MIE 271
LIMIT, FITS AND TOLERANCE
Md. Abdus Shabur
Lecturer, Dept. of MIE
CUET
Outline
Basics of Limit and Fit
Interchangeable manufacturing
Different types of fit
Tolerance
Tolerance Calculation
Basics
Generally in engineering, any component
manufactured is required to fit or match with
some other component.
The correct and prolonged functioning of
the two components in match depends upon
the correct size relationships between the
two, i.e., the parts must fit with each other in
a desired way
Basics
if a shaft is to rotate in a hole, there must
be enough clearance between the shaft
and hole to allow the oil film to be
maintained for lubrication.
If the clearance is too small, excessive
force would be required in rotation of
shaft.
If clearance is too wide, there would be
vibrations and rapid wear and ultimate
failure.
Interchangeable manufacturing
In the early days the majority of components were
actually mated together, their dimensions being
adjusted until the required type of fit was obtained.
But the interchangeable production and continuous
assembly require some standard procedure to be
followed. In order to obtain various fits, it is possible to
vary the hole sizes and shaft size.
The aim of any general system of standard limits and fits
should be to provide guidance to the users in selecting
basic functional clearance and interference for a given
application or type of fit that minimize cost and labor.
Interchangeable manufacturing
For a system of limits and fits to be successful,
following conditions must be fulfilled :
I. It must be based on some standard so that
everybody understands alike, and a given
dimension has the same meaning at all places.
II. The range of sizes covered by the system
should be sufficient for most purposes.
III. Number of limits and fits should be reduced
and be adequate to cover all applications.
IV. Each basic size of hole and shaft must have a
range of tolerance values for each of the
different fits.
Interchangeable manufacturing
V. Both unilateral and bilateral methods of tolerance,
and hole basis or shaft basis systems should be
acceptable.
VI. For any basic size it must be possible to select
from a carefully designed range of fits the most
suitable one for a given application.
VII. The fundamental deviation required to give a
particular fit must increase with the basic size so
that a particular fit or same assembly
characteristics are obtainable for smaller as well
as bigger sized parts.
VIII. The system should cover work from high class tool
and gauge work up to rough work.
Size Designations
Nominal Size: It is the designation used for general
identification and is usually expressed in common
fractions.
Basic Size or Basic dimension: It is the theoretical
size from which limits of size are derived by the
application of allowances and tolerances.
Actual Size: is the measured size of the finished
part.
Deviation: It is the algebraic difference between
actual size and corresponding basic size
Size Designations
Upper Deviation: The algebraic difference
between the maximum limit of size and the
corresponding basic size.
Lower Deviation: The algebraic difference
between the minimum limit of size and the
corresponding basic size.
Fundamental Deviation: It is equal to either upper
or lower deviation which is closer to the zero line.
Allowance: It is the intentional difference
between the Maximum Material Limit(MML) of
mating parts. For shaft, the maximum material
limit will be its high limit and for the hole the
maximum material limit will be its low limit
Size Designations
Size Designations
Size Designations
Fit
Fit is the general term used to signify the
range of tightness or looseness that may
result from the application of a specific
combination of allowances and tolerances
in mating parts.
Three types of fit
Clearance Fit
Interference Fit
Transition Fit
Hole and Shaft Basis System
HOLE BASED SYSTEM:
Size of hole is kept
constant, shaft size is
varied to get different fits.
SHAFT BASED SYSTEM:
Size of shaft is kept
constant, hole size is
varied to get different fits.
Hole and Shaft Basis System
A fit is indicated by the basic size common
to both components, followed by symbol
corresponding to each component, the hole
being quoted first.
E.g. 45 H8/g7
Tolerance
Tolerance can be defined as the magnitude of
permissible variation of a dimension or other
measured or control criterion from the
specified value.
The primary purpose of tolerances is to
permit variation in dimensions without
degradation of the performance beyond the
limits established by the specification of the
design.
For Example a dimension given as 1.625 ±
.002 means that the manufactured part may
be 1.627” or 1.623”, or anywhere between
these limit dimensions.
International Tolerance Grade (IT)
They are a set of tolerances that varies according to the
basic size and provides a uniform level of accuracy within
the grade.
Representation of Tolerance Tolerance Grade defines range
2) Number or Grade of dimensions (dimensional
IT01, IT0, IT1,….IT16 variation)
There are manufacturing
constraints on tolerance grade
chosen
Standard Tolerances
There are eighteen grades of tolerances provided with
designations ITO1, ITO, IT1, IT2, … IT16. These are known as
standard tolerances. For grades 5 to 16, the values of the
tolerances are determined in terms of the standard tolerance
factor i ,
Where,
Standard Tolerances
The selection of letter freezes
one limit of hole / shaft
Representation of
(how much away from Basic Tolerance
size)
1) Letter Symbol
Basic Size
45 E8/e7
One can have different
possible combinations; e.g.
45H6/g7, 45H8/r6, 45E5/p7
E.S. – upper deviation
E.I. – lower deviation
H : lower deviation of
hole is zero
h : upper deviation of
shaft is zero
Hole limits are
identical with the
shaft limits of the
same
symbol(letter and
Grade)but
disposed on the
other side of the
zero line
EI=Upper
deviations of shaft
of the same letter
symbol but of
opposite sign
Tolerance Calculation
Determine the tolerance for 25H8/d9 Fit for
diameter step 18mm and 30mm
First Determine the Geometric mean of diameter
Calculate Fundamental Tolerance Unit
Find Tolerance Grade and Standard Tolerance for
Hole and Shaft.
Find Fundamental Deviation for Hole(H) and
Shaft(d)
Tolerance Calculation
For Hole H: Fundamental Deviation is Zero
For d shaft: Fundamental Deviation
Find Upper Limit and Lower Limit
Find Tolerance
Tolerance and Limit
Tolerance and Limit
That’s all for today
