Springs in Autobiles, types, strutcures, design criteraisa

Design of Springs

1. Mechanical Springs Prof.Sachin P Dhavane Mech Engg Dept

2. About Spring A spring is defined as an elastic body, whose function is to distort when loaded and to recover its original shape, when the load is removed. Applications : Springs in clutches, brakes, spring loaded valves, toys, clocks, vehicle suspension systems. Prof.Sachin P Dhavane Mech Engg Dept

3. Types Of Springs Prof.Sachin P Dhavane Mech Engg Dept

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6. HELICAL SPRINGS The helical springs are made up of a wire coiled in the form of a helix and is primarily intended for compressive or tensile loads. The cross-section of the wire from which the spring is made may be circular, square or rectangular. The two forms of helical springs are Compression Helical Spring as shown in Fig. ( a ) and Tension Helical Spring as shown in Fig.( b ). Prof.Sachin P Dhavane Mech Engg Dept

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8. The helical springs have the following advantages Easy to manufacture. Available in wide range. Reliable. Constant spring rate. Performance can be predicted more accurately. Characteristics can be varied by changing dimensions. Prof.Sachin P Dhavane Mech Engg Dept

9. CONICAL AND VOLUTE SPRINGS The conical spring, is wound with a uniform pitch. The volute springs, are wound in the form of paraboloid with constant pitch and lead angles. Used in special applications where a telescoping spring or a spring with a spring rate that increases with the load is desired. The major stresses produced in conical and volute springs are also shear stresses due to twisting Prof.Sachin P Dhavane Mech Engg Dept

10. Conical And Volute Springs Prof.Sachin P Dhavane Mech Engg Dept

11. The helical type may be used only in applications where the load tends to wind up the spring and are used in various electrical mechanisms. The spiral type is also used where the load tends to increase the number of coils and when made of flat strip are used in watches and clocks. The major stresses produced in torsion springs are tensile and compressive due to bending. Door hinges, automobile starters ,watches ,clocks TYPES OF SPRINGS-TORSION SPRINGS Prof.Sachin P Dhavane Mech Engg Dept

12. Helical And Spiral Torsion Springs Prof.Sachin P Dhavane Mech Engg Dept

13. LAMINATED OR LEAF SPRINGS The laminated or leaf spring (also known as flat spring or carriage spring ) consists of a number of flat plates (known as leaves) of varying lengths held together by means of clamps and bolts. These are mostly used in automobiles. The major stresses produced in leaf springs are tensile and compressive stresses Prof.Sachin P Dhavane Mech Engg Dept

14. Prof.Sachin P Dhavane Mech Engg Dept Laminated Or Leaf Springs

15. DISC OR BELLEVILE SPRINGS These springs consist of a number of conical discs held together against slipping by a central bolt or tube. These springs are used in applications where high spring rates and compact spring units are required. The major stresses produced in disc or Belleville springs are tensile and compressive stresses . Prof.Sachin P Dhavane Mech Engg Dept

16. . Disc Or Bellevile Springs . Prof.Sachin P Dhavane Mech Engg Dept

17. TERMS USED IN COMPRESSION SPRINGS… 1.Solid Length (Ls): When the spring is compressed until the coils touch each other . Prof.Sachin P Dhavane Mech Engg Dept

18. 2. Free Length (Lf) : A free length is the length when spring is in uncompressed condition . 3. Active Coils : Which contributes to the spring action 4. Inactive Coils : When the ends coils which are in contact with seat do not contribute spring action Prof.Sachin P Dhavane Mech Engg Dept

19. 5. Spring index. The spring index is defined as the ratio of the mean diameter of the coil to the diameter of the wire. Mathematically, C = D / d Where D = Mean diameter of the coil, and d = Diameter of the wire. 6. Spring rate : The spring rate (or stiffness or spring constant) is defined as the load required per unit deflection of the spring. Mathematically, k = W / δ where W = Load δ = Deflection of the spring. Prof.Sachin P Dhavane Mech Engg Dept

20. 7. Pitch : The pitch of the coil is defined as the axial distance between adjacent coils in uncompressed state. Mathematically, Pitch of the coil, p = Free length / n′ – 1 The pitch of the coil may also be obtained by using the following relation, i.e. where Lf = Free length of the spring, L S = Solid length of the spring, n' = Total number of coils, and d = Diameter of the wire. Prof.Sachin P Dhavane Mech Engg Dept

