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8-6 FITSANDALLOWANCES

THE FIT BETWEENTWOMATINGPARTSISTHERELATIONSHIPBETWEENTHEMWRTTHEAMOUNTOFCLEARANCEORINTERFERENCEPRESENTWHENTHEYAREASSEMBLED.

Clearance fit: A fit between mating parts having limitsof size so prescribed that a clearance always results inassembly.

Interference fit: A fit between mating parts havinglimits of size so prescribed that an interference always

results in assembly

Transition fit: A fit between mating parts having limitsof size so prescribed as to partially or wholly overlap,so that either a clearance or an interference may resultin assembly.

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ALLOWANCE: ANINTENTIONALDIFFERENCEBETWEENTHEMAXIMUMMATERIALLIMITSOFMATINGPARTS. ITISTHEMINIMUM

CLEARANCE (POSITIVEALLOWANCE) ORMAXIMUMINTERFERENCE

(NEGATIVEALLOWANCE) BETWEENPARTS.

Basic size: The size to which limits or deviations are designed. The basic

size if the same for both members of a fit.

Deviation: The algebraic difference between a size and the corresponding

basic size.

Upper deviation: The algebraic difference between the maximum limit of the

size and the corresponding basic size.

Lower deviation: The algebraic difference between the minimum limit of size

and the corresponding basic size.

Tolerance: The difference between the maximum and minimum size limits on

a part.

Tolerance zone: A zone representing the tolerance and its position in relationto the basic size.

Fundamental deviation: The deviation closest to the basic size.

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DESCRIPTIONOFFITSRUNNINGANDSLIDINGFITS: RUNNINGANDSLIDINGFITSFOR

WHICHTOLERANCEANDCLEARANCESAREGIVENINTHEAPPENDIXREPRESENTASPECIALTYPEOFCLEARANCEFIT. THESEAREINTENDEDTOPROVIDEASIMILARRUNNINGPERFORMANCE, WITHSUITABLELUBRICATIONALLOWANCE,THROUGHOUTTHERANGEOFSIZES.

LOCATIONFITS: AREINTENDEDTODETERMINEONLYTHELOCATIONOFMATINGPARTS.

CLEARANCEFITS: INTENDEDFORPARTSTHATARENORMALLYSTATIONARYBUTCANBEFREELYASSEMBLEDORDISASSEMBLED.

TRANSITIONFITS: AREACOMPROMISEBETWEENCLEARANCEANDINTERFERENCEFITSWHENACCURACYOFLOCATIONISIMPORTANTBUTASMALLAMOUNTOFCLEARANCEORINTERFERENCEISPERMISSIBLE.

INTERFERENCEFITS: AREUSEDWHENACCURACYOFLOCATIONISOFPRIMEIMPORTANCE.

Drive and force fits: constitute a special type ofinterference fit, normally characterized by amaintenance of constant bore pressure throughout theranges of sizes.

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INTERCHANGEABILITYOFPARTS: THEBASIS

FORMASSPRODUCTION

No part can be manufactured to exact dimensions as tool wear, machine

variations, and human factors contribute to deviation from perfection.

Three basic approaches to manufacturing:

1. The completely interchangeable assembly. Parts are designed with

tolerances.

2. The fitted assembly. Mating features are fabricated either simultaneously orwrt one another. Individual members of mating features are not

interchangeable.

3. The select assembly. Parts are mass produced, but member of mating parts

are individually selected to provide the required relationship to one another.

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STANDARD INCHFITS: AREDESIGNATEDFOR

DESIGNPURPOSESANDNOTSHOWNONSHOP

DRAWINGS.

RC Running and sliding fit

LC locational clearance fit

LT locational transition fit

LN locational interference fit FN force or shrink fit

These letter symbols are used with numbers to

represent classes of fit. For example FN4represents class 4 force fit.

Appendix tables 43-47 in the textbook give the

limits of size for the mating parts.

