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Spring design

Spring design. Types Factors in spring design Materials Torsional. Types of Springs. Types of spring cont. Types of springs cont. Types of springs cont. Factors in spring design. High strength High yield Modulus may be low for energy storage Cost Environmental factors

phelan-franklin
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Spring design

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  1. Spring design • Types • Factors in spring design • Materials • Torsional

  2. Types of Springs

  3. Types of spring cont.

  4. Types of springs cont.

  5. Types of springs cont.

  6. Factors in spring design • High strength • High yield • Modulus may be low for energy storage • Cost • Environmental factors • Temperature resistance (e.g. valve springs) • Corrosion resistance

  7. Common materials for springs

  8. Influence of diameter on ultimate stress

  9. Influence of diameter on ultimate stress cont.

  10. Design of helical compression springs • Length nomenclature • Free • Assembled • Solid or shut height • Working deflection

  11. Stresses in Helical Spring

  12. Stresses in Helical springs cont. At the inside of the spring Substituting for Gives 4<C<12 Defining the spring index Therefore the stress is Equation(1)

  13. Effect of curvature on Stress • Equation (1) is based on the wire being straight • However the curvature increases the stress on the inside of the wire • For static stress the effect of curvature can be neglected • For fatigue the effect of curvature is important

  14. Effect of curvature cont. Wahl factor Bergstrasser factor The results of the two equations differ by less than 1%. Bergstrasser factor is preferred due to simplicity

  15. Deflection • The external work done on an elastic member in deforming it is transformed into strain, or potential, energy. If the member is deformed a distance y, and if the force-deflection relationship is linear, this energy is equal to the product of the average force and the deflection, or • This equation is general in the sense that the force F can also mean torque, or moment, provided, that consistent units are used for k.

  16. Deflection cont.. • By substituting appropriate expressions for k, strain-energy formulas for various simple loadings may be obtained. For tension and compression and for torsion,

  17. Deflection of a helical spring • Using Castigliano’s theorem, strain energy is equal to • Substituting

  18. Deflection cont. • Using the spring index • Spring scale is

  19. Spring design – end treatment • End details affect active coils • Plain ends • Squared ends • Squared • Ground

  20. Number of active coils

  21. Stability of a column Euler Formula

  22. Stability of a spring • We know a column will buckle when the load is too large • A compression coil spring will also buckle • ycr is the deflection corresponding to onset of instability

  23. Deflection cont. Is called the effective slenderness ratio Alpha = end condition constant Lo is the spring length D is the Coil diameter

  24. Instability cont. • End constraint alpha given by

  25. Instability cont. • For absolute stability • For steels it turns out • For square and ground ends

  26. Static design flow chart

  27. Flow chart cont.

  28. Recommended design conditions Figure of merit (fom)

  29. Materials for springs • Yield strength for static loading • Depends on set • Before set removed use Wahl factor • After set removed no stress concentration used

  30. Properties for fatigue • Fatigue Strength • Torsion is relevant loading- could use von Mises stress • Materials testing specific to helical compression springs is available, however • Correct for temp., reliability, environment

  31. Properties - endurance • Endurance Strength (steels) unlimited cycles • For high ultimate strengths, endurance limits max out at 45 kpsi (unpeened) and 67.5 kpsi (peened) • Small wires have high ultimate strength • Tests have been done specific to spring wire • Temperature may require compensation • Corrosion • Reliability

  32. S-N and Modified Goodman

  33. Designing springs Requirements Design Choices • Functionality • Stiffness • Lengths • Diameter • Forces • Reliable operation • Static factor of safety • Fatigue factor of safety • Buckling and surge • Manufacturability • Index C • Material • Wire and coil diameter • Number of turns • End treatment and constraint • Set and shot peen Constraints (other) • Bend radius

  34. Helical extension spring • Similar in most ways to compression springs • Usually wound to be closed coil at zero force • Thus a preload is required to stretch any, i.e. y=k(F-Fi ) • Spring hook is a source of failure in bending and torsion • No set is used • One coil not considered active

  35. End stresses Bending stress: Torsional stress:

  36. Design for fatigue • Data available for springs with loading from zero to some compresion value • Application often has preload… how to use? • First construct (or find) S-N curve • Next construct Mod-Goodman chart • Apply load line for given preload and design stress • Find factor of safety to failure point

  37. Goodman curve

  38. A word about torsional springs • The wire in a torsional spring is primarily in bending • Spring constant is rotary M=k • Loading should act to wind up coil • Design process resembles compression springs

  39. Torsional

  40. Homework • Read chapter 10 of Shigley

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