Remote Operated Screw Jack1

8/2/2019 Remote Operated Screw Jack1

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REMOTE OPERATEDSCREW JACK


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INTRODUCTION

Fabrication of a motorized screw jack is easy especially

when the parts are avaialble in the market. This mechanical

engineering project can be easily completed by integrating an

electric motor with a screw jack.

A screw jack or a Jackscrew is operated by turning a lead screw.

The height of the jack is adjusted by turning the lead screw. This

can be done either manually or by integrating an electric motor with

it. This integration is our project.

The difficult part in the project may be finding a low speed motor

that is able to work at 12V. This is because the battery output of anautomobile is 12V, and the electricity needed for the operation of

the screw jack is taken from this battery. Another problem will be

regarding speed reduction. 12V motors usually operate at higher

speeds, likely at 4000 or 5300 rpm. So reducing this high rpm to

the required lower rpms for the operation of screw jack without

bulky accessories or power loss can be challenging. But still this is

one of the easiest projects in mechanical.


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Spur gears

1. Introduction

2. Reduction factor

3. Gear ratio

Introduction

Power transmission is the movement of energy from its place of generation to alocation where it is applied to performing useful work

A gear is a component within a transmission device that transmits rotational force

to another gear or device

Gears are wheels with teeth. Gears mesh together and make things turn. Gears

are used to transfer motion or power from one moving part to another.


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Gears are generally used for one of four different reasons:

To reverse the direction of rotation

To increase or decrease the speed of rotation

To move rotational motion to a different axis

To keep the rotation of two axis synchronized

Drawing gears

It would be very difficult to draw gears if you had to draw all the teeth every time you

wanted to design a gear system. For this reason a gear can be represented by drawingtwo circles.

CIRCLES OVERLAP WHERE TEETH MESH


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Types of gears

According to the position of axes of the shafts.

a. Parallel

1. Spur Gear

2. Helical Gear

3. Rack and Pinion

b. Intersecting

Bevel Gear

c. Non-intersecting and Non-parallelWorm and worm gears

Spur gears

Teeth is parallel to axis of rotation

Transmit power from one shaft to another parallel shaft

Used in Electric screwdriver, oscillating sprinkler, windup alarm clock, washing

machine and clothes dryer


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External and internal spur gears

NOMENCLATURE OF SPUR GEARS


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1. Pitch surface: The surface of the imaginary rolling cylinder (cone, etc.) that the

toothed gear may be considered to replace.

2. Pitch circle: A right section of the pitch surface.

3. Addendum circle: A circle bounding the ends of the teeth, in a right section of thegear.

4. Root (or dedendum) circle: The circle bounding the spaces between the teeth, in

a right section of the gear.

5. Addendum: The radial distance between the pitch circle and the addendum circle.

6. Dedendum: The radial distance between the pitch circle and the root circle.

7. Clearance: The difference between the dedendum of one gear and the addendum

of the mating gear.

8. Face of a tooth: That part of the tooth surface lying outside the pitch surface.

9. Flank of a tooth: The part of the tooth surface lying inside the pitch surface.

10.Circular thickness (also called the tooth thickness): The thickness of the tooth

measured on the pitch circle. It is the length of an arc and not the length of a

straight line.

11.Tooth space: pitch diameter The distance between adjacent teeth measured on

the pitch circle.

12.Backlash: The difference between the circle thickness of one gear and the tooth

space of the mating gear.

13.Circular pitch (Pc) : The width of a tooth and a space, measured on the pitch

circle.

14. Diametral pitch (Pd): The number of teeth of a gear unit pitch diameter. The

diametral pitch is, by definition, the number of teeth divided by the pitch diameter.

That is,

Where

N

D

cP

D

N

dP


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Pd = diametral pitch

N = number of teeth

D = pitch diameter

15. Module (m): Pitch diameter divided by number of teeth. The pitch diameter is

usually specified in inches or millimeters; in the former case the module is the

inverse of diametral pitch.

m = D/N

VELOCITY RATIO OF GEAR DRIVE

d = Diameter of the wheel

N = Speed of the wheel

= Angular speed

2

1

1

2

1

2

d

d

N

N


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GEAR TRAIN

1. A gear train is two or more gear working together by meshing their teeth andturning each other in a system to generate power and speed

2. It reduces speed and increases torque

3. Electric motors are used with the gear systems to reduce the speed and increase

the torque

Types of gear train

1) Simple gear train

2) Compound gear train

3) Planetary gear train

Simple Gear Train

1. The most common of the gear train is the gear pair connecting parallel

shafts. The teeth of this type can be spur, helical or herringbone.

2. Only one gear may rotate about a single axis

Compound gear train

1. For large velocities, compound arrangement is preferred

2. Two or more gears may rotate about a single axis


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1. What is power transmission?

2. Why gear drives are called positively driven?

3. What is backlash in gears?

4. What are the types of gears available?

5. What is gear train? Why gear trains are used?

6. Why intermediate gear in simple gear train is called idler?

7. What is the advantage of using helical gear over spur gear?

8. List out the applications of gears

9. Define the term module in gear tooth

10. What is herringbone gear?


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LEADSCREW

A leadscrew (or lead screw), also known as a power

screw or translation screw,[2]is a screw designed to translate turning motion into linear

motion. Common applications are machine slides (such as in machine

tools), vises, presses, and jacks.Lead screws are manufactured in the same way as other thread forms.

