INTRODUCTION TO HYDRAULICS
Hydraulics in Mechanical Engineering:
Hydraulics is the branch of physics having to do
with the mechanical properties of water and other liquids and the application
of these properties in engineering.
The word "hydraulics" comes from the
Greek words "Hydro" means water and Aulics" means pipes.
Hydraulics pertains to power transmitted and controlled through the use of a
pressurized liquid. So hydraulics can be defined as a means of transmission and
control of forces and movement of fluids.
Basic Elements of Hydraulic Power Systems:
A hydraulic system can be broken down into five
main divisions. First, the power device; second, hydraulic power source using
fluid; third, the control valves; fourth, the lines in which the fluid power
flows and fifth, actuators devices.
Advantages of Using Hydraulic Power:
- Hydraulic power provides flexibility in the control of machines.
- Hydraulic power provides an efficient method of multiplying forces.
- Hydraulic power provides constant torque at infinitely variable speeds in either direction with smooth reversals.
- Hydraulic power is compatible with other means of control, such as electrical, electronic, or Mechanical.
- Hydraulic power is accurate.
- Hydraulic power gives fast response to controls.
- Small forces can be amplified to control large forces.
- Oil fluid power provides automatic lubrication for lesser wear. .
- Hydraulic power provides freedom in machine design.
- Hydraulic power is simple and provides ease of installation and maintenance.
- Hydraulic power is economical.
- Hydraulic power is efficient and dependable.
- Hydraulic power gives predictable performance.
- Hydraulic power is readily available.
Field of Application of Hydraulics:
- Machine tools Press construction D Vehicle construction
- Aircraft construction
- In industry ( i.e. aviation , agriculture, chemicals, petroleum, food processing, marine Construction, steel and mining etc.,)
Hydraulic fluids:
The fluid, which is selected for use in
hydraulically actuated equipment, will have a considerable effect on its
performance, maintenance costs and service life. The primary
function of the liquid in a hydraulic system is to transmit power to perform useful work.
The hydraulic fluid must transmit an applied force from one part of the system
to another and must respond to reproduce any change in magnitude or direction
of the applied force. In- order to perform this primary function, the fluid
must be relatively incompressible and have flow ability.
Three types of fluids are
generally accepted as a adaptable for use in hydraulic circuits:
- Petroleum oils
- Synthetic fluids
- Water
Water does not meet all the
requirements for general hydraulic equipment use as a hydraulic fluid, water
presents many problems, such as. rust and corrosion, ineffective lubrication,
temperature variations and the hazard of foreign matter in the water itself which might
cause an abrasive action on the smooth interior surfaces. The petroleum oils
and synthetic fluids more nearly meet all internal requirements for perfect
hydraulic performance.
It is well to remember that a
hydraulic fluid must not only transmit power; it must also act as a sealant and
a lubricant to perform these functions satisfactorily, the fluid must have
certain basic properties which might be called "service properties"
Service properties:
There are six service
properties, which are important to the specifier and user of hydraulic fluids:
- Viscosity
- Viscosity index
- Demulsibility
- Oxidation stability
- Lubricity or lubricating value
- Rust and corrosion preventive qualities
Viscosity:
The most important property
of the fluid to be used in hydraulic equipment is its viscosity. Viscosity is
the measure of the fluid's internal resistance to shear or flow at a definite
temperature and pressure. It is defined as the shearing force required to move
two plane surfaces relative to one another with a film of fluid between them.
Viscosity is affected y temperature and must be expressed as a certain value at
certain temperature. As temperature increases, these oils thin out, i.e., their
viscosity decreases; as temperature decrease, these oils thicken and their
viscosity increases.
Viscosity index:
It might be said that one of
the properties of an ideal hydraulic fluid would be that•of retaining the same
viscosity under, all temperature and pressure conditions to which it is
subjected. The rate of change of viscosity with temperature is indicated on an
arbitrary scale called the viscosity index (V.I).
Demulsibility:
The service property of a
hydraulic fluid which enables it to separate rapidly and completely from
moisture and to successfully resist emulsification is known as demulsibility.
