thread and gear measurement.ppt

7/25/2019 thread and gear measurement.ppt

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Unit-3

Screw threadand

Gear measurement


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Screw thread measurement: Elements of measurement-

errors in screw threads-measurement of effective

diameter, angle of thread and thread pitch

Gear Measurement: Gear measuring instruments, Gear

tooth profile measurement. Measurement of diameter,

pitch, pressure angle and tooth thickness.


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Screw thread- defnitionA screw thread is a continuous helical groove of

specied cross-section produced on theexternal or internal surface of a cylinder or acone.A screw thread formed on a cylinder is knownas straight or parallel screw thread while theone formed on a cone is known as tapered

threads.


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Types o thread

External thread! a thread formed onoutside of a work piece is known asexternal thread."xample! on #olts or studs etc.

Internal thread! a thread formed on

inside of a work piece is known asinternal thread."xample! on a nut


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Screw thread - use

Screw threads are used: $o hold parts together-act as fasteners with

the help of nuts #olts and studs

"x! %-threads $o transmit motion & power with the help of

lead screw in a lathe or in 'y press.

"x! S(uare Acme threads


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$erminology of screwthreads


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Screw Threadterminology

PitchCrest

Root

Flank

ThreadAngle

Pitch line

Axis of thread

Axial thickness

Addendum

Dedendum

Flankangle

Major dia Pitch dia Minor dia

EXTERNAL THREAD TERMINOLOGY


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BCpitch


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Crest ! crest is the prominent part of the thread i.e top surface )oining the two sides of threadRoot! *oot is the #ottom of the groove #etween thesides of two ad)acent threads.

Flan! $he straight surface #etween the crest and rootwhich connects the crest with the root is called the

'ank.

Screw Thread

terminology

!itch! $he distance from a point on a screw thread toa corresponding point on the next thread measured

parallel to the axis.

"ead! $he distance a screw thread advances in oneturn. +or a single start threads lead,pitch

+or dou#le start lead,xpitch & so on.


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#a$or %iameter! $his is the diameter of animaginary cylinder co-axial with the screw which

)ust touches the crests of an external thread or rootsof an internal threads.

t is also called as /0ominal diameter1

#inor diameter: $his is the diameter of animaginary cylinder co-axial with the screw which

)ust touches the roots of an external thread or the

crest of an internal thread.

$his is also referred to as /root1 or /core diameter1.


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E&ecti'e diameter or !itch diameter!

2n a straight screw thread the diameter of animaginary cylinder where the widths ofthe threadsmetal4 and the widths of the spaces

#etween the threads are e(ual5 each #eing halfthe pitch.


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crew reaterminologyThread angle(Included angle )*! ncluded

angle #etween sides of thread measured inaxial plane.

Flan angle()+,*! Angle made #y the 'ank

of a thread with the perpendicular to the

thread axis is called 'ank angle. t is half the

included angle of the thread.

elix angle(.*! 2n a straight thread the

helix angle is the angle made #y the helix of

the thread at the pitch line with the axis. 6elix

angle is complementary of lead angle.

Tan . / p+0d1 where p/pitch1 d/mean


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%epth o thread! t is the distance #etween

crest and root measured perpendicular to axisof screw.

2ddendum! t is the distance #etween the

crest and the pitch line measured

perpendicular to axis of the screw.

%edendum: t is the distance #etween the

pitch line & the root measured perpendicular

to axis of the screw.



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#ultiple start thread: $his is produced #yforming two or more helical grooves e(ually

spaced and similarly formed in an axialsection on a cylinder. $his gives a (uicktraverse without sacricing core strength.

"ead! $he distance ascrew thread advancesin one turn. +or a singlestart threads

lead,pitch

+or dou#le start

lead,xpitch & so on.


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Errors in threads

$here are 7 important elements of a threadand errors in any one of these may causere)ection.

8.9a)or diameter.9inor diameter3.":ective diameter;.


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Errors in #a$or and #inor diameters:

!ill cause interferencewith the mating thread. "ue to errors in

these elements, the root section and wallthickness will #e less,also the flank contact will #e reduced and ultimatel$ the

component will #e weakin strength.

Errors on the e&ecti'e diameter will also result inweakening of the assem#l$ due to interference #etween the

flanks. More force is re%uired for fitting.

