thread and gear measurement.ppt
Transcript of 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! $he distance a screw thread advances in oneturn. +or a single start threads lead,pitch
+or dou#le start lead,xpitch & so on.
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Screw Threadterminology
#a$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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Screw Threadterminology
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.
Screw Threadterminology
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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. $he wires may #eheld in hand or hung from a stand.
+rom the g 9,diameter over the wires
", e:ective diameter to #e found4
d, diameter of wires h,height of wire
center a#ove the pitch line r,radius of
Three >ire method
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Three >ire method
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ssume
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Three >ire method
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Three >ire method
out"found*ecanEkno#n$isdasM$ofluecorrect %athefindingAfter
a*o%e"deri%edformulaeusing%aluesaltheoretic#ith thecom+are
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?EST >IRE SI@E
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?EST >IRE SI@E
P
/2
A
P62
P6!
Pitch line
)E7T 789E F .8RE
)
?EST >IRE SI@E
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?EST >IRE SI@E
2
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thread"theof+itchtheisP#here
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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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Tool maers microscope:
;am+
(ollo# *ase
Collimator lens
)ase
Column
E-e +iece+tical head
Mirror
#ork ta*le
#ith carriage
Tool maers microscope:
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Tool maers microscope:
Tool maers microscope:
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Tool maers microscope:
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
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*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