressurized (B)
control

(Note) Detailed symbol, Mnemonic symbol

Hydraulics Basic Training Material 2

● Pressure control valves 66 Flow control valves (2) Input system
57 Basic display (with pressure 1Spring offset
compensation)
or
(1) Normally closed (1) Fixed type 2Spring-centered

3Detent
(2) Normally open (2) Variable types
or 1 Variable type
(basic signal)

58 Press relief valves 2With relief (3) Solenoid operated

and safety valves systems
(1) Internal pilot control 1No spring
system 3With temperature
(2) External pilot control compensation 2Spring offset
system
3Spring-centered
59 Proportional 67 Flow dividing
pressure relief valve 4Push button
valve

60 Unloading valve (4) Solenoid pilot systems

1No spring

● Directional-control valves
61 Sequence valve 68 Basic display
(1) Internal pilot control (1) Two-port
system two-position directional
control valve
(2) External pilot control 2Spring offset
system (2) Four-port
62 Pressure- three-position
reducing valves directional control valve

(1) Internal pilot control (3) Four-port

system without relief restrictor

(2) External pilot control

3Spring-centered
system without relief Graphic symbols for
central positions
(3) Internal pilot control A B
system with relief Closed PR port
center connection
(4) External pilot control
P R(T)
system with relief Open PAB port
center connection
63 Fixed differential 4Pressure-centered
reducing valve Open
BR port two position
center with
connection
restrictor
ABR port
64 Proportional connection PA port
pressure with connection
reducing valve restrictor

ABR port AR port 5Pressure-centered

connection connection three position

● Flow control valves PAR port

65 Variable throttles connection

(1) Input system (Note) R is expressed as T. (UTS)

(Note) Restrictor means a valve which can
consecutively switch flow directions.
(2) Mechanical system 69 Control systems 70 Expressions for
(illustration of roller central positions
system) (1) External pilot control
systems (1) Two-position directional
1No spring control valve

2Spring offset (2) Three-position

directional control valve
3Spring-centered When the transitional
central positions need to be
4Pressure-centered indicated, use dotted
partition lines.

(Note) Detailed symbol, Mnemonic symbol

Hydraulics Basic Training Material 3

 71 Restrictors 83 Pressure source

(1) Two-port restrictor

(2) Three-port restrictor

84 Filters
(3) Four-port restrictor
(1) Reservoir type filter
(4) Electro-hydraulic
servo-valve (2) Line filter
One-stage direct
acting type
85 Thermostat

● Non-return valves
72 Non-return valve 86 Cooler
(check valve) (oil cooler)

When indicating
73 Pilot pipings for
controlled check coolant
valves

(1) When opened by a 87 Heater

control signal (oil heater)

(2) When closed by a

control signal
88 Pressure gauge

74 Check valve with

fixed restrictor

89 Pressure gage
with a contact
75 Shuttle valve point

90 Thermometer (oil
temperature
76 Rapid exhaust gauge)
valve
(ISO)
91 Flow meter

(1) Momentary indicator

system
● Auxiliary components (2)
Integrated indicator
77 Oil tank
system
(1) Release tank

(2) Pilot pressure tank

● Simplified graphics
78 Stop valve or cock 92 Gauges

79 Pressure switch 93 Valves

80 Accumulator

81 Motor

82 Internal
combustion
engine
Other heat
engines

(Note) Detailed symbol, Mnemonic symbol

Hydraulics Basic Training Material 4

(2) Other symbols

1) Directional-control valves 2) Non-return valves, shuttle valves

1 Two-port valve Detailed symbol General symbol 1 Non-return
Detailed symbol Mnemonic symbol
• Normally closed valve
• Variable throttle (check valve) (1)
(1) Without spring

