y l p p u s r e w o p y l c p i l p u a s r r d e y w h o e p h c T i l u 8 a . r o d y N h t i e n h U T 8 . o N t i n U
UNITS IN THIS COURSE UNIT 1
BASIC PRINCIPLES OF PNEUMATICS
UNIT 2
THE BASIC PNEUMATIC SYSTEM
UNIT 3
HOW THE AIR COMPRESSOR DOES ITS JOB
UNIT 4
PRESSURE SUPPLY COMPONENTS COMPONENTS
UNIT 5
CONTROLS AND END DEVICES
UNIT 6
HOW TO READ PNEUMATIC PNEUMATIC SCHEMATICS
UNIT 7
BASIC PRINCIPLES OF HYDRAULICS HYDRAULICS
UNIT 8
THE HYDRAULIC HYDRAULIC POWER SUPPLY
UNIT 9
HYDRAULIC HYDRAULIC SYSTEM OPERATION
UNIT 10
HYDRAULIC SYSTEM COMPONENTS COMPONENT S
UNIT 11
HYDRAULIC CONTROLS AND END DEVICES
UNIT 12
HOW TO READ HYDRAULIC HYDRAULIC SCHEMATICS
TABLE OF CONTENTS Para s c i l u a r d y h d s n c a i l s u c i a t r a d y m h u e d n P a : s c 8 i . t a o N m u e l e u n d P o : M 8 . o N e l u d o M
P a ge
7.0
COURSE OBJECTIVES
3
7.1
INTRODUCTION
4
7.2
WHY DO WE USE HYDRAULICS?
4
7.3
WHAT DO WE MEAN WHEN W E SAY "HYDRAULICS?"
6
7.4
HOW DOES THE HYDRAULIC SYSTEM DO WORK?
7
7.4.1
Hy Hydraulic Power
7
7.4.2
Pascal's Law
7
7.4.3
Viscosity
11
7.5
HOW HYDR YDRAULIC ULIC SYSTE YSTEM MS DIFFE IFFER R FROM PNEU PNEUM MATIC ATIC SYST SYSTE EMS
7.0
OBJECTIVES
13
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The trainee will be able to define hydraulic principles and describe how hydraulic pressure can be made to do work. The trainee will be able to demonstrate a knowledge of fluid dynamics and describe the differences between pneumatics and hydraulics. The trainee will be able to define Pascal's Law and demonstrate an understanding of the forces in hydraulic systems containing different size pistons.
INTRODUCTION In this Unit 7, Basic Principles of Hydraulics, you will learn what we mean by the term hydraulic power, what it is, and how it can do work in much the same way as pneumatic power. You will also learn the basic principles of fluid dynamics and gain an understanding of their forces.
WHY DO WE USE HYDRAULICS? As you learned in Unit 1, we can use air (a gas) to do work by storing energy in it. That energy is stored in the molecules and the atoms that make up those molecules by pushing them together in a process we call compression. (Refer to Unit 3 and see figures 7-1 and 7-2.) When the energy in that compressed air is released its force can be made to do work.
Figure 7-1. Atoms in a Gas Are Far Apart s c i l u a r d y h d n a s c i t a m u e n P : 8 . o N e l u d o M
Figure 7-2. Atoms in a Liquid Are Close Together Page 2/13
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Liquids cannot store energy. Hydraulic liquid is not compressible, so it cannot store energy. Hydraulic fluid can only transmit energy . The energy comes from the pump in the hydraulic system. That energy can be made to do work. This important property of liquids results in a hydraulic system that can be controlled very accurately. It also gives a smooth operating system. In figure 7-3 you can see how easily and quickly energy can be transferred when we try to compress a liquid.
Figure 7-3. Energy Is Easy to Transfer Through a Fluid s c i l s u c a i l r d u y a r h d d y n h a d s n c a i t s a c i m t u a e m n u P e : n 8 P . : o 8 N . e o l N u d e l o u M d o M
Remember, the molecules.of the fluid push equally in all directions. The force from the hammer transfers energy into the molecules of the fluid. The fluid can not be compressed, so they must send that energy to the jar. The jar cannot expand enough to absorb the energy (as air, a gas, could), so the resulting force breaks the jar. This is a good example of the way liquids can be made to do work. In this case, breaking the jar was the work done. You can also see by this example how easily the energy can be sent through the molecules of the fluid. This energy is transmitted (sent) with very little loss due to expansion or friction. Therefore, it is goo for doing work that needs more energy than pneumatic air pressure can supply.
Hydraulic fluid pressure can be made to lift or push much heavier things than Page 3/13
pneumatic air pressure. (See figure 7-4).
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Figure 7-4. A Hydraulic Brake System
7.3
WHAT DO WE MEAN WHEN WE SAY "HYDRAULICS?" The science of hydraulics includes the physical properties of liquids as well as the flow of liquids. Some of the hydraulic systems used in plants are: •
Hydraulic lifts that include jacks.
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Valve actuators.
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Control systems.