21. Prof.Sachin P Dhavane Mech Engg Dept Types Of Ends of Helical Compression Springs

22. Let, n’ = Total no of coils or turns n = no of active coils or turns p = pitch , mm d = wire diameter. Prof.Sachin P Dhavane Mech Engg Dept Types Of Ends of Helical Compression Springs

23. Material For Helical Springs The springs are mostly made from oil-tempered carbon steel wires containing 0.60 to 0.70 per cent carbon and 0.60 to 1.0 per cent manganese. Music wire is used for small springs. Non-ferrous materials like phosphor bronze, beryllium copper, monel metal, brass etc., may be used in special cases to increase fatigue resistance, temperature resistance and corrosion resistance. The helical springs are either cold formed or hot formed depending upon the size of the wire. Wires of small sizes (less than 10 mm diameter) are usually wound cold whereas larger size wires are wound hot. The strength of the wires varies with size, smaller size wires have greater strength and less ductility, due to the greater degree of cold working . Prof.Sachin P Dhavane Mech Engg Dept

24. Material Properties For Helical Springs… The material of the spring should have high fatigue strength, high ductility, high resilience and it should be creep resistant . It largely depends upon the service for which they are used i.e. severe service, average service or light service. Severe service Average service Light service Prof.Sachin P Dhavane Mech Engg Dept

25. Severe service means rapid continuous loading where the ratio of minimum to maximum load (or stress) is one-half or less, as in automotive valve springs. Average service includes the same stress range as in severe service but with only intermittent operation, as in engine governor springs and automobile suspension springs . Light service includes springs subjected to loads that are static or very infrequently varied, as in safety valve springs. Material Properties For Helical Springs … Prof.Sachin P Dhavane Mech Engg Dept

26. The values of allowable shear stress, modulus of rigidity and modulus of elasticity for various materials used for springs. Prof.Sachin P Dhavane Mech Engg Dept Material For Helical Springs …

27. STRESSES IN HELICAL SPRINGS OF CIRCULAR WIRE Consider a helical compression spring made of circular wire and subjected to an axial load W. Let D = Mean diameter of the spring coil, d = Diameter of the spring wire, n = Number of active coils, G = Modulus of rigidity for the spring material, F = Axial force on the spring τ = Maximum shear stress induced in the wire, C = Spring index = D/d, p = Pitch of the coils, and δ = Deflection of the spring, as a result of an axial load F . Prof.Sachin P Dhavane Mech Engg Dept

28. Prof.Sachin P Dhavane Mech Engg Dept STRESSES IN HELICAL SPRINGS OF CIRCULAR WIRE

29. Prof.Sachin P Dhavane Mech Engg Dept STRESSES IN HELICAL SPRINGS OF CIRCULAR WIRE

30. Stresses Induced In Spring Wire : Torsional Shear Stress: Torsional Shear Stress induced in a spring wire due to torsional moment T=FD/2 2) Direct Shear Stress: Due to the direct shear force F Prof.Sachin P Dhavane Mech Engg Dept

31. Prof.Sachin P Dhavane Mech Engg Dept Stresses In Spring Wire

32. Stresses Induced In Spring Wire : Prof.Sachin P Dhavane Mech Engg Dept 3) Resultant Shear Stresses

33. Stresses In Spring Wire Prof.Sachin P Dhavane Mech Engg Dept

34. 4. Result Shear Stress with Curvature Effect : Curvature of wires increases the shear stress on inner surface of spring and decreases it slightly on outer surface. Wahl shear stress factor is used to consider effects of direct shear stress and curvature effect stresses in addition to torsional shear stress. The maximum shear stress induced in a spring wire is Prof.Sachin P Dhavane Mech Engg Dept

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36. Deflection Of Helical Springs Spring Stiffness is given by K = F / δ where F = Load δ = Deflection of the spring   Prof.Sachin P Dhavane Mech Engg Dept

37. Spring Stiffness is given by K = F / δ Prof.Sachin P Dhavane Mech Engg Dept

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40. Prof.Sachin P Dhavane Mech Engg Dept Problem 1 . Design a close coiled helical compression spring with following data: Service load range = 2250 N to 2750 N Axial deflection of spring for load range= 6 mm Spring Index = 5 Permissible shear stress for spring=420 N/mm2 Modulus of rigidity for spring material = 84 KN/mm2 Neglect the effect of stress concentration.( Assume Square and Ground ends) Draw a dimensional sketch of spring. Given Data : Fmin = 2250 N Fmax=2750 N G=84 KN/mm2 C=5

