basic principles of hydraulics

7/27/2019 Basic Principles of Hydraulics

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The trainee will be able to define hydraulic principles and describe howhydraulic pressure can be made to do work.

The trainee will be able to demonstrate a knowledge of fluid dynamics anddescribe the differences between pneumatics and hydraulics.

The trainee will be able to define Pascal's Law and demonstrate anunderstanding of the forces in hydraulic systems containing different sizepistons.

7.1 INTRODUCTION

In this Unit 7, Basic Principles of Hydraulics, you will learn what we mean by theterm hydraulic power, what it is, and how it can do work in much the same way aspneumatic power. You will also learn the basic principles of fluid dynamics and gainan understanding of their forces.

7.2 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 thosemolecules by pushing them together in a process we call compression. (Refer toUnit 3 and see figures 7-1 and 7-2.) When the energy in that compressed air isreleased its force can be made to do work.

Figure 7-1. Atoms in a Gas Are Far Apart

Figure 7-2. Atoms in a Liquid Are Close Together

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pneumatic air pressure. (See figure 7-4).

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 theflow of liquids.

Some of the hydraulic systems used in plants are:

Hydraulic lifts that include jacks.

Valve actuators.

Control systems.

Impact and torque (tightening) tools.

Dead weight testers for calibrating pressure devices.

The advantages of hydraulics are the ease of control, as well as the making andsending of large forces and power through the use of small units. Hydrauliccylinders and hydraulic motors can be started from a position at rest with maximumpower. They can also reverse direction quickly through remote control. Hydraulicequipment is self-lubricating (by the hydraulic fluid) and has long service life.

7.4 HOW DOES THE HYDRAULIC SYSTEM DO WORK?

7.4.1 Hydraulic Power

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Hydraulic power is the ability of the movement of fluid to do work. The work is doneby 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 onlysmall losses. Very large output forces can be produced by much smaller inputforces.

A fluid system that is adjusted properly gives smooth action. It is not affected byload changes. Over-pressure conditions are easy to control with automaticpressure release devices.

Hydraulic power systems can provide both rotary and straight line powertransmission. They are closed systems, so they are economical to operate. In aclosed system, the fluid does not exhaust like the air in a pneumatic system. Thereis very little need to add more hydraulic fluid.

7.4.2 Pascal's Law

The basis of modern hydraulics and pneumatics was developed in 1653 by BlaisePascal of France. Mr. Pascal discovered that pressure on a fluid acts equally in alldirections. This discovery is called Pascal's Law. (See figure 7-5).

Figure 7-5. Pascal's Law

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When a liquid is under pressure, all surfaces of the container in contact with theliquid receive equal pressure. Solids press in one direction only. Liquids press on allthe surfaces they contact. Gases press in all directions because the container isalways completely filled with gas. (See figures 7-6 and 7-7).

Figure 7-6 Exertion of Pressure

This physical property is important to hydraulic action. It is this property of pressurethat makes it possible to transmit forces through a hydraulic system.

Liquids expand when heated. Expansion of a substance by heat is called thermalexpansion. As liquids in hydraulic systems are pressurised, the temperature rises.Hydraulic systems usually have devices to protect the equipment from thermalexpansion 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 isapplied to an object. If a hydraulic system is to operate, force must be applied at alltimes.

Figure 7-8 shows a hydraulic system with force being applied by a pump. The pumpapplies a force on the hydraulic fluid. The fluid transfers the force to the piston inthe cylinder. The piston rod applie the force to the lever. Work is performed in the

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movement of the lever.

Figure 7-8. Force in a Hydraulic System

Figure 7-9 shows a hydraulic system with two cylinders. The cylinder on the left hasa cross sectional area of 1 cm2 . The cylinder on the 2 right has a cross sectionalarea 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 rodand piston down. The force of the 1 kg. weight is spread evenly over the area ofthe small cylinder. This force causes a pressure of 1 kg. per 1 cm2 (1 kg/cm2 ) onthe fluid in the cylinder.

Pascal's Law as seen in figure 7-5, shows that fluid pressure is transmitted equallythroughout a fluid system. The size or shape of the container does not affect thepressure. The pressure at all points in the left cylinder is 1 kg / cm 2 . The samepressure is found in the small connecting pipe. The pressure at all points in thelarger cylinder on the right is also 1 kg/ cm2.

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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 thecylinder on the right ten times that of the left cylinder, ten times as much weightcan be lifted.

The 1 kg. weight pushing down on the piston in the left cylinder is able to lift a 10kg. 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 pistonon the left would have to move ten times further than it moves the large piston onthe 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 highviscosity. Viscosity is affected by changes in temperature. As the temperature of aliquid rises, the viscosity decreases. A warm liquid flows more easily than a cold

liquid.

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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 possibleover the operating temperature range of the system. Hydraulic fluid that flows tooeasily will leak around pistons and pumps. The close fitting parts in hydraulicdevices depend on the fluid to seal the parts. If the fluid leaks around the partwithout 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 issaid to be sluggish. The power necessary to do the work will increase. Theefficiency of the system will decrease. Efficiency is a comparison of the amount ofwork done to the amount of input power needed.

1. 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 isa problem

Uses a compressor to make pressure Uses a pump to make pressure

Gas (air) used is compressible. Liquid used (hydraulic fluid) isincompressible

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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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basic principles of hydraulics

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