Hydraulics

Contents:
• Introduction
• Basic Physics
• Hydro-mechanics (Pascal’s Law)
• Hydro-kinetics
 Discharge and Flow volume
 Losses in pipe flow
 Flow types (Laminar & Turbulent)
• Hydraulic Systems
• Pressure Fluids
 Tasks of Pressure fluids
 Types of Pressure fluids
 Technical data of Pressure fluids
• Hydraulic Pumps
 Types of pumps

 Gear pumps & Types of Gear pumps
 Axial piston pumps & Types of Axial Piston pumps
• Hydraulic Motors
 Types of Hydraulic Motors
 Gear motor
 Annular gear motor
 Radial Piston motors
• Valves
 Check valves
 Simple check valves
 Hydraulic releasable check valves
 Twin check valves
 Directional valves
 Directional Gate Valves
 Directional Seat Valves
 Pressure valves
 Pressure limitation valves (PLV)

 Pressure switch-on valves
 Pressure cut-off valves
 Pressure reduction valves
 Flow valves
 Throttle valves
 Flow regulation valves
• Hydro-pneumatic reservoir
 Classification & P – V characteristics
 Tasks
 Diaphragm reservoirs
 Bubble Reservoirs
• Hydraulic filters
 Suction filters
 Line filters / pressure filters
 Ancillary / return flow filters
 Fillers and ventilation filters

Introduction
• The word "Hydraulic" derives from the Greek word “HYDRO“ (which
means water) and, in a scientific sense, determines the theory
governing stationary and moving liquids (hydrostatics and
hydrodynamics).
• When referring to hydraulics in machines and vehicles, this involves
the practical application of physics concerning power transmission
as well control and regulation technology.
• In order to divide this vast range of applications, the terms
"stationary hydraulics" and "mobile hydraulics" are commonly used.
"Mobile hydraulics" propels itself on wheels or crawler units,
whereas "stationary hydraulics" is located in one position.
• Although the equipment for these two systems differs enormously,
their boundaries cannot be clearly marked.
• NOTE: A machine based on mobile hydraulics can consist of mobile
hydraulics and stationary hydraulics.

Basic Physics
• Mass (m): Amt. of matter possessed by body
• Unit: Kilograms (kg)
• Force (F): Quantity resembled by product of mass(m) and
acceleration(a), where generally, ‘a’ being replaced by ‘g’.
• Unit: Newton (N)
• Pressure (p): Force per unit transverse area
• Unit: Pascal (Pa) *generally measured in ‘bar’
• Energy (E): Capacity of object to independently carryout work
• Unit: Joule (J=Nm)
• Power (P): Rate of work done
• Unit: Watt (W) *generally measured in ‘HP’
• Velocity (v): Rate of distance travelled
• Unit: Metre/sec (m/s)

Hydro-mechanics (Pascal’s Law)
• Pascal’s Hydrostatic law states that, Pressure acting on
a stationary liquid is transmitted equally throughout
the liquid in all directions over its active surface.
• Thus for a force F1 acting on surface A1 of a liquid column connected
to surface A2 by the same liquid channel transmits equal pressure
p1=p2=p over the area A2.
• Therefore, F2=p*A2
• The forces inside this system have
the same behavior as the surfaces
(F2/F1)=(A2/A1)
• The pressure inside such a system always behaves according to the
size of the force F and the active area A. This means that the
pressure inside a system will increase until the pressure is larger
than the force of the resistance which is active against the pressure.
• The pressure inside a hydraulic system can only be correctly
measured when the counter pressure against the pressure is larger
than the pressure to be measured.

Hydro-kinetics
• Hydro-kinetics is the theory of the rules governing the movement of
liquids and the thereby generated forces.
Discharge (Q): It is the rate of volume of fluid flowing through.
 Unit: m3/s *generally measured in ‘ltrs/min’
• NOTE: Identical volumes flow through a pipe during an identical
period i.e. Discharge at any point inside a pipe/hose is equal.
• This equality is maintained by velocity compensation over varying
cross-sections of the pipe.
• Discharge/flow volume: Q=v*A
• And for any two cross-sections
A1 & A2, Q1=Q2 i.e. v1*A1=v2*A2
• NOTE: However velocity is also
influenced by the various losses
in fluid flow.

 Losses in pipe flow
• Hydraulic energy cannot be transmitted without losses through
pipes or hoses. These losses are classified as Major & Minor losses.
• Major Losses (Friction & Pressure Losses):
• Friction occurs against the inner walls and inside the fluids themselves.
The friction during transmission of hydraulic energy is converted into
heat. It is governed by the following factors:
1. Length of pipes/hoses
2. Cross-section of pipes/ hoses over the length
3. Flow speed
4. Flow viscosities
• The loss of pressure is depicted as ‘ΔP’ and is directly
proportional to velocity heads (v2/2g).
• Minor Losses (Fitting losses): These losses comprise
of losses at Valves, Elbows, Tees, Entrance,
Sudden Enlargement & Sudden Contraction.
Darcy-Weisbach
equation:

 Flow types
• Laminar Flow: Smooth, orderly movement of a fluid, in which there
is no turbulence, and any given sub current moves more or less in
parallel with any other nearby sub current. Laminar flow is common
in viscous fluids, especially those moving at low velocities.
Couette Flow Plane Poiseuille Flow Haygen-Poiseuille Flow
• Turbulent Flow: Movement of a fluid in which sub currents in the
fluid display turbulence, moving in irregular patterns, while the
overall flow is in one direction. Turbulent flow is common in non-
viscous fluids moving at high velocities.
NOTE: It should always be an objective
to avoid turbulent flow.