A lead screw can be used in conjunction with split nut.
http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari202-1http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari202-1http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari202-1http://en.wikipedia.org/wiki/Screwhttp://en.wikipedia.org/wiki/Turning_motionhttp://en.wikipedia.org/wiki/Linear_motionhttp://en.wikipedia.org/wiki/Linear_motionhttp://en.wikipedia.org/wiki/Machine_toolhttp://en.wikipedia.org/wiki/Machine_toolhttp://en.wikipedia.org/wiki/Vise_(tool)http://en.wikipedia.org/wiki/Mechanical_presshttp://en.wikipedia.org/wiki/Jack_(device)http://en.wikipedia.org/wiki/Thread_formhttp://en.wikipedia.org/wiki/Split_nuthttp://en.wikipedia.org/wiki/Split_nuthttp://en.wikipedia.org/wiki/Thread_formhttp://en.wikipedia.org/wiki/Jack_(device)http://en.wikipedia.org/wiki/Mechanical_presshttp://en.wikipedia.org/wiki/Vise_(tool)http://en.wikipedia.org/wiki/Machine_toolhttp://en.wikipedia.org/wiki/Machine_toolhttp://en.wikipedia.org/wiki/Linear_motionhttp://en.wikipedia.org/wiki/Linear_motionhttp://en.wikipedia.org/wiki/Turning_motionhttp://en.wikipedia.org/wiki/Screwhttp://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari202-1


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Types

Power screws are classified by the geometry of their thread. V-threads are less suitable

for leadscrews than others such as Acme because they have more friction between the

threads. Their threads are designed to induce this friction to keep the fastener fromloosening. Leadscrews, on the other hand, are designed to minimize

friction.[4]Therefore, in most commercial and industrial use, V-threads are avoided for

leadscrew use. Nevertheless, V-threads are sometimes successfully used as

leadscrews, for example on microlathes and micromills.[5]

Square thread

Main article:Square thread form

Square threads are named after their square geometry. They are the most efficient,having the least friction, so they are often used for screws that carry high power. But

they are also the most difficult to machine, and are thus the most expensive.

Acme thread
http://en.wikipedia.org/wiki/Screw_threadhttp://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari203-3http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari203-3http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari203-3http://en.wikipedia.org/wiki/Microlathehttp://en.wikipedia.org/wiki/Leadscrew#cite_note-4http://en.wikipedia.org/wiki/Leadscrew#cite_note-4http://en.wikipedia.org/wiki/Leadscrew#cite_note-4http://en.wikipedia.org/wiki/Square_thread_formhttp://en.wikipedia.org/wiki/Mechanical_efficiencyhttp://en.wikipedia.org/wiki/Frictionhttp://en.wikipedia.org/wiki/File:Acme_thread.jpghttp://en.wikipedia.org/wiki/File:Acme_thread.jpghttp://en.wikipedia.org/wiki/Frictionhttp://en.wikipedia.org/wiki/Mechanical_efficiencyhttp://en.wikipedia.org/wiki/Square_thread_formhttp://en.wikipedia.org/wiki/Leadscrew#cite_note-4http://en.wikipedia.org/wiki/Microlathehttp://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari203-3http://en.wikipedia.org/wiki/Screw_thread


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An Acme screw

Main article:Acme thread form

Acme threads have a 29thread angle, which is easier to machine than square threads.

They are not as efficient as square threads, due to the increased friction induced by the

thread angle.[3]

Buttress thread

Main article:Buttress thread

Buttress threads are of a triangular shape. These are used where the load force on the

screw is only applied in one direction.[6]They are as efficient as square threads in these

applications, but are easier to manufacture.

Characteristics

leadscrew nut and screw mate with rubbing surfaces, and consequently they have a

relatively high friction and stiction compared to mechanical parts which mate with rolling

surfaces and bearings. Leadscrew efficiency is typically between 25 and 70%, with

higher pitch screws tending to be more efficient. A higher performing but more

expensive alternative is the ball screw.

The high internal friction means that leadscrew systems are not usually capable of

continuous operation at high speed, as they will overheat. Due to inherently high

stiction, the typical screw is self-locking (i.e. when stopped, a linear force on the nut will

not apply a torque to the screw) and are often used in applications where backdriving is

unacceptable, like holding vertical loads or in hand cranked machine tools.

Leadscrews are typically used well greased, but, with an appropriate nut, they may be

run dry with somewhat higher friction. There is often a choice of nuts, and

manufacturers will specify screw and nut combination as a set.

The mechanical advantage of a leadscrew is determined by the screw pitch and lead.

For multi-start screws the mechanical advantage is lower, but the traveling speed is

better.[7]

Backlash can be reduced with the use of a second nut to create a static loading force

known as preload; alternately, the nut can be cut along a radius and preloaded by

