7452998 Hydraulics and Pneumatics

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    Hydraulics &Pneumatics

    TechnologyDMM3623

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    What is Fluid

    Power ?

    Technology that deals with generation, control andtransmission of power, using pressurized fluids

    example: brake automobile, launches spacecraft,mines coal, drives machine tools even drills teeth.

    Hydraulics when the fluid is liquid

    - petroleum oils, synthetic oils and

    water. Pneumatics when the fluid is gas

    - air (very abundant and can be readilyexhausted into the atmosphere after completing

    task)HYDRAULICS

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    Hydraulic Basics

    Hydraulics is the science of transmitting force and/ormotion through the medium of a confined liquid. In ahydraulic device, power is transmitted by pushing on aconfined liquid. The transfer of energy takes placebecause a quantity of liquid is subject to pressure. Tooperate liquid-powered systems, the operator should havea knowledge of the basic nature of liquids. This chaptercovers the properties of liquids and how they act underdifferent conditions.

    Hydraulics

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    Fluid System

    Fluid transportDelivery of a fluid from one location to another toaccomplish some useful purpose.

    Fluid powerDesigned specifically to perform work.

    Work-accomplished by a pressurized fluid bearing directlyon an operating fluid cylinder or fluid motor.Fluid cylinder-produces a force in linear motion.Fluid motor-produces a torque in rotary motion.

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    Hydraulic chain saw

    Dextrous hand

    EXAMPLES

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    Advantages of using

    Hydraulics

    1. Very high force despite small components (if

    pneumatics, bigger components are needed forhigh force, thus, incur high cost as well aswastage of compressed air)

    2. Very slow and linear movements (if

    pneumatics, too slow will create jerking on thecylinder)

    3. Very exact positioning (stops exactly on apoint). Compared to pneumatics, theres atolerance and if a load is given, it will move a

    little.HYDRAULICS

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    1. Start-up under heavy loads (Possible to startthe motor & process a job with a load

    attached to it. This is because there is oil inthe actuator though the pump is idling).

    3. Simplicity, safety, economy

    In general, fluid power systems use fewermoving parts than comparable mechanical orelectrical systems. Thus, they are simpler tomaintain and operate. This, in turn,maximizes safety, compactness and

    reliability.

    Advantages of usingHydraulics

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    oceanography

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    Disadvantages of using

    Hydraulics

    1. Slow movement of actuators (Pneumatics are faster)

    2. Messy (Leakage may occur and if not bothered, thesurrounding area could be messy. However, constantcleaning could avoid messy.)

    3. Dangerous!

    i. It produces extreme pressure and if theres a

    leakage, oil will be forced out. Stopping the flow withbare hand would result in a punctuated wound.

    ii. While in process, oil became hot and flammable(though not easily happened). Therefore, it isadvisable to control the temperature by limiting it(turning off the machine if the limit is exceeded).H

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    General Application of

    Hydraulics

    Mobile Hydraulics Stationary Hydraulics

    Using manualhydraulics to control

    Crane

    Excavator

    Back-hoe

    Using electro-hydraulics to control

    Press machine

    Molding machine

    Lathe machine

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    HYDRAULICS

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    excavator

    boom

    stick

    bucket

    4 hydraulic cylinders

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    Sky tram is unique. It is self-propelled and travels on astationary cable. Because the tram moves instead of the

    cable, the operator can stop, start, and reverse any one carcompletely independently of any other car in the tram system.Integral to the design of the Sky-tram drive is a pump (drivenby a standard eight-cylinder gasoline engine) which suppliespressurized fluid to four hydraulic motors.

    Sky-tram

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    forklift

    Tilt Cylinders - are hydraulic cylinders that are mounted to the truck frame and themast. The tilt cylinders pivot the mast to assist in engaging a load.

    The Mast - is the vertical assembly that does the work of raising and lowering the load.

    It is made up of interlocking rails that also provide lateral stability. The interlocking rails

    may either have rollers or bushings as guides. The mast is either hydraulically operated

    by one or more hydraulic cylinders or it may be chain operated with a hydraulic motor

    providing motive power. It may be mounted to the front axle or the frame of the forklift.HYDRAULICS

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    Pressure Transfer Medium

    Form of liquid used

    Oil, when surrounding is not hot (car brake system)

    5. Water, when surrounding is hot / very hot. To

    avoid corrosion and to have better lubrication,water is usually mixed with some additives (foodprocessing, semiconductor process, papermanufacturing, construction works, medical andwelfare appliances).

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    Pressure Transfer Medium

    Vs.Gas Liquid

    Compressible, both intheory and practical

    Pressure is storable

    Not compressible intheory, however, iscompressible up to0.7% in practical (dueto small quantity of

    air in the liquid)

    Pressure is notstorable. Releasedwhen the pump stops.

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    Common Fluid in Hydraulic System

    Base stock may be either castor oil, glycol, ester,ethers, mineral oil, chutte etc.

