gear measurement and basic concepts

7/28/2019 gear measurement and basic concepts

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By

K.VARUN

ROLL NO 123511

GEAR MEASUREMENT


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INTRODUCTION

A gear is a rotating machine part having cut teeth, or cogs,

which mesh with another toothed part in order to transmit torque.

Geared devices can change the speed, torque, and direction of a

power source. The most common situation is for a gear to mesh

with another gear; however, a gear can also mesh with a non-

rotating toothed part, called a rack, thereby producing translation

instead of rotation.


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HISTORY

The earliest known reference to gears was circa A.D. 50 by Hero

of Alexandria, but they can be traced back to the Greek

mechanics of the Alexandrian school in the 3rd century B.C. and

were greatly developed by the Greek polymath Archimedes(287212 B.C.).

The Antikythera mechanism is an example of a very early and

intricate geared device, designed to calculate astronomical

positions. Its time of construction is now estimated between 150

and 100 BC.


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DEFINITION

Gears are toothed members which transmit power / motion

between two shafts by meshing without any slip.

A wheel with teeth around its rim that mesh with the teeth of

another wheel to transmit motion.

In any pair of gears, the smaller one is called pinion and the

larger one is called gear immaterial of which is driving the other.


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CLASSIFICATION OF GEARS

Gears are classified according to the shape of the tooth pair and

disposition into

spur

helical

bevel

worm


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SPUR GEAR

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 clothesdryer


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Helical Gear The teeth on helical gears are cut at an angle to the face of the gear

This gradual engagement makes helical gears operate much more

smoothly and quietly than spur gears

One interesting thing about helical gears is that if the angles of the

gear teeth are correct, they can be mounted on perpendicular shafts,

adjusting the rotation angle by 90 degrees


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Helical Gear


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Types of Helical gear1. Cross helical gears

Crossed Helical Gears are a type of Helical Gears only. They are

the typical example of two helical gears applied for transmitting

power between non parallel and non-intersecting shafts.


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2. Double Helical Gears

Double Helical Gear or Herringbone Gear is of cylindrical form. It

has two sections of teeth, right hand and the other left hand. They

operate by engaging themselves simultaneously with the teeth of an

identically designed mating gear.


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Contd

To avoid axial thrust, two helical

gears of opposite hand can be

mounted side by side, to cancel

resulting thrust forces

Herringbone gears are mostly used

on heavy machinery.


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Rack and pinion

Rack and pinion gears are used to

convert rotation (From the pinion)

into linear motion (of the rack)

A perfect example of this is the

steering system on many cars


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Bevel gears

Bevel gears are useful when the direction of a shaft's rotation needs

to be changed

They are usually mounted on shafts that are 90 degrees apart, but

can be designed to work at other angles as well

Based on the arrangement of teeth on bevel gears can be divided

into straight, spiral & hypoid gears.


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Straight bevel gears

A bevel gear is shaped like a right circular cone with most of its

tip cut off.

Wide application of the straight bevel drives is in automotive

differentials, right angle drives of blenders and conveyors.

Straight bevel gears are generally used only at speeds below

5 m/s or, for small gears, 1000 r.p.m.

A typical application of straight bevel used in differential

application in automobiles.


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Contd.

. Bevel gears with equal numbers of teeth andshaft axes at 90 degrees are called miter gears.


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spiral bevel gear

Spiral bevel gear teeth are curved along the tooth's length and set

at an angle, analogously to the way helical gear teeth are set at an

angle compared to spur gear teeth.

They operate smoother than straight bevel gears and have higher

load capacity.

spiral bevel gear are extensively used in portable power tools,

gear boxes, machine tools, cooling towers, power plants, steel

plants, railway track inspection machines, etc.


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Their efficiency is slightly lower than straightbevel gears.

They have the same advantages and

disadvantages relative to their straight-cut

cousins as helical gears do to spur gears.


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Hypoid bevel gears

Hypoid gears resemble spiral bevel gears except the shaft axes

do not intersect.

These gears are also used for right angle drive in which the axes

do not intersect.

This permits the lowering of the pinion axis which is an added

advantage in automobile in avoiding hump inside the automobile

drive line power transmission.


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WORM GEAR

Worm and worm gear pair consists of a worm, which is very

similar to a screw and a worm gear, which is a helical gear. They

are used in right-angle skew shafts.