clamping that cut back together.
http://en.wikipedia.org/wiki/Acme_thread_formhttp://en.wikipedia.org/wiki/Thread_anglehttp://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley-2http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley-2http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley-2http://en.wikipedia.org/wiki/Buttress_threadhttp://en.wikipedia.org/wiki/Leadscrew#cite_note-5http://en.wikipedia.org/wiki/Leadscrew#cite_note-5http://en.wikipedia.org/wiki/Leadscrew#cite_note-5http://en.wikipedia.org/wiki/Nut_(hardware)http://en.wikipedia.org/wiki/Frictionhttp://en.wikipedia.org/wiki/Stictionhttp://en.wikipedia.org/wiki/Ball_screwhttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Mechanical_advantagehttp://en.wikipedia.org/wiki/Screw#Mechanical_analysishttp://en.wikipedia.org/wiki/Lead_(engineering)http://en.wikipedia.org/wiki/Leadscrew#cite_note-6http://en.wikipedia.org/wiki/Leadscrew#cite_note-6http://en.wikipedia.org/wiki/Leadscrew#cite_note-6http://en.wikipedia.org/wiki/Backlash_(engineering)http://en.wikipedia.org/wiki/Preloadhttp://en.wikipedia.org/wiki/Preloadhttp://en.wikipedia.org/wiki/Backlash_(engineering)http://en.wikipedia.org/wiki/Leadscrew#cite_note-6http://en.wikipedia.org/wiki/Lead_(engineering)http://en.wikipedia.org/wiki/Screw#Mechanical_analysishttp://en.wikipedia.org/wiki/Mechanical_advantagehttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Ball_screwhttp://en.wikipedia.org/wiki/Stictionhttp://en.wikipedia.org/wiki/Frictionhttp://en.wikipedia.org/wiki/Nut_(hardware)http://en.wikipedia.org/wiki/Leadscrew#cite_note-5http://en.wikipedia.org/wiki/Buttress_threadhttp://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley-2http://en.wikipedia.org/wiki/Thread_anglehttp://en.wikipedia.org/wiki/Acme_thread_form


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A lead screw will back drive, whereby forces on the nut applied parallel with the lead

screw will cause a free-moving leadscrew to begin to rotate. A leadscrew's tendency to

backdrive depends on its thread helix angle, coefficient of friction of the interface of the

components (screw/nut) and the included angle of the thread form. In general, a steel

acme thread and bronze nut will back drive when the helix angle of the thread is greater

than 20.

Advantages & disadvantages

The advantages of a leadscrew are:[2]

Large load carrying capability

Compact

Simple to design

Easy to manufacture; no specialized machinery is required

Large mechanical advantage

Precise and accurate linear motion

Smooth, quiet, and low maintenance

Minimal number of parts

Most are self-locking

The disadvantages are that most are not very efficient. Due to the low efficiency they

cannot be used in continuous power transmission applications. They also have a highdegree for friction on the threads, which can wear the threads out quickly. For square

threads, the nut must be replaced; for trapezoidal threads, a split nut may be used to

compensate for the wear.[4]

Alternatives

Alternatives to actuation by leadscrew include:

Ball screws and roller screws (sometimes categorized as types of leadscrew rather

than in contradistinction)
http://en.wikipedia.org/wiki/Helix_anglehttp://en.wikipedia.org/wiki/Coefficient_of_frictionhttp://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari202-1http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari202-1http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari202-1http://en.wikipedia.org/wiki/Split_nuthttp://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari203-3http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari203-3http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari203-3http://en.wikipedia.org/wiki/Ball_screwhttp://en.wikipedia.org/wiki/Roller_screwhttp://en.wikipedia.org/wiki/Roller_screwhttp://en.wikipedia.org/wiki/Ball_screwhttp://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari203-3http://en.wikipedia.org/wiki/Split_nuthttp://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari202-1http://en.wikipedia.org/wiki/Coefficient_of_frictionhttp://en.wikipedia.org/wiki/Helix_angle


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Fluid power (i.e., hydraulics and pneumatics)

Gear trains (e.g., worm drives, rack-and-pinion drives)

Electromagnetic actuation (e.g., solenoids)

Piezoelectric actuation

Mechanics

Diagram of an "unwrapped" screw thread

The torque required to lift or lower a load can be calculated by "unwrapping" one

revolution of a thread. This is most easily described for a square or buttress thread as

the thread angle is 0 and has no bearing on the calculations. The unwrapped thread

forms a right angle triangle where the base is dm long and the height is the lead

(pictured to the right). The force of the load is directed downward, the normal force is

perpendicular to the hypotenuse of the triangle, the frictional force is directed in the

opposite direction of the direction of motion (perpendicular to the normal force or along

the hypotenuse), and an imaginary "effort" force is acting horizontallyin the direction

opposite the direction of the frictional force. Using this free-body diagram the torque

required to lift or lower a load can be calculated:[8][9]

Coefficient of friction for leadscrew threads[10]

Screw materialNut material

Steel Bronze Brass Cast iron

Steel, dry 0.150.25 0.150.23 0.150.19 0.150.25

Steel, machine oil 0.110.17 0.100.16 0.100.15 0.110.17
http://en.wikipedia.org/wiki/Fluid_powerhttp://en.wikipedia.org/wiki/Hydraulicshttp://en.wikipedia.org/wiki/Pneumaticshttp://en.wikipedia.org/wiki/Gearhttp://en.wikipedia.org/wiki/Worm_drivehttp://en.wikipedia.org/wiki/Rack_and_pinionhttp://en.wikipedia.org/wiki/Electromagnetismhttp://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Free-body_diagramhttp://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley402-7http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley402-7http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley402-7http://en.wikipedia.org/wiki/Leadscrew#cite_note-9http://en.wikipedia.org/wiki/Leadscrew#cite_note-9http://en.wikipedia.org/wiki/Leadscrew#cite_note-9http://en.wikipedia.org/wiki/File:Lead_angle.svghttp://en.wikipedia.org/wiki/File:Lead_angle.svghttp://en.wikipedia.org/wiki/File:Lead_angle.svghttp://en.wikipedia.org/wiki/File:Lead_angle.svghttp://en.wikipedia.org/wiki/File:Lead_angle.svghttp://en.wikipedia.org/wiki/File:Lead_angle.svghttp://en.wikipedia.org/wiki/File:Lead_angle.svghttp://en.wikipedia.org/wiki/File:Lead_angle.svghttp://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley402-7http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley402-7http://en.wikipedia.org/wiki/Free-body_diagramhttp://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Electromagnetismhttp://en.wikipedia.org/wiki/Rack_and_pinionhttp://en.wikipedia.org/wiki/Worm_drivehttp://en.wikipedia.org/wiki/Gearhttp://en.wikipedia.org/wiki/Pneumaticshttp://en.wikipedia.org/wiki/Hydraulicshttp://en.wikipedia.org/wiki/Fluid_power


