limits,fits and tolerances

design of machines (limits,fits,and tolerances).

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

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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

16. 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

17. Es = Upper deviation Ei = Lower Deviation

18. Es = Upper deviation Ei = Lower Deviation

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20. 20 H6- e7 24

21. 20 H6- e7 25

22. Together (Letter & Grade) on both mating components decide quality of fit H7 : Tol Grade 7 mean 21 μ variation (H means Lowe Deviation is zero ) Representation of Fit 0.021 0.022 0.013 Φ 30.000 Φ 30.021 Φ 30.022 Φ 30.035 p6 : Tol Grade 6 means 13 μ variation (p means Lower Deviation is 22 μ ) INTERFERENCE FIT

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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

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

33. 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

34. Solution 20 H6- e7 38

35. Solution 20 H6- e7 39

36. 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

37. Solution 20 H7- s6 41

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