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Introduction:

A hydraulic drive system is a drive or transmission system that uses
pressurized hydraulic fluid to power hydraulic machinery. The term hydrostatic
refers to the transfer of energy from flow and pressure, not from the kinetic
energy of the flow.

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A hydraulic drive system consists of three parts: The
generator (e.g. a hydraulic pump), driven by an electric
motor, a combustion engine or
a windmill;
valves, filters, piping etc. (to guide and control the system); and the
actuator (e.g. a hydraulic motor or hydraulic cylinder) to drive the machinery.

Pascal’s law is the basis of hydraulic
drive systems. As the pressure in the system is the same, the force that the
fluid gives to the surroundings is therefore equal to pressure × area. In such
a way, a small piston feels a small force and a large piston feels a large
force.

The same principle applies for a hydraulic pump with a
small swept volume that asks for a small torque,
combined with a hydraulic motor with a large swept volume that gives a large
torque. In such a way a transmission with a certain ratio can be built.

Most hydraulic drive systems make use of hydraulic
cylinders. Here the same principle is used — a small torque can be transmitted
into a large force.

By throttling the fluid between the generator part and
the motor part, or by using hydraulic pumps and/or motors with adjustable swept
volume, the ratio of the transmission can be changed easily. In case throttling
is used, the efficiency of the transmission is limited. In case adjustable
pumps and motors are used, the efficiency, however, is very large. In fact, up
to around 1980, a hydraulic drive system had hardly any competition from other
adjustable drive systems.

Currently, electric drive systems using electric
servo-motors can be controlled in an excellent way and can easily compete with
rotating hydraulic drive systems. Hydraulic cylinders are, in fact, without
competition for linear forces. For these cylinders, if hydraulic systems are
available, it is easy and logical to use this system for the rotating drives of
the cooling systems.

An important advantage of a hydraulic drive is its
highpower density: the mass of a hydraulic drive is several times smaller than
the mass of an electric drive of the same power.

 

 

1.   
Classification:

Hydraulic drives
are traditionally divided into three classes.

 

·       
Mobile hydraulics

·       
Aircraft hydraulics

The
classification is basically due to the fact that components are classified in
these categories, although some overlap exists between industrial and mobile
hydraulics, aircraft hydraulics components are highly specialized due to
extreme requirements on weight and certification.

 

 

                         Fig.1.1 – Componenets
of Hydraulic drives

2.   
Hydraulic pump:

Hydraulic pumps supply fluid to the components
in the system. Pressure in the system develops in reaction to the load. Hence,
a pump rated for 5,000 psi is capable of maintaining flow against a load of
5,000 psi.

      Fig.1.2 – basic components

 

Pumps have a power density about
ten times greater than an electric motor (by volume). They are powered by an
electric motor or an engine, connected through gears, belts, or a
flexible elastomeric coupling to reduce vibration.

Common types of hydraulic pumps to hydraulic machinery
applications are;

3.   
Gear pump:

cheap, durable (especially in g-rotor form), simple. Less
efficient, because they are constant (fixed) displacement, and mainly suitable
for pressures below 20 MPa (3000 psi).

4.    Vane pump:

cheap and simple, reliable. Good for higher-flow
low-pressure output.

5.    Axial piston
pump:

many designed with a variable displacement mechanism, to
vary output flow for automatic control of pressure. There are various axial
piston pump designs, including swashplate (sometimes referred to as a
valveplate pump) and checkball (sometimes referred to as a wobble plate pump).
The most common is the swashplate pump. A variable-angle swashplate causes
the pistons to reciprocate a greater or lesser distance per rotation, allowing
output flow rate and pressure to be varied (greater displacement angle causes
higher flow rate, lower pressure, and vice versa).

6.    Radial piston pump:

normally used for very high pressure at small flows.

Piston pumps are more expensive than gear or vane pumps,
but provide longer life operating at higher pressure, with difficult fluids and
longer continuous duty cycles. Piston pumps make up one half of a hydrostatic transmission.

Control valves:

Directional control valves route
the fluid to the desired actuator. They usually consist of a spool inside
a cast iron or steel housing.
The spool slides to different positions in the housing, and intersecting
grooves and channels route the fluid based on the spool’s position.

The spool has a central (neutral) position maintained
with springs; in this position the supply fluid is blocked, or returned to
tank. Sliding the spool to one side routes the hydraulic fluid to an actuator
and provides a return path from the actuator to tank.

When the spool is moved to the opposite direction the
supply and return paths are switched. When the spool is allowed to return to
neutral (center) position the actuator fluid paths are blocked, locking it in
position.

Directional
control valves are usually designed to be stackable, with one valve for each
hydraulic cylinder, and one fluid input supplying all the valves in the stack.