&imilarl$ pitch and angle errors are also not desira#le as

the$ cause a progressive tightening and interference onassem#l$. 'hese two errors have a special significance as the$

can #e precisel$ related to the e&ecti'e diameter.


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34!itchError


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%runen thread: 'his is the one having erraticpitch, in which the advance of the heli( is irregular in one

complete rotation.

'hread drunkenness is a particular case of a periodic

pitch errorrecurring at intervals of one pitch.

)n such a thread, the pitch measured parallel to thethread a(is will alwa$s #e correct, the onl$ error #eing that

the thread is not cut to a true heli(.

)f the development of the thread is taken then thedrunken can #e visualised.


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)t is ver$ difficult to determine such errors and moreoverthe$ do not have an$ great effect on the working, unless

the thread is of ver$ large si*e.

'he heli( will #e a curve in the case of drunken thread

and not a straight line as shown in fig.

Tan . /p+0d

'hread micrometer is the onl$ instrument to identif$ the

drunken thread

!it h i th d


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!itch errors in screw threads: Generall$ thethreads are generated #$ a point cutting tool.

+or pitch to #e correct, the ratio of the linear velocit$ of tool

and angular velocit$ of the work must #e correctand this ratio

must #e maintained constant, otherwise pitch errors will

occur.

'he total pitch error in overall length of the thread #eing called

cumulative error.

!rogressi'e pitch error: 'his error occurs when thetool work velocit$ ratio is incorrect though it ma$ #e constant.

)t can also #e caused due to pitch errors in the lead screw of thelathe or other generating machine.

'he other possi#ilit$ is #$ using an incorrect gear or an

appro(imate gear train#etween work and lead screw.


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rogressive pitch error eriodic pitch error

Effect: n error in pitch virtuall$ increases the effective diameter

of a #olt and decreases the effective diameter of a nut.


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!eriodic pitch error: 'his repeats itself at regularintervals along the thread. )n this case, successive portions of

the thread are either longer or shorter than mean.

'his t$pe of error occurs when the tool work velocit$ ratiois

not constant. 'his t$pe of error also results when a thread is

cut from a lead screw which lacks s%uareness.

'hus the errors due to these cases are c$clic and pitch

increases to a ma(imum, then reduces through normal value

to a minimum and so on.

'he graph #etween the cumulative pitch error and length of

threads for this error will, therefore, #e a sinusoidalform.

'hread drunkenness is a particular case of a periodic

pitch errorrecurring at intervals of one pitch.


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Irregular errors:'hese arise from distur#ances in themachining set-up, variations in the cutting properties of

materials etc. 'hus the$ have no specific causes and

correspondingl$ no specific characteristics also.

Errors could #e summarised as:

54Erratic pitch: 'his is the irregular error in pitch andvaries irregularl$ in magnitude over different lengths of

thread.

,4!rogressi'e error: !hen the pitch of a screw is

uniform, #ut is shorter or longer than its nominal value, it issaid to have progressive error

64!eriodic error: )f the errors var$ in magnitude andrecur at regular intervals, when measured from thread to

thread along the screw are referred to as periodic errors.


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742ngle

error


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ngle errors on threads ma$ #e either due to errors on one

or #oth flanks. n$ error in angle of thread results in

interference #etween the #olt and nut.

'hus like pitch errors, the angle errors also increase the

effective diameter of a #oltand decreasethat of a nut.

ssuming that one of the pairs is correct, it is possi#le tosatisfactoril$ assem#le the thread pairs #$ modif$ing the

effective diameter.


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#E2S8RE#E9T F ;2RI8SE"E#E9TS F TRE2%

$o nd out the accuracy of a screwthread it will #e necessary to measure

the following!

84 9a)or diameter.

4 9inor diameter.34 ":ective or


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#easurement o e&ecti'ediameter

":ective diameter measurement iscarried out #y following methods.

8.9icrometer method. 2ne wire two wire three wiremethod

'hread micrometer


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$hread micrometermethod:)t has special contacts to suit the screw thread form that is to#e checked. )n this micrometer, the end of the spindle ispointed to the ee-thread form with a corresponding ee-

recess in the fi(ed anvil.

)f correctl$ ad/usted, this micrometer gives the pitch

diameter.

