HYDRAULICS AND PNEUMATICS
COMPONENTS














System overview
Pumps
Compressors,
Valves,
Cylinders ,
Motors
Turbines
Accumulators,
Filters,
Reservoirs,
Coolers,
Connectors,
Seals,
Fluids.
OUTLINE
System overview
Types of Hydraulic Systems
i. Open-Loop Hydraulic Systems
ii. Closed-Loop Hydraulic Systems
1. Open-Loop Hydraulic Systems
 In open-loop hydraulic systems, the pump supply oil
comes from the reservoir (tank). The pump then sends it to
a directional control valve, where it either returns to the
tank or is sent to an actuator then from actuator back to
reservoir through DCV and the process is repeated.
 The loop is considered to be open because the oil can
return to the tank from several sources (directional control
valve, actuators, pressure relief valves, etc.).
System overview
Schematic of a basic Open-Loop Hydraulic Circuit
 Unlike hydraulic systems, Pneumatic system is 'open'; the
fluid is obtained free, used and then vented back to
atmosphere.
2. Closed-Loop Hydraulic System
 In a closed-loop system the pump directly feeds a rotary
actuator (motor), which negates the need for a directional
System overview
Types of Hydraulic Systems
2. Closed-Loop Hydraulic System
 The pump outlet sends fluid to the actuator inlet, and the
return from the actuator goes directly back to the pump
inlet. To reverse direction of the motor, the pump has to
reverse flow direction, so it must be a bidirectional pump
Closed-Loop Hydraulic circuit
CLHS: Found in compact system e.g Skid steer, Doser
POWER TRANSFER IN A HYDRAULIC SYSTEM
COMPONENTS OVERVIEW
 Prime Mover -A machine’s prime mover is used for the
primary power source for a variety of machine systems,
such as drivetrain, electric, and hydraulic. Prime mover
examples; diesel engine, electric motor etc
 Reservoir - A hydraulic reservoir is a tank used to store the
system fluid.
 Hydraulic Pumps -A hydraulic pump converts mechanical
rotary motion from the prime mover (the electric motor or
internal combustion engine) to hydraulic power to operate
the system.
COMPONENTS OVERVIEW
 A hydraulic motor, or actuator, is similar in construction
to a hydraulic pump. But whereas pumps convert
mechanical rotary motion to hydraulic power, the rotary
actuator reverses the process by using the hydraulic power
to create rotary mechanical motion
 Hydraulic Valves - They can be assembled in blocks or
modules, or can work alone. Whether used to relieve
pressure or to meter hydraulic fluid to actuators, each valve
is critical to the operation of all hydraulic equipment.
 Hydraulic Cylinders - Hydraulic cylinders, or rams,
receive hydraulic fluid from a directional control valve and
transfer fluid in motion into linear mechanical motion.
COMPONENTS OVERVIEW
 Lines and Fittings- Lines and fittings carry the fluid from
the reservoir to the pump and on to the remaining system
components through a combination of rigid steel tubing
and flexible hose assemblies.
 Accumulators -A hydraulic accumulator is capable of
storing hydraulic energy.
 Compressor- A compressor increases air pressure by
reducing its volume
 Heaters and Coolers - Heaters and coolers help to regulate
hydraulic fluid temperature.
COMPONENTS OVERVIEW
 Hydraulic Filters - Filters are an important part of a
hydraulic system because they remove damaging
contaminants from the hydraulic fluid.
 Fluid conductors, connectors and seals: pump(s),
filter(s), actuators(s), a reservoir, and a directional control
valve, need to be connected so that hydraulic fluid at
operating temperature can pass between them at maximum
pressure and flow without leaking internally or externally.
 Fluids- is the liquid in a hydraulic system used to convey
energy and produce the required force at the actuators.
 These components should be matched for both flow and
pressure capacity to ensure system efficiency, smooth
operation, and maximum longevity.
HYDRAULIC RESERVOIRS
 The reservoir’s primary function is to store the system
fluid to provide a constant supply of hydraulic fluid for the
system’s pump(s) and components.
