TOLERANCES
AND ALLOWANCES FOR FIT
Fit: It is a 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.
Clearance fit: It is one
having limits of zero so specified that a clearance always results when
mating parts are assembled.
Interface fit: It is one
having limits of zero so specified that an interface always result when mating
parts are assembled.
Transition fit: It is one
having limits of size so specified, that either a clearance or an interface may
result when mating parts are
assembled.
Size: It is a designation
of magnitude.
Basic size: It is that
size from which the limits of size are derived by the application of
allowances and tolerances.
Designed size: It is the
basic size with allowance applied, from
which the limits of size are derived by
the application of tolerances.
Where there is no allowance, the design size is the same as the basic
size.
Nominal size: The nominal
size is the designation which is used for the purpose of general
identification.
Actual size: It is a
measured size.
Limits of size: They are the
applicable maximum and minimum sizes.
Dimension: It is a geometrical characteristic
such as diameter, length, angle, or center distance.
Allowance: It is a prescribed
difference between the maximum material limits of mating parts. It is a minimum
clearance or
maximum interference between such parts.
Tolerance: It is the total
permissible variation of a size. The
tolerance is the difference between the limits
of size.
Tolerance limit:
It is the variation, positive or negative, by which a size is permitted to
depart from the design size.
Unilateral tolerance:
It is the tolerance in which variation is permitted in on direction only from
the design size.
Bilateral tolerance:
It is the tolerance in which variation is permitted in both directions from the
design size.
Basic hole system:
A system of fits in which the design size of the hole is the basic size and the
allowance, if any ,is applied to the
shaft.
Basic shaft system:
A system of fits in which the design size of the shaft is the basic size and
the allowance, if any is applied to the
hole.
FITS:
-Running and Sliding fits (Rc)
-Rc1: close sliding fits are intended for
accurate location of parts which must assemble without perceptive play.
-Rc2: sliding fits are intended for
accurate location, but with greater maximum clearance than class rc1. Parts
made to this fit move
and
turn easily but are not intended to run freely, and the larger sizes may seize
with small temperature changes.
-Rc3: precision running fits are about
the closest fits which can be expected to run freely, and are intended for precision
work at slow
speeds and light journal pressures, but are not suitable where
appreciable temperature differences are likely to be encountered.
-Rc4: close running fits
are intended chiefly for running fits on accurate machinery with moderate surface
speeds journal pressures,
where accurate location and minimum play is desired.
-Rc5 & -Rc6:
medium running fits are intended for higher running speeds, or heavy journal
pressures, or both.
-Rc7: free running fits
are intended for use where accuracy is not essential, or where large
temperature variations are likely to be
encountered, or under both these conditions.
-Rc8 & -Rc9:
loose running fits are intended for use where wide commercial tolerances may be
necessary, together with an
allowance,
on the external member.
-Lc: locational clearance
fits are for parts which are normally
stationary, but which can be freely assembled or disassembled. They
range from snug fits for parts requiring accuracy of location, through the medium clearance fits for parts
such as spigots, to the
looser fastener fits where freedom of assembly is of prime importance.
-Lt: locational
transition fits are compromise between clearance and interference fits, for
application where accuracy of location is
important, but either a small amount of clearance or interference is
permissible.
-Ln: locational
interference fits are used where accuracy of location is of prime importance,
and for parts requiring rigidity and
alignment with no special requirements for bore pressure.
-Fn1: light drive fits,
require light assembly pressures, and produce more or less permanent
assemblies.
-Fn2: medium drive fits,
suitable for ordinary steel parts, or for shrink fits on light sections.
-Fn3: heavy drive fits,
suitable for heavier steel parts or for shrink fits in medium sections.
-Fn4 & -fn5:
force fits are suitable for parts which can be highly stressed or for shrink
fits where the heavy pressing forces required
are
impractical.