LIMITS,FITS AND TOLERANCE
BY
PROF. KHAN AAMIR
TERMINOLOGY
BASIC SIZE: It is the standard size of a part , with
reference to which all the limits of variations of size
are determined.
ZERO LINE: the line corresponding to basic size is
called as zero line.
SHAFT AND HOLE: These terms are used to
designate all the external and internal features of any
shape and not necessarily cylindrical.
HOLE DESIGNATION: By upper case letters from
A, B, ... Z, Za, Zb, Zc (excluding I, L, O, Q, W and
adding Js, Za, Zb, Zc) - 25 nos. Indian Stds.
SHAFT DESIGNATION: By lower case letters from
a, b, ... z, za, zb, zc (excluding i, l, o, q, w and adding
js, za, zb, zc) - 25 nos.
DEVIATION
UPPER DEVIATION:The algebraic difference
between the maximum limit of size (of either hole or
shaft) and the corresponding basic size
LOWER DEVIATION:The algebraic difference
between the minimum limit of size (of either hole or
shaft) and the corresponding basic size
FUNDAMENTAL DEVIATION:It is one of the two
deviations which is chosen to define the position of
the tolerance zone
Tolerance:
The algebraic difference between upper and lower
deviations. It is an absolute value.
Types of tolerance
• Unilateral tolerance : if the tolerance is
allowed on one side of the basic size, the
system of tolerance is said to be unilateral.
• Bilateral tolerance : if the tolerance is allowed
on both side of the basic size, the system of
tolerance is said to be unilateral.
LIMITS
The limits are two extreme permissible sizes of a part between
which, the actual size of that part is contained. They are fixed
with reference to the basic size of that dimension.
 Basic shaft: it is the shaft, whose upper deviation is
zero or whose max. limit of size is equal to basic
size .
 Basic hole: it is the hole, whose lower deviation is zero or
whose min. limit of size is equal to basic size .
Allowance
 Allowance is the prescribed difference between the hole dimension
and shaft dimension for any type of fit.
 It is the intentional difference between the lower limit of the hole
and higher limit of the shaft.
Fits
 The relation between the two parts, where one is inserted into the
other with a certain degree of tightness or looseness is known as fit.
 Fit is the degree of tightness or looseness between two mating parts
to perform a definite function .
 fit is the relation between dimensions of two mating parts
before their assembly.
 Classification of fits
Clearance fit
In this type of fit, the largest permitted shaft diameter is smaller than the
Diameter of the smallest hole, so that, the shaft can rotate or slide
through, with Different degrees of freedom according to the purpose of
the mating members
Transition fit
• In a fit of this type, the diameter of the largest allowable hole is
greater than that of the smallest shaft, but the smallest hole is
smaller than the largest shaft, so that, small positive or negative
allowance between the shaft and hole members are employable.
• In this type of fit, the size limits of mating (shaft and hole) parts are
so selected that, either clearance or interference may occur
depending upon the actual size of the parts.
Interference fit
 In this type of fit, diameter of minimum allowable shaft is greater
than that of Maximum allowable hole.
 In this type of fit, the sizes of the mating parts are so selected that,
interference Or negative allowance will always occur.
Hole Basis System
• In this system, the design size of hole, whose lower deviation
(fundamental deviation) is zero, is assumed as basic size and
different class of fits are obtained by varying the limits of the shaft
only.
• In other words, the limits of the hole are kept constant and those of
the shaft are varied so as to obtain the necessary fit.
Shaft Basis System
• In this system, the design size of a shaft, whose upper deviation
(fundamental deviation) is zero, is assumed as basic size and
different class of fits obtained by varying the limits of the hole only.
• In other words, the limits of shaft are kept constant and limits of
holes are varied to obtain the necessary type of fit.
Designation of fit
 To describe completely a hole or a shaft, its basic size followed by
appropriate letter and the number of tolerance grade is given .
 Holes are designated by capita letter
 Shafts are designated by small letter.
Example,
 20 mm hole ‘H' with tolerance grade IT7 is designated as 20H7.
 20. mm 'f' shaft with tolerance grade IT8 is designated as 20f8.
for shafts
a to h -clearance fit,
j to n - transition it,
p to z -interference fit.
Grades of Tolerance
Grade of Tolerance: It is an indication of the level
of accuracy. There are 18 grades of tolerances –
IT01, IT0, IT1 to IT16
IT01 to IT4 - For production of gauges, plug
gauges, measuring instruments
IT5 to IT 7 - For fits in precision engineering
applications
IT8 to IT11 – For General Engineering
IT12 to IT14 -For Sheet metal working or press
working 14
IT12 to IT14 –For Sheet metal working or press
working
IT15 to IT16 – For processes like casting, general
cutting work
Designation of fits
example:
 50 H7g7 : is a fit indicated by its basic size 50mm,
followed by symbols representing the limits of hole (H7
i.e hole having basic size 50mm and tolerance grade IT7)
and shaft (g7 i.e shaft having basic size 50mm and
tolerance grade IT7) the type of fit system is hole basis
and obtained type of fit is clearance fit.
 35 H8j7 :