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TERM PAPER

HYDRAULICS ACTUATION SYSTEM

Caterpillar 797B mining truck. Source: Caterpillar

Submitted By:

RAJESH KUMAR P2009ME1100

LALIT AGGARWAL P2009ME 1088

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Abstract

Table Of Contents

List of Figures

List of Tables

Objective

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Fluid Power

Fluid power is the transmission of forces and motions using a confined pressurized fluid. In hydraulic fluid power systems the fluid is oil or water. Fluid power is ideal for high speed, high force, and high power applications. Compared to all other actuation technologies, including electric motors, fluid power is unsurpassed for force and power density and is capable of generating extremely high forces with relatively lightweight cylinder actuators. Fluid power systems have a higher bandwidth than electric motors and can be used in applications that require fast starts, stops and reversals, or that require high frequency oscillations. Because oil has a high bulk modulus, hydraulic systems can be finely controlled for precision motion applications. Major advantage of fluid power lies in its compactness and flexibility. Fluid power cylinders are relatively small and light for their weight and flexible hoses allows power to be snaked around corners, over joints and through tubes leading to compact packaging without sacrificing high force and high power. A good example of this compact packaging is Jaws of Life rescue tools for ripping open automobile bodies to extract those trapped within. But there are some disadvantages also.

1. Hydraulic systems can leak oil at connections and seals. 2. Hydraulic power is not as easy to generate as electric power and requires a heavy,

noisy pump. 3. Hydraulic fluids can cavitate and retain air resulting in spongy performance and loss

of precision. Hydraulic can become contaminated with particles and require careful filtering.

4. The physics of fluid power is more complex than that of electric motors which makes modelling and control more challenging.

Research is going on not only to overcome these challenges but also to open fluid power to new applications, for example tiny robots and wearable power-assist tools.

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Some main Applications Of Fluid Power (Hydraulics)

Fluid power is extensively used throughout industry and throughout the world because of its

major advantages and here are some examples.

Earth moving machines such as excavators

Winches on cranes and boats

Rams in forging and extrusion processes

Automated production lines

Aeroplane controls

Automated assembly units

Machine tools

Braking system

Roller coaster

Earthquake simulators

Figures showing application of fluid power

Caterpillar 797B mining truck. Source: Caterpillar

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40,000 ton forging press. Source: Shultz Steel

MAST Laboratory for earthquake simulation. Source: MAST Lab.

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Caterpillar 345C L excavator. Source: Caterpillar.

Hypersonic XLC roller coaster with hydraulic lanuch assist. Source: Wikipedia image

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Introduction to Hydraulics

Hydraulics refers to the means and mechanisms of transmitting power through liquids.

Hydraulic Actuators, as used in industrial process control, employ hydraulic pressure to drive

an output member. These are used where high speed and large forces are required. The

fluid used in hydraulic actuator is highly incompressible so that pressure applied can be

transmitted instantaneously to the member attached to it.

In fluid power, hydraulics is used for the generation, control, and transmission of power by

the use of pressurized liquids. Hydraulic topics range through most science and engineering

disciplines, and cover concepts such as pipe flow, dam design, fluidics and fluid control

circuitry, pumps, turbines, hydropower, computational fluid dynamics, flow measurement,

river channel behaviour and erosion.

It was not, however, until the 17th

century that the branch of hydraulics with which we are

to be concerned first came into use. Based upon a principle discovered by the French

scientist Pascal, it relates to the use of confined fluids in transmitting power, multiplying

force and modifying motions.

Then, in the early stages of the industrial revolution, a British mechanic named Joseph

Bramah utilized Pascal’s discovery in developing a hydraulic press.

Principle Behind

Pascal’s Law

“Pressure applied to a confined fluid at any point is transmitted undiminished and equally throughout the fluid in all directions and acts upon every part of the confining vessel at right angles to its interior surfaces.”

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Amplification of Force or Hydraulic “Leverage”

As the pressure in the system is the same, the force that the fluid gives to the surroundings

is therefore equal to pressure multiplies by area. In such a way, a small piston feels a small

force and a large piston feels a large force.