+or correct results it is necessar$ to use a separate thread

micrometer for ever$ si*e of screw thread to #e gauged,

otherwise there will alwa$s #e a small amount of errorinherent in thread micrometer.

'his is the onl$ method which shows the variation for the

drunkenthread.

2 i h d


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2ne wire method!one wire is placed #etween two

threads at one side and on the

other side the anvil of themeasuring micrometer contacts the

crests as shown in +ig.

+irst the micrometer reading isnoted on a standard gauge whose

dimension is nearl$ same as to #e

o#tained #$ this method.

ctual measurement over wire onone side and threads on other side

si*e of gauge 0 difference in two

micrometer readings.


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Two wire method:=ires of exactly known identical diameters are

chosen such that they contact the 'anks ofthe thread as shown.f the si>e of the wire is such it contacts the

'anks at the pitch line it is called the /


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Two wire method:

9

#-%imension o'er the wire


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Two wire method:


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Two wire method:

T i th d


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Two wire method:

A


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Two wire method:

constant, which depends on the dia. of wire and pitch of the thread1 2

p pitch

E M - 1d 3 12 or M E31d-12


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Floating Carriage machine

Th >i th d


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Three >ire method

Three >ire method


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Three >ire methodME31h31r

M E31d-12 4in 1 wire

&o, d h3 r 3 2

Three >ire method


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Three >ire method

$his method is more accurate than twowire method as it ensures alignment ofmicrometer faces parallel to the threadaxis.

6ere three wires of exactly knowndiameters are used one on one side & thetwo on the other side. $he wires may #eheld in hand or hung from a stand.

+rom the g 9,diameter over the wires

", e:ective diameter to #e found4

d, diameter of wires h,height of wire

center a#ove the pitch line r,radius of

Three >ire method


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Three >ire method

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Three >ire method


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Three >ire method

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?EST >IRE SI@E

P

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P62

P6!

Pitch line

)E7T 789E F .8RE

)

?EST >IRE SI@E


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?EST >IRE SI@E

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!itch measurement


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!itch measurement

$he most commonly used methods formeasuring the pitch are

8.$ool makers microscope.


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Tool maers microscope:ccurac$: 7.778 mm, for angles 87 sec of arc

Tool maers microscope:


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;am+

(ollo# *ase

Collimator lens

)ase

Column

E-e +iece+tical head

Mirror

#ork ta*le

#ith carriage



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8. =orkta#le is placed on the #ase of theinstrument.. $he optical head is mounted on a vertical columnit can #e moved up and down.3. =ork piece is mounted on a glass plate.;. A light source provides hori>ontal #eam of light

which is re'ected from a mirror #y B degreeupwards towards the ta#le.7. mage of the outline of contour of the work piecepasses through the o#)ective of the optical head.

C. $he image is pro)ected #y a system of threeprisms to a ground glass screen.D. $he measurements are made #y means of crosslines engraved on the ground glass screen.E. $he screen can #e rotated through 3CBF.. @i:erent types of graduated screens and

u


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umachine(!itter*

Spring loaded head permits the stylus to mo'e up the Aan othe thread and down into the next spaceas it is mo'ed along4Screw is held stationary


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When the pointer is accurately placed in position, themicrometer reading is noted. The stylus is then movedalong into the next thread space, by rotation of themicrometer, and a second reading taken.

The dierence between the two readings is the pitch ofthe thread. Readings are taken in this manner until thewhole length of the screw thread has been covered.

&t$lus point should make contact on or near the effective diameter


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#atrix !itch #easuring

machine:

&imilar to itter machine.9o#ustin construction and sensitivein measurementccurac$:7.771 mm over a distance of 7mm for all

thread formsMicrometer head is provided on the headstock which is

fi(ed to the #ase . 'he rotation of micrometer head produces

movement of the longitudinal carriage along the #ed of the

#asenother carriage carr$ing the indicating and amplif$ing

units comprising st$lus.'he st$lus is traversed along the thread, pitch #$ pitch

reading #eing taken each time fiducial indicator is set at

*ero.


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Matrix pitch measuring machine


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rofile ro/ector


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rofile ro/ector

Profile Projector(optical profile pro/ector;


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Profile Projector(optical profile pro/ector;

B$ using lenses and #eams of light, profiles of small shapes

can #e magnified. 'he enlarged image can #e compared with

accurate drawing made to the scale of magnification.