This property is particularly important since the general design, construction,
and operation of a hydraulic system is conductive to the forming of moisture
and of stable water in oil emulsions within the system. Use of a hydraulic fluid without
effective demulsification qualities is, as a rule, reflected in the destruction
of the lubricating value and sealant properties. Not only does such a condition
greatly reduce the life expectancy of the fluid, it also reduces the service
life of the working surfaces. Such a fluid is also more susceptible to water
absorption.
Oxidation stability:
Here is another service
property which is exceedingly important in judging hydraulic oil. It is defined
as the fluid's ability to resist oxidation and deterioration for long periods.
It is based on inherent chemical structure of the oil itself, which can be
further fortified by additives developed to help resist breakdown. When
oxidation of oil takes place, sludge is formed. As sludges are formed, certain
changes in the physical and chemical properties of the oil take place. The oil
becomes progressively darker in color, heavier in viscosity, and organic acids
are formed as evidenced by an increase in the neutralization number. Therefore
the fluid should resist oxidation.
Lubricating' value:
Lubricity, or oiliness, is another factor not
determinable by specification. The ability of oil to perform satisfactorily,
i.e., to minimize friction and wear, is due to that property termed
"lubricating value." The lubricating value is directly related to its
chemical nature and its chemical affinity for metals. Lubricating value depends
on oiliness, extreme pressure or anti-weld properties, and on the composition
of rubbing surfaces. Oiliness agents are sometimes used to improve that
property.
Film strength:
Seldom included in the hydraulic oil specifications
because the lubrication function of such oil has been generally considered as
secondary. Nevertheless, film strength can be important; for lack of it will
result in excessive wear of moving parts and eventually in increased clearance.
Adequate film strength will minimize wear.
Rust and Corrosion preventive qualities:
Freedom from corrosion is another factor to be
considered. The neutralization number of the oil (indicating its degree of
acidity or alkalinity), when new, may be satisfactory, but after use the oil
may tend to develop corrosive tendencies as it begins to deteriorate. Another
form of corrosion is rusting, which must be combated in a different manner.
Many systems are idle for lengthy periods after a run at elevated temperatures.
This permits moisture to be condensed in the system, resulting in rust. Rust preventive
additives, usually synthetic chemicals, are often used in hydraulic oils.
Standard Specifications:
It should be pointed out that
there are other specifications which are also used as a basis for selecting the
proper fluid, such as: color, gravity, pour point, flash and fire points,
carbon residue and neutralization number.
Colour: The colour of oil is usually an indication of the type
of crude from which it has been obtained.
Gravity: Gravity is of no particular
importance to the user of hydraulic oils. In any one viscosity range, oils
obtained from different base stocks will vary in API gravity readings.
Pour pdint: the pour point of oil
indicates the temperature below which the oil will not flow freely. Except for
oils used in low temperature equipment, the pour point is not an important
characteristic for hydraulic systems.
Flash and Fire points: flash and fire points
indicate the temperature at which the oil begins to volatilize. The former is
the oil temperature at which the oil will give off sufficient vapors to ignite
momentarily; the later is the oil temperature
at which sufficient vapors are given off to ignite and continue to burn
when exposed to spark or flame.
Carbon Residue: the carbon residue test is a
measure of the percentage of carbon that remains in a sample of the oil after
the volatile content has been driven off by means of heat.
Neutralization Number: the neutralization number of
oil denotes its degree of acidity or alkalinity.
Qualities of Good Hydraulic
Oil:
Here are the
"musts" and "must nots" which good hydraulic oil should
meet:
- It must be of the correct viscosity to provide the right seal so that working pressures can be developed and maintained.
- It must have a high viscosity index to resist changes in fluidity as temperature change
- It must not permit wear on working parts.
- It must not foam.
- It must be stable, resisting oxidation.
- It must not precipitate gums or sludges on working parts.
- It must retain all of its original properties in use.
- It must have a long service life.
- It must protect parts against rust.
Without some of these properties, the hydraulic oil
used in an expensive mechanism can shorten the life of equipment and spoil fine
work.
The Hydraulic Power Unit
The basic hydraulic power unit consists a hydraulic
pump, an oil reservoir with a cover, a suction strainer or filter, a motor
coupling, an electric motor, pressure gage, pressure valves, hydraulic oil, and
the necessary internal piping.
The Hydraulic Pump:
The heart of the power unit is the hydraulic pump.
