3 - TOLERANCES AND ALLOWANCES ME 418
TOPIC NO. 2
TOLERANCES ANDDEFINITIONS
ALLOWANCES
This topic includes the following: NOMINAL SIZE. The nominal size of a part is
the designation used for the purpose of
2.1 Tolerances
general identification.
2.2 Allowances
TOLERANCE. Tolerance is the total
DIMENSIONAL CONTROL. Two important
permissible variation in size. The tolerance is
facts in the control of dimensions of parts
the difference between the limits of size. For
that come in contact or near contact are that
instance, for a shaft of nominal size 1 ½ in.
(1) the more exact the dimensions of the
in diameter, the size may be designated as
parts, the more satisfactory will be the
1.500±0.010, which indicates a tolerance of
characteristics of the machine, and (2) the
0.020 in.
more exact the dimensions of the parts, the
higher will be the production costs of the
BILATERAL TOLERANCE. Tolerance in which
machine.
case variation in size is permitted so that the
part maybe either larger or smaller than the
The exactness of dimensions may be
nominal size.
fixed by such requirements as space,
leakage, assembly of interchangeability,
UNILATERAL TOLERANCE. Tolerance which
limits to impact loads, expansion or
are usually used in specifying fits for
contraction, deflection, and pressure + 0.000
interchangeable parts. For example, 1.500 - 0.010
required in force or shrink fits.
ALLOWANCE. An allowance is an intentional
In addition to establishing correct
difference between the maximum material
dimensions for the parts of a machine being
limits of mating parts. It is a minimum
designed, it is necessary for the mechanical
clearance (positive allowance) or maximum
design engineer to select tolerances for the
interference (negative allowance) between
dimensions of the various parts. It has been
mating parts.
aptly said that tolerances make or break a
machine. Too large tolerances may affect
FIT. Fit is the general term used to signify
performance and life expectancy. Too close
the range of tightness that may result from
tolerances may mean excessive cost. ( From
the application of a specific combination of
Paul H. Black and O. Eugene Adams, Jr.,
allowances and tolerances in the design of
“Machine Design” 3rd edition, McGraw-Hill
mating parts.
Kogakusha, Ltd.,1968)
MACHINE DESIGN 1 Page 24
TOPIC NO. 2
TOLERANCES ANDDEFINITIONS
ALLOWANCES
This topic includes the following: NOMINAL SIZE. The nominal size of a part is
the designation used for the purpose of
2.1 Tolerances
general identification.
2.2 Allowances
TOLERANCE. Tolerance is the total
DIMENSIONAL CONTROL. Two important
permissible variation in size. The tolerance is
facts in the control of dimensions of parts
the difference between the limits of size. For
that come in contact or near contact are that
instance, for a shaft of nominal size 1 ½ in.
(1) the more exact the dimensions of the
in diameter, the size may be designated as
parts, the more satisfactory will be the
1.500±0.010, which indicates a tolerance of
characteristics of the machine, and (2) the
0.020 in.
more exact the dimensions of the parts, the
higher will be the production costs of the
BILATERAL TOLERANCE. Tolerance in which
machine.
case variation in size is permitted so that the
part maybe either larger or smaller than the
The exactness of dimensions may be
nominal size.
fixed by such requirements as space,
leakage, assembly of interchangeability,
UNILATERAL TOLERANCE. Tolerance which
limits to impact loads, expansion or
are usually used in specifying fits for
contraction, deflection, and pressure + 0.000
interchangeable parts. For example, 1.500 - 0.010
required in force or shrink fits.
ALLOWANCE. An allowance is an intentional
In addition to establishing correct
difference between the maximum material
dimensions for the parts of a machine being
limits of mating parts. It is a minimum
designed, it is necessary for the mechanical
clearance (positive allowance) or maximum
design engineer to select tolerances for the
interference (negative allowance) between
dimensions of the various parts. It has been
mating parts.
aptly said that tolerances make or break a
machine. Too large tolerances may affect
FIT. Fit is the general term used to signify
performance and life expectancy. Too close
the range of tightness that may result from
tolerances may mean excessive cost. ( From
the application of a specific combination of
Paul H. Black and O. Eugene Adams, Jr.,
allowances and tolerances in the design of
“Machine Design” 3rd edition, McGraw-Hill
mating parts.
Kogakusha, Ltd.,1968)
MACHINE DESIGN 1 Page 24