&uch a comparison can reveal an$ deviations in the si*es and

contours of the o#/ects and to get a numerical assessment of

such deviations, measurements can #e made on the enlargedshadow.


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S it h


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Screw pitch gauge

)& ? the ga ges hich help to identif the thread pitch


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)&:?, the gauges which help to identif$ the thread pitch

of )&@ metric screw threads in the pitch range 7.1 to >.7 mm

are made in the form of 1< #lades made of suita#le tool steel

sheet and 7. mm thick.

1Afor checking the pitches and

onehaving an )&@ profile of >77

ll these #lades are assem#led in a protective sheath.

'hese are suita#le hingedin the sheathwith screw and nut

arrangement, and can #e easil$ removed and rotated.

f h d l l k


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9easurement of screw thread angle +lankangle4

'ool makers microscope

rofile ro/ector protractor

#easurement o ma$or


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#easurement o ma$ordiameter

$he instruments which are used to nd the

ma)or diameter are #yench micrometer


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rdinary micrometer:


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rdinary micrometer:

$he ordinary micrometer is (uite suita#le for

measuring the external ma)or diameter.

t is rst ad)usted for appropriate cylindrical si>eS4 having the same diameter

approximately4.$his process is known as / gaugesetting1 .After taking this reading / * the micrometer isset on the ma)or diameter of the thread and thenew reading is /*

#easurement


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ymicrometer:

Clam+

Fiducial8ndicator

MeasuringAn%ils (olding centres

Micrometer head

7u++orts

)E


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ymicrometer:

+or getting the greater accuracy the #ench micrometer is

used for measuring the ma)or diameter.n this process the variation in measuring


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ymicrometer:

$hen the cylinder is replaced #y the threaded work piece andthe new reading is taken

#easurement


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ymicrometer:

#easurement


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ymicrometer:

(olding centre

Measuring an%il

(olding centre

Measuring an%il

7tandardC-

linder

7cre#

Threa

d

Measurement of Major diameter


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#easurement o the ma$ordiameter o an Internal thread:

An indirect approach of measuring internal dia

is obtained by obtaining the cast of theThread. The main art thus lies in obtaining aperfect cast.


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#easurement o the ma$ordiameter o an Internal thread:

#easurement o #inor


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diameter$he minor diameter is measured #y acomparative method #y using 'oatingcarriage diameter measuring machine andsmall / % pieces which make contact with the

root of the thread.

$hese % pieces are made in several si>eshaving suita#le radii at the edges.

% pieces are made of hardened steel.

$he 'oating carriage diameter-measuring

machine is a #ench micrometer mounted on

#easurement o #inor


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diameter

#easurement o #inor


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$he threaded work piece is mounted #etween thecentres of the instrument and the % pieces are placedon each side of the work piece and then the reading isnoted.

After taking this reading the work piece is thenreplaced #y a standard reference cylindrical settinggauge.

#easurement o #inordiameter

# t #i


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#easurement o #inordiameter o Internal

threads:

$he 9inor diameter of nternal

threads are measured #y8. Using taper parallels. Using *ollers.

#easurement o #inor


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diameter o Internal

threads:54 8sing taper parallels: +or diameters less than BBmm the use of $aperparallels and micrometer is very common.

$he taper parallels are pairs of wedges having

reduced and parallel outer edges. $he diameter across their outer edges can #echanged #y sliding them over each other.

#easurement o #inor


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diameter o Internal

threads:8sing rollers:+or more than BBmm diameter this methodis used.


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$wo wire method

GEAR MEASUREMENT


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GEAR MEASUREMENT

A gear is a com+onent #ithin a transmission de%ice that

transmits rotational force to another gear or de%ice

.hen t#o s+ur gears of different si:es mesh together$ the

larger gear is called a #heel$ and the smaller gear is called a

+inion"

TYPES OF GEARS


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TYPES OF GEARS

1" According to the+osition of axes of the shafts"

a" Parallel

1"7+ur =ear

2"(elical =ear

0"Rack and Pinion

*" 8ntersecting

)e%el =ear

c"


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,4 2ccording to the ormso Teeth

n actual practice following are the two types ofteeth commonly used

1. Involute teeth. . Cycloidal teeth

Parallel


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Parallel

1.SPUR GEAR

Teeth is +arallel to axis of rotation

Transmit +o#er from one shaft to another +arallel shaft


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External and Internal spr Gear!