The same principle applies for a hydraulic pump with a small swept volume that asks for a

small torque, combined with a hydraulic motor with a large swept volume that gives a large

torque. In such a way a transmission with a certain ratio can be built.

Note : Pressure remains same everywhere only force changes due to change in area and

work done in the process remains same i.e. input = output.

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The illustration given below shows that 1 lb. of force exerted on a 1 sq. in. piston, moved 10

in. will lift 10 lbs. a distance of 1 in. with a 10 sq. in. piston. The larger piston will move a

shorter distance, but provides the mechanical advantage to lift a much heavier load. This

mechanical workforce advantage is hydraulic leverage.

Where hydraulic actuation should be used?

Hydraulic actuation should be used for any installations where there are a number of

systems that can be operated from a single system; installations where the gates, valves,

and actuators must be submerged; and installations where the system must operate in a

power failure or other emergency. Hydraulic actuators are particularly desirable where

equipment is to be operated frequently, where loads are high, where the speed of

operation is high or must be varied during operation, and where they are located in a

hazardous area requiring explosion-proof and intrinsically safe equipment.

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Components of Hydraulic Actuation Systems

1. Hydraulic Fluid

Hydraulic fluid must be essentially incompressible to be able to transmit power

instantaneously from one part of the system to another. At the same time, it should

lubricate the moving parts to reduce friction loss and cool the components so that the heat

generated does not lead to fire hazards. It also helps in removing the contaminants to filter.

Figure below shows the role played by hydraulic fluid films in lubrication and sealing.

2. The Fluid Delivery Subsystem

It consists of the components that hold and carry the fluid from the pump to the actuator. It

is made up of the following components.

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3. Reservoir

It holds the hydraulic fluid to be circulated and allows air entrapped in the fluid to escape.

This is an important feature as the bulk modulus of the oil, which determines the stiffness of

hydraulic system, deteriorates considerably in the presence of entrapped air bubbles. It also

helps in dissipating heat.

4. Filter

The hydraulic fluid is kept clean in the system with the help of filters and strainers. Metal particles are continually produced by mechanical components and need to be removed along with other contaminants, which can cause blocking of the orifices of servo-valves or cause jamming of spools.

The graphical symbol for Reservoirs and Filters

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5. Line

Pipe, tubes and hoses, along with the fittings or connectors, constitute the conducting lines

that carry hydraulic fluid between components. Lines convey the fluid and also dissipate

heat. There are various kinds of lines in a hydraulic system. The working lines carry the fluid

that delivers the main pump power to the load. The pilot lines carry fluid that transmits

controlling pressures to various directional and relief valves for remote operation or safety.

Lastly there are drain lines that carry the fluid that inevitably leaks out, to the tank.

The various kinds of lines in a hydraulic system

Connection Arrangement of Filter and Lines with a Reservoir

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6. Fittings and Seals Various additional components are needed to join pipe or tube sections, create bends and

also to prevent internal and external leakage in hydraulic systems. Although some amount

of internal leakage is built-in, to provide lubrication, excessive internal leakage causes loss of

pump power since high pressure fluid returns to the tank, without doing useful work.

External leakage, on the other hand, causes loss of fluid and can create fire hazards, as well

as fluid contamination. Various kinds of sealing components are employed in hydraulic

systems to prevent leakage. A typical such component, known as the O-ring is shown below

in Figure.

Sealing by O-rings

7. Hydraulic Pumps The pump converts the mechanical energy of its prime-mover to hydraulic energy by

delivering a given quantity of hydraulic fluid at high pressure into the system. Generally, all

pumps are divided into two categories, namely, hydrodynamic or non-positive displacement

and hydrostatic or positive displacement. Hydraulic systems generally employ positive

displacement pumps only.

The graphical symbol for Pumps

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Different types of pumps

Hydrostatic or Positive Displacement Pumps These pumps deliver a given amount of fluid for each cycle of motion, that is, stroke or

revolution. Their output in terms of the volume flow rate is solely dependent on the speed

of the prime-mover and is independent of outlet pressure notwithstanding leakage. These

pumps are generally rated by their volume flow rate output at a given drive speed and by

their maximum operating pressure capability which is specified based on factors of safety

and operating life considerations. In theory, a pump delivers an amount of fluid equal to its

displacement each cycle or revolution. In reality, the actual output is reduced because of

internal leakage or slippage which increases with operating pressure. There are various

types of pumps used in hydraulic systems as described below.

