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Hydraulics & Pneumatics Technology DMM3623

hydraulics and pneumatics

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Page 1: hydraulics and pneumatics

Hydraulics & Pneumatics Technology

DMM3623

Page 2: hydraulics and pneumatics

What is Fluid Power ?

Technology that deals with generation, control and transmission of power, using pressurized fluidsexample: 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 readily exhausted into the atmosphere after completing task)H

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Hydraulic BasicsHydraulics is the science of transmitting force and/or motion through the medium of a confined liquid. In a hydraulic device, power is transmitted by pushing on a confined liquid. The transfer of energy takes place because a quantity of liquid is subject to pressure. To operate liquid-powered systems, the operator should have a knowledge of the basic nature of liquids. This chapter covers the properties of liquids and how they act under different conditions.

Hydraulics

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Fluid System

Fluid transport Delivery of a fluid from one location to another to accomplish some useful purpose.

Fluid powerDesigned specifically to perform work. Work-accomplished by a pressurized fluid bearing directly on 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 for high force, thus, incur high cost as well as wastage of compressed air)

2. Very slow and linear movements (if pneumatics, too slow will create jerking on the cylinder)

3. Very exact positioning (stops exactly on a point). Compared to pneumatics, there’s a tolerance and if a load is given, it will move a little.

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4. Start-up under heavy loads (Possible to start the motor & process a job with a load attached to it. This is because there is oil in the actuator though the pump is idling).

5. Simplicity, safety, economyIn general, fluid power systems use fewer moving parts than comparable mechanical or electrical systems. Thus, they are simpler to maintain and operate. This, in turn, maximizes safety, compactness and reliability.

Advantages of using Hydraulics

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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, the surrounding area could be messy. However, constant cleaning could avoid messy.)

3. Dangerous!

i. It produces extreme pressure and if there’s a leakage, oil will be forced out. Stopping the flow with bare hand would result in a punctuated wound.

ii. While in process, oil became hot and flammable (though not easily happened). Therefore, it is advisable 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 manual hydraulics to control

•Crane

•Excavator

•Back-hoe

Using electro-hydraulics to control

•Press machine

•Molding machine

•Lathe machine

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excavator

boom

stick

bucket

4 hydraulic cylinders

Page 12: hydraulics and pneumatics

Sky tram is unique. It is self-propelled and travels on a stationary cable. Because the tram moves instead of the cable, the operator can stop, start, and reverse any one car completely independently of any other car in the tram system. Integral to the design of the Sky-tram drive is a pump (driven by a standard eight-cylinder gasoline engine) which supplies pressurized fluid to four hydraulic motors.

Sky-tram

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forkliftTilt Cylinders - are hydraulic cylinders that are mounted to the truck frame and the

mast. 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. HYD

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Pressure Transfer Medium

Form of liquid used

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

1. Water, when surrounding is hot / very hot. To avoid corrosion and to have better lubrication, water is usually mixed with some additives (food processing, semiconductor process, paper manufacturing, construction works, medical and welfare appliances).

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Pressure Transfer Medium

Vs. Gas Liquid

•Compressible, both in theory and practical

•Pressure is storable

• Not compressible in theory, however, is compressible up to 0.7% in practical (due to small quantity of air in the liquid)

•Pressure is not storable. Released when 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 fluids include:

•Durad®

•Fyrquel®

•Houghton-Safe®

•lubritherin®HYD

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A hydraulic fluid has the following 4 primary functions:-1. Transmit power2. Lubricate moving parts3. Seal clearance between mating parts4. Dissipate heat

In addition a hydraulic fluid must be inexpensive and readily available. A hydraulic fluid should have the following properties:-

1. Good lubricity2. Ideal viscosity3. Chemical stability4. Fire resistance5. Low density6. Foam resistance7. Nontoxicity8. Low volatility

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 a

definite volume independent of the shape of its container.

- A free surface is also formed in the case of a body 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 shape

of the container

Compressibility Incompressible for most Readily compressibleengineering 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 can cause an expansion, or resistance to compression, of a fluid that is being squeezed. A fluid is any liquid or gas (vapor). Force is anything that tends to produce or modify (push or pull) motion and is expressed in pounds.

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Pressure

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The earth has an atmosphere of air extending 50 miles up, and this air has weight. This air creates a head of pressure that is called atmospheric pressure. A column of air 1 square inch in cross section and the height of the atmosphere 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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Continue…

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Force

Example of P at 100 kPa and A at 20 m²,

F = P x A, so F = 100 kPa x 20 m²

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 have a system pressure of 75 bar. How large does the piston surface need to be?

Solution:P = F/AA = 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 m2 (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 x s1 = 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 difference in the pressure at two points. In a hydraulic system, flow is usually produced by the action of a hydraulic pump-a device used to continuously push on a hydraulic fluid. The two ways of measuring flow are velocity and flow rate.

a. Velocity. Velocity is the average speed at which a fluid's particles move past a given point, measured in feet per second (fps). Velocity is an important consideration in sizing the hydraulic lines that carry a fluid between the components.

b. Flow Rate. Flow rate is the measure of how much volume of a liquid passes a point in a given time. Flow rate determines 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 an automobile tire. As a tire is inflated, more air is squeezed into it than it can hold. The air inside a tire resists the squeezing by pushing outward on the casing of the tire. The outward push of the air is pressure. Equal pressure throughout a confined area is a characteristic of any pressurized fluid. For example, in an inflated tire, the outward push of the air is uniform throughout. If it were not, a tire would be pushed into odd shapes because of its elasticity.

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Energy is the ability to do work. The three forms of energy are potential, kinetic, and heat. Work measures accomplishments; it requires 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 vertical distance above the earth's surface. For example, water held back by a dam represents potential energy because until it is released, the water does not work. In hydraulics, potential energy is a static factor. When force is applied to a confined liquid, as shown in Figure 1-4, potential energy is present because of the static pressure of the liquid. Potential energy of a moving liquid can be reduced by the heat energy released. Potential energy can also be reduced in a moving liquid when it transforms into kinetic energy. A moving liquid can, therefore, perform work as a result of its static pressure and its momentum.