2 Two-port valve Detailed symbol General symbol

• Normally opened
(2) (2) With spring
• Variable throttle

3 Three-port Detailed symbol General symbol

2 Pilot controlled (1)
Detailed symbol Mnemonic symbol
valve • Normally opened check valve • Close the valve
• Variable throttle (1) with pilot control.
• Without spring
4 Two-port (2)
manually operated • Two positions (2)
• Open the valve with
directional control • Shut off valve pilot control.
valve • With spring
5 Three-port • Two positions 3 High pressure • The inlet of the
prioritized type Detailed symbol Mnemonic symbol higher pressure
manually operated • One transient
directional control position shuttle valve side is connected
valve • Solenoid controlled to the outlet, and
spring return the inlet of the
lower pressure
6 Five-port side is closed.
pilot directional • Two positions
control valve • Two-way pilot
controlled
3) Pressure control valves
7 Four-port Detailed symbol
solenoid • Main valve 1 Pressure relief
controlled pilot Three position valve • Direct-acting or
operated Spring-centered general symbol
directional Internal pilot
control valve
• Pilot valve 2 Unloading
Four-port valve • Direct-acting or
Three positions general symbol
Mnemonic symbol Spring-centered Solenoid • Internal drain
controlled (single-acting
solenoid) 3 Sequence • Direct-acting or
Manual overrideexternal valve general symbol
drain with control • External pilot valve
• External drain
8 Four-port Valve center position underlap 4 Pressure-
restrictors
reducing valves • Direct-acting or
• Three positions general symbol
• Spring-centered
Valve center position overlap • Non-step intermediate
5 Counterbalance
position
valve

9 Servo-valve
• Representative
example
4) Flow control valves
1 Restrictor valve • The mnemonic
Detailed symbol Mnemonic symbol
Variable throttle symbol does not
express the control
method and the
state of the valve.
• |Normally, it is not
fully closed.
2 One way • With variable
restrictor valve throttle
Speed control • Free flow in one
valve direction, and
controlled flow in the
opposite direction.
3 Flow dividing
valve • Arrows indicate
pressure
compensation.

Hydraulics Basic Training Material 5

 biodegradable oil [p. 95]
Reference material 2 Hydraulic terms (index) A hydraulic fluid which is degraded with bacterium or
microbes in nature.
The page number in brackets shows the location the term is
mentioned in this text. bladder accumulator
An accumulator in which liquid and air are separated by a
A flexible bladder.

bleed-off restrictor [p. 76]

absolute pressure A pressure-compensated flow control valve installed in a
An intensity of pressure based on a perfect vacuum. bypass circuit connected to a circuit on the bottom side of a
cylinder.
actuator [p. 58]
A device which transforms fluid energy into mechanical bleed-off system [p. 76]
work. A system to control operation speed by diverging a part of
the flow which is flowing into the actuator to a tank.
aeration
A state or phenomena in which air is mixed or being mixed
in fluid as fine bubbles. C
all ports blocked
cavitation [p. 95]
A flow pattern which all ports are closed in the center
position of the switch valve. A phenomenon when the pressure of flowing fluid is locally
reduced, causing vapors to form from reaching the saturated
A B
vapor pressure or saturated air pressure, and form voids
when the dissolved air separates. When they burst, very
P R A1144589 high pressure may occur locally, and in many cases it makes
noise.
all ports open
A flow pattern which all ports are mutually connected in the chattering
center position of the switch valve. An automatic oscillation phenomenon in relief valves, etc.
where they make relatively loud noises when striking valve
A B
seats. It is also referred to as beep sound generation.

P R A1144588
check valve [p. 69]
axial piston pump [p. 36] (non-return valve)
(axial plunger pump) A valve that allows flow in one direction only, and prevents
flow in the opposite direction.
A piston pump (plunger pump) whose piston’s or plunger’s
direction of reciprocating motion is almost parallel to the choke [p. 64]
central axis of a cylinder block. A restrictor of flow when, in a passage which the area
is reduced, the length of the passage is relatively longer
B compared to the section size. In this case, the pressure
drops are greatly affected by fluid viscosity.

back pressure [p. 73, 75] connector

The pressure which works on the return side of a hydraulic A general term for removable splicing fittings which have a
circuit or the back of a pressure working surface. flow path for connecting pipelines or attaching to containers.

back pressure valve [p. 59] contaminant [p. 102]

(counterbalance valve) Foreign objects, such as dirt or metal chips, mixed in the
A pressure control valve which maintains the back pressure hydraulic fluid.
to prevent the load from dropping.
counterbalance valve
biodegradability [p. 99] (back pressure valve)
A capacity of degradation where 78% or more of the A pressure control valve which maintains the back pressure
hydraulic fluid is degraded in 28 days by bacterium or to prevent the load from dropping.
microbes. In cases where the hydraulic fluid leaks into the
ground or water, its affect on the environment is extremely cracking pressure
low. Microbes in the air are not of a sufficient number to (cracking power)
cause biodegradability, and cannot degrade hydraulic fluid in A pressure with a certain flow rate observed when a valve is
the air or operating oil tank. In addition, when the hydraulic opening, such as a non-return valve or a relief valve, when
fluid is deteriorated, the capacity of biodegradability does the pressure is increasing.
not change.