 The reservoir holds excess hydraulic fluid to accommodate
volume changes from cylinder extension and contraction,
temperature-driven expansion and contraction, and minor
leaks.
 The reservoir is also designed to aid in conditioning the
fluid by (heating, cooling, dehydration, deaeration, and
separating of contaminants from the fluid).
Types of Hydraulic Reservoirs
1. Vented reservoirs
In a vented reservoir, also called a breathing reservoir, the
reservoir is open to the atmospheric pressure so that, as the
fluid level changes because of the operation of the system
actuators, atmospheric air enters and leaves the reservoir.
Types of Hydraulic Reservoirs
2. Pressurized Reservoirs
 A pressurized reservoir is a sealed compartment that
doesn’t need a large breather. Therefore, atmospheric air
doesn’t usually enter the reservoir as the fluid level
changes during system operation.
 Hydraulic reservoirs are made from different materials, but
the most common ones are formed plate steel or molded
plastic.
Hydraulic Reservoir Components
1. External Reservoir Components
 Filler Cap - The filler cap is where fluid is added to the
reservoir; it is usually located on the top of the reservoir
but could be located on its side.
 Breather Filter - The breather filter prevents
contamination from entering the reservoir with air that
enters as the reservoir “breathes.”
Hydraulic Reservoir Components
1. External Reservoir Components
 Sight Glass - The sight glass or oil level gauge allows for
quick and easy checking of the fluid level.
 Access Cover - The access, or cleanout, cover can be
removed to inspect and clean the inside of the reservoir
Hydraulic Reservoir Components
1. External Reservoir Components
 Dished Bottom - Sometimes called the sump, the dished,
or tapered, bottom allows water and solids to settle to the
lowest point in the reservoir and is located away from the
suction tube.
 Drain Plug - The drain plug is located at the lowest point
of the reservoir and allows settled water to be drained or
the reservoir to be emptied completely, if needed. Many
larger reservoirs feature an ecology drain valve for draining
fluid in a controlled manner to prevent spills.
Hydraulic Reservoir Components
2. Internal Reservoir Components
 Suction Tube - The suction tube provides fluid flow
from the reservoir to the pump. It should be well below the
minimum fluid level but far enough above the floor of the
reservoir that it will not allow settled dirt and water to be
picked up and carried to the pump. It should be located
away from the return to prevent oil from going directly into
the pump from the return.
 Suction Screen - A suction screen is a coarse screen on
the end of the suction line that provides the first line of
filtration for the fluid going to the pump.
Hydraulic Reservoir Components
2. Internal Reservoir Components
 Return Tube The return tube directs fluid from the
system back to the reservoir, normally terminating well
below the reservoir fluid level but slightly above the
bottom of the tank. It should also be located away from the
suction inlet. The tube is often cut at a 45-degree angle to
slow the fluid as it enters the reservoir and prevent
churning and foaming.
 Return Screen - Return screens can be found inside a
reservoir and, in the oil returning to the tank from the
system, are designed to stop any large contamination from
entering the tank and reaching the pump inlet. See Figure
Hydraulic Reservoir Components
2. Internal Reservoir Components
 Return Filter Many larger machines have hydraulic tanks
with return filters mounted inside the tank.
Hydraulic Reservoir Components
2. Internal Reservoir Components
 Baffle The baffle is a barrier separating the return line from
the pump suction line. It is designed to force the fluid to
stay in the reservoir longer so that the reservoir functions
of cooling, deaeration, dehydration, and contaminant
settling can take place.
RESERVOIR
Hydraulic Reservoir Components
2. Internal Reservoir Components
 Hydraulic Fluid Level Sensor Many newer and larger
machines have fluid level sensors in their tanks. Its purpose
is to alert the operator when the tank fluid level is low.
These sensors could trigger a light on the dash, log a fault
code, prevent the machine from starting, or derate it and
put into a safe shutdown mode.