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where

T=

torque

F= load on the screw dm= mean diameter

= coefficient of friction (common values are found in the table to the right)

l= lead

= friction angle

= lead angle

Based on the Tlower equation it can be found that the screw is self-locking when

the coefficient of friction is greater than the tangent of the lead angle. An

equivalent comparison is when the friction angle is greater than the lead angle

( > ).[11]When this is not true the screw will back-drive, or lower under the

weight of the load.[8]

The efficiency, calculated using the torque equations above, is:[12][13]

For screws that have a thread angle other than zero, such as a trapezoidal

thread, this must be compensated as it increases the frictional forces. The

equations below takes this into account:

where is one half the thread angle.

If the leadscrew has a collar in which the load rides on then the

frictional forces between the interface must be accounted for in

Bronze 0.080.12 0.040.06 - 0.060.09
http://en.wikipedia.org/wiki/Friction_anglehttp://en.wikipedia.org/wiki/Leadscrew#cite_note-10http://en.wikipedia.org/wiki/Leadscrew#cite_note-10http://en.wikipedia.org/wiki/Leadscrew#cite_note-10http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley402-7http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley402-7http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley402-7http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley403-11http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley403-11http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley403-11http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley403-11http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley403-11http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley402-7http://en.wikipedia.org/wiki/Leadscrew#cite_note-10http://en.wikipedia.org/wiki/Friction_angle


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the torque calculations as well. For the following equation the

load is assumed to be concentrated at the mean collar diameter

(dc):[12]

where c is the coefficient of friction between the collar on

the load and dc is the mean collar diameter. For collars that

use thrust bearings the frictional loss is negligible and the

equation can being ignored

equation can being inored.[15]

Safe running speeds for various nut materials and loads on a steel screw[16]

Nut material Safe loads [psi] Speed

Bronze 25003500 Low speed

Bronze 16002500 10 fpm

Cast iron 18002500 8 fpm

Bronze 8001400 2040 fpm

Cast iron 6001000 2040 fpm

Bronze 150240 50 fpm

Coefficient of friction for thrust collars[15]

Material combination Starting c Running c

Soft steel / cast iron 0.17 0.12

Hardened steel / cast iron 0.15 0.09

Soft steel / bronze 0.10 0.08

Hardened steel / bronze 0.08 0.06
http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley403-11http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley403-11http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley403-11http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari213-14http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari213-14http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari213-14http://en.wikipedia.org/wiki/Leadscrew#cite_note-15http://en.wikipedia.org/wiki/Leadscrew#cite_note-15http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari213-14http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari213-14http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari213-14http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari213-14http://en.wikipedia.org/wiki/Leadscrew#cite_note-15http://en.wikipedia.org/wiki/Leadscrew#cite_note-bhandari213-14http://en.wikipedia.org/wiki/Leadscrew#cite_note-shigley403-11


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DC MOTOR FUNDAMENTALS

There are different kinds of D.C. motors, but they all work on the same principles. Tounderstand what goes on inside a motor, here is an example.


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When a permanent magnet is positioned around a loop of wire that is hooked up to

a D.C. power source, we have the basics of a D.C. motor. In order to make the loop of

wire spin, we have to connect a battery or DC power supply between its ends, and

support it so it can spin about its axis. To allow the rotor to turn without twisting the

wires, the ends of the wire loop are connected to a set of contacts called the

commutator, which rubs against a set of conductors called the brushes. The brushes

make electrical contact with the commutator as it spins, and are connected to the

positive and negative leads of the power source, allowing electricity to flow through

the loop. The electricity flowing through the loop creates a magnetic field that

interacts with the magnetic field of the permanent magnet to make the loop spin.

The DC motor used in this project is Direct Current permanent magnet motors

operated at a constant voltage. Motor characteristics vary considerably from type to

type, and their performance characteristics can be altered by the way electrical

power is supplied. can be quite different than those covered here. Few physical

parameters associated with DC motors are

1. TORQUE

a. A force that produces or tends to produce rotation or torsion (an

automobile engine delivers torque to the drive shaft); also: a measure of

the effectiveness of such a force that consists of the product of the force

and the perpendicular distance from the line of action of the force to the

axis of rotation.

b. A turning or twisting force.

c. The quantitative measure of the tendency of a force to cause or change

rotational motion is called torque.

Torque (also called a moment) is the term we use when we talk about forces that act

in a rotational manner. You apply a torque or moment when you turn a dial, flip a


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light switch, drill a hole or tighten a screw or bolt.

As shown in the picture of a ratchet, a vertical force applied at the end of the handle

creates a torque. The force, F, applied to the ratchet as shown causes a tendency to

rotate about point O. The force can be broken down into two components: a radial

component, Frad, parallel to the ratchet handle that does not contribute to the

torque, and a tangential component, Ftan, perpendicular to the handle that does

contribute to the torque. The distance from point O to the point of action of F is

described by the direction vector, r. The moment arm, l is the perpendicular distance

between point O and the line of action of F.

