1. Presented By
Mr.S.Saravanakumar, M.E.,
Assistant Professor
Easwari Engineering College

2. TERMINOLOGY

3. 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.

4. 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

5. The algebraic difference between upper and lower
deviations. It is an absolute value.

6.  Unilateral tolerance : if the tolerance is allowed on one
side of the basic size, the system of tolerance is said to
be unilateral.

7.  Bilateral tolerance : if the tolerance is allowed on both
side of the basic size, the system of tolerance is said to
be unilateral.

8. 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.

9.  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 .

10.  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.

11.  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

12. 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

13.  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.

14.  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.

15.  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.

16.  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.

17.  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.

18. 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

20. 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 :

28. Endurance Limit
• Whenever a cyclic (repetitive) load is applied on the
material, If the material shows no evidence of fracture then
this property of the material is called, “Endurance Limit”
• While if the material shows any evidence of fracture
during the loading this property is called, “Fatigue Limit”
• The conventional fatigue testing has been concerned
primarily with the testing of specimens with smooth
surfaces under the conditions of rotating-bending or uni
axial tension compression cycling.
• The results of these tests are presented in the form of plots
of stress verses the number N of stress cycles required to
cause the fracture.

30. • These plots are called δ-N diagrams, it was suggested by
“WhOler”. These diagrams are also called, “S-N
diagrams” in some literatures. It is shown in the figure.
Usually in mild steel or certain other steels, an endurance
limit is observed.
• Actually when the cyclic load is applied on the material
stress decreases and once a point is arrived where the
stress becomes constant, means there is no further
• decrease in stress with the increase of N. However many
materials do not exhibit a clear cut endurance limit, but δ-
N curves continues downward as N increases.

31. • Most of mechanical components has irregularities in structure
and abrupt change in cross section, which increase stress
concentration at such changes. Some materials are more
sensitive than others to stress raising notches under fluctuating
loads , to account for this effect a parameter called notch
sensitivity factor is found for each material....
• Notch sensitivity of a material is measure of how sensitive
material is for notches or geometric irregularities...
• Notch Sensitivity: Degree to which the theoretical effect of
stress concentration has actually reached.
q=(kf-1)/(kt-1)
q - Notch Sensitivity Factor
kf - Theoritical Stress Concentration Factor for Axial or
Bending Loading.
kt - Theoritical Stress Concentration Factor for Torsional or
Shear Loading.
Notch Sensitivity Factor