 Hydraulic Fluid Temperature Sensor temperature
sensors to monitor fluid temperature in the tank. They are
used to alert the operator when the hydraulic oil is too hot
or cold and could derate the machine
Hydraulic Reservoir Location
 Ideally the tank should be mounted above the pump inlet to
take advantage of head pressure the oil height creates, but
this is sometimes hard to accomplish.
 The other main consideration is for the distance from the
reservoir to the pump inlet to be kept as short as possible.
 Other factors such as accessibility for checking fluid levels
and servicing are also considerations for designers.
Hydraulic Oil Change
 The following procedure describes a generic hydraulic oil
change for a large excavator:
Hydraulic Oil Change
 The following procedure describes a generic hydraulic oil
change for a large excavator:
Hydraulic Tank Cleanout
 This may never be part of a regular maintenance interval,
but should be part of the procedure after a catastrophic
hydraulic component failure.
Hydraulic Tank Cleanout
Hydraulic Pumps Specifications
1. Pump Pressure Rating
 Pumps must be designed to match with the system
requirements in terms of flow output and pressure
capacity.
 The pump pressure rating is expressed in psi, bar, mPa, or
kg/cm2.
2. Pump Displacement
 This is a calculated theoretical number that states the
volume of oil a pump will move or displace during one
revolution of the pump’s driveshaft.
 Pump displacement is stated in cubic inches per revolution
(CIR) or cubic centimeters per revolution (CCR).
Hydraulic Pumps Specifications
1. Pump Flow Output
 This is the flow a pump produces per unit of time.
 It can be calculated using the pump’s displacement figure
and a simple formula.
 If D is in cubic centimeters, then unit conversion factor is
1,000
 If D is in cubic inches, then the unit conversion factor is
231
 The flow can be measured in (U.S.) gallons per minute
(gpm) or liters per minute (lpm).
Hydraulic Pumps
Types and Construction Features of Hydraulic
Pumps
 There are two general categories of hydraulic pumps:
1. Nonpositive displacement pumps (also known as dynamic
pumps) and
2. Positive displacement pumps.
Hydraulic Pumps
Nonpositive Displacement Pumps
 This Pump is designed with a loose-fitting rotating
component (impeller) inside its housing.
 When the impeller rotates as the pump shaft is driven, it
creates a low pressure at its inlet that directs inlet flow to
the center of the impeller.
 The impeller then moves the fluid out of the pump housing
at a high velocity.
Hydraulic Pumps
Nonpositive Displacement Pumps
 Because there is a lot of clearance between the impeller
fins and the pump housing, if the pump outlet were
blocked, flow would stop while the impeller kept turning.
 This is where the term “nonpositive” comes from.
Hydraulic Pumps
Positive Displacement Pumps
 PDP build-up of pressure at the outlet has little effect on
the output flow rate of the pump.
 The flow out of the pump is constantly pushed downstream
by more flow, and a constant flow is created as the pump is
continuously driven.
 Because of the tight clearances inside positive
displacement pumps, fluid does not have the chance to leak
back through the pump.
Hydraulic Pumps
Positive Displacement Pumps
 If pump flow output of a positive displacement pump were
blocked, a serious pump failure would occur or the prime
mover would stop turning.
 Remember that fluid is virtually incompressible, and if a
pump is rotating, fluid has to keep moving through the
pump or else serious consequences occur.
 There are three main categories of positive displacement
pumps: gear pumps, vane pumps, and piston pumps,
Fixed Displacement
 It “displaces” the same amount of oil for every pump
rotation.
 The only way to change the flow output of a fixed
displacement pump is to change the speed of its driveshaft
 The main types of fixed displacement pumps are gear,
vane, and piston.
 Both vane- and piston-type pumps have variations that can
be either fixed or variable displacement pumps.
Variable Displacement
 Variable displacement pumps are able to change the
amount of fluid they pump per revolution, independently of
the speed they are turning.
 They have mechanisms that can alter the pump
displacement to make them more efficient
 Piston pumps are the most common type of variable
displacement
Gear pumps
 Pressures up to
150 bar
As the teeth come out of mesh at the centre, a partial
vacuum is formed which draws fluid into the inlet
chamber.