" H l# l G


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".Hel#$al Gear The teeth on helical gears are cut at an angle to the face of the

gear

This gradual engagement makes helical gears o+erate much

more smoothl- and >uietl- than s+ur gears

Hel#$al Gear %&ntd..


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' R ( d # #


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'.Ra$( and p#n#&n

Ra$( and p#n#&n )earsare used to con%ert rotation 'From the+inion& into linear motion 'of the rack&

A +erfect exam+le of this is the steering s-stem on man- cars

*. Stra#)+t and Sp#ral *e,el Gears -Interse$t#n)


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% /ORM AND /ORM GEAR


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%. /ORM AND /ORM GEAR

-N&n0#nterse$t#n) N&n0parallel

Forms o Teeth n actual practice following are the two types of teeth


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n actual practice following are the two types of teethcommonly used

1. Involute teeth. . Cycloidal teeth

Comparison ?etween In'olute and Cycloidal Bears n actual practice the involute gears are more commonly used as

compared to cycloidal gears due to the following advantages !


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compared to cycloidal gears due to the following advantages !

Advantages of involute gears

$he most important advantage of the involute gears is that the centre

distance for a pair of involute gears can #e varied within limits withoutchanging the velocity ratio. $his is not true for cycloidal gears whichre(uires exact centre distance to #e maintained.

n involute gears the pressure angle from the start of the engagement ofteeth to the end of the engagement remains constant.t is necessary forsmooth running and less wear of gears. ut in cycloidal gears the pressure

angle is maximum at the #eginning of engagement reduces to >ero atpitch point starts decreasing and again #ecomes maximum at the end ofengagement. $his results in less smooth running of gears.

$he face and 'ank of involute teeth are generated #y a single curve whereas in cycloidal gears dou#le curves i.e. epi-cycloid and hypo-cycloid) arerequired for the face and ank respectively. $hus the involute teeth are

easy to manufacture than cycloidal teeth. n involute system the #asicrack has straight teeth and the same can #e cut with simple tools.

0ote ! $he only disadvantage of the involute teeth is that the interferenceoccurs with pinions having smaller num#er of teeth. $his may #e avoided#y altering the heights of addendum and dedendum of the mating teeth orthe angle of o#li(uity of the teeth.

Advantages of cycloidal gears


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+ollowing are the advantages of cycloidal gears !

Since the cycloidal teeth have wider 'anks therefore thecycloidal gears are stronger than the involute gearsfor the samepitch. @ue to this reason the cycloidal teeth are preferredspecially for cast teeth.

n cycloidal gears the contact takes place #etween a convex'ank and concave surfacewhereas in involutegears the convexsurfaces are in contact. $his condition results in less wear incycloidal gears as compared to involute gears. 6owever thedi:erence in wear is negligi#le.

n cycloidal gears the interference does not occur at all. $houghthere are advantages of cycloidal gears #ut they are outweighed#y the greater simplicity and 'exi#ility of the involute gears.


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NOMEN%LATURE OF SPUR GEARS


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NOMEN%LATURE OF SPUR GEARS

ace ! It is the surface of the gear tooth above the pitchsurface.


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lank ! It is the surface of the gear tooth below the pitchsurface.

Top land! It is the surface of the top of the tooth.ace width! It is the width of the gear tooth measured parallel

to its ais.

"ro#le! It is the curve formed by the face and ank of thetooth.


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ke$ parameter is the pitch circle, which indicates where the teeth should mesh,

and is used in calculating the Center "istance. The Pressure angle indicates

the angle at which the contact force between gear teeth occur


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"itch circle. It is an imaginary circle which by purerolling action would give the same motion as the actual


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rolling action would give the same motion as the actualgear.

"itch circle diameter. It is the diameter of the pitchcircle. !he si"e of the gear is usually specied #y thepitch circle diameter. t is also known aspitch diameter.

"itch point. It is a common point of contact betweentwo pitch circles of gears in mesh.

