Gear [Chapter 4]

7/24/2019 Gear [Chapter 4]

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UNIT 4 GEAR

JJ513 ENGINEERING DESIGN


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DEFINITION: GEAR

A wheel with evenly sized and spaced teeth

machined or ormed aro!nd its parameter"#

Mechanism and Mechanical Devices Sourcebook

$y : Neil %clater


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Applications: Gear

&" Transmit motion/powerrom one point to

another"

'" (han)in) the rotation speed"

*" (han)in) rotational direction"

+" (onvertin) rotational to linear motion"


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(lassiication o Gears

E,ternal )earTeeth on the o!tside o the dis- or wheel

Internal )ear

Teeth on the inside s!race o a rin).cylinder


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Types o Gears

a/ %p!r )ear

0/ 1elical

c/ 1errin)0onesd/ $evel

e/ 2orm

/ Rac- and pinion)/ Ann!al

h/ Face


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Gear: %p!r )ear

%p!r )ears have teeth that r!n

perpendic!lar to the ace o the )ear

Not as smooth.3!ite as spiral 0evel )ear

4ermit medi!m load capacity

4rovide moderate tor3!e transmission

Application: oot pedal )ear

motorcycle.)ear0o, o a!tomo0iles"

Fi)!re 5"& : %p!r )ear


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Gear: 1elical1elical )ears are very similar to sp!r )ears

e,cept the teeth are not perpendic!lar to the

ace"

The teeth are at an an)le to the ace )ivin)

helical )ears more tooth contact in the same

area"

1elical )ears can also 0e !sed on non6parallel

shats to transmit motion"

1elical )ears tend to r!n 3!ieter and smootherthan sp!r )ears d!e to the increased n!m0er o

teeth in constant contact at any one period o

time"

Application: cement mills 7 cr!sher

Fi)!re 5"': 1elical )ear


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Gear: 1errin)0ones

1errin)0one )ears resem0le two helical)ears that have 0een placed side 0y side"

They are oten reerred to as 8do!0le

helical8"

One 0eneit o herrin)0one )ears is that

it helps to avoid iss!es related to side

thr!st created with the !se o helical

)ears"

Application: cement mills 7 cr!sher

Fi)!re 5"*: 1errin)0ones )ear


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Gear: $evel Gear

$evel )ears are !sed mostly in

sit!ations that re3!ire power to 0etransmitted at ri)ht an)les 9or

applications that are not parallel/"

$evel )ears can have dierent an)les oapplication 0!t tend to 0e ;


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Gear: 2orm )ear

2orm )ears are !sed to transmit power at;< and where hi)h red!ctions are re3!ired"

The worm resem0les a thread that rides in

concaved or helical teeth"

Application: material

handlin).transportation machinery

Fi)!re 5"5: 2orm )ear


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Gear: Rac- and pinion

A rac- is 0asically a strai)ht )ear !sed to transmit power and motion in a

linear movement"

Application: drive mechanism"

Fi)!re 5"=: Rac- and pinion )ear


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Gear: Ann!al )ear

Internal )ears typically resem0le inverted sp!r )ears 0!t are occasionally c!t

as helical )ears"

Application: a!tomatic transmission o a!tomo0iles.step6!p drives o wind

mills.red!ction )ear0o,es o cement mills

Fi)!re 5">: Ann!al )ear


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Gear: Face )ear

Face )ears transmit power at 9!s!ally/ ri)ht

an)les in a circ!lar motion"

Face )ears are not very common in ind!strialapplication"

Fi)!re 5"?: Face )ear


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Gear Terminolo)y

Fi)!re 5" Gear Terminolo)y


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Terms and Deinitions&" Addend!m 9A/

Radial distance between the pitch circle andoutside diameter

'" Dedend!m 9D/

Radial distance between the pitch circle to thebottom of the tooth space.

*" 4itch diameter 9d/

Diameter of the pitch circle which is equal to theoutside diameter minus 2 addendums.


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Terms and Deinitions

Diametral 4itch 9D4/ @ n!m0er o teeth.pitch dia

For a D4 which is in inch !nit ONB"

Example

I the pitch diameter o a )ear @ = inchC and

the )ear has n!m o teeth @ =; so the )ear the

)ear D4 is &;"


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od!l 9/

Ratio of the pitch diameter in mm to the

number of teeth.

2here m @ d.T or A @

d @ pitch diameter

T@ n!m o teeth


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4itch circle 9(4/

Circle throuh the point havin its centre at

the axis of the ear.

