8.6 Fits and Allowances - Ivy Tech -

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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.
ALLOWANCE:
AN INTENTIONAL DIFFERENCE BETWEEN
THE MAXIMUM MATERIAL LIMITS OF MATING PARTS.
IT IS THE
MINIMUM CLEARANCE (POSITIVE ALLOWANCE) OR MAXIMUM
INTERFERENCE
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(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.
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.
INTERCHANGEABILITY OF PARTS:
THE
BASIS FOR MASS PRODUCTION
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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 or wrt 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.
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 43-47 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 +.0020, .0025,
and -.0012 given; hence the limits are
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holeФ1.000 +.0020
-.0000
 Shaft Ф.9975 +.0000
-.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 +.0020, .0025, .0012 (see previous example)
 Hole: Ф1.0025 +.0020
-.0000
 Shaft: Ф 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
of the tolerance symbol wrt basic size
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Tolerance position letters give this fundamental
deviation.
 Internal dimension: Capital letter
 External dimension: lower case letter
Example: 40H8 international tolerance grade
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Basic size
Fundamental deviation (internal)
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
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Example: Ф25H8/f7 fit Use the table 48 in appendix
Hole limits Ф25.000-25.033
Shaft limits Ф24.959-24.980
Min interference -0.020 (fit)
Max interference -0.074 (fit)
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: Ф16.095-16.205
Shaft limits: Ф15.890-16.000
Min clearance 0.095 (fit)
Max clearance 0.315 (fit)
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