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46. Solution: Wire diam. d= 9.58 mm Ls= ( n+2 ) d=112.32 mm Mean coil diam. D=48 mm Max. Deflection=33 mm Spring stiffness K=83.33N/mm Lf=Ls + Max Defle . + 0.15 Max Defle No of turns n=9.7 turns =150.27 mm Total no of turns n’= n+2 =11.7 turns Lf= pn +2d p= 13.5 mm Problem 1 Prof.Sachin P Dhavane Mech Engg Dept

47. Problem 2 . A safety valve of 60mm diameter is to blow off at a pressure of 1.2 Mpa . It is held on its seat by closed coil helical spring. The maximum lift of the valve is 10 mm. Design a suitable compression spring of spring index 5 and providing an initial compression of 35 mm. The maximum shear stress in the material of the wire is limited to 500 N/mm2 while the modulus of rigidity of spring material is 80000 N/mm2.Calculate 1) diameter of spring wire. 2) mean coil diameter 3) no of active turns 4) pitch of coil. Assume Square and Ground ends. Given Data : dv =60 mm Pmax = 1.2 N/mm2 Solution: d=11 mm D=55 mm n=11.67 turns p=15.42 mm Prof.Sachin P Dhavane Mech Engg Dept

48. Springs In Series Consider two springs connected in series as shown in Fig. W = Load carried by the springs, δ1 = Deflection of spring 1, δ2 = Deflection of spring 2, k 1 = Stiffness of spring 1 = W / δ1, and k 2 = Stiffness of spring 2 = W / δ2 A little consideration will show that when the springs are connected in series, then the total deflection produced by the springs is equal to the sum of the deflections of the individual springs. Prof.Sachin P Dhavane Mech Engg Dept

49. Springs In Series Prof.Sachin P Dhavane Mech Engg Dept

50. Springs In Parallel Consider two springs connected in parallel as shown in Fig Let W = Load carried by the springs, W 1 = Load shared by spring 1, W 2 = Load shared by spring 2, k 1 = Stiffness of spring 1, and k 2 = Stiffness of spring 2. Prof.Sachin P Dhavane Mech Engg Dept

51. A little consideration will show that when the springs are connected in parallel, then the total deflection produced by the springs is same as the deflection of the individual springs. We know that W = W 1 + W 2 or δ. k = δ. k 1 + δ. k 2 ∴ k = k 1 + k 2 where k = Combined stiffness of the springs, and δ = Deflection produced. Springs In Parallel Prof.Sachin P Dhavane Mech Engg Dept

52. Concentric Or Composite Springs To obtain greater spring force within a given space. To insure the operation of a mechanism in the event of failure of one of the springs. The concentric springs for the above two purposes may have two or more springs and have the same free lengths as and are compressed equally. Such springs are used in automobile clutches, valve springs in aircraft, heavy duty diesel engines and rail-road car suspension systems. Prof.Sachin P Dhavane Mech Engg Dept

53. 4. Sometimes concentric springs are used to obtain a spring force which does not increase in a direct relation to the deflection but increases faster. 5. Such springs are made of different lengths .The shorter spring begins to act only after the longer spring is compressed to a certain amount. 6.These springs are used in governors of variable speed engines to take care of the variable centrifugal force. Concentric Or Composite Springs Prof.Sachin P Dhavane Mech Engg Dept

54. Prof.Sachin P Dhavane Mech Engg Dept A Carriage Suspension System Showing The Use Of Concentric Springs

55. Concentric Or Composite Springs Prof.Sachin P Dhavane Mech Engg Dept

56. Concentric Or Composite Springs Prof.Sachin P Dhavane Mech Engg Dept

57. Consider a concentric spring as . W = Axial load, W 1 = Load shared by outer spring W 2 = Load shared by inner spring d 1 = Diameter of outer spring, d 2 = Diameter of inner spring, D 1 = Mean diameter of outer spring, D 2 = Mean diameter of inner spring δ1 = Deflection of outer spring, δ2 = Deflection of inner spring, n 1 = Number of active turns of outer spring n 2 = Number of active turns of inner spring. Prof.Sachin P Dhavane Mech Engg Dept Concentric Or Composite Springs

58. Prof.Sachin P Dhavane Mech Engg Dept Problem 3 : A concentric spring consists of two helical compression springs one inside the other. The free length of the outer spring is 15 mm greater than the inner spring. The wire diameter and mean coil diameter of inner spring are 5 mm and 30 mm respectively, while wire diameter and mean coil diameter of outer spring are 8 mm and 52 mm resp. Assume same material for two spring and modulus of rigidity of spring is 81370 N/mm2. If the composite spring is subjected to maximum axial force of 1000 N. The inner spring and outer spring has 8 and 10 active coils. calculate The compression of each spring The force transmitted by each spring; and The maximum torsional shear stress induced in each spring .