Hydraulics Basic Training Material 6

cushioned cylinder [p. 54] F
A hydraulic cylinder with functions to absorb shocks at the
end of strokes.
filling valve [p. 65]
Normally, automatic restriction mechanisms are installed to
(prefill valve)
slow down strokes at the end by reducing the hydraulic flow
A valve used in large type presses, etc. that allows flow from
from the cylinder outlet to prevent shocks.
a tank to a hydraulic cylinder in a rapid forwarding process,
cut-in prevents reverse flow from the hydraulic cylinder to the tank
The act of loading a pump by unloading valves, etc. The in a pressing process, and allows free flow in a returning
pressure limit is called the cut in pressure or reloading process.
pressure.
filter [p. 15]
cut-out (strainer)
The act of unloading a pump by unloading valves, etc. The A component which removes solid materials from fluid
pressure limit is called the cut out pressure or unloading through filtration.
pressure.
fire-resistant hydraulic fluid
cylinder [p. 47] A hydraulic fluid which is fire-resistant and prevents fire as
An actuator that provides linear motion of which cylinder much as possible.
force is proportionate to an effective sectional area and a
fixed displacement motor [p. 20]
differential pressure.
(fixed motor)
A hydraulic motor which cannot alter the theoretical inflow
D per one rotation.

fixed displacement pump [p. 31]

deceleration valve [p. 64]
(fixed pump)
A valve which slowly reduces the flow rate by controlling a
A pump which cannot alter the theoretical discharge amount
cam, etc. in order to reduce the speed of an actuator.
per one rotation.
decompression valve [p. 59]
fixed motor
The act of reducing shocks of a circuit that cause damage to
(fixed displacement motor)
machines by slowly relieving pressure in a hydraulic cylinder
A hydraulic motor which cannot alter the theoretical inflow
in a press, etc.
per one rotation.
detent [p. 67]
fixed pump
A mechanism which holds a valve element in a set place
(fixed displacement pump)
with artificial resistance. Movement to a different position
is performed by applying a higher pressure than the A pump which cannot alter the theoretical discharge amount
resistance. per one rotation.

directional-control valve [p. 65] flange connector

A generic term for valves that control flow direction. A connector which uses flanges.

displacement flow control valve [p. 64]

Geometric volume of displacement per one rotation of a (pressure-compensated flow control valve)
positive displacement pump or a motor. A flow control valve which maintains the flow rate to at
• Amount of oil discharged by one axial rotation: pump a certain value with a pressure compensation function
• Amount of oil required for one rotation of an axis: hydraulic regardless of changes in inlet pressures or back pressures.
motor”
flow control valve [p. 64]
dither A generic term for valves that control flow rate.
An oscillation of relatively high frequency given to improve
flow dividing valve [p. 64]
characteristics by reducing the effect of friction or locking in
(flow divider valve)
spool valves, etc.
A valve in which flows fluid in a set ratio regardless of the
double-acting cylinder [p. 51] pressures in each pipeline when dividing the flow into two or
A hydraulic cylinder whose structure is able to supply fluid to more hydraulic lines by a hydraulic power source.
both sides of a piston.
flow-combining valve
drain A valve which combines flows so that a predetermined outlet
A phenomenon where liquids or fluids to be returned to flow rate is maintained regardless of the pressures of the
a tank or a manifold return from the path or pipeline of a two inflow pipelines.
component.

Hydraulics Basic Training Material 7

fluid output J
(fluid power)
The power that fluid has in the outlet side of a component. In jumping phenomenon
hydraulic pressure, it is practically expressed as a product of
A phenomenon in a control valve with pressure
flow rate and pressure.
compensation where the flow rate exceeds a set value
full cut-off transiently, such as the start of flow.
A state when a pump is cut-off, the flow rate becomes zero
(0). L

G leakage
A relatively small amount of flow running where it normally
gasket valve should not run or in undesirable places.
A valve used by attaching it a manifold or a subplate.
load sensing restrictor
gauge pressure A flow control valve installed on directional-control valves
An intensity of pressure based on the atmospheric pressure. that controls the flow of hydraulic pumps.