Hydraulic Systems
Inside hydraulic systems, mechanical energy is transformed into
hydraulic energy, where after it is transported, controlled & regulated.
Afterwards, hydraulic energy is converted back into mechanical energy.
Characteristics of hydraulic systems:
• Transmission of large forces (torque) while using relatively small component
dimensions
• Its functions under full load are immediately available from a machine that
was previously stationary
• Infinitely variable regulation and control of speeds and torque
• Simple overload protection
• Realization of fast and extremely slow movements
• Energy storage with gases
• Simple and centralized drive systems
• Non-central transformation of hydraulic energy into mechanical energy is
easily possible

Drive Control Drive Machine
Electric-motor
I.C. engine-motor
Or by hand
Hydro-pump
Hydraulic
Control and
Control Valves
Hydro-
cylinder
Hydro-motor
To rugged
switching
devices work
item
Electric energy
Thermal energy
Hydraulic Energy
Mechanical
work
Mechanical
energy
Mechanical
energy
BLOCK DIAGRAM OF A HYDRAULIC SYSTEM

Schedule of components Symbol by DIN standards

Pressure Fluids
• Hydraulic fluids or pressure fluids, are the medium by which power
is transferred in hydraulic machinery.
• The correct function, life-expectancy, operational reliability and
economy of a hydraulic system is decisively influenced by the choice
of a suitable pressure fluid.
• In general, the applied fluids are based on mineral oil, and are a
called hydraulic oils. Apart from these oils, liquids that are difficult
to ignite are also used.
Tasks for Pressure Fluids:
 Lubrication of moving parts, such as pistons, valves, bearings, etc
 Anti-corrosion protection
 Transmission of the pump's hydraulic output
 Flushing out contamination
 Conduction of frictional heat

 Types of Pressure fluids
Following are Mineral-based hydraulic oils defined according to
DIN standards:
Designation Description
H  Hydraulic oils without additives.
 Characteristics comparable with C lubricants.
 Rarely used in hydraulics nowadays.
HL  Hydraulic oils with additive to increase anti-corrosion and
life-expectancy.
 Used in systems with pressures up to approx.200 bar.
 Fulfill general thermal load requirements.
HLP  Hydraulic oils with special high-pressure additives to offer
high protection against wear.
 Used in systems with pressures exceeding 200 bar.
HV  Hydraulic oils of particularly low viscosity and
temperature dependency.
 Other characteristics are like HLP oils.

 Technical data of Pressure fluids
 Viscosity: Viscosity defines the toughness of a pressure fluid.
Following table depicts few viscosity grades and their applications. It also
offers rough idea of resulting effects on filling system with unsuitable oil.
Starting viscosity
Application: ZRP with diesel motor
Starting viscosity
Application: RKP with electric motor
Recommended range for continues operation
Ideal viscosity
Reference point for measurements
Minimal value for pump
(Lubrication problem)
Viscosity of water
NOTE:
 The viscosity of hydraulic
oils increase with increase
in pressure
 At pressure of 400 bar, the
viscosity is approximately
double of that at 200 bar.

 Lubricating capability:
o A pressure liquid must be capable of sufficiently lubricating and creating
surface adhesion on moving parts of a hydraulic system; in particular the
inner components like pumps and motors.
o If the lubrication film should collapse due to insufficient viscosity or
excessive surface pressure, the moving parts will come into contact with
each other. This can cause extreme wear on the contoured surfaces and
then result in a function fault of hydraulic components.
 Air Exclusion capability:
o Under normal atmospheric conditions, hydraulic oils contain
approximately 90% depleted air. This is not detrimental for a hydraulic
system. The depletion rate will increase when the pressure and
temperature increases.
o When a pressure decrease occurs the air saturation limit will drop rapidly
and the depleted air inside the hydraulic oil will be released as air
bubbles.
o Air can also enter through leaking suction lines or shaft seals. This
released air, which is visible as foam, is a particular hazard for hydraulic
systems because it can cause the dreaded CAVITATIONS. As cavitation
can lead to material abrasions, pressure impacts and noise.

NOTE: Development of foam must be avoided! This can be achieved by
a sufficiently dimensioned hydraulic oil tank from which any released
air can escape and the hydraulic oil must be capable of allowing the air
bubbles inside the tank to float to the surface and then explode.
 Durability:
o Durability of a pressure fluid means its resistance to chemical changes
under high temperatures and to catalytic (non-ferrous) metals.
o In general, its durability at a temperature above 70° Celsius will be
halved by every further 10 °C increase of temperature.
o This ageing processing is also accelerated by contamination due to
abrasions and rust, etc.
NOTE: Old hydraulic oil will darken and appears resinous, which can
lead to malfunction of hydraulic valves!

Hydraulic Pumps
Hydraulic pumps have the task of transforming mechanical energy into
hydraulic energy. This involves transforming torque (mechanical) into
flow volume (hydraulic).
Factors to be considered while choosing Hydraulic pumps:
 Operational media
 Required operational pressure
 Expected speed range
 Minimum and maximum operational temperatures
 Installation situation
 Drive type (coupling)
 Expected durability
 Maximum noise level
 Service convenience
 Costs

• Single pump cannot perfectly fulfill every criteria. This is why various
pump designs are available.
• All designs have one thing in common; they function according to
the displacement principle.
• They are equipped with mechanically sealed chambers in which the
fluid is transported from the input side (suction connection) to the
output side (pressure connection).
• As there is no direct connection between these two connections,
pumps of displacement principle are suitable for high system
pressures and are therefore ideal for hydraulic applications.
*(check!)NOTE: Displacement Principle: An object that sinks displaces
an amount of fluid equal to the object's volume.

Gear Pumps
• A gear pump uses the meshing of gears to pump fluid by displacement. They are
used to pump high viscosity fluids. Gear pumps are positive
displacement (or fixed displacement), meaning they pump a constant amount of
fluid for each revolution.
• Basic gear pump uses spur gear but othervariations with lobe and rotatory vane
pump are also possible.
• Theory of operation:
 As the gears rotate they separate on the intake side of the pump, creating a void
and suction which is filled by fluid. The fluid is carried by the gears to the
discharge side of the pump, where the meshing of the gears displaces the fluid.
The mechanical clearances are small— in the order of 10 μm. The tight clearances,
along with the speed of rotation, effectively prevent the fluid from leaking
backwards.
 The rigid design of the gears and houses allow for very high pressures and the
ability to pump highly viscous fluids.
 An external precision gear pump is usually limited to a maximum working pressure
of 210 bars (21,000 kPa) and a maximum speed of 3,000 rpm.
 Suction and pressure ports need to interface where the gears mesh.
 Pump formulae:
o Flow rate in US gal/min = Fluid Density X Pump Capacity X rpm
o Power in hp = US gal/min X (lbf/in³)/1714
Lobe pump
Rotatory vane pump

Gear pumps are broadly classified as:
• External Gear Pump:
Inside the displacement chambers, the air inside the suction line is firstly transported from the
suction side to the pressure side. This will generate a vacuum inside the suction line. When the
vacuum increases, the fluid will be extracted from the tank into the suction line until it reaches the
pump. The gear chambers will fill with oil and then displace the oil around the outside to the
pressure side. The combing of the gear teeth will prevent the oil from flowing back. Characteristics:
 Relatively high pressure of approx. 300 bar with small installation dimensions
 Low price
 Large speed range (500 – 6000 rpm)
 Large temperature and viscosity range
 Typesof Gear pumps