Tolerances
are very tight in order to handle the high pressure and avoid leaking, spools
typically have a clearance with the
housing of less than a thousandth of an inch (25 µm). The valve block will
be mounted to the machine’s frame with a three point pattern to avoid
distorting the valve block and jamming the valve’s sensitive components.

The spool
position may be actuated by mechanical levers, hydraulic pilot pressure,
or solenoids which push the spool left or right. A seal allows part of the spool to protrude outside the
housing, where it is accessible to the actuator.

The main
valve block is usually a stack of off the shelf directional control
valves chosen by flow capacity and performance. Some valves are designed to be
proportional (flow rate proportional to valve position), while others may be
simply on-off. The control valve is one of the most expensive and sensitive
parts of a hydraulic circuit.

·       
Pressure relief valves are used in
several places in hydraulic machinery; on the return circuit to maintain a
small amount of pressure for brakes, pilot lines, etc.. On hydraulic cylinders,
to prevent overloading and hydraulic line/seal rupture. On the hydraulic
reservoir, to maintain a small positive pressure which excludes moisture and
contamination.

·       
Pressure regulators reduce the supply
pressure of hydraulic fluids as needed for various circuits.

·       
Sequence valves control the sequence
of hydraulic circuits; to ensure that one hydraulic cylinder is fully extended
before another starts its stroke, for example.

·       
Shuttle
valves provide a logical or function.

·       
Check
valves are one-way valves, allowing an accumulator to charge and
maintain its pressure after the machine is turned off, for example.

·       
Pilot controlled check valves are
one-way valve that can be opened (for both directions) by a foreign pressure
signal. For instance if the load should not be held by the check valve anymore.
Often the foreign pressure comes from the other pipe that is connected to the
motor or cylinder.

·       
Counterbalance valves are in fact a
special type of pilot controlled check valve. Whereas the check valve is open
or closed, the counterbalance valve acts a bit like a pilot controlled flow
control.

·       
Cartridge valves are in fact the
inner part of a check valve; they are off the shelf components
with a standardized envelope, making them easy to populate a proprietary valve
block. They are available in many configurations; on/off, proportional,
pressure relief, etc. They generally screw into a valve block and are
electrically controlled to provide logic and automated functions.

·       
Hydraulic
fuses are in-line safety devices designed to automatically seal off a
hydraulic line if pressure becomes too low, or safely vent fluid if pressure
becomes too high.

·       
Auxiliary valves in complex
hydraulic systems may have auxiliary valve blocks to handle various duties
unseen to the operator, such as accumulator charging, cooling fan operation,
air conditioning power, etc. They are usually custom valves designed for the
particular machine, and may consist of a metal block with ports and channels drilled.
Cartridge valves are threaded into the ports and may be electrically controlled
by switches or a microprocessor to route fluid power as needed.

7.   
Actuators:

·       
Hydraulic cylinder

·       
Swashplates are used in ‘hydraulic motors’ requiring
highly accurate control and also in ‘no stop’ continuous (360°) precision
positioning mechanisms. These are frequently driven by several hydraulic
pistons acting in sequence.

·       
Hydraulic motor (a pump plumbed in reverse)

·       
Hydrostatic transmission

·       
Brakes

8.   
Reservoir:

The hydraulic fluid reservoir holds excess hydraulic
fluid to accommodate volume changes from: cylinder extension and contraction,
temperature driven expansion and contraction, and leaks. The reservoir is also
designed to aid in separation of air from the fluid and also work as a heat
accumulator to cover losses in the system when peak power is used. Design
engineers are always pressured to reduce the size of hydraulic reservoirs,
while equipment operators always appreciate larger reservoirs. Reservoirs can
also help separate dirt and other particulate from the oil, as the particulate
will generally settle to the bottom of the tank. Some designs include dynamic
flow channels on the fluid’s return path that allow for a smaller reservoir.

9.   
Accumulators:

Accumulators are a
common part of hydraulic machinery. Their function is to store energy by using
pressurized gas. One type is a tube with a floating piston. On one side of the
piston is a charge of pressurized gas, and on the other side is the fluid.
Bladders are used in other designs. Reservoirs store a system’s fluid.

Examples
of accumulator uses are backup power for steering or brakes, or to act as a
shock absorber for the hydraulic circuit.

10. Hydraulic fluid:

Also known as tractor fluid, hydraulic fluid is the
life of the hydraulic circuit. It is usually petroleum oil with various
additives. Some hydraulic machines require fire resistant fluids, depending on
their applications. In some factories where food is prepared, either an edible
oil or water is used as a working fluid for health and safety reasons.

In addition to transferring energy, hydraulic fluid needs
to lubricate components, suspend contaminants and metal
filings for transport to the filter, and to function well to several hundred
degrees Fahrenheit or Celsius.

 

11. Filters:

Filters are an important part of hydraulic systems. Metal
particles are continually produced by mechanical components and need to be
removed along with other contaminants.

