DEFINITION OF ALLOWANCES AND
ACTUAL FIT: The actual fit between two mating parts is
the relation existing between them with respect to the amount of clearance or
interference which is present when they are assembled. (Fits are of three
general types; clearance, transition, and interference.)
ACTUAL SIZE: An actual size is a measured size.
ALLOWANCE: An allowance is an intentional difference between the maximum
material limits of mating parts. (See definition of Fit.) It is a minimum
clearance (positive allowance) or maximum interference (negative allowance)
between mating parts.
BASIC SIZE: The basic size is that size from which the limits of size are
derived by the application of allowances and tolerances.
BILATERAL TOLERANCE: A bilateral tolerance is a tolerance in which
variation is permitted in both directions from the design size.
CLEARANCE FIT: A clearance fit is one having limits of size so specified
that a clearance always results when mating parts are assembled.
DESIGN SIZE: The design size is that size from which the limits of size
are derived by the application of tolerances. When there is no allowance, the
design size is the same as the basic size.
DIMENSION: A dimension is a geometrical characteristic such as diameter,
length, angle, or center distance.
FIT: Fit is the general term used to signify the range of tightness which
may result from the application of a specific combination of allowances and
tolerances in the design of mating parts
INTERFERENCE FIT: An interference fit is one having limits of size so
specified that an clearance or interference fit may result when mating parts are
LIMITS OF SIZE: The limits of size are the applicable maximum and minimum
MAXIMUM MATERIAL LIMITS: A maximum material limit is the maximum limit of
size of an external dimension or the minimum limit of size of an internal
MINIMUM MATERIAL LIMIT: A minimum material limit is the minimum limit of
size of an external dimension or the maximum limit of size of an internal
NOMINAL SIZE: The nominal size is the designation which is used for the
purpose of general identification.
SIZE: Size is a designation of magnitude. When a value is assigned to a
dimension it is referred to as the size of that dimension. (It is recognized
that the words "dimension" and "size" are both used at times
to convey the meaning of magnitude).
TOLERANCE: A tolerance is the total permissible variation of a size. The
tolerance is the difference between the limits of size.
TOLERANCE LIMIT: A tolerance limit is the variation, positive or
negative, by which a size is permitted to depart from the design size.
TRANSITION FIT: A transition fit is one having limits of size so
specified that either a clearance or an interference may result when mating
parts are assembled.
UNILATERAL TOLERANCE: A unilateral tolerance is a tolerance in which
variation is permitted only in one direction from the design size.
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