H M

hydraulic accumulator [p. 2] manifold

A container to accumulate fluid in a pressurized state to use A block to form a path for piping internally and install two or
for things such as an energy source. more components externally.

hydraulic lock meter-in system [p. 73]

(spool lock) A system which controls the operation speed by reducing
A phenomenon in spool valves, etc. where action is disabled the flow rate in the inlet of an actuator.
because the spools are fixed to the valve body (or the meter-out system [p. 74]
sleeve) by being strongly pressed on due to unevenness
A system which controls the operation speed by reducing
of pressure distribution against the axises caused by
the flow rate in the outlet of an actuator.
inequalities of internal flow, etc.

hydraulic motor [p. 39] N

(oil motor)
An actuator which can continuously rotate, used in a
non-return valve [p. 69]
hydraulic circuit.
(check valve)
hydraulic power unit A valve that allows flow in one direction only, and prevents
A hydraulic power source system comprised of pumps, flow in the opposite direction.
driving motors, tanks and relief valves, or a hydraulic system
that adds control valves in addition to the hydraulic power normal position
source system. A valve position where no control force is working.

hydraulic tester [p. 62]

A measuring device used for measuring relief pressures. It normal position normal position
is composed of a hydraulic gauge, connecting nipples for A1144591

measuring hydraulic pressure, and connecting hoses.

O
hydrostatic power transmission
(hydrostatic transmission)
oil motor [p. 39]
Hydraulic power transmission using the pressure energy of
(hydraulic motor)
fluids. It uses positive-displacement pumps and actuators
(hydraulic cylinders or positive displacement motors). An actuator which can continuously rotate, used in a
hydraulic circuit.

I orifice [p. 64]

A restrictor of flow when, in a passage which the area is
reduced, the length of the passage is relatively shorter
intensifier
compared to the section size. In this case, pressure drops
A component used to transform inlet pressure into an outlet are not affected very much by fluid viscosity.
pressure almost proportionately as high.

Hydraulics Basic Training Material 8

overlap pressure relief valve [p. 60]
A state in which a lap allows fluids to flow through a spool A pressure control valve which maintains the pressure in a
valve or other valve when the valve is displaced a bit from its circuit to a set value by bypassing a part or all fluid to the
neutral position and the ports are opened for the first time. return side when the pressure of the circuit reaches a set
value of the valve.
override characteristics [p. 61]
A set of characteristics in which a pressure difference is pressure switch
generated between the pressure when starting oil relief and A device which switches electrical contacts when hydrostatic
a set pressure. pressure reaches a predetermined value.

override pressure pressure-compensated flow control valve [p. 64]

For a pressure control valve, the pressure increase from a (flow control valve)
specified minimum flow rate to a specified maximum flow A flow control valve which maintains the flow rate to at
rate. a certain value with a pressure compensation function
regardless of changes in inlet pressures or back pressures.

P pressure-reducing valve [p. 59]

(reducing valve)
pilot (controlled) directional control valve A pressure control valve which sets the outlet pressure
A directional control valve controlled by fluid pressure used to lower than the inlet pressure regardless on the inlet
as a pilot. pressure.

pilot controlled check valve [p. 70] proportional control valve

(pilot check valve) A valve which can control the output (pressure/flow)
A check valve which uses pilot pressure to open and close proportionate to input signals.
the valve.

port Q
An opening of a flow path that carries operating oil.
quick-action coupling
prefill valve [p. 65] (quick joint)
(filling valve) A connecting joint that can be attached and removed
A valve used in large type presses, etc. that allows flow from quickly, used for hoses and pipes.
a tank to a hydraulic cylinder in a rapid forwarding process,
prevents reverse flow from the hydraulic cylinder to the tank
in a pressing process, and allows free flow in a returning R
process.
radial piston pump [p. 37]
pressure center valve
(radial plunger pump)
A three position switch valve that when control forces are
A piston pump (plunger pump) whose piston’s or plunger’s
removed, it returns to the central position with hydraulic
direction of reciprocating motion is almost perpendicular to
force.
the driving axis.
pressure control valve [p. 58]
rated flow
A generic term for valves that control pressure.
A guaranteed flow rate in a certain condition.
pressure drop
rated pressure
Reduction of hydrostatic pressure due to flow.
The maximum pressure possible to be used continuously.
pressure override characteristics
rated speed of revolution
When pressure oil passes through a pressure control valve
The maximum revolution speed possible to be operated at
(relief valve), the degree which the valve seat member is
continuously by the rated pressure.
opened depends on the passing oil flow. Pressure override
characteristics are phenomena which the seat area changes reducing valve [p. 59]
with increasing flow, and to some extent the pressure of the (pressure-reducing valve)
P port increases with the change.
A pressure control valve which sets the outlet pressure
pressure pulsation [p. 37] to lower than the inlet pressure regardless on the inlet
A nearly periodical variation of pressure that occurs under pressure.
normal operating conditions. This excludes transient reseat pressure
pressure variation.
(reseat power)
The pressure in check valves or relief valves when the
leakage from the valve is reduced to a set amount when the
pressure of the inlet side of the valve drops, and the valve
begins to close.