• Internal Gear Pump:
Liquid enters the suction port between the rotor (large exterior gear) and idler (small interior
gear) teeth. Liquid travels through the pump between the teeth of the "gear-within-a-gear"
principle. The crescent shape divides the liquid and acts as a seal between the suction and
discharge ports. The pump head is now nearly flooded, just prior to forcing the liquid out of the
discharge port. Intermeshing gears of the idler and rotor form locked pockets for the liquid which
assures volume control. Rotor and idler teeth mesh completely to form a seal equidistant from the
discharge and suction ports. This seal forces the liquid out of the discharge port. Characteristics:
 Constant and even discharge regardless of pressure conditions
 Can be made to operate with one direction of flow with either rotation
 Low NPSH required
 Flexible design offers application customization

Axial Piston Pumps
• Axial piston pumps are positive displacement pumps inside which the pistons
are configured parallel to the rotary axis of a cylinder drum.
• They have a circular piston group which rotates against an angled swash plate.
• As the rotary group turns the pistons are pushed forwards and backwards, a
grooved timing plate at the at the top of the pistons controls the way the fluid is
drawn through the suction side of the pump and out through the pressure side.
• A fixed displacement piston pump has fixed swash plate but range of different
controllers can be used to control the position of and adjustable swash plate to
make a variable displacement version.
• Changing the angle of the swash plate will change the axial displacement of the
pistons and therefore the flow from the pump. The angle of swash can be
controlled manually although more commonly this a constant pressure, flow or
power control.
• Design: The bearing and piston feet have hydrostatic lubrication that requires a
constant leakage into the case. Draining leaking fluid and maintaining a low,
stable pressure in the pump casing is important for ensuring a long pump life.
• Unlike gear pumps, piston pumps do not generate much contamination as they
operate. But also piston pumps do not last as long if the fluid is dirty.

 Typesof Axial pistonpumps
Axial piston pumps are broadly classified as:
• Angled Plate Pump:
• The cylinder drum(1) is driven so that the pistons(2), which are guided inside it, are also driven.
The axial movements of the pistons are determined by an angled plate(3) inside the pump
housing which can be tilted away from right-angle position to the drive axis.
• The pistons move along an elliptical orbit against the stationary angled plate (deflection plate).
The generated friction is controlled by a friction disk(4) and the axial bearing. During the suction
phase, the pistons move outwards while being held against the deflection plate by retainer
devices. During the pressure phase, the pistons are forced inwards by the plate.
• The direction of oil flow for each piston, meaning either to a pressure or suction port, is
governed by control slots. They are located in a rigid control plate(5) against which the free end
of the cylinder drum will rotate.

• Tumbler Plate Pump:
• The shaft (1) drives a tumbler plate (2) which transmits its axial movement to a non-
rotating pistons. The pistons are pressed via springs against the tumbler plate. The forces
between the piston and the tumbler plate when pressing against each other are
transmitted by an axial bearing (3).
• The flow direction from each piston is governed either by control valves (6) or via control
slots in each piston.
• The angle of the rotating tumbler plate cannot be changed, which means that the
displacement volume of this type of pump remains constant.

• Angled Axle Pump:
• The cylinder drum (1) is driven by the pistons (5) which are themselves driven via a drive flange
(2). The cylinder drum is guided either by a central trunnion or by a needle bearing around its
circumference and it tilted away from the axis of the drive shaft. The displacement volume
varies according to the deflection angle. This principle enables these pumps to run in reverse
direction.
• The connection between the piston and drive flange is carried out by a ball-joint (6) which pulls
the piston inside its cylinder during the suction phase and pushes the piston during the pressure
phase. A further joint is necessary between the actual piston and ball-joint in order to equalise
any circular or elliptical orbits.
• The suction and pressure side of the pump are divided, just like the angled plate principle, by a
slotted plate. This flat or spherical control plate (7) deflects with the cylinder drum meaning
that the pressure port must pass through the deflection bearing or be connected to the control
plate via a sealed friction guide.

The working pistons extend in a radial direction symmetrically around the drive shaft, in
contrast to the axial piston pump. The stroke of each piston is caused by an eccentric drive
shaft or an external eccentric tappet. When filling the workspace of the pumping pistons
from "inside" (e.g., over a hollow shaft) it is called an Inside impinged (but outside braced)
radial piston pump. If the workspace is filled from "outside" it's called an Outside
impinged radial piston pump (but inside braced).
Inside Impinged Radial Piston Pump Outside Impinged Radial Piston Pump
Radial Piston Pump

• Working:
 The outer ring for bracing of the pumping pistons is in eccentric position to the hollow shaft
in the center. This eccentricity determines the stroke of the pumping piston.
 The piston starts in the inner dead center (IDC) with suction process. After a rotation angle
of 180° it is finished and the workspace of the piston is filled with the to moved medium.
The piston is now in the outer dead center (ODC). From this point on the piston displaces
the previously sucked medium in the pressure channel of the pump.
• Characteristics:
 very high load at lowest speed due to the hydrostatically balanced parts possible
 no axial internal forces at the drive shaft bearing
 high reliability
 high efficiency
 high pressure (up to 1,000 bar)
 low flow and pressure ripple
 low noise level

*Hydraulic Motors
• A hydraulic motor is a mechanical actuator that converts hydraulic
pressure and flow into torque and angular displacement (rotation).
• Conceptually, a hydraulic motor should be interchangeable with
a hydraulic pump because it performs the opposite function but most
hydraulic pumps cannot be used as hydraulic motors because they
cannot be back driven.
• Only very few hydraulic motors can cover a speed range from very slow
to over 1000 rpm. Therefore, hydraulic motors can be divided into fast-
running types (n = 500 to 10000 rpm) and slow-running types (n = 0.5
to 1000 rpm).
• The performance of a hydraulic motor depends on its flow volume and
the pressure difference inside the motor; its performance is
proportional to its rotation speed.
• Hydraulic motors usually have a drain connection for the internal
leakage, which means that when the power unit is turned off the
hydraulic motor in the drive system will move slowly if an external load
is acting on it. Thus, for applications such as a crane or winch with
suspended load, there is always a need for a brake or a locking device.