Gear Pumps

The construction of a Gear Pump

A gear pump develops flow by carrying fluid between the teeth of two meshed gears. One

gear is driven by the drive shaft and turns the other, which is free. The pumping chambers

formed between the gear teeth are enclosed by the pump housing and the side plates. A

low pressure region is created at the inlet as the gear teeth separate. As a result, fluid flows

in and is carried around by the gears. As the teeth mesh again at the outlet, high pressure is

created and the fluid is forced out. Figure shows the construction of a typical internal gears

pump; Most gear type pumps are fixed displacement. They range in output from very low to

high volume. They usually operate at comparatively low pressure.

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Vane Pumps In a vane pump a rotor is coupled to the drive shaft and turns inside a cam ring. Vanes are fitted

to the rotor slots and follow the inner surface of the ring as the rotor turns. Centrifugal force

and pressure under the vanes keep them pressed against the ring. Pumping chambers are

formed between the vanes and are enclosed by the rotor, ring and two side plates. At the pump

inlet, a low pressure region is created as the space between the rotor and ring increases. Oil

entering here is trapped in the pumping chambers and then is pushed into the outlet as the

space decreases.

Principle of Operation of Vane Pumps

Piston Pumps In a piston pumps, a piston reciprocating in a bore draws in fluid as it is retracted and expels it

on the forward stroke. Two basic types of piston pumps are radial and axial.

A radial pump has the pistons arranged radially in a cylinder block and in an axial pump the

pistons are parallel to the axis of the cylinder block. The latter may be further divided into in-line

(swash plate or wobble plate) and bent axis types.

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8. Motors Motors work exactly on the reverse principle of pumps. In motors fluid is forced into the

motor from pump outlets at high pressure. This fluid pressure creates the motion of the

motor, shaft and finally goes out through the motor outlet port and return to tank. All three

variants of motors, already described for pumps, namely Gear Motors, Vane Motors and

Piston motors are in use.

The graphical symbol for Motors

9. Accumulators

Unlike gases the fluids used in hydraulic systems cannot be compressed and stored to cater

to sudden demands of high flow rates that cannot be supplied by the pump. An accumulator

in a hydraulic system provides a means of storing these incompressible fluids under

pressure created either by a spring or compressed gas. Any tendency for pressure to drop at

the inlet causes the spring or the gas to force the fluid back out, supplying the demand for

flow rate.

Accumulator

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Types of Accumulators

Spring-Loaded Accumulators

In a spring loaded accumulator, pressure is applied to the fluid by a coil spring behind the

accumulator piston. The pressure is equal to the instantaneous spring force divided by the

piston area. The pressure therefore is not constant since the spring force increases as fluid

enters the chamber and decreases as it is discharged.

Spring loaded accumulators can be mounted in any position. The spring force, i.e., the

pressure range is not easily adjusted, and where large quantities of fluid are required spring

size has to be very large.

A spring-loaded accumulator

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Gas Charged Accumulator The most commonly used accumulator is one in which the chamber is pre-charged with an inert

gas, such as dry nitrogen. A gas charged accumulator should be pre-charged while empty of

hydraulic fluid. Accumulator pressure varies in proportion to the compression of the gas,

increasing as pumped in and decreasing as it is expelled.

A gas-charged accumulator

10. Cylinders

Cylinders are linear actuators, that is, they produce straight-line motion and/or force.

Cylinders are classified as single-or double-acting as illustrated in Figures with the graphical

symbol for each type.

Single Acting Cylinder: It has only one fluid chamber and exerts force in only one direction.

When mounted vertically, they often retract by the force of gravity on the load. Ram type

cylinders are used in elevators, hydraulic jacks and hoists.

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Cross Sectional View of Single-acting Cylinder

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Double-Acting Cylinder:

The double-acting cylinder is operated by hydraulic fluid in both directions and is capable of

a power stroke either way. In single rod double-acting cylinder there are unequal areas

exposed to pressure during the forward and return movements due to the cross-sectional

area of the rod. The extending stroke is slower, but capable of exerting a greater force than

when the piston and rod are being retracted.