Fluid is trapped between the outer teeth and the pump
housing, causing a continual transfer of fluid from inlet
chamber to outlet chamber where it is discharged to
Forms of Gear pumps
Vane pumps
Piston pumps
 In-line axial piston pumps: In in-line piston pumps, the
pistons operate parallel to the axis (driveshaft) of the pump.
 Bent axis piston pumps: In bent axis piston pumps, the
pistons operate at an angle to the axis (driveshaft) of the
pump.
 Radial piston pumps: In radial piston pumps, the pistons
operate perpendicular to the axis of the pump. The piston
arrangement is similar to that found in a radial internal
combustion engine.
Piston pumps
Piston pumps

Diagnosing Hydraulic Pump
Diagnosing or troubleshooting
a hydraulic system problem
Problems
is no different than any other system troubleshooting
procedure. The following steps should reaffirm a typical
diagnostic procedure for any system problem:
1.
2.
3.
4.
Gather information from the operator and/or machine.
Know the system.
Confirm the problem.
Following the service information troubleshooting
procedures for the

Diagnosing Hydraulic Pump
Once it is determined
that the pump is a possible legitimate
Problems
cause of the system problem, pump testing can begin.
Starting with simple visual checks, you would look for the
following:
 Pump housing discoloration, indicating overheating
 Oil condition (discoloration, indicating overheating or
contamination)
 Any loose fasteners for either the pump mount or pump
inlet plumbing
 A case drain filter; remove the filter and then cut it open.
Diagnosing Hydraulic Pump
preliminary checks
reveal nothing abnormal, further
Problems
 If
investigation could include running the machine and
listening for unusual noises or feeling for vibrations.
 The next stage of pump testing involves checking pump
output pressures and flows. Special tooling and equipment
could be required for this and must be inspected to ensure it
is in good condition before it is installed and used.
Testing Hydraulic Pumps
 Two important tests can be performed when testing
hydraulic pumps:
1. the no-load flow rate test and
2. the flow/pressure profile test.
 The following example is for a fixed displacement pump
mounted on a test stand, but the same test can be completed
with the pump on the machine.
 This is a generic example; always follow the pump or
machine manufacturer’s specific test procedures for the
pump you are testing.
Testing Hydraulic Pumps
Testing Hydraulic Pumps
Testing Hydraulic Pumps
Common Causes of Pump Failure
 Contaminated fluid (the most common cause of pump
failure)
 Cavitation
 Aeration
 Incorrect fluid
 Low oil level
 Excessive system pressures
 Restricted case drain lines
 Abuse and incorrect operating procedures such as stalling
implements and overheating oil, insufficient oil warm-up,
excessive pump speed, or incorrect adjustments).
Valves
 Three main classifications of valves are used to
control oil pressure, oil flow quantity, and oil flow
direction
1. Pressure control valves,
2. Flow control valves, and
3. Directional control valves
Principles of Operation of Pressure
Control Valves
 Oil pressure in both a circuit and the entire system has to be
controlled to specific pressure levels to prevent component
damage and allow the system to work as designed.
The main types of pressure control valves are as follows:
1. Pressure-relief valves: Pressure-relief valves limit the
maximum operating pressure in the system and provide a
safety valve to prevent system over-pressurization, which
could lead to component damage and or injury.
2. Unloading valves: Unloading valves are remotely piloted
(controlled by a pressure somewhere else in the system)
valves that divert pump flow to the tank so that the pump
operates at low pressure when certain pressure conditions
Principles of Operation of Pressure
Control Valves
The main types of pressure control valves are as follows:
3. Sequencing valves: Sequencing valves are remotely
piloted valves that are used to control the sequence, or
order of operation, of a series of actuators in the system.
4. Pressure-reducing valves: Pressure-reducing valves lower
the maximum pressure that can occur in a portion or
branch of a system.
5. Brake valves: Brake valves provide back pressure to limit
speed on a hydraulic motor operating an over-running load
(such as a piece of earth-moving equipment going
downhill).
