ME2303 DESIGN OF MACHINE ELEMENTS

1. ME2303DESIGN OF MACHINE ELEMENTS III YEAR V SEMESTER B.E MECHANICAL

2. STEEL

3. DESIGNATION OF STEELS

5. STEELS DESIGNATION – COMPOSITION BASIS

7. FITS AND TOLERANCES 1. Clearance fit. In this type of fit, the size limits for mating parts are so selected that clearance between them always occur, as shown in Fig. 3.5 (a). It may be noted that in a clearance fit, the tolerance zone of the hole is entirely above the tolerance zone of the shaft. In a clearance fit, the difference between the minimum size of the hole and the maximum size of the shaft is known as minimum clearance whereas the difference between the maximum size of the hole and minimum size of the shaft is called maximum clearance as shown in Fig. 3.5 (a).

8. 2. Interference fit. In this type of fit, the size limits for the mating parts are so selected that interference between them always occur, as shown in Fig. 3.5 (b). It may be noted that in an interference fit, the tolerance zone of the hole is entirely below the tolerance zone of the shaft. In an interference fit, the difference between the maximum size of the hole and the minimum size of the shaft is known as minimum interference, whereas the difference between the minimum size of the hole and the maximum size of the shaft is called maximum interference, as shown in Fig. 3.5 (b). • The interference fits may be shrink fit, heavy drive fit and light drive fit. • 3. Transition fit. In this type of fit, the size limits for the mating parts are so selected that either a clearance or interference may occur depending upon the actual size of the mating parts, as shown in Fig. 3.5 (c). It may be noted that in a transition fit, the tolerance zones of hole and shaft overlap. • The transition fits may be force fit, tight fit and push fit.

9. LIMIT SYSTEM

11. The system of limits and fits comprises 18 grades of fundamental tolerances i.e. grades of accuracy of manufacture and 25 types of fundamental deviations indicated by letter symbols for both holes and shafts (capital letter A to ZC for holes and small letters a to zc for shafts) in diameter steps ranging from 1 to 500 mm. • A unilateral hole basis system is recommended but if necessary a unilateral or bilateral shaft basis system may also be used. • The 18 tolerance grades are designated as IT 01, IT 0 and IT 1 to IT 16. These are called standard tolerances.

13. Failure Theories • Stress in machine components should be accurately computed. • Designer must understand material limits to ensure a safe design.

14. Design Factor • Factor of Safety (N) • Suitable values depend on inherent danger, certainty of calculations, certainty of material properties, etc.

15. Static Stresses - Brittle Materials • Percent elongation < 5% • for parts in tension • for parts in compression • for parts with general stress

16. Ø4.00” R0.25” Ø5.00” R0.25” Example The Gray Cast Iron (Grade 40) cylinder carries an axial compressive load of 75,000 lbs and a torque of 20,000 in lbs. Compute the resulting design factor.

17. Static Stresses - Ductile Materials • Percent elongation > 5% • Distortion Energy Theory • Define von Mises Stress • For nominal stress • For localized stress

18. Static Stresses - Ductile Materials • Percent elongation > 5% • Maximum Shear Stress Theory • For nominal stress • For localized stress

19. 450 5000 lbs Example Specify a diameter for the middle portion of the rod, if it is to be made from AISI 1040-hot rolled steel.

20. 400 lb 200 lb 20” 14” Example For the seat support shown, specify a standard structural tube to resist static loads shown. The tube has properties similar to AISI 1020 hot-rolled steel. Use a design factor of 3.

21. salt smean Repeated Loads

22. 1.25” 0.1” R .75” 1” Example The notched bar is machined from AISI 1020 steel. This bar is subjected to a load that varies from 2000 lb to 3000 lb. Determine the mean and alternating nominal stresses.