 Typesof Hydraulic motors
Gear Motor Angled-plate Axial motor
Planetary Motor
Angled axis axial motor

Radial Piston Motor Muti-stroke Radial Piston Motor
Multi-stroke Axial Piston Motor
(with static shaft)
Multi-stroke Axial Piston Motor
(with static hosing)

 Gear Motors
• A gear motor is designed very
Similar to a gear pump. The
differences can be found in the axial
pressure field as well as the fact that
gear motors are designed for changing
its sense of direction and therefore is equipped with a leakage oil connection.
• The pressurized fluid flowing into hydraulic motor is active on the gear wheels.
This generates a torque which is then transmitted to the motor output shaft.
• Gear motors are frequently installed in mobile hydraulic systems and agricultural
machinery to drive conveyor belts, cooling fans, screw conveyors or air blower
fans.
• Important nominal values for gear motors are its consumption volume (approx.
1 to 200 cm³), its maximum operational pressure and its speed range (500 to
10000 rpm).
• Gear motors and axial piston motors belong to the fast-running family. Fast-
running hydraulic motors operate as speeds higher than 500 rpm.

 Annular Gear Motor
• The main components of this motor
consist of a free-standing displacement
ring (1), its inner teeth made up of 7 rollers
and a moving gear wheel (3) equipped with
6 outer teeth. When the tooth cavities are
subjected to pressure, the gear wheel will rotate and simultaneously circulate in an
eccentric orbit around the center of the gear wheel.
• These two movements take place in opposite directions. With a tooth ratio of 7:6,
this will result in a transmission ratio of 6:1, which means that every time the shaft
rotates, each chamber is pressurized and relieved six times. This system produces a
down-gearing function. The motor is classified as a slow-running motor.
• The rotary movement of the gear wheel is guided by a jointed shaft (4) in order to
equalize the eccentric movements before the drive shaft (5). The input and output
flow is governed by control slots which are located on a rotating control bush (6). It is
then dog-driven by the drive shaft via a pin (7). Two non-return valves inside the
motor housing release the flow of leakage oil.

 Radial pistonmotor(Multi-stroke)
• With his type of motor, the that radial
configured pistons (3) are supported
on the stroke radius (4) by rollers (8).
The cylinder chamber is supplied with
fluid via axial bore holes in the control
unit (5). Per rotation, every piston is
pressurized and then relieved
according to the quantity of actuation cams.
• The resulting torque generated by the shape of the stroke ring is then
transmitted via teeth (6) from the rotor / piston group (3) to the drive shaft. A
conical roller bearing is integrated inside the housing (1) to absorb the high axial
and radial forces.
• It is technically possible to install a multi-disk brake (9) on an extending drive
shaft passing through the housing (2). If the air pressure inside the ring chamber
(10) falls below a certain value, the spring plate (11) will force the disk pack (12)
together and the brake will be actuated. If the air pressure exceeds a certain
value, the brake piston (13) will be forced against the spring plate. The disk pack
will then be relieved and the brake will be released.

Valves
• By far we have studied about the components necessary to carryout
required energy transformations, but other components are necessary to
govern and control the transformed energy. One of these component types
is the hydraulic control valve.
• Control valves are valves used to control conditions such
as flow, pressure, temperature, and liquid level by fully or partially opening
or closing in response to signals received from controllers that compare a
"set point" to a "process variable" whose value is provided by sensors that
monitor changes in such conditions. Control Valve is also termed as
the Final Control Element.
• Valves are classified on the basis of their control operations as:
 Check Valves
 Directional Valves
 Pressure Valves
 Flow Valves

 Check Valves
• The function of check valves in a hydraulic system is to stop the flow of
hydraulic fluid in one direction and to allow its flow in the other direction.
This is why check valves are also known as non-return valves.
• They function without causing a flow of leakage oil. Their shut-off elements
can consist of either balls, plates, cones or cones with shaft seals.
• Technical terminologies used:
 Cracking pressure - the inlet pressure at which the first indication of flow occurs
(the minimum upstream pressure at which the valve will operate).
 Reseal pressure - the pressure at which there is no indication of flow.
 Back pressure - the differential pressure between the inlet and outlet pressures.
Check valves can be divided into different groups according to their applications:
NOTE: Ball shut-off elements are economical to produce but cone shut-off elements are
more efficient and have longer life.
Symbol:

• This valve consists of a housing (1) and a
hardened piston (2) which is forced by a
spring (3) against a seat (4). When fluid
flows in the required direction, the fluid
pressure will force the piston cone away
from its seat to allow the fluid to pass
through. If the fluid flows in the other
direction, the spring and fluid pressure will force the
piston cone against its seat and stop the flow of fluid.
• The cracking pressure of the check valve depends on the force of the spring and therefore
cannot be influenced externally. Depending on their applications, the pre-tension of these
springs can be between 0.5 bar and 5.0 bar. If the check valve is not equipped with a spring, it
must always be installed vertically so that the weight of its internal closing element determines
its cracking pressure.
• Important nominal values for check valves are the nominal size (6 - 150), flow volume (up to
15000 l/min), operational pressure (up to 315 bar) and the cracking pressure without a spring
(0.5, 1.5, 3.0 or 5.0 bar).
• Applications:
 To bypass throttle
 To stop flow direction
 As bypass valve to bypass a return flow filter after a certain back-pressure is achieved to contamination.
 Simplecheck valves Symbol:
Reverse
Free Passage

• Contrary to simple check valves, the non-flow direction of releasable check valves can be
controlled.
• Applications:
 to hold the pressure inside operational circuits
 to secure raised components against lowering in the event of a pipe fracture
 to prevent creeping movements of involved consumers
• Types:
 Releasable check valves without leakage oil connection
 Releasable check valves with leakage oil connection
o Releasable check valves without leakage oil connection (valve type SV):
 Hydraulic releasable check valves
Symbol: In SV type the free volume flows from
A to B. The fluid is active on surface A1
of the main cone (1) and forces it
against the spring (3) away from its
seat. With a volume flow from B to A,
the flow is interrupted according to a
normal check valve.
 The flow release in direction B to A is
carried via the control piston (4). The
necessary control oil pressure coming
from connection X will move the
control piston to the right to open the
main cone (1).
 Nominal size: 6-150
 Flow volume up to 6400l/min
(approx.)
 Operation pressure up to 315
bar

 The necessary control pressure equals the surface area ratio between surface A1 and the opening control
piston. This ratio is usually 1:3.
 The total cross-section of surface A is instantly opened during the opening function. This could result in
pressure relief impacts, particularly when large volumes under pressure are released. Such relief impacts not
only produce noise, they also create peak loads in the total hydraulic system; in particular in hydraulic
connectors and moving parts.
In applications where these relief impacts are not compensated, the valve
goes through a pre-opening phase
o When control connection X is subjected to pressure, the control piston (4)
firstly forces the pre-opening ball (2) away before forcing the main
cone (1) from its seat.
o The pre-opening ball uncovers only a small cross-section. This enables
the cylinder to extend slowly before the main cones moves and uncovers
the total cross-section. Such designs enables dampened pressure relief
of the pressurized fluid.
o Releasable check valves with leakage oil connection (valve type SL):
 The difference to valve
type SV is the additional
leakage oil connection Y.
 The ring surface of the
control piston is separated
from connection A.
 Any pressure at
connection A is only
active on surface A4 of
the control piston.
Symbol:

