Limits Fits Tolerance

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LIMITS,FITS & TOLERANCES

In the production of engineering components also this fact is

applicable. They may found to be closely similar. Every process

is a combination of three elements namely MAN,MACHINE

and METHOD. A slight change in any of the three willconstitute a change in the PRODUCT . Thus when the process

is in control most of the item will be on or about the mean value

and other values, will be described in a more or less

symmetrical way about the mean value and remaining with in

expressible limits. It is therefore obvious that it is impossible to

produce a part to exact size and some allowances known as

TOLERANCE has to be allowed.

“NO TWO THINGS IN THE WORLD CAN BEIDENTICAL”



Such a system of giving tolerance to a dimension forthe manufacturing is known as LIMIT SYSTEM.

TERMINOLOGY FOR LIMITS AND FITS



A pair of mating components are generally calledSA!T and "LE.

SHAFT A term used #y convention to designate allthe e$ternal features of a part including those whichare not cylindrical. A shaft is commonly referred asmale component.Eg % plugs& a$les& spindles& locating spigots.

HOLE

A term used #y convention to designate all

the internal features of a part including those whichare not cylindrical. It is dented #y capital letters. Ahole is referred to the female component.Eg% #ore& recess.



SIZE A num#er e$pressing the numerical value of adimension in appropriate units is called si'e of acomponent.

BASIC SIZE or NOMINAL SIZE It is the decided si'eof a component from which the limits are divided #ythe application of tolerance. (asic si'e is same forthe hole and the shaft.

ACTUAL SIZE The si'e of a )nished componentwhich is actually measura#le with an instrument isknown as actual si'e.DEVIATION The alge#raic di*erence #etween theactual si'e of a component and its #asic si'e isknown as deviation. This can #e of two types.UPPER DEVIATION The alge#raic di*erence#etween the ma$imum limit si'e and corresponding#asic si'e. It is denoted #y +Es, for hole and +es, for a

shaft.



LOWER DEVIATION The alge#raic di*erence#etween the minimum limit si'e and corresponding#asic si'e is called lower deviation. It is denoted #y+Ei, for hole and +ei, for the shafts.

ZERO LINE The line which represents the #asic si'eof components in an assem#ly or a drawing isknown as 'ero line. In limits and )ts all the

deviations are shown with respect to the 'ero line.LIMITS

Limits are the e$treme permissi#ledimension of any part. There are two e$treme

permissi#le si'es for a dimension. The largestpermissi#le si'e is called upper or higher limits /where as the smallest si'e is known as the lowerlimit L/.

ALLOWANCE The di*. #0w the ma$imum shaft and

si'e and minimum hole si'e. It can #e either positive



TOLERANCE

Tolerance is the permissi#le variation in dimension ofa component.

Tolerance 1 2pper limit - Lower limit The di*erence #0w the upper limit dimension and the

lower limit dimension on the #asic si'e of thecomponent is termed as tolerance.

Eg% 33 4 5.3 mm.ere #asic si'e 1 33mm

33 6 5.3 1 33.3mm is the ma$. limit si'e.upper limitsi'e/33 - 5.3 1 35.7 mm is the min. limit si'e.lower limitsi'e/

8 33.3 - 35.7 1 5.9mm is the tolerance.



DIFFERENT WAYS OF EXPRESSING

TOLERANCE.

Tolerance is #asically speci)ed in to two forms.i/ 2nilateral tolerance.ii/ (ilateral tolerance.

UNILATERAL TOLERANCE.If all the tolerance is allowed to vary on one side

of the #asic si'e& the system is called asunilateral.

Ie. Tolerance lies wholly on one side of the #asicsi'e either a#ove or #elow it.

2nilateral system is preferred in interchangea#lemanufacturing & especially when precision )tsare re:uired.



BILATERAL SYSTEM

If all the tolerance is allowed on #oth the side of #asic

si'e& then the system is called as #ilateral system. The system is mainly used in mass production wheremachine setting is done for the #asic si'e.

WHY TOLERANCE IS SPECIFIED?

3/;ariation in the properties of the materials #eingmachined introduces error.

9/The production machines themselves have some inaccuracies to produce perfect parts.

</It is impossi#le for an operator to make perfectsettings. i.e in ad=usting tools and work pieces onthe machines some errors are likely to creep in./



USES OF TOLERANCE.

3/It reduces machine time.9/It is used in mass production</Avoids re=ection of components.>/?educes the cost of product.

Re!"#o$ %&' "oer!$(e !$) (o*".

If the tolerance are made closer thecost of production goes on increasing #ecause tomanufacture close tolerance parts we need&

@/recision machines& tools& and materials.

B/Trained and highly skilled operators.C/Tight inspection and more precise inspectiondevice.

D/lose supervision and control is essential.



Diference b/w TOLERANCE and ALLOWANCE.

TOLERANCE

• It is the permissi#levariation in dimension of apart either a hole or a

shaft.

• It is the di*erence #0whigher and lower limits ofa dimension or a part.

• Tolerance is given on thedimension of a part as it isnot possi#le to make apart to the e$act speci)eddimension

ALLOWANCE

• It is the prescri#eddimension of two mating

parts.• It is the intentional

di*erence #0w the lowerlimit of the hole andhigher limit of the shaft.

• Allowance is to #eprovided on the dimensionof mating parts to o#taindesired type of )t.



FIT !it may #e de)ned as a degree of

tightness or looseness #etween the twomating parts to perform a de)nitefunction when they are assem#ledtogether. There are three #asic types of)ts are possi#le to achieve #y varyingthe amount of allowance #etween thesi'e of two mating parts.

!ITS

CLEARNCE TRANSITION INTERFERANCE



CLEARANCE FIT

In clearance )t shaft has smallersi'e than the hole. i.e. the largest permissi#leshaft diameter is smaller than the smallesthole diameter. The resulting allowance isalways positive in value.

INTERFERANCE FIT

ere the shaft has a largersi'e than the hole si'e and an e$ternal force isre:uired to press the shaft to hole for )tting.

This )t is a permanent )t. The minimum permissi#leshaft diameter is larger than the ma$imumlimit of hole. The resulting allowance has

always a negative value. A hindrance will



TRANSITION FIT

In this type of )t&

the clearance )t is availa#le for matingparts with one set of dimension whengives in limit system and interference )t

is o#tained with other set of dimension. The allowance varies from positive tonegative corresponding to two given setof dimensions.

2sually this )tprovides either 'ero interference or aclearance. These are used where parts

can #e replaced without diFculty during



Basic Hole Ss!e" or Hole BasisGe)nition of the H(asic ole SystemH%

The Hminimum si'eH of the hole is e:ualto the H#asic si'eH of the )t.

learance 1 ole - Shaft

ma$ 1 ma$ - Sminmin 1 min - Sma$

(oth ma$ and min 5 - learance )t(oth ma$ and min J5 - Interference

)tma$ 5& min J 5 - Transition )t

Allowance 1 min K Sma$ i.e.& min/

Limits Fits Tolerance

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