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Overview of VDI 2230

Overview of VDI 2230. An Introduction to the Calculation Method for Determining the Stress in a Bolted Joint. Important Note.

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Overview of VDI 2230

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  1. Overview of VDI 2230 An Introduction to the Calculation Method for Determining the Stress in a Bolted Joint

  2. Important Note This summary of the VDI 2230 Standard is intended to provide a basic understanding of the method. Readers who wish to put the standard to use are urged to refer to the complete standard that contains all information, figures, etc.

  3. Definitions • Covers high-duty bolted joints with constant or alternating loads • Bolted joints are separable joints between two or more components using one or more bolts • Joint must fulfill its function and withstand working load

  4. Aim of Calculation Determine bolt dimension allowing for: • Strength grade of the bolt • Reduction of preload by working load • Reduction of preload by embedding • Scatter of preload during tightening • Fatigue strength under an alternating load • Compressive stress on clamped parts

  5. 1. Range of Validity • Steel Bolts • M4 to M39 • Room Temperature

  6. 2. Choice of Calculation Approach • Dependent upon geometry • Cylindrical single bolted joint • Beam connection • Circular plate • Rotation of flanges • Flanged joint with plane bearing face

  7. Cylindrical Single Bolted Joint • Axial force, FA • Transverse force, FQ • Bending moment, MB

  8. Beam Geometry, Ex. 1 • Axial force, FA • Transverse force, FQ • Moment of the plane of the beam, MZ

  9. Beam Geometry, Ex. 2 • Axial force, FA • Transverse force, FQ • Moment of the plane of the beam, MZ

  10. Rotation of Flanges • Axial force, FA(pipe force) • Bending moment, MB • Internal pressure, p

  11. Flanged Joint with Plane Bearing Face, Ex. 1 • Axial force, FA(pipe force) • Torsional moment, MT • Moment, MB

  12. Flanged Joint with Plane Bearing Face, Ex. 2 • Axial force, FA(pipe force) • Transverse force, FQ • Torsional moment, MT • Moment, MB

  13. Flanged Joint with Plane Bearing Face, Ex. 3 • Axial force, FA(pipe force) • Transverse force, FQ • Torsional moment, MT • Moment, MB

  14. 3. Analysis of Force and Deformation • Optimized by means of thorough and exact consideration of forces and deformations including: • Elastic resilience of bolt and parts • Load and deformation ratio for parts in assembled state and operating state

  15. 4. Calculation Steps • Begins with external working load, FB • Working load and elastic deformations may cause: • Axial force, FA • Transverse force, FQ • Bending Moment, MB • Torque moment, MT

  16. Determining Bolt Dimensions • Once working load conditions are known allow for: • Loss of preload to embedding • Assembly preload reduced by proportion of axial bolt force • Necessary minimum clamp load in the joint • Preload scatter due to assembly method

  17. Calculation Step R1 • Estimation of bolt diameter, d • Estimation of clamping length ratio, lK/d • Estimation of mean surface pressure under bolt head or nut area, pG • If pG is exceeded, joint must be modified and lK/d re-determined

  18. Calculation Step R2 • Determination of tightening factor, aA, allowing for: • Assembly method • State of lubrication • Surface condition

  19. Calculation Step R3 • Determination of required average clamping load, Fkerf, as either: • Clamping force on the opening edge with eccentrically acting axial force, FA Or • Clamping force to absorb moment MT or transverse force component, FQ

  20. Calculation Step R4 • Determination of load factor, F, including: • Determination of elastic resilience of bolt, dS • Evaluation of the position of load introduction, n*lK • Determination of elastic resilience of clamped parts, dP • Calculation of required substitutional cross-section, Aers

  21. Calculation Step R5 • Determination of loss of preload, FZ, due to embedding • Determination of total embedding

  22. Calculation Step R6 • Determination of bolt size and grade • For tightening within the elastic range, select bolt for which initial clamping load is equal to or greater than maximum initial clamping load due to scatter in assembly process • For tightening to yield, select bolt for which 90% of initial clamping load is equal to or greater than minimum initial clamping load due to scatter in assembly process

  23. Calculation Step R7 • If changes in bolt or clamping length ratio, lK/d, are necessary, repeat Steps R4 through R6

  24. Calculation Step R8 • Check that maximum permissible bolt force is not exceeded

  25. Calculation Step R9 • Determine alternating stress endurance of bolt • Allow for bending stress in eccentric load applications • Obtain approximate value for permissible stress deviation from tables • If not satisfactory, use bolt with larger diameter or greater endurance limit • Consider bending stress for eccentric loading

  26. Calculation Step R10 • Check surface pressure under bolt head and nut bearing area • Allow for chamfering of hole in determining bearing area • Tables provide recommendations for maximum allowable surface pressure • If using tightening to or beyond yield, modify calculation

  27. 5. Influencing Factors • Allow for factors depending upon: • Material and surface design of clamped parts • Shape of selected bolts and nuts • Assembly conditions

  28. Strength of the Bolt • Stress caused by: • Torsional and axial stresses during tightening • Working load • Should not exceed yield load

  29. Minimum Thread Engagement • Depends upon: • Thread form, pitch, tolerance, and diameter • Form of the nut (wrenching width) • Bolt hole • Strength and ductility of bolt and nut materials • Type of stress (tensile, torsional, bending) • Friction coefficients • Number of tightenings

  30. Thread Shear Strength • Bolt-Nut Strength Matching • Number for strength grade of nut is equivalent to first number of strength grade of bolt

  31. Calculation of Required Nut Height • Allows for geometry and mechanical properties of joint elements • Predicts type of failure caused by overloading • Considers: • Dimensional values (tensile cross-section of bolt thread, thread engagement length, etc.) • Thread form & nut form • Bolt clearance hole

  32. Bolt Head Height • Ensures that failure will occur in free loaded thread section or in the shank • Highest tensile stress in thread < Highest tensile stress in bolt head

  33. Surface Pressure at Bolt Head & Nut Bearing Areas • Calculation determines surface pressure capable of causing creep resulting in loss of preload • Surface pressure due to maximum load should not exceed compressive yield point of clamped material

  34. Tightening Factor, Alpha A • Allowance must be made for torsional stress caused by pitch and thread friction, and axial tensile stress • Scatter in friction coefficients and errors in method of controlling preload create uncertainty in level of tensile and torsional stress • Tightening factor, aA, reflects amount of required “over-design”

  35. Fatigue Strength • Design modifications to improve endurance limit of joint • Increase preload • Reduce pitch of screw thread • Reduction of modulus of nut material elasticity • Increase thread engagement

  36. Fatigue Strength -Continued • Design modifications to improve endurance limit of joint • Change form of nut • Reduce strength of nut material • Increase elastic resilience of bolt, lower elastic resilience of parts • Shift introduction of load toward interface

  37. Embedding • Caused by flattening of surface irregularities • Affects forces in joint • Reduces elastic deformation and preload

  38. Self-Loosening and Prevention • Preload drops due to: • Relaxation as a result of embedment or creep • Rotational loosening due to relative movements between mating surfaces

  39. 6. Calculation Examples • Ex. 1, Concentric Clamping and Concentric Loading • Ex. 2, Transverse Shearing Force • Ex. 3, Torsional Shearing Load • Ex. 4, Eccentric Clamping and Eccentric Loading • Ex. 5, Eccentric Clamping and Loading

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