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
24

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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