• Twin or double check valves are created by assembling two releasable check valves (1 and
2) inside one valve housing.
• In flow direction A1 to A2 or B1 to B2 the fluid flow is unrestricted, and in flow direction
A2 to A1 or B2 to B1 the fluid flow is interrupted.
• When, for examples, the fluid flows through the valve from A1 to A2, the control piston (3)
is move by the pressure to the right and pushes the cone (2) of the check valve against its
seat. Unrestricted flow is now possible from B1 to B2. This procedure functions in the
same manner when fluid flows from B1 to B2.
 Twincheck valves
Symbol:

 Directional Valves
• The term "directional valves" includes all valves that can start, stop or alter the flow
direction of a hydraulic pressure medium.
• Directional valves are designated according to the quantity of their switching
positions and connections, whereby the control connections are disregarded. A
directional valve with four consumer connections and three switching positions is
therefore designated as a 4/3 directional valve.
• The switching positions and the respective actuation elements are designated with
the small letter "a" and "b".
• When a valve has three positions, the central position is "neutral" and is usually
designated with figure "0". "Neutral" is the position where moving components
remain in non-active condition.
• When in horizontal position, the sequence of descriptions (switching positions) is
always carried out according to the alphabet from left to right.

Periodic directional switching using directional valves

 Directionalgate valves
• Direction gate valves are valves with a gate piston inside the bore hole of their
housing. Depending on the quantity of channels to be controlled, their housings
of cast graphite, steel or other suitable materials are equipped with two or more
cast or screw-in ring channels. These channels run concentric or eccentric
around a bore hole. This will result in control edges inside the housing which
function together with the edges of the control piston.
• The ring cavities inside the housing are separated or connected by displacing the
control piston. Sealing is carried out along a gap between the moving piston and
the housing. The sealing effect depends on the size of the gap, the viscosity of
the fluid and, in particular, the prevailing hydraulic pressure.
• Directional gate valves can be controlled directly or pre-controlled. Whether a
directional gate valve is directly or pre-controlled initially depends on the size of
the necessary actuation force an therefore on the component size (nominal
size) of the valve.

o Directlycontrolleddirectionalgatevalves
• The term "directly controlled directional gate valve" means a directional gate valve in
which its control piston is actuated either by a solenoid, pneumatic, hydraulic or via any
other mechanical device without assistance of an amplifier.
• Due to the static and dynamic forces generated inside directional gate valves as a result
of pressure and fluid flow, directly controlled directional gate valves are normally
available up to nominal size 10. This limit equals a performance of approximately 120
l/min with an operational pressure of 2350 bar, and applied in particular to directional
gate valves that are actuated by solenoids.
• Solenoid actuation is most common due to numerous automatic functions. Electro-
magnetic (solenoid) actuation is available in numerous designs:
 Dry solenoid - Direct current solenoids inside a chamber containing air.
 Wet or Pressure-sealed solenoid - Direct current solenoid inside a chamber containing oil. The
solenoid anchor moves in oil while the anchor chamber is connected to the T-channel.
 Alternating current solenoid inside a chamber containing air.
 Alternating current solenoid inside a chamber containing oil.
• Direct current solenoids are very reliable and enable soft and precise switching function.
They won’t burn out if piston jams while switching.
• Alternating current solenoids have a short switching phase.
NOTE: If the solenoid cannot fully switch to end position, the valve will burn out after
approximately 1 to 1.5 hours. Hence, direct current solenoids are preferred over alternating.

o Pre-controlleddirectionalgatevalves
• It is necessary to use directional gate valves of pre-controlled design when
controlling large hydraulic fluid volumes because of the higher force that is
necessary to move the control piston.
• Directional gate valves with a nominal size up to 10 are directly controlled.
Above the value, pre-control is used. An exception to this rule is manually
(mechanically) actuated directional gate valves that need not be pre-
controlled up to a nominal size of 32.
• A pre-controlled directional gate valve is normally equipped with a solenoid
valve as pre-control valve and a main valve. When the pre-control valve is
actuated, the main valve is subjected to pressure so that the control piston of
the main valve can be moved.
Main Valve
Pre-control Valve

 Directionalseat valves
• Directional seat valves are directional valves with one or more exactly fitting but moving
control pistons inside the bore(s) of the valve housing. These control pistons can be
designed as balls, cones or plates.
 No leakages.
 High Durability because there are
no leakage oil channels or throttle
gaps that can become clogged.
 Performance losses due to
insufficient pressure equalization.
 Locking function without additional locking elements.
 Suitable for high hydraulic pressures because no hydraulic jamming (deformations under
pressure) or leakages can occur.
 High pressure losses due to short strokes.
• Types: Directional seat valves are directly or indirectly (pre-control) actuated. Whether a
valve is actuated directly or with pre-control mainly depends on the nominal size of the
valve and, therefore, on the actuation force necessary to operate the valve.

o Directlycontrolleddirectionalseatvalves
• These are directional seat valves that are actuated by mechanically operated devices. An
example is an electrically actuated 4/2-way seat valve.
 The main valve is not operated. The
spring (5) holds the closing element (4)
on the seat (11). Port P is blocked and A
connected to T. One pilot line is
connected from A to the large area of the
pilot spool (8), which is thus unloaded to
the tank. The pressure applied via P now
pushes the ball (9) onto the seat (10).
Thus, P is connected to B, and A to T.
 When the main valve is operated, the
closing element (4) is shifted against the
spring (5) and pressed onto the seat (7).
During this, port T is closed, P, A, and B
are briefly connected to each other.
 P is connected to A. Because the pump
pressure acts via A on the large area of
the pilot spool (8), the ball (9) is pressed
onto the seat (12). Thus, B is connected
to T, and P to A. The ball (9) in the Plus-1
plate has a “positive spool overlap”.

 Pressure Valves
• "Pressure valves" is a general term for ALL valves that directly or
indirectly influence a part of or the total system pressure inside a
hydraulic system. This is carried out exclusively by changing the
throttle cross-section by means of either mechanical, hydraulic or
electrical control elements.
• When throttle cross-section is closed, all pressure valves can be
divided into gate valves and seat valves in a similar way to
directional valves. Pressure valves can be divided into sub-groups
according to their functions:
• Pressure limitation valves (PLV)
• Pressure switch-in valves
• Pressure reduction valves (PRV)
• Pressure cut-off valves
NOTE: These valves can be either directly
or pre-controlled.

