This document discusses tolerances, limits, and fits between machine elements. It defines tolerance as the permissible variation in a component's dimensions, with upper and lower deviation limits. Three common fits are defined - clearance fit which always provides space, transition fit which may provide clearance or interference, and interference fit which always overlaps. Hole and shaft basis systems are described for specifying tolerances. Different grades of fits are explained for various machine applications like loose running, precision, and press fits. Objective problems on fit selection and calculations are presented.

1. MEC223
Design of Machine Element -I

2. TOLERANCES ,LIMITS AND FITS
2

3. Tolerances
• Tolerance is defined as permissible
variation in the dimensions of the
component.
• The basic dimension is called the
normal or basic size.
• The two limits are some time called
upper and lower deviation
3

4. Tolerances
4

5. Max Hole size – Basic Size = Upper Deviation
Min Hole size – Basic Size = Lower Deviation
HOLE
Max shaft size – Basic Size = Upper Deviation
Min shaft size – Basic Size = Lower Deviation
SHAFT

6. Tolerances
• There are two systems of specification of
tolerance namely, unilateral and bilateral
6

7. Types of Fit
• When two parts are to be assembled, the
relationship resulting from the difference
between their sizes before assembly is
called a fit
• Clearance fit
• Transition fit
• Interference fit
7

8. Clearance fit
It is a fit, which
always provides a
positive clearance
between the hole
and shaft over the
entire range of
tolerance
8

9. Transition fit
It is a fit which may provide
the either a clearance or
interference, depending
upon the actual values of
the individual tolerances
of the mating
components.
9

10. Interference Fit
It is a fit which always
provides a positive
interference over
the whole range of
tolerances.
10

11. Types of Fit
• There are two basic systems for giving
tolerance
– the hole base system and
– the shaft base system
11

12. Hole Basis –Lower deviation is zero –
Fundamental Deviation is H

13. Shaft Basis- Upper deviation is zero-
Fundamental Deviation is h

14. 14

15. BIS System of Fits and Tolerances
• The description of tolerance consists of two
parts – fundamental deviation and magnitude
of tolerance
15
Gives location
of tolerance
zone w.r.t zero
line indicated
by an
alphabet
Designated
by a number
, called grade

17. The fit is indicated by the basic size common to both
components followed by symbols for tolerance of each
component.
50H8/g7 or 50H8-g7

19. Es = Upper deviation
Ei= Lower Deviation

21. Es = Upper deviation
Ei= Lower Deviation

23. 23

24. • 20 H6- e7
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25. • 20 H6- e7
25

26. Together (Letter & Grade)
on both mating
components decide quality
of fit
H7 : Tol Grade 7 mean 21μ variation
(H means Lowe Deviation is zero)
Representation of Fit
0.021
0.022
0.013
Φ30.000
Φ30.021
Φ30.022
Φ30.035
p6 : Tol Grade 6 means 13μ variation
(p means Lower Deviation is 22 μ)
INTERFERENCE
FIT

27. 27

28. Selection of Fits
• Loose Running fits :- H7- d8, H8 – d9 and H11 –
d11 and are used for plumber block, bearings
and loose pulleys
28

29. Selection of Fits
• Loose clearance fits :- H6- e7, H7 – e8 and H11
– e8 and are used for high speed bearings,
lubricated bearings and large electric motors
29

30. Selection of Fits
• Normal running fits :- H6- f6, H7 – f7 and H8 –
f8 and are used for shafts of gearboxes, small
electric motors and pumps
30

31. Selection of Fits
• Precision fits :- H6- g5, H7 – g6 and H8 – g7
and are used for pistons, slide valves and
bearing of accurate link mechanism
• Transition fits :- H6- j5, H7 – j6 and H8 – j7 and
are used for spigot and recess of the rigid
coupling and the composite gear blank
31

32. Selection of Fits
• Light Press fits :- H7- p6 or H7 – p7 results in
interference of parts which can be dismantled
easily as and when required. e.g. Fitting a
brass bush in the gear.
32

33. Selection of Fits
• Medium drive fits :- H6- r5 or H7 – r6 results in
interference of parts which can be dismantled
used for ferrous parts
• Permanent and semi permanent fits :- H6- s5 ,
H7 – s6 and H8 – S7 results in large
interference of parts which can be provide a
considerable gripping force. They are used in
valve seats and shaft collars.
33

34. Objective problems
• For a particular application, an H 7(0,25) fit has
been selected for the hole and a k 6(12,18) fit for
the shaft. The assembly shows
• A) Interference Fit
• B) Clearance Fit
• C) Transition fit
• D) Permanent fit
34

35. Objective problems
• On the drawing of the shaft the size is given as
the tolerance can be classified as
• A) unilateral tolerance
• B) Equal bilateral tolerance
• C) Unequal bilateral tolerance
• D) Positive tolerance.
35

36. Objective problems
• For following hole and shaft assembly ,the shaft
tolerance(in micron meter) is
• A) zero
• B) 40
• C) 60
• D) 20
36

37. Problem
• The main bearing of an engine is shown in fig
below. Calculate
• (i) the maximum and minimum diameters of
bush and crank pin; and
• (ii) the maximum and minimum clearances
between the crank pin and bush
37

38. Solution
• 20 H6- e7
38

39. Solution
• 20 H6- e7
39

40. Problem
• The Valve seat fitted inside the housing of a
pump is shown fig below. Calculate
• (i) the maximum and minimum diameters of
housing and valve seat; and
• (ii) the magnitude of maximum and minimum
interference between the housing and valve
seat
40

41. Solution
• 20 H7- s6
41

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43. 43

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