If we were to shorten the moment arm by applying the force closer to the head of

the ratchet, the magnitude of the torque would decrease, even if the force remained

the same. Thus, if we change the effective length of the handle, we change the

torque (see equation 1).


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UNITS of TORQUE

SI English

newton-meters {Nm} inch-pounds {inlb}

foot-pounds {ftlb}

inch-ounces {inoz}

1 Nm = 0.738 ftlb

1 Nm = 0.113 inlb

1 Nm = 141.61 inoz

1 inlb = 0.113 Nm

1 ftlb = 1.356 Nm

1 inoz = 7.062E-03 Nm


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2. SPEED

Speed (Angular Velocity)

The rate of rotation around an axis usually expressed in radians or revolutions per

second or per minute

Motors are devices that convert electrical energy into mechanical energy. The D.C.

motors that we have been dealing with here convert electrical energy into rotationalenergy. That rotational energy is then used to lift things, propel things, turn things,

etc. When we supply the specified voltage to a motor, it rotates the output shaft at

some speed. This rotational speed or angular velocity, is typically measured in

radians/second {rad/s}, revolutions/second {rps}, or revolutions/minute {rpm}.

o When performing calculations, be sure to use consistent units. In the English

system, calculations should be done in degrees/second, and radians/sec for SI

calculations.

NOTE:

1 revolution = 360

1 revolution = (2*) radians

1 radian = (180/)


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1 = (/180) radians

From the angular velocity, , we can find the tangential velocity of a point anywhere

on the rotating body through the equation tangential velocity,

v= r* , where r is the distance from the axis of rotation. This relation can be used

to compute the steady state (constant speed - no acceleration) speed of a vehicle if

the radius and angular velocity of a wheel is known, or a winch winds up the linear

speed of a rope as it.

3. POWER

Motive Power

a. Ability to act or produce an effect

b. A source or means of supplying energy; especially: ELECTRICITY

c. MOTIVE POWER the time rate at which work is done or energy emitted or

transferred

Power in Rotational Motion

When you pedal a bicycle, you apply forces to a rotating body and do work on

it. Similar things happen in real-life situations, such as a rotating motor shaft driving

a power tool or a car engine propelling the vehicle. We can express this work in terms

of torque and an angular displacement...

What about the power associated with work done by a torque acting on a rotating

body?

dW/dt is the rate of doing work, orpower P. When a torque T (with respect to the

axis of rotation) acts on a body that rotates with angular velocity W, its power (rate

of doing work) is the product of the torque and angular velocity. This is the analog of

the relation P = Fvfor particle motion.


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Power in rotational motion can be written as:

UNITS of POWER

SI English

Watts {W}

newton-meters per second {Nm/s}

1 W = 1 Nm/s

1 W = 0.738 ftlb/s

1 W = 1.341E-03 hp

foot-pounds per second {ftlb/s}

horsepower {hp}

1 ftlb/s = 1.818E-03 hp

1 ftlb/s = 1.356 W

2. Motor Characteristics

TORQUE/SPEED CURVES

In order to effectively design with D.C. motors, it is necessary to understand their

characteristic curves. For every motor, there is a specific Torque/Speed curve and

Power curve.


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The graph above shows a torque/speed curve of a typical D.C. motor. Note that

torque is inversely proportioal to the speed of the output shaft. In other words, there

is a tradeoffbetween how much torque a motor delivers, and how fast the output

shaft spins. Motor characteristics are frequently given as two points on this graph:

The stall torque, , represents the point on the graph at which the torque is a

maximum, but the shaft is not rotating.

The no load speed, , is the maximum output speed of the motor (when no

torque is applied to the output shaft).

The curve is then approximated by connecting these two points with a line, whose

equation can be written in terms of torque or angular velocity as equations 3) and 4):


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The linear model of a D.C. motor torque/speed curve is a very good approximation.

The torque/speed curves shown below are actual curves for the green maxon motor.

One is a plot of empirical data, and the other was plotted mechanically using a

device. Note that the characteristic torque/speed curve for this motor is quite linear.

This is generally true as long as the curve represents the direct output of the motor,

or a simple gear reduced output. If the specifications are given as two points, it is

safe to assume a linear curve.

Recall that earlier we defined power as the product of torque and angular velocity.

This corresponds to the area of a rectangle under the torque/speed curve with one

corner at the origin and another corner at a point on the curve (see figures below).

Due to the linear inverse relationship between torque and speed, the maximum

power occurs at the point where

= , and = .


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POWER/TORQUE and POWER/SPEED CURVES

By substituting equations 3 and 4 (torque and speed) into equation 2 (Power), we

see that the power curves for a D.C. motor with respect to both speed and torque

are quadratics, as shown in equations 5 and 6.

From these equations, we again find that maximum output power occurs at

= , and = respectively.


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H-BRIDGE

THEORY OF OPERATION

How do we make a motor turn on? You take a battery; hook the positive side to

one side of your DC motor. Then you connect the negative side of the battery to the

other motor lead. The motor spins forward. If you swap the battery leads the motor

spins in reverse. Ok, that's basic. Now lets say you want a Micro Controller Unit

(MCU) to control the motor, how would you do it? Well, for starters you get a

device that would act like a solid state switch, a transistor, and hook it up the

motor.