Hydraulics Basic Training Material 9

restrictor valve [p. 64] spool lock
(throttle valve) (hydraulic lock)
A valve which controls the flow rate with wire drawing. A phenomenon in spool valves, etc. where action is disabled
Normally it refers to valves without pressure compensators. because the spools are fixed to the valve body (or the
sleeve) by being strongly pressed on due to unevenness
restrictor [p. 64] of pressure distribution against the axises caused by
A mechanism which reduces the crosssectional area of flow inequalities of internal flow, etc.
and generates resistance in a pipeline or a fluid path. There
are choke restrictors and orifice restrictors. spool
A comb-like component which is inscribed in a cylinder
reynolds number sliding surface, and opens and closes the flow path by
A dimensionless quantity expressed as Rℓ = uD/v. moving in an axial direction.
Here, u = flow rate (m/s),
D = representative length that represents the size of an spring-centered valve
object (m) A three position switch valve that when control forces are
v = kinematic viscosity (m2/s) removed, it returns to the central position with the spring
force.
rotary joint [p. 110]
A connector which rotates relatively continuous. stack valve
A generic term for banked valves and modular stack valves.
Stack type valves.
S
starting torque
safety valve [p. 59] The minimum torque available from the motor when starting
A valve to limit the maximum pressure of a circuit to prevent from rest under a specified condition.
destruction of devices and pipes.
stick-slip
semi-rotary actuator A phenomena in which movements of sliding surfaced
(rotary motor) become intermittent.
An actuator that performs rotating reciprocal movements
strainer [p. 15]
where the angle of rotation is limited.
(filter)
sequence valve [p. 59] A component which removes solid materials from fluid
A valve which controls the operation order in a circuit with through filtration.
two or more branch circuits with the pressure, etc. in the
subplate
circuit.
A doubling plate to connect with a pipeline by attaching
servo-valve control valves using gaskets.
A valve which controls flow rates or pressure using input
surge pressure
signals such as electricity as a function.
(surge power)
shuttle valve [p. 70] The maximum pressure transitionally increased.
A valve with two inlets and one shared outlet where the
swivel joint [p. 110]
outlet is automatically connected to either inlet by the
influence of inlet pressures. There are two types. One has A rotatable connector.
the inlet of the higher pressure side connected to the outlet,
and the other has the lower pressure side connected to the T
outlet.

single-acting cylinder [p. 49] throttle valve [p. 64]

A hydraulic cylinder whose structure is able to supply fluid to (restrictor valve)
only one side of a piston. A valve which controls the flow rate with wire drawing.
Normally it refers to valves without pressure compensators.
solenoid (controlled) valve [p. 68]
(solenoid valve)
A generic term for solenoid controlled valves or solenoid U
pilot switch valves.
underlap
solenoid controlled pilot operated directional control A state in which a lap allows fluid to flow through a spool
valve valve or other valve when the valve is in neutral position the
A pilot controlled directional control valve with an integrated ports is already open.
solenoid controlled pilot valve.

Hydraulics Basic Training Material 10

unloader valve [p. 59] Z
(unloading valve)
A valve used to unload a pump in certain conditions. For zero lap
example, a pressure control valve that unloads a pump
A state in which a lap allows ports to open and fluids to
when the system pressure reaches a set value, and supplies
flow through a spool valve or other valve when the valve is
pressured fluid again to the system when the system
in the neutral point, the ports are closed, and there is any
pressure drops to a set value.
displacement.
unloading circuit [p. 72]
A circuit in a directional control valve which returns
discharged fluid from a hydraulic pump to an operating oil
tank.

unloading valve [p. 59]

(unloader valve)
A valve used to unload a pump in certain conditions. For
example, a pressure control valve that unloads a pump
when the system pressure reaches a set value, and supplies
pressured fluid again to the system when the system
pressure drops to a set value.

valve center position

The central position in a three position switch valve.

valve center
position
A1144590

valve stand
A control stand which combines valves, gauges, and other
components other than the pressure source.

variable displacement motor

(variable motor)
A hydraulic motor which can alter the theoretical inflow per
one rotation.

variable displacement pump [p. 31]

(variable pump)
A pump which can alter the theoretical discharge amount
per one rotation.

variable motor
(variable displacement motor)
A hydraulic motor which can alter the theoretical inflow per
one rotation.

variable pump [p. 31]

(variable displacement pump)
A pump which can alter the theoretical discharge amount
per one rotation.

vent port
An outlet open to the air.

Hydraulics Basic Training Material 11