6. Counterbalance valves: Counterbalance valves provide a
back pressure to hold a vertical load in place until certain
1. Pressure-relief valves configuration
2. Unloading valves configuration
3. Sequencing valves configuration
 A sequence valve is very similar to a pilot-operated
pressure-relief valve, except that instead of dumping flow
to the tank when it opens, it sends oil to another actuator.
3. Sequencing valves configuration
The figure shows two sequence valves are
used to sequence the operation of two
double-acting cylinders.
 When the DCV is actuated to its rightenvelope mode, the bending cylinder
(B) retracts fully and then the clamp
cylinder (A) retracts.
 This sequence of cylinder operation is
controlled by sequence valves.
 This hydraulic circuit can be used in a
production operation such as drilling.
 Cylinder A is used as a clamp cylinder
and cylinder B as a drill cylinder.
 Cylinder A extends and clamps a work
piece. Then cylinder B extends to drive
a spindle to drill a hole.
 Cylinder B retracts the drill spindle and
then cylinder A retracts to release the
work piece for removal.
4. Pressure-reducing valves configuration
 Some machines may have an auxiliary circuit that operates
at a lower pressure than the main system, and a pressurereducing valve could be used there as well.
 Unlike pressure-relief valves that are normally closed
valves, pressure-reducing valves sense the pressure at their
outlet and use that pressure to close the valve mechanism.
 This is the only pressure control valve that operates in this
way.
 A pressure-reducing valve can be used to limit the pressure
available to a single actuator or to an entire branch of a
system.
4. Pressure-reducing valves configuration
5. Brake valve configuration
 Brake valves provide back pressure to limit speed on a
hydraulic motor operating with an overrunning load (such
as a piece of earth-moving equipment going downhill).
 A brake valve uses pressure inputs from both upstream
and downstream of the hydraulic motor to adjust the
position of the valve’s moving mechanism and thus control
the flow of oil through the valve.
 This action limits the rotating speed of the motor and uses
it as a hydraulic brake to control the speed of the machine.
5. Brake valve configuration
6. Counterbalance valve configuration
 These are load control valves
 used to limit actuator movement when the load on the
actuator tries to create unwanted and/or uncontrolled
actuator movement.
 Common applications for these valves are crane boom lift
circuits, where the load tries to create a boom drift. aerial
work platform lift circuits
 Load control valves are sometimes referred to as lock
valves because they can hydraulically lock a cylinder in
place.
6. Counterbalance valve configuration
 These valves are used in circuits with open-center
directional control valves and vertical cylinders, either to
hold a load in place without risk of drifting or to precisely
control the descent when lowering the load.
 Counterbalance valves are placed as closed as possible to
the cylinder or directly on the cylinder.
6. Counterbalance valve configuration
A counterbalance valve is
applied to create a back
pressure or cushioning
pressure on the underside
of a vertically moving
piston to prevent the
suspended load from free
falling because of gravity
while it is still being
lowered.
6. Counterbalance valve operation
1. Valve Operation (Lowering)
 The pressure setting on the counterbalance valve is set
slightly higher than the pressure required to prevent the
load from free falling.
 Due to this back pressure in line A, the actuator piston must
force down when the load is being lowered.
 This causes the pressure in line A to increase, which raises
the spring- opposed spool, thus providing a flow path to
discharge the exhaust flow from line A to the DCV and
then to the tank.
 The spring-controlled discharge orifice maintains back
pressure in line A during the entire downward piston
stroke.
6. Counterbalance valve operation
2. Operation (Lifting)
 As the valve is normally closed, flow in the reverse
direction (from port B to port A) cannot occur without a
reverse free-flow check valve.
 When the load is raised again, the internal check valve
opens to permit flow for the retraction of the actuator.
3. Valve Operation (Suspension)
 When the valve is held in suspension, the valve remains
closed. Therefore, its pressure setting must be slightly
higher than the pressure caused by the load.
 Spool valves tend to leak internally under pressure. This
makes it advisable to use a pilot-operated check valve in
addition to the counterbalance valve if a load must be held
in suspension for a prolonged time.
