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8-6 Fits and allowances The FIT between two mating parts is the relationship between them wrt the amount of clearance or interference present when they are assembled. Clearance fit: A fit between mating parts having limits of size so prescribed that a clearance always results in assembly. Interference fit: A fit between mating parts having limits of size so prescribed that an interference always results in assembly Transition fit: A fit between mating parts having limits of size so prescribed as to partially or wholly overlap, so that either a clearance or an interference may result in assembly.

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**Illustration of definitions**

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Allowance: an intentional difference between the maximum material limits of mating parts. It is the minimum clearance (positive allowance) or maximum interference (Negative allowance) between parts. 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 relation to the basic size. Fundamental deviation: The deviation closest to the basic size.

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Description of fits Running and sliding fits: Running and sliding fits for which tolerance and clearances are given in the appendix represent a special type of clearance fit. These are intended to provide a similar running performance, with suitable lubrication allowance, throughout the range of sizes. Location fits: are intended to determine only the location of mating parts. Clearance fits: intended for parts that are normally stationary but can be freely assembled or disassembled. Transition fits: are a compromise between clearance and interference fits when accuracy of location is important but a small amount of clearance or interference is permissible. Interference fits: are used when accuracy of location is of prime importance. Drive and force fits: constitute a special type of interference fit, normally characterized by a maintenance of constant bore pressure throughout the ranges of sizes.

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**Interchangeability of parts: the basis for mass production**

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: The completely interchangeable assembly. Parts are designed with tolerances. The fitted assembly. Mating features are fabricated either simultaneously or wrt one another. Individual members of mating features are not interchangeable. 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 Inch fits: are designated for design purposes and not shown on shop 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 FN4 represents class 4 force fit. Appendix tables in the textbook give the limits of size for the mating parts.

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**Running and sliding fits: 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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Locational clearance fits: Intended for parts that are normally stationary but that can be freely assembled or disassembled 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 Transitional fits are a compromise between clearance and interference fits**

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 Fits are used where accuracy of location is of prime 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 light alloys and more rigid plastics

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Force or shrink fits are normally characterized by maintenance of constant bore pressure throughout the range of sizes 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 the heavy pressing forces required are impractical.

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**Basic Hole system is recommended for general use**

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 maximum interference and the tolerance will be minus as given in the tables in the Appendix Example: 1-in RC7 fit, values of , .0025, and given; hence the limits are holeФ -.0000 Shaft Ф -.0012

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Basic Shaft system: the basic size is the design size for the shaft and the design size for the hole is found by adding the minimum clearance or subtracting the maximum interference from the basic size Symbol:the letter S following the fit symbol RC7S Example: 1-in RC7S fit, values of , .0025, (see previous example) Hole: Ф -.0000 Shaft: Ф -.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 of the 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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**Tolerance symbol: 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: Ф25H8/f7 fit Use the table 48 in appendix Hole limits Ф Shaft limits Ф Min interference (fit) Max interference (fit)

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**Tolerance symbol: metric**

Shaft basis fits system: Basic size is the maximum shaft size Appendix 49 in book gives information Example: Ф16C11/h11 fit Hole limits: Ф Shaft limits: Ф Min clearance (fit) Max clearance (fit)

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