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RUNNINGANDSLIDINGFITS: RC

RC1 precision sliding fit

RC2 Sliding fit

RC3 Precision running fit

RC4 Close running fit RC5 and RC6 Medium running fits

RC7 Free running fit

RC8 and RC9 Loose running fits

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LOCATIONALCLEARANCEFITS: INTENDEDFOR

PARTSTHATARENORMALLYSTATIONARYBUTTHATCANBEFREELYASSEMBLEDORDISASSEMBLED

LC1 to LC4 minimum zero clearance (theoretical)

LC5 and LC6 small minimum clearance

(LC1-LC6 nonrunning parts)

LC7 and LC11 loose clearance for assembly of

bolts and similar parts

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LOCATIONAL TRANSITIONALFITSAREA

COMPROMISEBETWEENCLEARANCEAND

INTERFERENCEFITS

LT1 and LT2 slight clearance. Assembly by

pressure or light hammer blows

LT3 and LT4 virtually no clearance also referred to

as easy keying fit for shaft keys. Assembly is by

pressure or hammer blows.

LT5 and LT6 slight interference. These fits are

useful for heavy keying, for ball race fits subject to

heavy duty and vibration, and as light press fits for

steel parts.

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LOCATIONAL INTERFERENCE FITSAREUSED

WHEREACCURACYOFLOCATIONISOFPRIME

IMPORTANCE.

LN1 and LN2 Light press fits

LN3 heavy press fit

LN4 to LN6 press fits primarily intended for press

fits for more elastic or soft materials such as lightalloys and more rigid plastics

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FORCEORSHRINKFITSARENORMALLY

CHARACTERIZEDBYMAINTENANCEOF

CONSTANTBOREPRESSURETHROUGHOUTTHERANGEOFSIZES

FN1 light drive fit requiring light assembly pressure

and produces more or less permanent assemblies

FN2 medium drive fit

FN3 heavy drive fit

FN4 and FN5 force fits suitable for parts that can be

highly stressed and/or for shrink fits where theheavy pressing forces required are impractical.

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BASIC HOLESYSTEMISRECOMMENDEDFOR

GENERALUSE

The basic size is the design size of the hole

The tolerance will be plus

The design size for the shaft will be the basic size

minus the minimum clearance or plus the maximuminterference and the tolerance will be minus as

given in the tables in the Appendix

Example: 1-in RC7 fit, values of +.0020, .0025, and

-.0012 given; hence the limits are

hole1.000 +.0020

-.0000

Shaft .9975 +.0000

-.0012

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BASIC SHAFTSYSTEM: THEBASICSIZEIS

THEDESIGNSIZEFORTHESHAFTANDTHE

DESIGNSIZEFORTHEHOLEISFOUNDBYADDINGTHEMINIMUMCLEARANCEOR

SUBTRACTINGTHEMAXIMUMINTERFERENCE

FROMTHEBASICSIZE Symbol:the letter S following the fit symbol RC7S

Example: 1-in RC7S fit, values of +.0020, .0025, -.0012 (see previous example)

Hole: 1.0025 +.0020-.0000

Shaft: 1.0000 +.0000

-.0012

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PREFERRED METRIC LIMITS & FITS

International tolerance grade for internal & external

dimensions

Grade 1: precise Grade 16 course

A fundamental deviation establishes the position ofthe tolerance symbol wrt basic size

Tolerance position letters give this fundamental

deviation.

Internal dimension: Capital letter

External dimension: lower case letter

Example: 40H8 international tolerance grade

Basic size Fundamental deviation (internal)

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TOLERANCESYMBOL: METRIC

Combination of IT grade number and tolerance

position letter

International tolerance grade table in the appendix

Hole basis fit: basic size is minimum size of hole Example: 25H8/f7 fit Use the table 48 in appendix

Hole limits 25.000-25.033

Shaft limits 24.959-24.980

Min interference -0.020 (fit)

Max interference -0.074 (fit)

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TOLERANCESYMBOL: METRIC

Shaft basis fits system: Basic size is the maximum

shaft size

Appendix 49 in book gives information

Example: 16C11/h11 fitHole limits: 16.095-16.205

Shaft limits: 15.890-16.000

Min clearance 0.095 (fit)

Max clearance 0.315 (fit)