2. It is impossible to maintain dimensions , geometrical
properties & surface roughness of components with
absolute accuracy in production process.
Limits ,fits and tolerance are necessary to ensure
INTERCHANGEBILITY of mating parts , coupled with the
desired degree of tightness or looseness of assembly.
Lack of tolerance leads to improper fits,it will also add to
delay & higher cost.
4. The maximum and minimum sizes
permitted for a feature.
There are two extreme possible
sizes of a component.
The largest permissible size for a
component is called upper
limit and smallest size is
called lower limit.
5. MAXIMUM LIMIT : The greater value is taken
as maximum limit.
MINIMUM LIMIT : The smaller value is taken
as minimum limit.
Eg. If basic size is Ø=30. then
9. 1) Geometric tolerances are different from the tolerances
allowed for the size of feature, they specify the
allowable variation of the shape of a feature.
2) There are 3 basic types:
1.Form 2.Orientation 3.Position tolerances
3) Geometric tolerances are specified using a control frame
consisting of a tolerance symbol, a tolerance value &
optional datum planes.
11. Fit is the degree of looseness
or tightness between two
When FITS are combined
between shaft & hole they
can be classified into 3
12. Always has a gap between two mating parts.
Shaft size is small , compared to hole size.
Generally in this type of fit, the lower limit size of the hole is
greater or at least equal to the upper limit size of the shaft.
Use:- pivots , latches fit of parts exposed to corrosive
13. It is a fit where both clearance and interference may occur
in the coupling.
Here tolerance zones of the hole and shaft are partly or
Use:- Pulleys and bushing, Flushed bolts etc.
14. It always overlap & are used mainly for press fits where the
two parts are pushed together, and require no other
The upper limit size of the hole is smaller or at least equal to
the lower limit size of the shaft.
Use:-In bearing bushings,flanges etc.
16. • The geometrical characteristics of a surface include,
2. Surface waviness, and
• The surface roughness is evaluated by the height, Rt and
mean roughness index Ra of the micro-irregularities.
17. SURFACE ROUGHNESS NUMBER
• Represents the average departure of the surface
from perfection over a prescribed sampling length,
(usually selected as 0.8 mm)
• Surface roughness number (Ra) is expressed in
• Ra = (h1+h2+-----+hn)/n
• The measurements are usually made along a line,
running at right angle to the general direction of tool
marks on the surface.
18. INDICATION OF SURFACE TEXTURE
The basic symbol consists of two legs of
unequal length inclined at approximately 60’
to the line representing the considered
The symbol must be represented by thin line
If the removal of material by machining is
required, a bar is added to the basic symbol,
If the removal of material is not permitted,
a circle is added to the basic symbol.
When special surface characteristics have to
be indicated, a line is added to the longer arm of
any of the above symbols,
19. Generally to indicate the surface roughness, the symbol is used instead of value.
The relation is given in following table.
21. SELECTON OF FITS AND
1.CONDITIONS OF ROTATION
c)Direction of load in determinant
2. MAGNITUDE OF THE LOAD
d)very heavy load
3. BEARING INTERNAL CLEARANCE
5. DESIGN AND MATERIAL OF THE SHAFT AND HOUSING
6.EASE OF MOUNTING AND DISMOUNTING
24. For this example , we have selected m6 fit.
m- interference fit IT- 6grade
Shaft size= 70mm