Operating Principles of Hydraulic Flow
Control Valves
 Flow control components reduce flow below the pump
output flow rate or split it into two or more circuits.
 Flow control valves can be used in three different locations
relative to the actuator to control the actuator speed: meterin, meter-out, and bleed-off. See FIGURE 26-24 for the
different locations where flow controls can be found.
Flow Control Component Types
 These can be categorized into three groups:
noncompensated flow controls, pressure-compensated flow
controls, and flow dividers.
Noncompensated Flow Control Orifices
Pressure-Compensated Flow Control
Valves
Pressure-compensated flow control valves feature a movable
internal mechanism (spool or piston) that attempts to maintain
a constant flow through the valve regardless of the input
pressure or the actuator load pressure.
Flow Dividers
 Flow dividers can be placed in a circuit to split or divide
oil flow into two or more paths
 One example of where you might find a flow divider is a
tree harvester that must have separate functions that
are supplied from the same circuit run at the same speed
Principles of Operation of Hydraulic
Directional Control Valves
 Directional control valves are needed to direct pump oil
flow to actuators and direct return oil to the tank from the
actuators.
 They allow operators to control actuator movement speed
and direction by directing oil flow to one or more actuators
Check Valves
 Check valves are used to prevent flow in one direction but
allow free flow in the opposite direction.
 They are considered to be directional control valves
because they affect the flow direction of oil in the circuit
they are in.
 Shuttle check valves have three ports: two inlet and one
outlet. They have a ball that moves freely in a housing and
seals one of two ports when it is seated by the higher
pressure oil flow from one of the inlets.
Check Valves
In-Line Poppet–Type Check Valves
 Some circuits require having a check valve to allow flow in
one direction but prevent it from flowing in the opposite
direction.
 They have a light spring in them to assist with seating the
poppet.
Check Valves
 Pilot-operated check valves are often referred to as lock
valves because a common application for them is to lock a
cylinder in place in order to eliminate a drift problem.
 This type of check valve has a piston that moves a rod to
open the check valve and hold it open as long as pilot
pressure is applied to the piston.
Directional Control Valve Positions
 DCVs have movable spools or pistons that cover and
uncover ports internally as they are moved to different
positions.
 As the internal ports are covered and uncovered, the oil
flow changes its path through the valve.
 Different configurations of valves are based on the number
of positions the spool can move to. Two, three, four and
five positions might be possible.
 Valves with three or more positions have a neutral position
that determines whether the valve is open center (pump oil
goes to the tank) or closed center (pump oil is blocked).
Directional Control Valve Positions
 FIGURE 26-43 demonstrates how a three-position DCV
can redirect oil when shifted.
Directional Control Valve Positions
 Symbols for DCVs use individual boxes to represent the
number of positions they have. Therefore, a three-position
DCV symbol would have three boxes. The symbols in
FIGURE 26-44 represent one-, two-, and three-position
valves.
Directional Control Valve WAYS
 The number of ports that one section of a DCV has is
another way to describe it.
FILTERS
FILTERS
 Contamination of the fluid may result in wear, corrosion,
poor performance and ultimately component failure
 Filtration is about the removal of solid contaminants from
the fluid
 Most filters work on the principle of trapping the particles in
small holes or poles.
 The fluid may pass through but particles above a certain size
become trapped
 There are two main types of filter elements namely: Surface
and Depth
1. SURFACE FILTERS
 Surface filters are normally constructed from thin sheets of
material folded into many sections and then turn into a multi
pointed star as shown in figure below
1. SURFACE FILTERS
 They may be in form of replaceable cartridge or permanently
fitted inside a throw away bowl.
 The thin sheet is full of pores which trap the solid particles as
the fluid passes through them.
 Typical materials are
• Cellulose (paper).
• Woven steel fibres.
• Woven nylon fibres.
 Paper filter elements are ruined by water which makes them
become soggy and the pores close.
 Another design is the use of a single filament of metal
wound into a cylinder.
