Definition of Limit, Fit and Tolerance:
The extreme permissible values of a dimension are known as limits. The degree of tightness or
looseness between two mating parts that are intended to act together is known as the fit of the parts.
The character of the fit depends upon the use of the parts. Thus, the fit between members that move
or rotate relative to each other, such as a shaft rotating in a bearing, is considerably different from the
fit that is designed to prevent any relative motion between two parts, such as a wheel attached to an
axle'.
Tolerance on the other hand is the total amount that a specific dimension is permitted to vary. It is
the difference between the maximum and the minimum limits for the dimension.
 The extreme permissible values of dimensions of a mating part are known as limit.
 Due to the inevitable inaccuracy of manufacturing methods, a part
cannot be made precisely to a given dimension, the difference between maximum and
minimum limits of size is the tolerance.
 When two parts are to be assembled, the relation resulting from the difference
between their sizes before assembly is called a fit.
HOLE
SHAFT
Max Hole size – Basic Size = Upper Deviation
Max shaft size – Basic Size = Upper Deviation
Min Hole size – Basic Size = Lower Deviation
Min shaft size – Basic Size = Lower Deviation
CLEARANCE FIT
Maximum shaft dimension < Minimum hole dimension
Condition of looseness or tightness between two mating parts being assembled
together
INTERFERANCE FIT
Maximum Hole size < Minimum Shaft size
TRANSITION FIT
Obtained by overlapping of tolerance zones of shaft and hole
Does not guarantee neither clearance nor interference fit
To obtain different types of fits, it is general practice to
vary tolerance zone of one of the mating parts
HOLE BASED SYSTEMSize of hole is kept constant,
shaft size is varied
to get different fits.
SHAFT BASED SYSTEMSize of shaft is kept constant,
hole size is varied
to get different fits.
Allowance:

Allowance is calculated as the difference between the maximum shaft size
and minimum hole size is known as allowance.

It is a amount of designed (intentional) deviation between two mating parts of
dimensions in a fit.

In a clearance fit it is have a minimum clearance and is positive allowance.

In an interference fit it is have a maximum interference and is negative
allowance.

Allowance is the difference between the dimensions of hole and shaft.
The selection of letter freezes
one limit of hole / shaft
(how much away from Basic
size)
Representation of
Tolerance
1) Letter Symbol
Basic Size
45 E8/e7
One can have different
possible combinations; eg.
45H6g7, 45H8r6, 45E5p7
E.S. – upper deviation
E.I. – lower deviation
H : lower deviation of
hole is zero
h : upper deviation of
shaft is zero
Representation of Tolerance
2) Number or Grade
IT01, IT0, IT1,….IT16
Tolerance Grade defines range of
dimensions (dimensional variation)
There are manufacturing
constraints on tolerance grade
chosen
D =√ (D1×D2 (Geometrical mean diameter)
Standard or fundamental tolerance unit
Tolerance Grade
IT5
IT6
IT7
IT8
IT9
IT10
IT11
IT12
IT13
IT14
IT15
7i
10i
16i
25i
40i
64i
100i
160i
250i
400i
640i
or Zone
Tolerance
Magnitude
Tolerance Magnitude
RANGE or Distribution IN A GIVEN TOLERANCE GRADE
Upper and lower deviations
Tolerance on Components
Example 3. A medium force fit on a 75 mm shaft requires a hole tolerance
and shaft tolerance each equal to 0.225 mm and a maximum interference
of 0.0375 mm. Determine the proper hole and shaft dimension with the
basis hole standard.
Minimum diameter (lower limit) of hole:
Dmin = 75 mm
Maximum diameter (higher limit) of hole:
Dmax = 75 +0.225 = 75.225 mm
Minimum diameter (lower limit) of shaft:
dmin = Dmax + maximum interference
dmin = 75.225 + 0.0375 = 75.2625 mm
Maximum diameter (higher limit) of shaft:
dmax = dmin + tolerance = 75.2625 + .225 = 75.4875 mm
H8-f7
f7
= 59.97- 0.030 = 59.94 mm
Example 5. In a limit system, the following limits are
specified to give a clearance fit between a shaft and a hole:
Determine:(a)basic size (b) shaft and hole tolerance (c) the maximum and
minimum clearance
(a) Basic Size: 30 mm
(b) Shaft tolerance:
Tolerance = upper limit – lower limit = -0.005 – (-0.018) = 0.013 mm
Hole Tolerance = 0.02 mm
(c) Shaft Limit: High limit (dmax) = 30-0.005 = 29.995 mm
Low Limit(dmin) = 30-0.018 = 29.982 mm
Hole Limit: High Limit (Dmax) = 30+0.02 = 30.02 mm
Low Limit (dmin) = 30 mm
(d) Maximum Clearance = Dmax- dmin = 30.02 – 29.982 = 0.038 mm
Minimum Clearance = Dmin – dmax = 30 – 29.995 = 0.005 mm
Example 6. A hole and shaft have a basic size of 25 mm, and are to have a
clearance fit with a maximum clearance of 0.02 mm and a minimum clearance
of 0.01 mm. The hole tolerance is to be 1.5 times the shaft tolerance.
Determine: limit for both hole and shaft (a) using a hole basis system (b) using
a shaft basis system.