Tolerance
• The replacement of parts should be easy.
Dimensions are important for
interchangeability.
• Exact size is difficult to manufacture.
• Exact size is not required.
• To accept variation is size is called tolerances.
Maximum & minimum allowable dimensions are called
Limit Dimensions.
The difference in limit dimensions is called tolerance.
Shaft: A member which fit in another member
Hole: A member in which external member fit.
Clearance = Max Hole – Min Shaft
Allowance = Min Hole – Max Shaft (MMC)
Required to fit properly (not
interchangeable)
Size Designations
• Nominal Size: Used for general identification.
Calling name.
• Basic Size: This is the size on which
tolerances are applied.
• Actual size: measured size of the finished
parts.
• Allowance: Minimum clearance (MMC)
Fit between mating parts
• Clearance fit:
• Both allowance and Maximum clearance
positive.
Interference fit:
Both allowance and maximum
clearance are negative
Max Clearance = 1.2506 – 1.2513 = -0.0007
Allowance = 1.2500 – 1.2519 = -0.0019
Transition fit:
One negative and one positive
Max Clearance = 1.2506 – 1.2503 = +0.003
Allowance = 1.2500 – 1.2509 = -0.0009
Line fit
• Allowance is zero with positive maximum
clearance.
• This results in a surface contact.
• Selective Assembly:Pars are so classified and
grouped that small hoes matches with small
shaft, medium with medium and large with
large.
Basic Hole system:
When basic dimensions are for hole.
Basic Shaft system: When basic dimensions are for shaft.
Limit Dimensions: The maximum and
minimum values are specified.
Plus or minus dimensions: Basic size followed
by plus or minus tolerance.
Unilateral: variation in only one direction. One
value should be zero, another may be all + or -.
Bilateral tolerance:
allows variation in both
direction of basic size.
Symmetry in bilateral dimension
Single limit dimension: some time
one limit for example 0.5R MAX
Angular Tolerance: usually bilateral
and in degree minute or in second.
It is best to dimension to each surface so that it is
affected by only one dimension. This can be
done by referring all dimensions to a single
datum surface.
Decimal Inch system
Metric system of tolerances and fit
• Specified by International Organization for
Standardization (ISO).
• Basic Size:
The size where limits are
assigns.
• Deviation:
Different between the basic and
hole or shaft size.
Upper Deviation: Difference between the basic and
maximum size.
Lower Deviation: Difference between the basic and
minimum size.
Fundamental deviation
• The deviation which is closest to the basic
size.
• For Shaft, upper deviation is the fundamental
deviation
• For Hole, Lower deviation is the fundamental
deviation
Basic Hole System
•
•
•
•
•
For Hole,
Lower Deviation = Min Hole – basic size
Upper Deviation = Max Hole – basic size
Fundamental Deviation is Lower deviation
Fundamental deviation represents in capital letters, A, B, C
,…..H, G, ……AA,……
• Each letter has its own number can be seen in hand books. H
means zero.
• If Fundamental deviation is H, it means minimum hole size is
the basic size and the system is called basic hole system.
Basic Shaft System
• For shaft,
•
•
•
•
Lower Deviation = basic size – min shaft
Upper Deviation = basic size – max shaft
Fundamental Deviation is upper deviation
Fundamental deviation represents in small letters, a, b, c ,…..h,
g, ……aa,……
• Each letter has its own number can be seen in hand books. h
means zero.
• If Fundamental deviation is h, it means maximum shaft size is
the basic size and the system is called basic shaft system.
IT Grade for Tolerances
• Tolerances:
Difference between maximum
and minimum size.
• International Tolerance Grade (IT) Set of
tolerances that varies according to the basic
size and provide uniform level of accuracy.
• Total 18 IT grade, IT01, IT0, IT1,…IT16.
Example of IT Grade (p-a33)
• 40h6
shaft, basic size 40, Tolerance 0.016
• Max size = 40, Min size = 39.984
• 30H10 Hole, basic size 30, Tolerance 0.084
• Min size = 30, Max size = 30.084
Basic Size = 50, Basic Hole System
Max
Min
Tol
Hole
50.039
50.000
0.039
Shaft
49.975
49.950
0.025
Clearance = 0.089, Allowance = 0.025, Clearance fit.
From Hand out, for hole, +39 & 0, for shaft, -25. -50.
Another example, 45N7/h6
• Basic size = 45 and Basic shaft system.
Max
Min
Tol
Hole
44.992
44.967
0.025
Shaft
45.000
44.984
0.016
Clearance = 0.008 and allowance = -0.033
Transition fit
From hand out, for hole, -8,and -33and for shaft,
0, and -16