"ressure angle or angle of obli$uity. It is the anglebetween the common normal to the two gear teeth atthe point of contact and the common tangent to the twopitch circles at the pitch point.

t is usually denoted #y . $he standard pressure anglesare

53 5+, D and ,D4

Addendum. It is the radial distance of a tooth from thei h i l h f h h


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pitch circle to the top of the tooth.

%edendum.It is the radial distance of a tooth from the

pitch circle to the bottom of the tooth. Addendum circle. It is the circle drawn through the

top of the teeth and is concentric with the pitch circle.

%edendum circle. It is the circle drawn through the

bottom of the teeth. It is also called root circle4

9ote :

ase circle diameter

,


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tooth to the corresponding point on the next tooth. t isusually denoted #y #c$9athematically

#c, Km m, module

A little consideration will show that the two gears willmesh together correctly if the two wheels have thesame circular pitch.

9ote : f%8 and %, are the diameters of the twomeshing gears having the teeth T5 and T,respectively then for them to mesh correctly1

%iametral pitch! It is the ratio of number of teeth to the pitchcircle diameter in millimeters. t is denoted #y pd.9athematically


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9athematically

'odule! It is the ratio of the pitch circle diameter inmillimeters to the number of teeth. t is usually denoted #ym. It is reciprocal of %iametral pitch &athematically$

Total depth! It is the radial distance between theaddendum and the dedendum circles of a gear. It is

equal to the sum of the addendum and dedendum.

Theaddendum circleis that which contains the tops of the teeth and its

diameter is the outside or blank diameter


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Circular tooth thicknessis measured on the tooth around the pitch circle!

that is! it is the length of an arc

Circular pitchis the distance from a point on one tooth to the

corresponding point on the next tooth! measured around the pitch circle

The moduleis the pitch circle diameter di"ided b# the number of teeth

TheDiametrical pitchis the number of teeth per inch of pitch circle

diameter This is a ratio

Thepitch pointis the point of contact between the pitch circles of two

gears in mesh

The dedendum or root circleis that which contains the bottoms of the

tooth spaces and its diameter is the root diameter


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The line of action:Contact between the teeth of meshing gears ta$es place

along a line tangential to the two base circles This line passes throughthe pitch point and is called the line of action

The pressure angle:The angle between the line of action and the common

tangent to the pitch circles at the pitch point is the pressure angle

The tooth faceis the surface of a tooth abo"e the pitch circle! parallel to

the axis of the gear

The tooth flankis the tooth surface below the pitch circle! parallel to the

axis of the gear %f an# part of the flan$ extends inside the base circle it

cannot ha"e in"olute form %t ma# ha"e ant other form! which does notinterfere with mating teeth! and is usuall# a straight radial line


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"raw thepitch circle, diameter D, a#out the centre of the gear,O. 'his is shown in red.

"raw the outside diametergreen; #$ drawing a circle a#out O with a radius greater than

the pitch circle #$ the value of the addendum.

"raw the root diameterlight #lue; #$ drawing a circle a#out O with a radius smaller than

the pitch circle #$ the value of the dedendum.


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8."raw a tangentto the pitch circle pink;.

1."raw a line orange; through the intersection of the tangent with the

pitch circle which makes an angle e%ual to the pressure angle! ! with

the tangent. 'his line is theline of action

A."raw a circle a#out Owhich is tangent to the line of action. 'his is

thebase circleand is shown in dark #lue.


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ressure ngle

Errors ma$ #e present


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Errors ma$ #e present

8.9un out1.itch

A.rofile


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$

'his is measured using Gear eccentricit$ testers.

'he gear is held in the mandrel in the centres. 'he dialindicator of the tester possesses special tip depending upon

the module of the gear to #e checked.

'he tip is inserted in #etween the tooth spaces. 'he gear isrotated tooth #$ tooth.

'he ma(imum variation is noted from the dial indicator

reading which gives the runout of the gear.

'he runout is twice the eccentricit$.

9unout: 'he composite deviation of a circular part during one full rotation of A>77

Eccentricit$ is the distance of the a(is from the geometric center.

Gear eccentricit$ testers.


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ccentricit#: )t is half the radial runout.

)Pitch measurement(*ase pitch+Errors in the tooth spacing or pitch of gear ma$ #e

d # it h i i t t # i th


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measured #$ pitch measuring instrument #$ measuring the

distance from a point on one tooth to a point on the ne(t

tooth.