Pumps are used in hydraulic system to convert mechanical energy into hydraulic
energy. The hydraulic energy delivered to the system by the pumps is in the
form of a fluid flow.
There are two general classifications of pumps
which are being used for transmission of hydraulic power:
- Non-positive displacement pumps.
- Positive displacement pumps.
Positive displacement pumps are most generally used
for hydraulic power. The positive displacement pumps are generally four types:
- Gear type
- Vane type
- Piston type
- Screw type
Gear Type Pumps:
The gear type pump may be either of the external
gear design or the internal gear design. The principle upon which this type of
pump operates is that of a pair of gears being driven by an external means, the
oil moving into the pump as a partial vacuum is formed and then being
discharged by the meshing of the gear teeth on the discharge side of the pump.
Oil is actually squeezed or forced out as the gear mesh. Various gear designs,
such as helical, spur, spiral, or herringbone may be employed for the rotating
element. Depending upon the type of service expected, the gears may be made of
bronze, cast iron, or heat treated steel.
Advantages of gear type pumps are:
- They are inexpensive
- Have few moving parts, and
- Feature simplicity in design-and construction.the cam ring contour forces them back into the rotor and the hydraulic fluid must go out the discharge side of the pump.
There are several advantages
to the vane type design:
- Simplicity in construction, high efficiency, and low cost.
- It is compact in design.
Vane type pump
Piston Pumps:
These are two types
- Rotary piston pumps and
- Reciprocating piston pumps.
In rotary piston pumps the mechanism which actuates
the piston has a rotary motion rather than a back and forth motion as in the
reciprocating pumps. Rotary pumps may be of two types - radial type and axial
type.
In the
radial type, a number of pistons and cylinders are arranged radially around the
rotor hub, while in the axial type they are located in a parallel position with
respect to the rotor shaft.
In the
radial type, each piston rides in its cylinder with the base of the piston
pressing out against an eccentric ring or the eccentric surface of the casing.
As the rotor revolves, the eccentricity of the ring or casing causes an in
and-out or pumping motion of the pistons. In the axial type, the base of each
piston is connected by a piston rod to the driving plate. The driving plate is
free to tilt along any diameter through its center. Across the surface of this
driving plate rotates a wobble or cam plate which is tilted at an angle to the
shaft. The rotation of the wobble plate, produces an in-and-out motion of the
pistons in their cylinders. Constant displacement rotary piston pumps contain
no means of changing the volume of oil discharged at any given speed. Variable
displacement rotary piston pumps, on the other hand, contain means of varying
the length of piston stroke and hence the volume of discharge for a given speed
by changing the-eccentricity or angularity of the devices which
actuate the piston plungers or connecting rods.
The
advantages of the rotary piston pumps are;
- It is capable of delivering high operating pressures.
- It will handle oils in a wide viscosity range.
- It will provide a variable delivery of oil.
Rotary piston pump
Screw Pumps:
There are three types of
screw pumps which are commercially available. These three types are the
single-screw, the two-screw, and the three-screw pumps. A single screw pump
consists of a spiraled rotor which rotates eccentrically in an internal stator.
A two-screw pump consists of two parallel rotors with intermeshing threads
rotating in a closely machined housing. These pumps use external or internal
timing gears. A three screw pump consists of a central drive rotor with two
meshing idler rotors; the three rotors are surrounded by a closely machined housing.
The flow through a screw pump
is axial in direction of the power rotor. When the inlet side of the pump is
flooded with a hydraulic fluid, a certain volume of the liquid that surrounds
the rotors is caught as the rotors rotate. This fluid is pushed uniformly with
the rotation of the rotors along the axis and is forced out the other end. In
operation, when the power rotor is turning clockwise, the idler rotors are
turning counter clockwise.
Other applications include
hydraulic systems on submarines and other uses where noise must be controlled.
The Hydraulic Reservoir:
The hydraulic reservoir
should contain enough fluid that its working level is always maintained during
the system's operation. It should also have additional capacity to hold ,pll
the fluid in the system during shutdowns. The reservoir capacity is generally
between two and three times the capacity of the hydraulic pump. If a hydraulic
system has a great number of cylinders operating simultaneously, more careful
consideration should be given to the proper reservoir size by calculating the
entire system's volume.