Filters may be positioned in many locations. The filter
may be located between the reservoir and the pump intake. Blockage of the
filter will cause cavitation and possibly failure of
the pump. Sometimes the filter is located between the pump and the control
valves. This arrangement is more expensive, since the filter housing is
pressurized, but eliminates cavitation problems and protects the control valve
from pump failures. The third common filter location is just before the return
line enters the reservoir. This location is relatively insensitive to blockage
and does not require a pressurized housing, but contaminants that enter the
reservoir from external sources are not filtered until passing through the
system at least once.filters are used from 7 micron to 15 micron depends upon
the viscosity grade of hydraulic oil.

 

12. Tubes, pipes and
hoses:

Hydraulic tubes are seamless steel precision pipes, specially
manufactured for hydraulics. The tubes have standard sizes for different
pressure ranges, with standard diameters up to 100 mm. The tubes are
supplied by manufacturers in lengths of 6 m, cleaned, oiled and plugged. The
tubes are interconnected by different types of flanges (especially for the
larger sizes and pressures), welding cones/nipples (with o-ring seal), several
types of flare connection and by cut-rings. In larger sizes, hydraulic pipes
are used. Direct joining of tubes by welding is not acceptable since the
interior cannot be inspected.

Hydraulic pipe is used in case standard hydraulic tubes are not
available. generally these are used for low pressure. They can be connected by
threaded connections, but usually by welds. Because of the larger diameters the
pipe can usually be inspected internally after welding. Black pipe is non-galvanized and suitable for welding.

Hydraulic hose is graded by pressure, temperature, and fluid
compatibility. Hoses are used when pipes or tubes cannot be used, usually to
provide flexibility for machine operation or maintenance. The hose is built up
with rubber and steel layers. A rubber interior is surrounded by multiple
layers of woven wire and rubber. The exterior is designed for abrasion
resistance. The bend radius of hydraulic hose is carefully designed into the
machine, since hose failures can be deadly, and violating the hose’s minimum
bend radius will cause failure. Hydraulic hoses generally have steel
fittings swaged on the ends. The weakest part of the high pressures
hose is the connection of the hose to the fitting. Another disadvantage of
hoses is the shorter life of rubber which requires periodic replacement,
usually at five to seven years intervals.

Tubes and
pipes for hydraulic applications are internally oiled before the system is
commissioned. Usually steel piping is painted outside. Where flare and other
couplings are used, the paint is removed under the nut, and is a location where
corrosion can begin. For this reason, in marine applications most piping is
stainless steel.

 

13. Seals, fittings
and connection:

Components of a hydraulic system sources (e.g. pumps),
controls (e.g. valves) and actuators (e.g. cylinders) need connections that
will contain and direct the hydraulic fluid without leaking or losing the
pressure that makes them work. In some cases, the components can be made to
bolt together with fluid paths built-in. In more cases, though, rigid tubing or
flexible hoses are used to direct the flow from one component to the next. Each
component has entry and exit points for the fluid involved (called ports) sized
according to how much fluid is expected to pass through it.

There are a number of standardized methods in use to
attach the hose or tube to the component. Some are intended for ease of use and
service, others are better for higher system pressures or control of leakage.
The most common method, in general, is to provide in each component a
female-threaded port, on each hose or tube a female-threaded captive nut, and
use a separate adapter fitting with matching male threads to connect the two.
This is functional, economical to manufacture, and easy to service.

Fittings serve several purposes;

1.    To join components with ports of different sizes.

To bridge different standards; O-ring
boss to JIC, or pipe threads to face seal, for example.

3.    To allow proper orientation of components, a 90°, 45°,
straight, or swivel fitting is chosen as needed. They are designed to be positioned
in the correct orientation and then tightened.

4.    To incorporate bulkhead hardware to pass the fluid
through an obstructing wall.

5.    A quick
disconnect fitting may be added to a machine without modification
of hoses or valves

A typical piece of machinery or heavy equipment may have
thousands of sealed connection points and several different types:

·       
Pipe fittings, the fitting is screwed in until
tight, difficult to orient an angled fitting correctly without over or under
tightening.

·       
O-ring boss, the fitting is screwed into a boss
and orientated as needed, an additional nut tightens the fitting, washer
and o-ring in place.

·       
Flare
fittings, are metal to metal compression seals deformed with a cone
nut and pressed into a flare mating.

·       
Face seal, metal flanges with a groove and
o-ring seal are fastened together.

·       
Beam seals are costly metal to metal seals used
primarily in aircraft.

·       
Swaged seals, tubes are connected with fittings that are
swaged permanently in place. Primarily used in aircraft.

Elastomeric
seals (O-ring boss and face seal) are the most common types of seals in heavy
equipment and are capable of reliably sealing 6000+ psi (40+ MPa) of fluid pressure.

 

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