59. Prof.Sachin P Dhavane Mech Engg Dept A concentric spring consists of two helical compression springs one inside the other. The free length of the outer spring is 15 mm greater than the inner spring. The wire diameter and mean coil diameter of inner spring are 5 mm and 30 mm respectively, while wire diameter and mean coil diameter of outer spring are 8 mm and 52 mm resp. Assume same material for two spring and modulus of rigidity of spring is 81370 N/mm2. If the composite spring is subjected to maximum axial force of 1000 N. The inner spring and outer spring has 8 and 10 active coils. calculate The compression of each spring The force transmitted by each spring; and The maximum torsional shear stress induced in each spring .

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63. Prof.Sachin P Dhavane Mech Engg Dept Problem 4 : Two helical springs are arranged in a concentric manner with one inside each other. Both the springs have same free length and carry a total load of 2000 N. The outer spring has 8 active coils with mean coil diam. Of 80 mm and wire diam. Of 10 mm. The inner spring has 12 active coils with mean coil diam. Of 64 mm and wire diam of 8mm .G=81370 N/mm2. determine Max. load carried by each spring. Total deflection of each spring. Max. stresses in each spring. Solution: i )F1= 1304.3 N F2=695.7 N ii)def.=52.33 iii)314.6 and 262.19 N/mm2

64. Leaf springs (also known as flat springs ) are made out of flat plates. The advantage of leaf spring over helical spring is that the ends of the spring may be guided along a definite path as it deflects to act as a structural member in addition to energy absorbing device. Thus the leaf springs may carry lateral loads, brake torque, driving torque etc., in addition to shocks. Prof.Sachin P Dhavane Mech Engg Dept Leaf Springs…

65. Leaf Springs… Prof.Sachin P Dhavane Mech Engg Dept

66. Nipping: Process of pre stressing by giving different radii of curvature before assembly . The Gap C between the full length leaf and the graduated leaf before assembly is called Nip . Nip is adjusted so that under maximum load condition the stresses in all leaves are equal. Nip is adjusted to give stress in full length leaves slightly less than the graduated leaves. Prof.Sachin P Dhavane Mech Engg Dept Nipping of Leaf Springs…

67. Prof.Sachin P Dhavane Mech Engg Dept Nipping Of Leaf Springs

68. Surge In Springs When one end of a helical spring is resting on a rigid support and the other end is loaded suddenly, then all the coils of the spring will not suddenly deflect equally , because some time is required for the propagation of stress along the spring wire. A little consideration will show that in the beginning, the end coils of the spring in contact with the applied load takes up whole of the deflection and then it transmits a large part of its deflection to the adjacent coils. In this way, a wave of compression propagates through the coils to the supported end from where it is reflected back to the deflected end . Prof.Sachin P Dhavane Mech Engg Dept

69. This wave of compression travels along the spring indefinitely. If the applied load is of fluctuating type as in the case of valve spring in internal combustion engines and if the time interval between the load applications is equal to the time required for the wave to travel from one end to the other end, then resonance will occur. This results in very large deflections of the coils and correspondingly very high stresses. Under these conditions, it is just possible that the spring may fail. This phenomenon is called surge. Surge In Springs Prof.Sachin P Dhavane Mech Engg Dept

70. Prof.Sachin P Dhavane Mech Engg Dept Failure Of Valve Spring Due To Surge In Springs

71. Under fatigue loading of springs the poor surface finish reduces the endurance strength. So spring surface becomes irregular which acts as source for stress concentration. The fatigue cracks begins at such points and propogates leading to failure of springs. In order to reduce the chances of failure due to surface cracks , a residual compressive stress is induced in the surface of spring wire . Prof.Sachin P Dhavane Mech Engg Dept Shot Peening …

72. Prof.Sachin P Dhavane Mech Engg Dept Shot Peening In Springs

73. Prof.Sachin P Dhavane Mech Engg Dept Shot Peening In Springs

74. Shot peening is used for inducing the residual compressive stress. The surface is bombarded with high velocity iron or steel shots (balls) discharged from a rotating wheel or pneumatic nozzle. It increases the area of exposed surface which creates a layer of residual compressive stress on the surface. Shot peening improves the fatigue strength. Prof.Sachin P Dhavane Mech Engg Dept Shot Peening …

75. Prof.Sachin P Dhavane Mech Engg Dept InSem QP Backlog 2015

76. Prof.Sachin P Dhavane Mech Engg Dept InSem QP Backlog 2015