NOTE: If you connect up these relay circuits, remember to put a diode across the

coil of the relay. This will keep the spike voltage (back EMF), coming out of the coil

of the relay, from getting into the MCU and damaging it. The anode, which is the

arrow side of the diode, should connect to ground. The bar, which is the Cathode

side of the diode, should connect to the coil where the MCU connects to the relay.

If you connect this circuit to a small hobby motor you can control the motor with a

processor (MCU, etc.) Applying a logical one, (+12 Volts in our example) to point A

causes the motor to turn forward. Applying a logical zero, (ground) causes the

motor to stop turning (to coast and stop).


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Hook the motor up in this fashion and the circuit turns the motor in reverse when you apply a

logical one (+12Volts) to point B. Apply a logical zero, which is usually a ground, causes the

motor to stop spinning.

If you hook up these circuits you can only get the motor to stop or turn in one direction, forward

for the first circuit or reverse for the second circuit.

Motor SpeedYou can also pulse the motor control line, (A or B) on and off. This powers the motor in short

burst and gets varying degrees of torque, which usually translates into variable motor speed.

But if you want to be able to control the motor in both forward and reverse with a processor, you

will need more circuitry. You will need an H-Bridge. Notice the "H"-looking configuration in the

next graphic. Relays configured in this fashion make an H-Bridge. The "high side drivers" are

the relays that control the positive voltage to the motor. This is called sourcing current.

The "low side drivers" are the relays that control the negative voltage to sink current to the

motor. "Sinking current" is the term for connecting the circuit to the negative side of the power

supply, which is usually ground.


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So, you turn on the upper left and lower right circuits, and power flows through the

motor forward, i.e.: 1 to A, 0 to B, 0 to C, and 1 to D.

Then for reverse you turn on the upper right and lower left circuits and power flows

through the motor in reverse, i.e.: 0 to A, 1 to B, 1 to C, and 0 to D.

CAUTION: You should be careful not to turn on both circuits on one side or the

other, or you have a direct short which will destroy your circuit; Example: A and C

or B and D both high (logical 1).


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SEMICONDUCTOR H-BRIDGES

we can better control our motor by using transistors or Field Effect Transistors

(FETs).Most of what we have discussed about the relays H-Bridge is true of thesecircuits. You don't need diodes that were across the relay coils now. You should use

diodes across your transistors though. See the following diagram showing how they

are connected.These solid state circuits provide power and ground connections tothe motor, as did the relay circuits. The high side drivers need to be current

"sources" which is what PNP transistors and P-channel FETs are good at. The low

side drivers need to be current "sinks" which is what NPN transistors and N-channel

FETs are good at.

If you turn on the two upper circuits, the motor resists turning, so you effectively

have a breaking mechanism. The same is true if you turn on both of the lower

circuits. This is because the motor is a generator and when it turns it generates a

voltage. If the terminals of the motor are connected (shorted), then the voltage

generated counteracts the motors freedom to turn. It is as if you are applying a

similar but opposite voltage to the one generated by the motor being turned. Vis--

vis, it acts like a brake. To be nice to your transistors, you should add diodes to


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catch the back voltage that is generated by the motor's coil when the power is

switched on and off. This flyback voltage can be many times higher than the supply

voltage! If you don't use diodes, you could burn out your transistors.

Transistors, being a semiconductor device, will have some resistance, which causes

them to get hot when conducting much current. This is called not being able to sink

or source very much power, i.e.: Not able to provide much current from ground or

from plus voltage.

Mosfets are much more efficient, they can provide much more current and not get

as hot. They usually have the flyback diodes built in so you don't need the diodes

anymore. This helps guard against flyback voltage frying your MCU. To use Mosfets

in an H-Bridge, you need P-Channel Mosfets on top because they can "source"

power, and N-Channel Mosfets on the bottom because then can "sink" power. N-

Channel Mosfets are much cheaper than P-Channel Mosfets, but N-Channel Mosfets

used to source power require about 7 volts more than the supply voltage, to turnon. As a result, some people manage to use N-Channel Mosfets, on top of the H-

Bridge, by using cleaver circuits to overcome the breakdown voltage.

It is important that the four quadrants of the H-Bridgecircuits be turned on and off

properly. When there is a path between the positive and ground side of the H-

Bridge, other than through the motor, a condition exists called "shoot through".

This is basically a direct short of the power supply and can cause semiconductors to

become ballistic, in circuits with large currents flowing. There are H-bridge chips

available that are much easier, and safer, to use than designing your own H-Bridge

circuit.


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H-Bridge Devices

The L 293 has 2 H-Bridges, can provide about 1 amp to each and occasional peak

loads to 2 amps. Motors typically controlled with this controller are near the size of

a 35 mm film plastic canister.The L298 has 2 h-bridges on board, can handle 1amp and peak current draws to

about 3amps. You often see motors between the size a of 35 mm film plastic

canister and a coke can, driven by this type H-Bridge. The LMD18200 has one h-

bridge on board, can handle about 2 or 3 amps and can handle a peak of about 6

amps. This H-Bridge chip can usually handle an average motor about the size of a

coke. There are several more commercially designed H-Bridge chips as well.

DC GEARED MOTOR SPECIFICATION

1. OPERATING VOLTAGE 12V dc

2. 30 RPM

3. TORQUE 3kgcm


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BATTERY


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Battery construction

The word batterysimply means a group of similar components. In military vocabulary, a"battery" refers to a cluster of guns. In electricity, a "battery" is a set of voltaic cellsdesigned to provide greater voltage and/or current than is possible with one cell alone.