The sliding action prevalent in the system while resulting in

quieter operation produces considerable frictional heat.

Efficiency of these gears is low anywhere from 90% to 40 %.


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Internal Gear

Internal gears are used for transmitting power between two

parallel shafts.

In these gears, annular wheels are having teeth on the inner

periphery. This makes the drive very compact.

In these drives, the meshing pinion and annular gear are running

in the same direction.


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Forms OF Gear Tooth

INVOLUTE:

The tooth is derived from the trace of point on a

straight line, which rolls without slipping around a

circle i.e base circle .


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Cycloidal

The tooth is derived from curve which is locus ofpoint on a circle rolling on pitch circle of gear.


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Gear tooth terminology


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NOMENCLATURE.

Pitch surface: The surface of the imaginary rolling cylinder

(cone, etc.) that the toothed gear may be considered to replace.

Pitch circle: A right section of the pitch surface.

Addendum circle: A circle bounding the ends of the teeth, in a

right section of the gear.

Root (or dedendum) circle: The circle bounding the spacesbetween the teeth, in a right section of the gear.


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Contd

Addendum: The radial distance between the pitch circle and theaddendum circle.

Dedendum: The radial distance between the pitch circle and the

root circle.

Clearance: The difference between the dedendum of one gear

and the addendum of the mating gear.

Face of a tooth: That part of the tooth surface lying outside the

pitch surface.

Flank of a tooth: The part of the tooth surface lying inside the

pitch surface.


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Contd. 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.

Tooth space: pitch diameter The distance between adjacent teeth

measured on the pitch circle.

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

the tooth space of the mating gear.

Circular pitch (Pc) : Distance from one face of a tooth to the

corresponding face of an adjacent tooth on the same gear, measured

along the pitch circle. N

DPc


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Contd.

Diametral pitch (Pd): The number of teeth of a gear unit pitchdiameter. The diametral pitch is, by definition, the number ofteeth divided by the pitch diameter. That is,

WherePd = diametral pitch

N = number of teeth

D = pitch diameter

Module (m): Pitch diameter divided by number of teeth. Thepitch diameter is usually specified in inches or millimeters; in theformer case the module is the inverse of diametral pitch.

m = D/N

D

NP

d


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Importance of gear measurement

Measurement helps us in three main ways:accuracy, unreliability of senses, and

standardization. In order that the rotary motion of

driven shaft be perfectly uniform relative to the

driving shaft, it is essential that both gears be ofperfectly uniform relative to the driving shaft, it is

essential that both gears be of perfect

geometrical form and be perfectly mounted on

perfect shafts running in perfect bearings. Forcloser control over the accuracy of manufacture,

precision measurement of gears plays a vital role.


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Problem set

1) What is Positive Drives? Why Gears are said as Positive Drives?

Ans : Positive drive means movement without slip. Gears are said to

be 'POSITIVE DRIVE" because there is no slippage between the

input and output.

2) If quietness of operation is required? Which gears are preferred

among following?

a)Spur gears

b)Helical gears

c)Herringbone gears


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3) The DRIVER has 60 teeth and because it is the largest wesay that it revolves once. The DRIVEN gear has 30 teeth.What is gear ratio?

Ans :

Gear ratio =no: of teeth in driven /no of teeth in driver=1:2

4) A pair of spur gears consists of a 20 teeth pinion meshingwith 120 teeth gear. The module is 4mm .calculate

i) Centre distance ii) pitch circle diameters of pinion and gear

Ans :Centre distance =

Dp=m.Zp=4(20)=80mm

Dg=m.Zg=4(120)=480mm


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Conclusion

Gears are used in many machines. Many are

machines we use in our everyday life, such as

cars; windup toys, clocks, mixers, bikes, oil rigs,

vacuum etc. Gears make our lives easier in

many ways. They allow us to change direction

and speed of movement, measure time andcreate a great force mechanical output with a

little force input. Properly designed & properly

maintained gear system can run over decades.


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References:

1. Engineering Metrology. By R. K. Jain.2. http://en.wikipedia.org/wiki/Gear.

3. http://machinedesign.com/article/gear-tooth-

form-1115

4. design of machine elements by V.B.Bhandari


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THANK YOU

gear measurement and basic concepts

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