 Pressurelimitationvalves (PLV)
• A PLV has the task of limiting a pressure to a certain value. If the
required pressure is exceeded, the PLV will be actuated and directs
the excessive flow volume out of the hydraulic circuit back to the
oil tank.
• According to its task it can also be called a safety valve. It is always
located in a bypass line.
• Principle common to all pressure limitation valves is that the input
pressure is guided onto a surface which is subjected to a force. As
long as the spring force is higher than the force of pressure, the
control element will remain on its seat. If the force of the pressure
exceeds the spring force, the excessive hydraulic fluid will be
directed back to the tank. When flowing back to the tank, the
hydraulic energy inside the fluid will be transformed into heat.
Symbol:
Seat Valve Gate Valve
Simple circuit of Pressure
Limitation valve

o Pre-controlledpressurelimitationvalve
• Pre-controlled pressure limitation valve consists
of a main (1) and a pre-control stage (2),
whereby the latter involves a simple pressure
limitation valve of seat design.
• It is a measuring unit within a system because
the setting of its spring (3) is decisive for the
actuation pressure of the total valve. The input
pressure reaches the lower end of the valve
and, via a throttle (4), to the upper end. From
here, there is a connection to the pre-control
valve. As long as this valve is not actuated, the
pressure is balanced and its closed switching
position can be maintained by the relatively
weak spring (5).
• When the opening pressure is achieved at the
input of the of the valve, a small control
pressure will flow through the throttle and the
pre-control valve. This will generate a pressure
drop at the throttle - therefore a force
difference between the upper and lower ends
of the valve - which will cause the throttle to be
forced upwards against its spring and open a
connection between the input and the output.

 Pressureswitch-onvalves
• Pressure switch-on valves are installed in the main flow of a hydraulic system and are actuated
when a certain pressure is achieved to switch on another hydraulic system.
• It is possible to use pressure limitation valves as a replacement for pressure switch-on valves. A
requirement for this is that the pressure inside channel T (with directly controlled PLV) or inside
channel B (with pre-controlled PLV) cannot change the pre-selected pressure setting.
• This can be achieved when the leakage oil of a directly controlled pressure limitation valve, or
the control oil of a pre-controlled pressure limitation valve, is returned externally to the
hydraulic tank.
• Settings of the switch-on pressure are carried out on the adjuster element (4). The compression
spring (3) will hold the control piston (2) at starting position. The valve is closed.
Directly controlled pressure switch-on valve

• The pressure inside channel P passes through the control line (6) and is active on surface (8) of
control piston (2), therefore against the spring force (3). If the pressure in channel P exceeds the
value of the spring, the control piston (2) will be forced away against the spring (2). The
connection between channel P and channel A is opened. The hydraulic system connected to
channel A will be switched in without any pressure loss in channel P.
• The control signal is received internally via the control line (6) and the nozzle (7) from channel P,
or externally via connection B (X). Depending on the application, the leakage oil is returned via
connection T (Y), or internally via connection A. In order to ensure an unrestricted return flow
from channel A to channel P, it is possible to install a check valve. The manometer connection
(1) is designed to check the switch-on pressure.

 Pressurecut-offvalves
• Pressure cut-off valves, also known as reservoir charging
valves, are mainly used in hydraulic systems equipped
with a pressure reservoir. Their task is to switch over the
flow volume to pressure-free circulation when the
pressure reservoir has achieved its nominal pressure.
Pressure cut-off valves are also used in hydraulic
systems equipped with high-pressure and low-pressure
pumps (twin circuit systems). In this case, the low-
pressure is switched over to pressure-free circulation
when the pressure range of the high-pressure has been
achieved.
• A pressure cut-off valve mainly consists of a main valve (1) with a main piston unit (3), a pre-
controlled valve (2) with a pressure adjuster element (16) and a check valve (4). With valves of
nominal size 10, the check valve is installed in the main valve. With valves exceeding nominal
size 10, the check is located in a separate intermediate plate.
• The hydraulic pumps feed the flow volume via the check valve (4) in the hydraulic system. The
pressure in channel A passes through control line (5) to the control piston (6). At the same time,
the pressure is active in channel P via the nozzles (7) and (8) on the spring-loaded side of the
main piston (3) and the ball (9) inside the pre-control valve. As soon the pre-selected cut-off
pressure of the pre-controlled valve is achieved, the ball will move from its seat against the
spring (10). The fluid will then flow via the nozzles (7) and (8) into the spring chamber (11).
From here, the fluid flows internally or externally via the control line (12) and channel T back to
the tank.

 Pressurereductionvalves
• Contrary to pressure limitation valves, pressure
reduction valves have the task of reducing the input
pressure in certain sections of a hydraulic system.
The reduction of the input pressure (primary
pressure) and maintaining the output pressure
(secondary pressure) is carried out at a value which
is lower than the value of the pressure inside the
main hydraulic circuit. It is therefore possible to use
a pressure reduction valve to reduce the pressure
inside a certain section of a hydraulic system.
• In order to reduce and maintain the output pressure
at a certain level, the pressure input pressure works
against the end of the control valve (piston or cone)
where it is compared with the force of the regulation
spring. If the hydraulic force pA * AK exceeds the
spring force, the piston will move upwards towards
closing position. While regulating, the control gate is
in a balance of pressure. The average cross-section
that is necessary to hold pA at a constant value is
regulated according to the flow volume Q and input
pressure pE.
Symbol:

In principle, directly controlled pressure reduction valves are produced in 3-way design, meaning
that the pressure safeguarding of the secondary circuit is carried out via the adjuster element. The
design of the adjuster element can vary according to the customer or application requirements.
• The valve is open at starting position; meaning that the flow volume can flow unrestricted from
channel P to channel A. The pressure in channel A is simultaneously active via the control line
(2) and the piston surface against the spring (3). When the pressure inside channel A increases
above the pressure value of the spring (3), the control piston (4) will move to regulation position
in order to hold a constant pressure inside channel A. The signal and control flow volume is
monitored internally via the control line (2) from channel A.
• If the pressure increases in channel A due to external influences on a consumer, the control
piston (4) will adjust even further against the spring (3). Channel A will then be connected to the
tank via the control edge (5) of the control piston (4). The necessary quantity of fluid will flow
back to the tank to prevent the pressure from increasing.
• The leakage oil will flow out of the spring chamber (6) via channel T(Y). If required, a check valve
can be installed to allow the fluid to flow freely back from channel A to channel P. The
manometer connection (8) is designed to monitor the reduced pressure.