Pitch measuring instrument


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'his instrument has Atips. @neis the fi(edmeasuring tip, other

one is the sensitive tip whose position can #e ad/usted #$ a


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p p / $

screw and the further movement of it is transmitted through a

leverage s$stem to the dial indicator and the third tip is the

supplementar$ ad/usta#le stop which is meant for the sta#ilit$ ofthe instrument and its position can also #e ad/usted #$ a screw.

'he distance #etween the fi(ed and sensitive tip is set to #e

e%uivalent to the #ase pitch of the gear with the help of slipgauges.

'he properl$ set-up instrument is applied to the gear so that all

the three tips contact the tooth profile.

'he reading on dial indicator is the error in the #ase pitch

distance from one face of a tooth to the corresponding face of

an ad/acent tooth on the same gear, measured along the #ase

circle;

Pitch errors Contd


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& ,ngular indexing: 'he simplest method of determining

pitch errors is to set a dial gauge against a tooth and note

the reading.

)f gear is not inde(ed through the angular pitch, the reading

differs from the original reading.

'he difference #etween these is the cumulative pitch error.

)t is necessar$ to use suita#le inde(ing device to o#tain

accurate results.


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itch measurement using angular indexing

'wo dial gauges on ad/acent teeth with the gear mounted in

)Pitch comparator


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'wo dial gauges on ad/acent teeth with the gear mounted in

centres.

'he gear is inde(ed through successive pitches to give a

constant reading on dial .

n$ change in the reading on dial B indicates that pitch errors

are present.

'he actual error can #e determined #$ deducting the individual

reading on dial B from the mean of the readings.


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Pitch comparator

-Profile: 'o check the involute profile of a spur gear


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g

.ptical Projection method: 'he profile of the gear under

test is magnified #$ optical means and pro/ected on thescreen. )t is then compared with master profile.

'his method is %uick and suita#le for checking the profile of

small instrument gears.

/*ac$lash : Backlash in gears is the pla$ #etween mating

tooth surfaces Backlash is defined as the amount #$ which


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tooth surfaces. Backlash is defined as the amount #$ which

tooth space e(ceeds the thicknessof an engaging tooth.

'ight mesh is o#/ectiona#le, #ecause of gear sound, increased

power loss, over heating and rupture of lu#ricating film,

overloaded #earing and premature gear failure.

5ence some #acklash is necessar$.1 t$pes- circumferential and normal #acklash.

Backlash is determined as follows: one of the two gears of

the pair is locked, while the other is rotated #ackwards andforward as far as possi#le, the ma(imum displacement

recorded #$ a comparator.

74 #easurement o tooth thicness(w*

$he tooth thickness is generally measured at pitch circle andi th f th it h li thi k f th t th + ll i


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is therefore the pitch line thickness of the tooth. +ollowingmethod is used for measuring the gear tooth thickness !

#easurement o tooth thicness


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- @ne of the #enefits using a caliper is that the gear does not

have to #e removed from the machine

- 'he caliper consists of two ad/usta#le verniers, that

reference two dimensions on the gear and provide a

measurement

- ertical scale: Measures the depthof the teeth from the top

of the pitch line

- 5ori*ontal scale: 'his is used to measure the Chordal

'hicknessof the gear tooth


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0Concentricit#: 'he centre a#out which the gear is

mounted should #e coincident with the centre from which the


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mounted should #e coincident with the centre from which the

gear is generated. @therwise, the gear wheel will not function

correctl$ #ecause of eccentricit$of mounting.

'he concentricit$ of the gear ma$ #e readil$ checked #$

mounting the gear #etween centres and measuring the

variation in height of a roller placed #etween the successiveteeth.

B$ rotating the gear, tooth #$ tooth, dial gauge readings over

the rollerscan #e noted and plotted in the form of a graph.

'he variation in reading o#tained will #e a function of the

eccentricit$ present and also of an$ variation which ma$ #e

there in the tooth thickness.

1,lignment: 'he alignment of gear w.r.t the a(is of

mounting ma$ #e checked #$ placing a parallel bar #etween

the gear teeth 'he gear #eing mounted #etween centres


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the gear teeth. 'he gear #eing mounted #etween centres.

9eadings are taken at the two ends of the #ar. "ifferences inthe readings i.e height of either end of the parallel #ar will

indicate presence of misalignment.