2. DEPTH FILTERS
 Depth filters are constructed with a thick layer of material
with small passages through which the fluid must pass (like a
bed of sand for example).
 The particles become trapped in the passages. The passages
may be formed from granules compacted into a thick
cylindrical layer or fibres compacted into a tube.
2. DEPTH FILTERS
Typical materials are
• Cellulose (paper).
• Synthetic fibres.
• Metal fibres.
• Glass fibres.
• Sintered metal granules.
CONTAMINANTS
The main contaminants are
• Welding scale and beads.
• Sealing tape shreds.
• Shards of screw threads.
• Bits of seal material.
• Grinding chips.
• Wear particles.
• Sludge and varnish from oxides oil.
• Rust.
• Water.
• Dirt from the atmosphere.
• Biological material such as hair, skin scales, flies and so
on.
CONTAMINANTS
 Solid contaminants get into the clearance spaces between
moving parts such as piston and valve spools.
 They cause wear and damage and may jam the component.
 Solid particles are removed in the filter by trapping them in
tiny passages in the filter element. Particles down to 5
microns (0.005 mm) may be trapped.
 Water damages paper filters and causes corrosion and lack of
lubrication.
 Water should be removed by settling the fluid in the tank
and draining it off.
FILTER SIZE AND EFFICIENCY
 Filters do not have uniform holes and passages so it cannot
be guaranteed that all the particles larger than the nominal
size are trapped.
 The efficiency of the filter determines how much of the
contaminant is removed. The level of contamination in a
fluid is covered in standards ISO 4406 (BS5540).
 Two range numbers must be stated. The first number is the
range of particles in 1 millilitre larger than 5 microns and the
second the range for particles larger than 15 microns.
FILTER SIZE AND EFFICIENCY
BETA RATING (β)
 The Beta rating is the percentage of particles of a stated size
trapped by a filter in a multi-pass test.
 For example a β15 of 80% means that 80% of particles larger
than 15 microns are trapped.
 The absolute rating is widely based on 98% of particles larger
than the stated size being trapped by the filter. This is more
likely to be given for surface filters because the size of the
pores is more consistent.
FILTER LOCATION
 A full flow filter is designed to filter the full output from a
pump.
 It may be placed before the pump (suction filter) or after the
pump in which case it must be capable of withstanding the
full pressure of the system.
 It may also be placed in the return line to the tank in which
case it does not have to withstand high pressure.
 Sometimes a separate pump is used solely for the purpose of
 circulating the fluid through a filter with no great pressure on
it.
 The system in which the fluid may be pumped through a
filter at any time independent of the main system is called
OFF LINE FILTERING.
FILTER LOCATION
 The check valve prevents reverse flow and draining of the
filter and pump.
 High precision components such as spool valves are
sometimes protected by using a filter adjacent to or part of
the component.
 These would be high quality filters of 5 or 10 microns.
FILTER CLOGGING
 A clogged filter will cause excess suction and cavitation
which will damage the pump.
 The state of a filter is often indicated either on the filter or
on a control panel.
 The indicator may be a small pressure gauge showing the
build up of pressure.
 It might also be a pressure switch as shown for setting off an
alarm.
FILTER CLOGGING
 The full flow filter shown below has a spring loaded element
which moves as it clogs and pressure builds up.
 The movement moves a pointer on the outside which
indicates the state.
FILTER CLOGGING
BYPASS
 If a filter is not changed when clogged, it is better to allow
unfiltered oil through the system than to run with it clogged.
 In this emergency situation, the oil automatically bypasses
the filter element.
 This may be done with a simple spring loaded valve which is
opened by the back pressure.
 In the diagram of the full flow filter above, the filter element
moves against the spring as back pressure builds up and
uncovers a bypass hole in the central stem.
FILTER CLOGGING
BYPASS
 The symbol below shows a filter with bypass and clogging
indicator.
FILTER CLOGGING
BYPASS
 The full flow filter shown below has a bowl which screws into
a head fitting.
 This contains a bypass valve which opens automatically when
pressure builds up due to clogging.