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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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Terms and Deinitions
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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Terms and Deinitions
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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Terms and Deinitions
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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Terms and Deinitions
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"
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