2Composite errors:ariations in manufacturing conditions

ma$ lead to man$ t$pes of errors in gears.

&Pitch error: 'his is a source of gear noiseand the character

of noise will depend upon how pitch errors are produced and

how the$ are distri#uted.

)Tooth thic$ness error: )t is the value o#tained #$su#tracting the design tooth thickness from the actual tooth

thickness measured along the surface of the reference

c$linder.

- C#clic error: )t is the error occurring during each revolution of

the element under considerations.


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/ Periodic error: n error occurring at regular intervals not

necessaril$corresponding to one revolution of the component.

3'unout: )t is total range of reading of a fi(ed indicator with the

contact point applied to a surface rotated, without a(ial

movement , a#out a fi(ed a(is.

0'adialrunout: )t is the runout measured along perpendicular

to the a(is of rotation.

1,xial runout: )t is the runout measured parallelto the a(is of

rotation, at a specified distance from the a(is.

2ccentricit#: )t is half the radialrunout.

'he presence of these errors cause interference in efficient

operation of gears. 'hese result in non-smooth and nois$

ti hi h lti t l ff t th ki lif


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operation which ultimatel$ effect the working life.

'hese composite errors can #e checked #$ measuring thevariations of the centre distance when the gear under test is

rolledunder spring pressure against a master gear. 'he test is

generall$ known as rolling gear test or; functional test.

'otal composite variation is centre distance variation in one

complete revolution of the gear #eing inspected where as

tooth to tooth variation is the centre distance variation as the

gear is rotated through an increment of A>76F.

uniform tooth to tooth variation shows profile variation

while a sudden /ump indicates pitch variation.

M t G 'h d ith ffi i t


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Master Gears: 'hese gears are made with sufficient accurac$

and can #e used as the #asis for comparing the accurac$ of

other gears.

Master gears are mostl$ used in composite errors

determination in which the master gears are rotated in close

mesh.

'hese can also #e used for cali#ration of gear checking

instruments used in shop floor.

Master gears are made from chromium-manganese tool steel

or good %ualit$ gauge steel and are hardened and properl$

sta#ili*ed to relieve internal stresses.

Par$inson gear tester


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4e"eloped b# 5ames Par$inson Standard gear on fixed

spindle! gear to be tested on similar spindle mounting on a

sliding carriage Maintaining the gears in mesh b# spring


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sliding carriage Maintaining the gears in mesh b# spring

pressure

Mo"ements of the sliding carriage as the gears are rotated and

indicated b# a dial indicator! and these "ariations are a

measure of an# irregularities in the gear under test! alternati"el#

a recorder can be fitted! in the form of a waxed circular chart

and records made of the gear "ariation in accurac# of mesh

6hen the waxed paper recorder is fitted! the chart ma$es a

re"olution for each one of the gears mounted on the sliding

carriage

,s the chart mo"es and rotates! the line traced records the

mo"ements of floating carriage , circle is drawn at the same

time as the record! as shown in figure


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imitations of parkinson gear tester


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8.Ma(. A77 mm diameter gear, usuall$ 87mm or smaller

dia. Gears are tested

1.'he accurac$ of the order of 0 7.778mm

A.9olling test does not reveal all errors, since the device issensitive to cumulative position errors


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8."erive the e(pression for

a; Effective diameter of thread

#; "iameter of #est wire

1."escri#e the generali*ed measurement s$stem with #lock diagram

A.Classif$ the t$pes of errors in instruments


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8. +irst find the #lank diameter, @" #$ a vernier caliper and also count the

num#er of teeth ' of the spur gear.

1. Fe(t calculate pitch circle diameter "'(@";6'31;A. +ind addendum, clearance, pitch, module and dedendum as per the

formulae given in the theor$.

F.' C,T%.:

8. east count of caliper 7.71mm

1. Fum#er of teeth


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C,7C;7,T%.S:8. itch circle diameter, "'(@";6'31;

1. module, m"6' mm

A. ddendumm


142/142

*ase pitch Pb? Circular pitch Pcx cos ? m @ cos

!here ressure angleN

+or involute spur gear,

,ddendum ? reciprocal of diametral pitch ?4AT ?m

4edendum?,ddendum B clearance

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