•
Impact and torque (tightening) tools.
s c i Dead weight testers for calibrating pressure devices. l u a r The advantages of hydraulics are the ease of control, as well as the making and d y sending of large forces and power through the use of small units. Hydraulic h cylinders and hydraulic motors can be started from a position at rest with maximum d n power. They can also reverse direction quickly through remote control. Hydraulic a s equipment is self-lubricating (by the hydraulic fluid) and has long service life. c i t a m u e n 7.4 HOW DOES THE HYDRAULIC SYSTEM DO WORK? P : 8 . o 7.4.1 Hydraulic Power N e l u Page 4/13 d o M •
Hydraulic power is the ability of the movement of fluid to do work. The work is done by applying pressure to the fluid at one point in a system and transmitting (sending) the pressure through the fluid to another point. The different parts of a fluid power system can be located in widely separated areas. The fluid forces can be transmitted up, down or around corners with only small losses. Very large output forces can be produced by much smaller input forces.
y l p p u s A fluid system that is adjusted properly gives smooth action. It is not affected by r e load changes. Over-pressure conditions are easy to control with automatic w o pressure release devices. p c i l Hydraulic power systems can provide both rotary and straight line power u a transmission. They are closed systems, so they are economical to operate. In a r d closed system, the fluid does not exhaust like the air in a pneumatic system. There y h is very little need to add more hydraulic fluid. e h T 8 7.4.2 Pascal's Law . o N The basis of modern hydraulics and pneumatics was developed in 1653 by Blaise t i Pascal of France. Mr. Pascal discovered that pressure on a fluid acts equally in all n directions. This discovery is called Pascal's Law. (See figure 7-5). U
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Figure 7-5. Pascal's Law
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When a liquid is under pressure, all surfaces of the container in contact with the liquid receive equal pressure. Solids press in one direction only. Liquids press on all the surfaces they contact. Gases press in all directions because the container is always completely filled with gas. (See figures 7-6 and 7-7).
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Figure 7-6 Exertion of Pressure This physical property is important to hydraulic action. It is this property of pressure that makes it possible to transmit forces through a hydraulic system. Liquids expand when heated. Expansion of a substance by heat is called thermal expansion. As liquids in hydraulic systems are pressurised, the temperature rises. Hydraulic systems usually have devices to protect the equipment from thermal expansion of the fluid.
Figure 7-7 Liquids Transmit Applied Pressure in All Directions
Liquids can increase the amount of work force. Force is a push or a pull that is applied to an object. If a hydraulic system is to operate, force must be applied at all times. Figure 7-8 shows a hydraulic system with force being applied by a pump. The pump applies a force on the hydraulic fluid. The fluid transfers the force to the piston in the cylinder. The piston rod applie the force to the lever. Work is performed in the Page 6/13
movement of the lever.
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Figure 7-8. Force in a Hydraulic System
Figure 7-9 shows a hydraulic system with two cylinders. The cylinder on the left has a cross sectional area of 1 cm 2 . The cylinder on the 2 right has a cross sectional area of 10 cm . The two cylinders are connected by a small pipe. If a 1 kg weight is placed on the piston rod of the left cylinder, the force of the weight will push the rod and piston down. The force of the 1 kg. weight is spread evenly over the area of the small cylinder. This force causes a pressure of 1 kg. per 1 cm 2 (1 kg/cm2 ) on the fluid in the cylinder. Pascal's Law as seen in figure 7-5, shows that fluid pressure is transmitted equally throughout a fluid system. The size or shape of the container does not affect the pressure. The pressure at all points in the left cylinder is 1 kg / cm 2 . The same pressure is found in the small connecting pipe. The pressure at all points in the larger cylinder on the right is also 1 kg/ cm 2. s c i l s u c a i l r d u y a r h d d y n h a d s n c a i t s a c i m t u a e m n u P e : n 8 P . : o 8 N . e o l N u d e l o u M d o M
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Figure 7-9 Hydraulic Systems Can Increase Work Force
With equal pressure in both cylinders and with the cross sectional area of the cylinder on the right ten times that of the left cylinder, ten times as much weight can be lifted. The 1 kg. weight pushing down on the piston in the left cylinder is able to lift a 10 kg. weight on the piston in the right cylinder. The amount of force doing the work of lifting the 10 kg. weight has been increased ten times. However, the small piston on the left would have to move ten times further than it moves the large piston on the right. The small piston would go down 10 cm to raise the large piston 1 cm.
7.4.3 Viscosity One of the most important physical properties of hydraulic fluid is its viscosity. Viscosity is the resistance to flow. It is a measure of the thickness of a liquid. Gasoline which flows easily has a low viscosity. Tar which flows slowly has a high viscosity. Viscosity is affected by changes in temperature. As the temperature of a liquid rises, the viscosity decreases. A warm liquid flows more easily than a cold liquid. s c i l u a r d y h d n a s c i t a m u e n P : 8 . o N e l u d o M
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Figure 7-10. Viscosity Decreases as Temperature Increases It is important that the viscosity of hydraulic fluid remains as constant as possible over the operating temperature range of the system. Hydraulic fluid that flows too easily will leak around pistons and pumps. The close fitting parts in hydraulic devices depend on the fluid to seal the parts. If the fluid leaks around the part without making a seal, there is a loss of pressure and a loss of work force. If the viscosity of the fluid is too high, the moving parts will be slow. The system is said to be sluggish. The power necessary to do the work will increase. The efficiency of the system will decrease. Efficiency is a comparison of the amount of work done to the amount of input power needed.
1.
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HOW HYDRAULIC SYSTEMS DIFFER FROM PNEUMATIC SYSTEMS
PNEUMATIC
HYDRAULIC
Uses air (gas) to transfer energy.
Uses oil to transfer energy.
Cooling of air is a problem
Heating and cooling of hydraulic fluid is a problem
Uses a compressor to make pressure
Uses a pump to make pressure
Gas (air) used is compressible.
Liquid used incompressible
(hydraulic
fluid)
is
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Uses complex filtering elements
Uses simple filtering elements
System is noisy.
System is quiet
It's an open system.
It's a closed system
Transmits low forces
Transmits high forces
. . . .
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