limit, fits, tolerance

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Limit, Fits, Tolerance

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Limit, Fits, Tolerance

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INTRODUCTION

•  The components must be manufacturedto the specied size !o"e#er, it ispracticall$ impossible to achie#e this

e%actness due to man$ reasons such ashuman error, hi&h cost and lac' ofsophisticated measurin& instruments

 Therefore, in practice the #ariation "hichcan be tolerated in the size of acomponent is al"a$s &i#en

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•  This permitted #ariation in the size ofthe component is calledTOLERANCE.

•  Thus, the di(erence bet"een theupper and lo"er limit on a dimensionof a part is called the tolerance

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  Tolerance #aries accordin& to thede&ree of accurac$ necessar$ forthat particular "or'

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   The purpose of this chapter is to stud$the #arious aspect of limit and ts,tolerances, machinin& s$mbol "ith

special emphasis on its increasin& utilit$in the modern industr$ accordin& to )I*+Bureau of Indian Standards)

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Need of limit and ts

• ass production and specialization

• *tandardization

Interchan&eabilit$

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 TOL-R.NC-  The amount of #ariation permitted in the

size of a part is called tolerance  The di(erence bet"een the upper and

lo"er limit on a dimension of a part is

called the tolerance

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UNILATERAL AND BILATERAL

TOLERANCES

• Unilateral+ one "a$/

• )ilateral+t"o "a$/

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Unilateral+ one "a$/

• . unilateral tolerance is one in "hichthe #ariation is permitted in onedirection ie either plus of minus

from the desi&n size!ere, 01mm is the desi&nsizeUpper limit 2 01111

Lo"er limit 2 34450 Tolerance 2 01111634450

  2 1107mmIn this, the tolerance 1107 is all one direction

to"ards smaller size

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• . bilateral tolerance is one in "hichthe #ariation is permitted in both

directions plus and minus from thedesi&n size!ere, 01mm is the desi&n

size

Upper limit 2 01101Lo"er limit 2 34481 Tolerance 2 01101634481

  2 1191mmIf the tolerance 1191 species e:ual#ariations in both directions ,then the bilateraltolerance is "ritten as

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*I;- OF TOL-R.NC-

• .ctual size < The size of a part asma$ be found b$ measurement iscalled actual size

• )asic size< The basic size is the size"ith reference to "hich the limit ofsize is %ed +desi&n size/

 The basic size is same for bothmembers of a t

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• Limit of size < the t"o e%tremepermissible possible sizes bet"een"hich the actual size contained is

called limit of size

• a%imum limit of size< the &reater ofthe t"o limits of size is called

ma%imum limit of size•  inimum limit of size< the smaller of

the t"o limits of size is called

minimum limit of size

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  De#iation = a basic size is %ed to the partand each of the t"o limits is deed b$ itsde#iation from the size The ma&nitude and

si&n of the de#iation is obtained b$ subtractin&the basic size from the limit problem

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• Upper de#iation = the al&ebraicdi(erence bet"een the ma%imumlimit of size and the correspondin&basic size is called upper de#iation

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• lo"er de#iation = the al&ebraic

di(erence bet"een the minimum limitof size and the correspondin& basicsize is called lo"er de#iation

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• ;ero line< in a &raphical representation of

limits, a strai&ht line to "hich the de#iationsare referred is called zero line The zero line isthe line of zero de#iation and represents thebasic size

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  Tolerance Zone In a &raphical representationof tolerance, the zone bounded b$ the t"o limits

of size of the part is called the tolerance zone Itis dened b$ its ma&nitude +ie tolerance/ andb$ its position in relation to the zero line

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.LLO>.NC-•

 The di(erencebet"een thedimension of t"omatin& parts is called

allo"ance

•  The di(erencebet"een the smallest

hole and lar&est shaft.llo"ance2 smallest hole6lar&estshaft

2 0111 = 3445  2 113

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  Fit. The relationship e%istin&bet"een t"o parts, shaft and hole,"hich are to be assembled, "ith

respect to the di(erence in theirsizes before assembl$ is called t

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 T$pes of ts

  Dependin& upon the actual limits ofthe hole or shaft, the t ma$ bedi#ided into three main classes as

follo"s <

@ Clearance t

A Interference t

7 Transition t

Cl Fit

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Clearance Fits• . clearance t al"a$s has a &ap

bet"een the t"o matin& parts• In a clearance t , there is al"a$s a

positi#e di(erence bet"een the lar&estpossible shaft and the smallest possible

hole In this the shaft is smaller thanhole

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Clearance Fits

• Clearance Fits ma$ be <

@ *lide t

A -as$ slide t7 Runnin& t

3 *lab runnin& t

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Runnin& t•

In a runnin& t, one part can beassembled into the other so as torotate or slide freel$

-&< shaft freel$ rotatin& in a bearin&

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Bush t• In push t, one part can be assemble

in to other "ith li&ht hand pressureand there bein& no sucientclearance to allo" shaft to rotate

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Dri#in& t

• In a dri#in& t, one part can beassembled into other "ith a handhammer or b$ medium pressure

-&< pulle$ tted on a shaft "ith a 'e$

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Forced t or shrin' t

• In Forced t or shrin' t, onepart+shaft/ is assembled intoother+hole/ either "ith &reat pressure

or the hole is e%panded b$ heatin&,so as to shrin' on the shaft It isspeciall$ used "hen t"o parts are to

be %ed to&ether-&< cart "heels, rail"a$ and tram car

"heels are tted

I t f t

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Interference ts• Interference ts al"a$s o#erlap and are

used mainl$ for press ts "here the t"oparts are pushed to&ether, and re:uire noother fasteners

• In a interference t there is al"a$s a

ne&ati#e allo"ance bet"een the lar&esthole and the smallest shaft

• In this the shaft is lar&er than the hole

T iti l Fit

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Transitional Fits•  This t$pe of t ma$ result in

interference, or clearance

•  This t$pe of t can be used for itemssuch as forced t, push t

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For an$ basic size there are A0 di(erent holes These areobtained b$ pro#idin& a series of holes "hich are pro&ressi#el$

o#ersize and a series of holes "hich are pro&ressi#el$undersize The di(erence from basic size of the #arious holes&i#en b$ the fundamental de#iation, and it is these di(erencesin size "hich &i#e the t re:uired The A0 holes are dis&natedb$ the capital letters < . ), C, D, -, F, , !, E*, E, , , N, B, R, *,

 T, U, G, H, , ;, ;., ;), ;C

-ach of the A0 holes has a choice of @8 tolerances "hich aredesi&nated < IT 1@, IT 1, IT @, IT A up to and includin& IT @5 Thetolerance &rade decides the accurac$ of manufacture The se#ennest &rades +IT 1@ to IT 10/ co#er sizes up to 011 mm and the

ele#en coarsest &rades up to 7@01 mm The tolerance in each&rade depends on the size of shaftJhole

*imilarl$ for shafts, for an$ &i#en size there are A0 di(erentshafts desi&nated b$ small letters from a to zc .lso each shaft

has @8 &rades of tolerance &rades "hich are desi&nated as forthe holes

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