2here (4 @ d.T

d @ pitch diameter

T@ n!m o teeth


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Gear ratio 9G/

!he ratio of the number of teeth on ear to

that on the pinion.

2here G @ T.t

T@ n!m o teeth on )ear

t@ n!m o teeth on pinion


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4itch line velocity 9p/

2here p @

D @ pitch circle diameter

N @ rotatin) speed


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UNIT 4 GEAR

JJ513 ENGINEERING DESIGN


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Gear Drive

Fi)!re a0ove shows a )ear drive transmittin) power rom a motor to a p!mp"

2heel A -eyed to the motor shat is -nown as driver"

2heel $ -eyed to the p!mp shat is -nown as ollower or the driven"

2hen wheel A rotatesC its teeth transmit the orce and the tor3!e to wheel $"

Note that in the e,ternal )earin) the wheels rotate in opposite directions"

motor A

$p!mp

Fi)!re 5"&; E,ternal )earin)


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Gear Drive

I 0oth o the shat have to 0e rotated in the same directionC one way o doin) it is

to !se internal )earin)"

The lar)er wheelC -nown as ann!lar or internal )earC has its teeth c!t internally"

The smaller wheel is called the pinion"

motor A

$

p!mp

Fi)!re 5"&& Internal )earin)


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elocity Ratio

Gear drives are )enerally !sed to chan)e the rotational speed

o the driven shat"

velocity ratio deine as the ratio o a distance thro!)h which

any part o a machine moves to that which the drivin) part

moves d!rin) the same time"

R@Eort Distance.oad Distance


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elocity ratio

From Fi)!re 5"&'C we can derive the ollowin) notations

N& @ speed o the driverC rpm

T& @ n!m0er o teeth on the driver

D&@ pitch circle diameter o the driver

N' @ speed o the drivenC rpm

T' @ n!m0er o teeth on the driven

D' @ pitch circle diameter o the driven

otor

4!mp

Driver

Driven

Fi)!re 5"&' velocity ratio in inversely proportional to the n!m0er o

teeth


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elocity RatioAt the contact s!race the linear velocity is the sameC hence

D&N& @ D'N'Or

elocity ratio @

@

For meshin) )ears the module, m is the sameC Now

D& @ mT& and D' @ mT'

%!0stit!tin) the e3!ationC we )et

=

=


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elocity RatioExample 1

An electric motor r!nnin) at &5;; rpm has to drive ap!mp at a speed o *>5 rpm" The lar)est )ear availa0le

has ?+ teeth" I the mod!le is +mmC determine the

velocity ratioC the center distanceC the n!m0er o teethon the pinionC and the pitch line velocity"


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Gear Train

ost electric motorsC petrol en)ines and diesel en)ines

operate eiciently at certain speciic speeds" The driven

machinery s!ch as p!mpsC compressorsC machine tools etc"

need to r!n at their own optim!m speed or 0etter

perormance"

In s!ch sit!ationsC )ear trains are used to change output

speed even though the driving machines run at constant

speed.


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Gear Train: Deinition

Combination of three or more gears used to transmit power

from one shaft to another.

Gear train may classiied into + cate)ories

%imple )ear train

(ompo!nd )ear train

Reverted )ear train

Epicyclic )ear train


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Gear Train%imple Gear Train

@

=

2here

T& @ n!m0er o teeth in )ear &

T' @ n!m0er o teeth in )ear '

N' @ rpm o )ear '

N& @ rpm o )ear &

Driver

Driven


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Gear Trainet introd!ce another )ear in 0etween the driver anddriven"

2

1=

1

2

3

2=

2

3

Driver

Driven

Idler


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Gear train!ltiplyin) the two e3!ationsC we )et

2

1

3

2=

1

2

2

3

On simpliicationC we o0tain

3

1=

1

3


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Gear TrainExample 1

Desi)n a )ear 0o, to )et a velocity ratio o + !sin) asimple )ear train" The center distance 0etween the

inp!t shat and o!tp!t shat is *;; mm and )ears

mod!le = are !sed" 9Answ: T& @ ?;C T' @ ';/


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%!mmary

Deinition o )ear

Types 7 applications

Gear terminolo)y

elocity Ratio 7 Gear Train


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ReerencesitraC G" " 9';;&/" "andbook of #ear Desin Second Edition. New Delhi: Tata cGraw61ill"

Gear [Chapter 4]

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