Double-rod double-acting cylinders are used where it is advantageous to couple a load to

each end, or where equal displacement is needed on each end. With identical areas on

either side of the piston, they can provide equal speeds and/or equal forces in either

direction. Any double-acting cylinder may be used as a single-acting unit by draining the

inactive end to tank.

Cross Sectional View of Double-acting Cylinder

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11. Control Valve

Control valves are essential and appear in all fluid power systems. Valves are sometimes

categorized by function, which includes directional control valves for directing fluid flow to

one or the other side of a cylinder or motor, pressure control valves for controling the fluid

pressure at a point and flow control valves for limiting the fluid flow rate in a line, which in

turn limits the extension or retraction velocities of a piston.

On/off valves can only be in the states defined by their positions while proportional valves

are continuously variable and can take on any position in their working range. A servo valve

is a proportional valve with an internal closed-loop feedback mechanism to maintin precise

control over the valve behaviour.

Types of control valves Left to right: hand-operated directional valve for a log splitter, On-off

miniature, solenoid actuated pneumatic valve, Precision proportional pneumatic valves, High

precision, flapper-nozzle hydraulic servo valve.

Valve actuation symbols Left to right: push-button, lever, springreturn, solenoid, pilot-line.

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12. Hydraulic Circuit Drawings

Accurate diagrams of hydraulic circuits are essential to the technician who must diagnose and repair

possible problems. The diagram shows how the components will interact. It shows the technician

how it works, what each component should be doing and where the oil should be going, so that he

can diagnose and repair the system.

There are two types of circuit diagrams.

1. Cutaway Circuit Diagrams show the internal construction of the components as well as the

oil flow paths. By using colors, shades or various patterns in the lines and passages, they are

able to show many different conditions of pressure and flow.

2. Schematic Circuit Diagrams are usually preferred for troubleshooting because of their ability

to show current and potential system functions. A schematic diagram is made up of

consistent geometric symbols for the components and their controls and connections.

Schematic symbol systems: I.S.O. = International Standards Organization. A.N.S.I. = American National Standards Institute A.S.A = American Standards Association J.I.C. = Joint Industry Conference

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A Complete Hydraulic Schematic

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Different Kinds of Hydraulic Actuators

A hydraulic actuation system is a drive or transmission system that uses pressurized

hydraulic fluid to drive hydraulic machinery. The term hydrostatic refers to the transfer of

energy from flow and pressure, not from the kinetic energy of the flow.

All hydraulic systems are essentially the same regardless of the application. There are four

basic components required; a reservoir to hold the fluid; a pump to force the fluid through

the system; valves to control the flow; and an actuator (motor) to convert the fluid energy

into mechanical force to do the work.

1. Hydraulic Jack

The principle behind most hydraulic systems is similar to that of the basic hydraulic jack. Oil

from the reservoir is drawn past a check ball into the piston type pump during the piston's

up-stroke.

When the piston in the pump is pushed downward, oil will be directed past a second check

ball into the cylinder. As the pump is actuated up and down, the incoming oil will cause the

cylinder ram to extend. The lift cylinder will hold its extended position because the check

ball is being seated by the pressure against it from the load side of the cylinder. More liquid

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is pumped under a large piston to raise it. To lower a load, a third valve (needle valve)

opens, which opens an area under a large piston to the reservoir. The load then pushes the

piston down and forces the liquid into the reservoir.

Because the pump displacement is usually much smaller than the cylinder, each stroke of

the pump will move the cylinder a very small amount. If the cylinder is required to move at a

faster rate, the surface area of the pump piston must be increased and/or the rate which

the pump is actuated must be increased. Oil FLOW gives the cylinder ram its SPEED of

movement and oil PRESSURE is the work force that lifts the load.

2. Hydraulic brake

The hydraulic brake is an arrangement of braking mechanism which uses brake fluid, typically containing ethylene glycol, to transfer pressure from the controlling unit, which is usually near the operator of the vehicle, to the actual brake mechanism, which is usually at or near the wheel of the vehicle.