23. Motor Alternating Stress, sa Endurance Strength, sn 108 107 106 105 103 104 Cycles to Failure, N (log) Fatigue Strength • R.R. Moore Test

24. Endurance Strength • sn = Endurance strength • Listed in tables • If no information is available, use • sn 0.5 su (Steel) • sn 0.4 su (Aluminum)

25. Adjusted Endurance Strength • The data from the standard R.R. Moore test is adjusted for a particular application. • sn’ = Adjusted endurance strength = (Cs) (Cm) (Cst) (CR) (sn)

26. Size and Stress Type Factors • Cs = Size Factor • D< 0.4 in Cs = 1.0 • 0.4 < D 2.0 in Cs = (D/0.3)-0.068 • 2.0 < D 10.0 in Cs = D-0.19 For rectangular sections, D=.808(h b)1/2 • Cst = Stress Type Factor • = 1.0 for bending • = 0.80 for axial tension • = 0.50 for torsion

27. Material and Reliability Factor • Cm = Material Factor • = 1.0 for wrought steel • = 0.80 for cast steel • = 0.70 for cast iron • CR = Reliability Factor • 50% CR = 1.0 • 90% CR = 0.90 • 99% CR = 0.81 • 99.9% CR = 0.75

28. 1.25” 0.1” R .75” 1” Example The notched bar is machined from AISI 1020 steel. This bar is subjected to a load that varies from 2000 lb to 3000 lb. Determine the endurance limit of the material.

29. Repeated Stresses - Ductile Materials • Distortion Energy Theory • Define repeated von Mises Stress • Solderberg criterion

30. Repeated Stresses - Ductile Materials • Maximum Shear Stress Theory • ssy = 0.5 sy • s’sn = 0.5 sn

31. 1.25” 0.1” R .75” 1” Example The notched bar is machined from AISI 1020 steel. This bar is subjected to a load that varies from 2000 lb to 3000 lb. Comment on the robustness of the design.

32. 48” Example Comment on the robustness of a 1-1/4” round bar made from AISI 1213 C-D steel. It carries a constant tensile load of 1500 lbs, a bending load that varies from 0 to 800 lbs at the senter of the 48” length and a constant torque of 1200 in lbs.

33. Shafts • Connect power transmission components. • Inherently subjected to transverse loads and torsion.

34. Wt Wr T Shaft Forces • Gears As before

35. Ftight D T Fslack = 0 Shaft Forces • Chains

36. Shaft Forces • V-belts Ftight D T Fslack

37. Shaft Forces • Flat belts Ftight D T Fslack

38. Material Properties • For steady load (torsion) sys=.5sy • For fatique load ( bending) sn’=cs cR sn cT = 1 (bending) cm = 1 (wrought steel)

39. Stress Concentrations • Keyseats • Sled Runner Kt = 1.6 • Profile Kt = 2.0 • Woodruff Kt = 1.5

40. Stress Concentrations • Shoulders • Sharp, Bearing (r/d .03) Kt = 2.5 • Round, Gear Bore (r/d .17) Kt = 1.5 • Grooves • Retaining Rings Kt = 1.5 Try not to let Kt’s overlap. Leave .10 - .15” between

41. Strength Analysis • Bending stress • Torsion stress For round sections For round sections

42. Strength Analysis • Mohr’s circle and Solderberg • Suggested Design Factors: • N=2 smooth operation • N=3 typical industrial operation • N=4 shock or impact loading

43. Minimum Acceptable Diameter • The designer must size the shaft. • Solve for appropriate diameters

44. Example Determine a suitable diameter for a shaft made from AISI 1144 OQT 1000. It is subjected to a reversing bending moment of 3000 ft lbs and a steady torque of 1800 ft lbs. The shaft has a profile keyway.

45. Example The shaft shown is part of a grain drying system • At A, a 34 lb. propeller-type fan requires 12 hp when rotating at 475 rpm. • A flat belt pulley at D delivers 3.5 hp to a screw conveyor handling the grain. • All power comes to the shaft through the v-belt at C. Using AISI 1144 cold drawn steel, determine the minimum acceptable diameter at C.

46. E B D A C 12” 4” 10” 10” 150 Sheave C Sheave D Example

47. Shafts Accessories • Components used to securely mount power transmitting elements on a shaft. • Axial • Rotational

48. Keys • Allow torque to be transferred from a shaft to a power transmitting element (gear, sprocket, sheave, etc.)

49. H W L Key Design • Use a soft, low strength material (ie, low carbon steel) • Standard size H=W=1/4 D • Design length based on strength

50. H W Shaft Dia. (in) W (in) T S Standard Key Sizes