The symbol for a cell is very simple, consisting of one long line and one short line,parallel to each other, with connecting wires:

The symbol for a battery is nothing more than a couple of cell symbols stacked inseries:


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As was stated before, the voltage produced by any particular kind of cell is determinedstrictly by the chemistry of that cell type. The size of the cell is irrelevant to its voltage.To obtain greater voltage than the output of a single cell, multiple cells must beconnected in series. The total voltage of a battery is the sum of all cell voltages. Atypical automotive lead-acid battery has six cells, for a nominal voltage output of 6 x 2.2

or 13.2 volts:

The cells in an automotive battery are contained within the same hard rubber housing,connected together with thick, lead bars instead of wires. The electrodes and electrolytesolutions for each cell are contained in separate, partitioned sections of the batterycase. In large batteries, the electrodes commonly take the shape of thin metal grids orplates, and are often referred to as platesinstead of electrodes.

For the sake of convenience, battery symbols are usually limited to four lines,alternating long/short, although the real battery it represents may have many more cellsthan that. On occasion, however, you might come across a symbol for a battery withunusually high voltage, intentionally drawn with extra lines. The lines, of course, arerepresentative of the individual cell plates:

If the physical size of a cell has no impact on its voltage, then what does it affect? Theanswer is resistance, which in turn affects the maximum amount of current that a cellcan provide. Every voltaic cell contains some amount of internal resistance due to theelectrodes and the electrolyte. The larger a cell is constructed, the greater the electrodecontact area with the electrolyte, and thus the less internal resistance it will have.

Although we generally consider a cell or battery in a circuit to be a perfect source ofvoltage (absolutely constant), the current through it dictated solely by the externalresistance of the circuit to which it is attached, this is not entirely true in real life. Sinceevery cell or battery contains some internal resistance, that resistance must affect thecurrent in any given circuit:


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When connecting batteries together to form larger "banks" (a batteryof batteries?), theconstituent batteries must be matched to each other so as to not cause problems. Firstwe will consider connecting batteries in series for greater voltage:

We know that the current is equal at all points in a series circuit, so whatever amount ofcurrent there is in any one of the series-connected batteries must be the same for all theothers as well. For this reason, each battery must have the same amp-hour rating, orelse some of the batteries will become depleted sooner than others, compromising the

capacity of the whole bank. Please note that the total amp-hour capacity of this seriesbattery bank is not affected by the number of batteries.

Next, we will consider connecting batteries in parallel for greater current capacity (lowerinternal resistance), or greater amp-hour capacity:

We know that the voltage is equal across all branches of a parallel circuit, so we mustbe sure that these batteries are of equal voltage. If not, we will have relatively largecurrents circulating from one battery through another, the higher-voltage batteriesoverpowering the lower-voltage batteries. This is not good.

On this same theme, we must be sure that any overcurrent protection (circuit breakersor fuses) are installed in such a way as to be effective. For our series battery bank, one

fuse will suffice to protect the wiring from excessive current, since any break in a seriescircuit stops current through all parts of the circuit:


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With a parallel battery bank, one fuse is adequate for protecting the wiring against loadovercurrent (between the parallel-connected batteries and the load), but we have otherconcerns to protect against as well. Batteries have been known to internally short-circuit, due to electrode separator failure, causing a problem not unlike that wherebatteries of unequal voltage are connected in parallel: the good batteries will overpowerthe failed (lower voltage) battery, causing relatively large currents within the batteries'connecting wires. To guard against this eventuality, we should protect each and everybattery against overcurrent with individual battery fuses, in addition to the load fuse:

When dealing with secondary-cell batteries, particular attention must be paid to themethod and timing of charging. Different types and construction of batteries havedifferent charging needs, and the manufacturer's recommendations are probably thebest guide to follow when designing or maintaining a system. Two distinct concerns ofbattery charging are cyclingand overcharging. Cycling refers to the process of charginga battery to a "full" condition and then discharging it to a lower state. All batteries have afinite (limited) cycle life, and the allowable "depth" of cycle (how far it should bedischarged at any time) varies from design to design. Overcharging is the conditionwhere current continues to be forced backwards through a secondary cell beyond thepoint where the cell has reached full charge. With lead-acid cells in particular,

overcharging leads to electrolysis of the water ("boiling" the water out of the battery) andshortened life.

Any battery containing water in the electrolyte is subject to the production of hydrogengas due to electrolysis. This is especially true for overcharged lead-acid cells, but notexclusive to that type. Hydrogen is an extremely flammable gas (especially in thepresence of free oxygen created by the same electrolysis process), odorless andcolorless. Such batteries pose an explosion threat even under normal operating


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conditions, and must be treated with respect. The author has been a firsthand witnessto a lead-acid battery explosion, where a spark created by the removal of a batterycharger (small DC power supply) from an automotive battery ignited hydrogen gaswithin the battery case, blowing the top off the battery and splashing sulfuric acideverywhere. This occurred in a high school automotive shop, no less. If it were not for

all the students nearby wearing safety glasses and buttoned-collar overalls, significantinjury could have occurred.

When connecting and disconnecting charging equipment to a battery, always make thelast connection (or first disconnection) at a location away from the battery itself (such asat a point on one of the battery cables, at least a foot away from the battery), so that anyresultant spark has little or no chance of igniting hydrogen gas.

In large, permanently installed battery banks, batteries are equipped with vent capsabove each cell, and hydrogen gas is vented outside of the battery room through hoodsimmediately over the batteries. Hydrogen gas is very light and rises quickly. The

greatest danger is when it is allowed to accumulate in an area, awaiting ignition.