 Flow Valves
• Flow valves are designed to alter (increase or decrease) their throttle cross-
sections and thereby influence movement speeds of consumers. A flow
distributor has a special function in that is divides an incoming flow volume
into two or more outgoing flow volumes.
• Classification:

 Throttle Valves
The flow volume of throttle valves depends on the difference of pressure; which means that a
larger pressure difference will result in a larger flow volume. In numerous control systems where a
constant speed is not decisive, throttles are used because flow regulation valves would be too
expensive for such purposes. Throttle valves are used when:
• A constant working resistance is available.
• A speed change with a fluctuating load is not important or can be disregarded.
When the throttle distance is shorter, a change of viscosity will have less influence. It should be
noted that the flow volume will increase as the fluid viscosity decreases. Whether a valve is
dependent or not dependent on the fluid viscosity, this depends on the design of the throttle
distance.
1 – Throttle Unit
2 – Valve Seat
3 – Throttle Piston
4 – Adjuster Screw

• The fluid in channel A reaches consumer A2 via the throttle unit (1) consisting of the valve
seat (2) and the throttle piston (3). The throttle piston (3) can be axially adjusted via the
adjuster screw (4) in order to adjust the throttle cross-section (1).
• The fluid returning from consumer B2 will force the valve seat against the spring (5) in the
direction of the throttle piston (3) to enable an unrestricted return flow. Depending on its
installation configuration, the throttling effect will take place in the feed or return flow
direction.
o Twin throttle check valves
In order to alter the speed of a consumer (main flow limitation), a twin throttle check valve
is installed between the directional valve and the connection plate. With pre-controlled
directional valves, the twin throttle check valve can be utilized to set the switching time
(control flow limitation). It is then installed between the pre-control valve and main valve.
1 – Throttling point
2 – Valve seat
3 – Throttle spool
4 – Setscrew
① - Component side
② - Plate side
Symbol:

 Flow RegulationValves
• The task of a flow regulation valve is to maintain a constant selected flow volume independent
of any pressure fluctuations in the system. Apart from the adjustable measuring throttle (1),
this task is achieved by installing an additional moving throttle which works as a regulation
throttle (pressure balance) and is therefore has a comparison function in a circuit.
When these two throttles work together, the
pressure load is separated by the varying pressure
difference p1 – p3 into two branches:
 The inner and constant pressure difference p1-p2
at the adjustable measuring throttle
 the outer and varying pressure difference p1-p3
The flow regulation valve is designed as a regulator
with the following main components:
 Measuring throttle (1)
 Pressure balance (2) with spring (3)
When temperature or viscosity fluctuations occur,
the adjustable measuring throttle (1) will monitor a
change of pressure difference of p1 – p2. The shape
of the throttling point can counteract this influence.
The configuration of the pressure balance
determines the type of flow regulation valve. If it is
positioned in series with the measuring throttle,
the result is a 2-way flow regulation valve. If it is
positioned parallel to the measuring throttle, the
result is a 3-way flow regulation valve.
Flow regulation valve as Flow Regulator.

• In 2-way valves, the measuring aperture and pressure balance are configured in
series. The pressure balance in a 2-way flow regulation valve precedes or follows
behind is a matter of design and has no influence in practical use.
• Three basic installation configurations:
1. Input Control:
 The flow regulation takes place between the hydraulic pump and the respective
consumer. This type of regulation is recommended for hydraulic systems in which the
consumer generates a positive resistance (counter force) against the regulated flow
volume.
 Advantage: The pressure is active between the flow
regulation valve (1) and the pressure cylinder (2)
which results from the resistance of the pressure
cylinder. As the pressure against the cylinder seals is lower, the friction on the sleeves
in the cylinder is also lower.
 Disadvantage: The pressure limitation valve (3) in front of the flow regulation valve
must be adjusted according to the largest consumer. The hydraulic pump (4) therefore
always generates the maximum selected pressure even at times when the consumer
requires less pressure. The generated throttling heat is also transmitted to the
consumer, which can have a negative influence, for example, on hydraulic motors.
o Two-wayflowregulationvalve

Output Control Bypass Control
2. Output Control:
 With this type, the flow regulation valve (1) is installed in the line between the consumer
(2) and the tank. This type of regulation is recommended for hydraulic systems with
negative or precipitating (pulling) working loads, which have the tendency to move the
cylinder faster than is possible, by the displaced flow volume.
 Advantage: Counter valve is not necessary. The generated throttle heat is also directed
back to the hydraulic tank.
 Disadvantage: The pressure limitation valve (3) must be set according to largest consumer
pressure (heat development). Even when idling, all components of the cylinder are
subjected to the maximum operational pressure (high friction).
3. Input auxiliary control (bypass):
 the flow regulation valve (1) is installed parallel to the consumer. The flow regulation valve
can only regulate the flow volume to the consumer in that a selectable amount of the flow
volume is returned back to the hydraulic tank.
• Advantage: the necessary pressure is generated to move the load during the working
stroke. Therefore, less energy is transformed into heat. Only when the cylinder rests
against the buffer will the set pressure of the pressure limitation valve be achieved. The
throttling heat is also directed back to the hydraulic tank.

• The measuring aperture A1 and the regulation aperture A2 of
a 3-way flow regulation valve are installed parallel and not in
series.
• The pressure balance returns the excess flow volume via an
additional line back to the tank. To safeguard the maximum
pressure, a pressure limitation valve must be installed inside.
• As the excess flow volume QR is directed back to the tank, 3-
way flow regulation valves can only be installed in the feed or
leading lines. This type of valve also enables a relief
connection X for virtually free circulation.
• The working pressure of the hydraulic pump is only larger
than the consumer pressure according to the reduction by the
measuring aperture, whereby the hydraulic pump with 2-way
flow regulation valves must always generate the pressure,
which has been set on the pressure limitation valve.
• Advantage: less losses in the lines, offers a preferable system
efficiency and has lower heat development.
o Three-wayflowregulationvalve

Hydro-pneumatic Reservoir
• The task of a hydro-pneumatic reservoir is to store hydraulic energy coming from the
hydraulic pump and then release this energy later when it is required.
• Volume equalization and the respective storage of energy inside a hydro-pneumatic
reservoir is achieved by subjecting the fluid to the pressure of a weight, spring or gas.
The pressure of the fluid and the respective weight, spring or gas is always in
balance.
• Weight-loaded or spring-loaded reservoirs are only used for very special industrial
applications.
• Gas pressurized reservoirs without a separating diaphragm are rarely used because
hydraulic fluid will absorb gas.
• The hydro-pneumatic reservoirs (gas-spring reservoir) in the majority of hydraulic
systems are equipped with separating elements. They are further divided as bubble,
piston and diaphragm reservoirs according to the type of separating elements used.
 Reduced heat generation
 Application of smaller hydraulic pumps
 Lower installed output
 Simple maintenance and installation
NOTE: Above advantages are enhanced by damping pressure impacts and pulses, thereby
increasing the life expectancy of the hydraulic system.