Other features of a well
designed reservoir include the following
- Baffle plate
- Bottom drain
- Sight glass
- Filling cap and hole
- Breather air filter
- Clean out cover or door
- Return line
- Drain line
- Pump inlet line
- Drive mounting base
- Casters.
- Heat exchanger
The proper selection of a
reservoir for any hydraulic system depends largely on the field of application
and the type of duty under which the system must operate.
The Oil Filters:
The filter may be defined as "a device for the
removal of solids from a fluid wherein the resistance to motion of such solids
is in a tortuous path." Filters are listed under two types:
- Sump type
- Line type
The sump type or immersion type is placed in the
oil reservoir and is connected to the intake of the pump. The line type filter
is mounted outside of the tank either in the intake line or in the return line
from the system.
Some of the basic designs of the filters are
- T type
- Pot type
- Y type
- In Line type
There are several basic types of filter media, such
as paper, sintered metal powder, woven wire cloth, and certain kinds of ceramic
or plastic. In the selection of a filter media, it is important to consider the
type of fluid, chemical compatibility, temperature, and the ability to
withstand high flow rates.
Hydraulic Accumulators:
An
accumulator is essentially a pressure storage reservoir in which a noncompressible
hydraulic fluid is retained under pressure from an external source. The fluid,
under pressure, is readily available as a quick secondary source of fluid
power. Accumulators are used in conjunction with hydraulic systems on large
hydraulic presses, farm machinery, diesel engine, power brakes, and landing
gear mechanisms on airplanes, hatch covers on ships and other devices.
Accumulators are usually divided into three classes:
- The dead weight operated type
- The spring operated type
- The air operated type
Uses of Accumulator:
- As a Leakage compensator
- As secondary source of energy
- As fluid make-up device
- Synchronizing ram movements of two cylinders
- Provides emergency source of power
- Used as a holding device
- Used as shock suppressor
- Used in dual pressure circuit Used as lubricant dispenser
Hydraulic Actuators or Cylinders:
A hydraulic cylinder is a
device, which converts fluid power into linear mechanical force and motion. It
usually consists of a movable element, such as a piston and piston rod, plunger
or ram, operating within a cylindrical bore.
The operating principle of
the piston and piston rod type is that fluid entering one port drives the movable piston and
the rod assembly in one direction. Fluid from the opposite side is exhausted
back to the reservoir through a directional control valve.
The ram or plunger type
actuating cylinders have a movable element of the same cross-sectional area as
the piston rod. The ram or plunger type of cylinder is generally powered in
only one direction.
The opposed piston type
produces a high turning force at low pressure. As the pistons extend or
retract, they rotate the pinion gear, which is in mesh with each rack. It
produces the same turning force in both directions.
The piston chain type has two
cylinders in parallel. The large piston is used for powering the chain and
drive sprocket, while the smaller cylinder provides a seal. Fluid entering one
end operates against both pistons, but the large piston creates the greater
force, so it moves, causing rotation of the output shaft. Flow directed to the
other port reverses the operation.
Cushioning:
For the prevention of shock
due to stopping loads at the end of
the piston stroke,
cushion devices are used. Cushions may be applied at either end or both. They
operate on the principle that as the cylinder piston approaches the end of the
stroke, exhaust fluid is forced to go through an adjustable needle valve which
is set to control the escaping fluid at a given rate. This allows the
deceleration characteristic to be adjusted for different loads.
Gear Type Pump
Vane Type Pumps:
This is a rotary type and
also operates on the principle of increasing and diminishing volume. It
consists of a shaft, rotor, vanes, cam ring, pump housing, bearings, and seals.
Vane type hydraulic pumps may be either fixed delivery 'or variable volume
units.
As the rotor is turned,
centrifugal force causes the vanes to move out against the hardened and ground
contour of the cam-shaped ring. Fluid is trapped between the rotating vanes
when they pass over the inlet port. Because the vanes reciprocate in and out of
the rotor as they rotate, while passing over the inlet port the vanes are extended out of the rotor and carry a maximum
amount of fluid. As the vanes reach the outlet port.
for videos regarding hydraulics, follow the links below
http://youtu.be/YlmRa-9zDF8
http://youtu.be/mgkyabWnZFw