More modern lead-acid battery designs are sealed, using a catalyst to re-combine theelectrolyzed hydrogen and oxygen back into water, inside the battery case itself.Adequate ventilation might still be a good idea, just in case a battery were to develop aleak in the case.

Battery ratings

Because batteries create electron flow in a circuit by exchanging electrons in ionicchemical reactions, and there is a limited number of molecules in any charged battery

available to react, there must be a limited amount of total electrons that any battery canmotivate through a circuit before its energy reserves are exhausted. Battery capacitycould be measured in terms of total number of electrons, but this would be a hugenumber. We could use the unit of the coulomb (equal to 6.25 x 1018 electrons, or6,250,000,000,000,000,000 electrons) to make the quantities more practical to workwith, but instead a new unit, the amp-hour, was made for this purpose. Since 1 amp isactually a flow rate of 1 coulomb of electrons per second, and there are 3600 secondsin an hour, we can state a direct proportion between coulombs and amp-hours: 1 amp-hour = 3600 coulombs. Why make up a new unit when an old would have done justfine? To make your lives as students and technicians more difficult, of course!

A battery with a capacity of 1 amp-hour should be able to continuously supply a currentof 1 amp to a load for exactly 1 hour, or 2 amps for 1/2 hour, or 1/3 amp for 3 hours,etc., before becoming completely discharged. In an ideal battery, this relationshipbetween continuous current and discharge time is stable and absolute, but real batteriesdon't behave exactly as this simple linear formula would indicate. Therefore, when amp-hour capacity is given for a battery, it is specified at either a given current, given time, orassumed to be rated for a time period of 8 hours (if no limiting factor is given).


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For example, an average automotive battery might have a capacity of about 70 amp-hours, specified at a current of 3.5 amps. This means that the amount of time thisbattery could continuously supply a current of 3.5 amps to a load would be 20 hours (70amp-hours / 3.5 amps). But let's suppose that a lower-resistance load were connectedto that battery, drawing 70 amps continuously. Our amp-hour equation tells us that the

battery should hold out for exactly 1 hour (70 amp-hours / 70 amps), but this might notbe true in real life. With higher currents, the battery will dissipate more heat across itsinternal resistance, which has the effect of altering the chemical reactions taking placewithin. Chances are, the battery would fully discharge some time before the calculatedtime of 1 hour under this greater load.

Conversely, if a very light load (1 mA) were to be connected to the battery, our equationwould tell us that the battery should provide power for 70,000 hours, or just under 8years (70 amp-hours / 1 milliamp), but the odds are that much of the chemical energy ina real battery would have been drained due to other factors (evaporation of electrolyte,deterioration of electrodes, leakage current within battery) long before 8 years had

elapsed. Therefore, we must take the amp-hour relationship as being an idealapproximation of battery life, the amp-hour rating trusted only near the specified currentor timespan given by the manufacturer. Some manufacturers will provide amp-hourderating factors specifying reductions in total capacity at different levels of currentand/or temperature.

For secondary cells, the amp-hour rating provides a rule for necessary charging time atany given level of charge current. For example, the 70 amp-hour automotive battery inthe previous example should take 10 hours to charge from a fully-discharged state at aconstant charging current of 7 amps (70 amp-hours / 7 amps).

Approximate amp-hour capacities of some common batteries are given here:

Typical automotive battery: 70 amp-hours @ 3.5 A (secondary cell) D-size carbon-zinc battery: 4.5 amp-hours @ 100 mA (primary cell) 9 volt carbon-zinc battery: 400 milliamp-hours @ 8 mA (primary cell)

As a battery discharges, not only does it diminish its internal store of energy, but itsinternal resistance also increases (as the electrolyte becomes less and lessconductive), and its open-circuit cell voltage decreases (as the chemicals become moreand more dilute). The most deceptive change that a discharging battery exhibits isincreased resistance. The best check for a battery's condition is a voltage measurementunder load, while the battery is supplying a substantial current through a circuit.Otherwise, a simple voltmeter check across the terminals may falsely indicate a healthybattery (adequate voltage) even though the internal resistance has increasedconsiderably. What constitutes a "substantial current" is determined by the battery'sdesign parameters. A voltmeter check revealing too low of a voltage, of course, wouldpositively indicate a discharged battery:


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Fully charged battery:

Now, if the battery discharges a bit . . .

. . . and discharges a bit further . . .

. . . and a bit further until it's dead.


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Notice how much better the battery's true condition is revealed when its voltage ischecked under load as opposed to without a load. Does this mean that it's pointless tocheck a battery with just a voltmeter (no load)? Well, no. If a simple voltmeter checkreveals only 7.5 volts for a 13.2 volt battery, then you know without a doubt that it'sdead. However, if the voltmeter were to indicate 12.5 volts, it may be near full charge orsomewhat depleted -- you couldn't tell without a load check. Bear in mind also that the

resistance used to place a battery under load must be rated for the amount of powerexpected to be dissipated. For checking large batteries such as an automobile (12 voltnominal) lead-acid battery, this may mean a resistor with a power rating of severalhundred watts.


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Reference

1. R.S. Khurmi

2. J.K.Gupta

3. Naveen

4. www.wikipedia.com

5. www.scienceHPLORE.com

6. www.google.com

7. www.machinedesign.com
http://www.wikipedia.com/http://www.wikipedia.com/http://www.sciencehplore.com/http://www.sciencehplore.com/http://www.google.com/http://www.google.com/http://www.google.com/http://www.sciencehplore.com/http://www.wikipedia.com/

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