 Classificationand P-Vcharacteristics:
Weight-loaded
Reservoir
Spring-loaded
Reservoir
Gas-loaded Reservoir
Hydro-pneumatic Reservoir
With separating element between gas & liquid
Piston Bubble Diaphragm
Weight
-loaded
Spring
-loaded
Gas over
piston
Gas over
bladder
Without separating
element
With separating
element

 Tasks
Various tasks of Hydro-pneumatic reservoirs in a hydraulic system:
 Storing energy
 Reserve fluid
 Emergency actuation
 Force equalization
 Damping mechanical impacts
 Damping hydraulic impacts
 Leakage oil compensation
 Impact and oscillation damping
 Pulsation damping
 Vehicle suspension springing
 Recycling braking energy
 Maintaining a constant pressure
 Flow volume compensation (expansion vessel)

 Diaphragmreservoir
• An elastic diaphragm (2) is used inside the pressurized
reservoir (1) to separate the hydraulic fluid from the
nitrogen filling. A shut-off button (3) is installed in the base
of the diaphragm which will cover the connection port
when the diaphragm is fully expanded and thereby prevent
the expanded diaphragm from being forced into the
connection port. At the gas side of the diaphragm, a plug-
screw (4) enables the filling pressure of the reservoir to be
checked or re-filled via a testing and filling device.
• Technical Data
 V0=0.075 to 2.8 ltr
 Pmax =160 to 250 bar
Pressurizing and relieving a diaphragm reservoir:
p0 = Gas pre-load pressure
V0 = Nominal volume
p1 = min.Working pressure
V1 = Relieved volume
p2 = max.Wroking pressure
V2 = Pressurized volume
Initial position Unloading Loading

 Bubble reservoir
The elastic separating wall between the pressurized fluid and
nitrogen, which develops a bubble (3), is affixed inside the
reservoir (1) via the vulcanized gas valve element (4) and can
be removed or installed inside the reservoir through the
opening in the fluid valve (2). The fluid valve has the task of
closing the input port when the bubble is fully expanded,
thereby preventing the bubble from being pressed into the
opening. A damping device protects the valve against
hydraulic impacts when it is opened quickly.
Pressurizing and relieving a diaphragm reservoir:
p0 = Gas pre-load pressure
V0 = Nominal volume
p1 = min.Working pressure
V1 = Relieved volume
p2 = max.Wroking pressure
V2 = Pressurized volume
Initial position Unloading Loading
Sampling volume:
ΔV = V1 - V2

Hydraulic Filters
• In hydraulic systems, large volumes of hydraulic oil flow under high pressure
through narrow gaps. Such systems are more sensitive to contamination in
the oil, particularly solid particles, than other drive systems.
• Contamination causes negative effects on bearings, guides, rotors, pistons
and the flanks of gear wheels in hydraulic pumps and motors, as well as the
pistons, piston rods and bushes of hydraulic cylinders. The wear on friction
surfaces increases their gaps and can lead to leakages.
• Larger solid particles (< 0,050 mm²) will frequently cause sudden failure of
machine components, whereas smaller solid particles (> 0,010 mm²)
generally cause slowly developing damage and component faults.
• The task of a hydraulic oil filter is to reduce the contamination in the oil to
an acceptable minimum in order to protect the hydraulic components
against excessive wear.
• Types:
 Suction filters
 Line filters / Pressure filters
 Ancillary / return flow filters
 Filler and Ventilation filters

 Suctionfilters
• Suction filters are used when there is a high
risk of damage to a pump by large
contamination particles. This occurs when:
 Numerous hydraulic circuits use the same
hydraulic fluid
 Hydraulic tanks cannot be cleaned due to
their shape
• Pressure differences must not be too great
because pumps are particularly sensitive to
vacuums. This is why filters with large
filtering surfaces must often be installed. A
bypass valve and/or a contamination
indicator on the filter is also necessary.
• A suction filter will only safeguard the
function of the pump. The necessary
protection against wear must be achieved
by other filters.
Suction
connection
Cover for
filter element
Filter
housing

 Line/ Pressurefilters
• This type of filter has the task of safeguarding all
components that follow. It must be installed as close as
possible to the components that require protection.
• The following criteria is decisive for the application of line
filters:
 Components are particularly sensitive to contamination, or are
particularly important for the function of the hydraulic system
 Components are particularly expensive and are decisive for
the reliability of the hydraulic system
 Down-time costs are particularly high with this type of
hydraulic system
 Line filters can be installed as safety filters or as operational
filters
 Operational filters: Protection of components against
wear. Maintaining the required grade of fluid cleanliness
 Safety filters: Safeguarding the component function.
Safety filters are only installed together with operational
filters
NOTE: This type of line filter must be able to withstand
maximum system pressure.
1 – Upper section
2 – Screw-in lower section
3 – Filter element
The standard design is
without a bypass valve and
without a pressure
differential switch to
safeguard the filter.

 Ancillary / Return flow filters
• These filters are installed at the end of the return flow line and are generally
designed as tank ancillary filters. This means that all contamination that enter
the system or are generated inside the system are filtered out of the fluid before
it flows back into the tank. The size of the filter mainly depends on the size of the
flow volume.
• In order to prevent the production of foam inside the tank, it must always be
ensured that the return flow of fluid always enters the tank below the fluid level.
• This can be achieved by a pipe inside the tanks that extends below the fluid level.
Filter with electrical clogging indicator,
with bypass valve.
Filter with optical clogging indicator,
with bypass valve
Filter without clogging indicator
with bypass valve
Filter without clogging indicator, with
bypass valve, switchable
6 – Connection
3 – Tank lid
1 – Retainer flange
2 – Housing
4 – Dirt collection basket

 Filler / Breathers and Ventilation filters
• A large amount of contamination in a hydraulic system enters via the
surrounding air.
• Due to continuous fluctuations of the fluid level inside the tank, a vacuum
or pressure is generated against the air inside. It must therefore be ensured
that no contaminated air can enter the tank.
Symbol:
2 – Clogging indicator
4 – Filling housing
5 – Fastening screws
3 - Filling strainer

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