    Some of the trade names for hydraulics fluidsinclude:

    DuradFyrquel

    Houghton-Safe

    lubritherinHYDRAULICS

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    A hydraulic fluid has the following 4 primary functions:-2. Transmit power3. Lubricate moving parts

    4. Seal clearance between mating parts5. Dissipate heat

    In addition a hydraulic fluid must be inexpensive and readilyavailable. A hydraulic fluid should have the followingproperties:-

    2. Good lubricity3. Ideal viscosity4. Chemical stability5. Fire resistance6. Low density7. Foam resistance

    8. Nontoxicity9. Low volatilit

    Fluid Selection

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    Fluids: Liquids and Gases

    Liquids refer to both gases and liquids.

    - is a fluid that, for a given mass, will have adefinite volume independent of the shape of itscontainer.

    - A free surface is also formed in the case of abody of water, such as a lake.Gases fluids that are readily compressible. In addition,

    their volume will vary to fill the vessel containing them.- the gas molecules always fill the entire vessel.

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    Parameter Liquid Gas

    Volume Has its own volume Volume is determinedby container

    Shape Takes shape of container Expands to completely

    but only to its volume fill and take the shapeof the container

    Compressibility Incompressible for most Readily compressibengineering applications

    Physical differences between liquids and gases

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    Key

    Equation

    Specific weight

    Density

    Kinematic viscosity

    V

    W=

    V

    m=

    =

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    Pressure is force exerted against a specific area (force per unit

    area) expressed in pounds per square inch (psi). Pressure cancause an expansion, or resistance to compression, of a fluid thatis being squeezed. A fluid is any liquid or gas (vapor). Force isanything that tends to produce or modify (push or pull) motionand is expressed in pounds.

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    Pressure

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    The earth has an atmosphere of air extending 50 milesup, and this air has weight. This air creates a head ofpressure that is called atmospheric pressure. A column ofair 1 square inch in cross section and the height of theatmosphere would weigh 14.7 pounds at sea level. Thus,the earth's atmospheric pressure is 14.7 psi at sea level.The role of atmospheric pressure in most hydraulic

    systems is significant.

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    Force

    Example of P at 100 kPa and A at 20 m,

    F = P x A, so F = 100 kPa x 20 m

    F = 2000 kN.

    Pressure is defined as force per unit area.Hence, pressure is the amount of force acting

    over a unit area, as indicated by

    Pressure (P) = Force (F) / Area(A)

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    Example

    A lifting is to lift a load of 15kN and is to havea system pressure of 75 bar. How large does

    the piston surface need to be?

    Solution:

    P = F/A

    A = F/P= 15000N/(75x105 Pa)

    = 0.002 m2

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    EXAMPLE

    F2=10 kN

    d2=3 m

    d1=0.5 m

    S2=0.5m

    F1 = ?

    S1 = ?

    S1

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    Answer

    F1/A1 = F2/A2

    A1 = 0.1963 m2

    (1 mark)A2 = 7.0686 m

    2 (1 mark)

    F1 = (F2/A2) x A1

    = (10 kN / 7.0686 m2) x 0.1963 m2

    = 0.278 kN (3 marks)

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    Answer

    W = F.s

    F1s1 = F2s2

    0.278 xs1 = 10 x 0.5

    s1 = 17.986 m (5 marks)

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    Sky-tram

    backhoe

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    FlowFlow is the movement of a hydraulic fluid caused by a differencein the pressure at two points. In a hydraulic system, flow isusually produced by the action of a hydraulic pump-a deviceused to continuously push on a hydraulic fluid. The two ways ofmeasuring flow are velocity and flow rate.

    a. Velocity. Velocity is the average speed at which afluid's particles move past a given point,

    measured in feet per second (fps). Velocity is animportant consideration in sizing the hydraulic lines thatcarry a fluid between the components.

    b. Flow Rate. Flow rate is the measure of how muchvolume of a liquid passes a point in a given time. Flow ratedetermines the speed at which a load moves and, therefore,

    is important when considering power.

    Pressure. An example of pressure is the air (gas) that fills anautomobile tire. As a tire is inflated, more air is squeezed into itthan it can hold. The air inside a tire resists the squeezing bypushing outward on the casing of the tire. The outward push of the

    air is pressure. Equal pressure throughout a confined area is acharacteristic of any pressurized fluid. For example, in an inflatedtire, the outward push of the air is uniform throughout. If it werenot, a tire would be pushed into odd shapes because of itselasticity.

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    Energy is the ability to do work. The three forms of energy arepotential, kinetic, and heat. Work measures accomplishments; itrequires motion to make a force do work. Power is the rate of

    doing work or the rate of energy transfer.

    a. Potential Energy. Potential energy is energy due to position.An object has potential energy in proportion to its verticaldistance above the earth's surface. For example, water held backby a dam represents potential energy because until it is released,the water does not work. In hydraulics, potential energy is astatic factor. When force is applied to a confined liquid, as shown

    in Figure 1-4, potential energy is present because of the staticpressure of the liquid. Potential energy of a moving liquid can bereduced by the heat energy released. Potential energy can alsobe reduced in a moving liquid when it transforms into kineticenergy. A moving liquid can, therefore, perform work as a resultof its static pressure and its momentum.