Construction

The most common arrangement of hydraulic brakes consists of the following:

Brake pedal or lever A pushrod (also called an actuating rod) A master cylinder assembly containing a piston assembly (made up of either one or

two pistons, a return spring, a series of gaskets/ O-rings and a fluid reservoir) Reinforced hydraulic lines Brake calliper assembly usually consisting of one or two hollow aluminum or

chrome-plated steel pistons (called caliper pistons), a set of thermally conductive brake pads and a rotor (also called a brake disc) or drum attached to an axle.

The system is usually filled with a glycol-ether based brake fluid (other fluids may also be

used).

System Operation

Within a hydraulic brake system, as the brake pedal is pressed, a pushrod exerts force on the piston(s) in the master cylinder causing fluid from the brake fluid reservoir to flow into a pressure chamber through a compensating port which results in an increase in the pressure of the entire hydraulic system. This forces fluid through the hydraulic lines toward one or more callipers where it acts upon one or two calliper pistons sealed by one or more seated O-rings which prevent the escape of any fluid from around the piston. The brake calliper

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pistons then apply force to the brake pads. Subsequent release of the brake pedal/ lever allows spring(s) to return the master piston(s) back into position. This relieves the hydraulic pressure on the calliper allowing the brake piston in the calliper assembly to slide back into its housing and the brake pads to release the rotor. The hydraulic braking system is designed as a closed system: unless there is a leak within the system, none of the brake fluid enters or leaves it, nor does it get consumed through use.

Advantages and disadvantages of Hydraulic Actuators Hydraulic actuators are widely used in many systems such as drives of machine tools, rolling

mills, pressing, road and building machines, transport and agricultural machines. There are

great advantages of hydraulic actuator which differentiate them from mechanical and

electric transfers which explain such their widespread application.

Advantages:

1. Power-to-weight ratio: Hydraulic components, because of their high speed and pressure capabilities, can provide high power output with vary small weight and size, say, in comparison to electric system components. It is one of the reasons that hydraulic equipment finds wide usage in aircrafts, where dead-weight must be reduced to a minimum.

2. Infinitely variable control of gear-ratio in a wide range and an opportunity to create

the big reduction ratio which can be used in high power appliances for example to

open of the gate of a canal or a dam.

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3. Tall Condition and Overload Protection: A hydraulic actuator can be stalled without damage when overloaded, and will start up immediately when the load is reduced. The pressure relief valve in a hydraulic system protects it from overload damage. During stall, or when the load pressure exceeds the valve setting, pump delivery is directed to tank with definite limits to torque or force output. The only loss encountered is in terms of pump energy. On the contrary, stalling an electric motor is likely to cause damage. Likewise, engines cannot be stalled without the necessity for restarting.

4. Variable Speed and Direction: Most large electric motors run at adjustable, but constant

speeds. It is also the case for engines. The actuator of a hydraulic system, however, can be driven at speeds that vary by large amounts and fast, by varying the pump delivery or using a flow control valve. In addition, a hydraulic actuator can be reversed instantly while in full motion without damage. This is not possible for most other prime movers

Disadvantages: It is also necessary to highlight the disadvantages of hydraulic actuators:

1. Efficiency: Efficiency of a volumetric hydraulic actuator is a little bit lower, than efficiency of mechanical and electric transfers, and during regulation it is reduced.

2. Conditions of operation: It’s operational condition influence its characteristics. 3. Hydraulic system is susceptible to contaminations & foreign object damage (FOD). 4. Mishandling and constant exposure to hydraulic fluid and its gas fumes without proper

equipment and precautions is a health risk.

Conclusion Modern robotic systems are difficult. drives are a mechanical part of this systems. Three types of drives are basically used now: electric, pneumatic and hydraulic. Each type has its own advantages and disadvantages.

References Artemieva T.V., Lisenko T.M. Hydraulic, hydromachines and hydropneumoactuator, Moscow, 2005 4. Majumdar S.R. Oil Hydraulic Systems : Principles and Maintenance, McGraw-Hill, 2001

Ian C. Turner, Engineering Applications of Pneumatics and Hydraulics, Butterworth- Heinemann, 1995

Appendices

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