Bolted-joint preload
Why proof load and target preload are starting points, not a substitute for joint-specific tightening control.
ENGINEERING PRINCIPLE
Preload clamps the joint before service loading. The tensile stress area and proof strength establish an upper material-based reference load.
Fproof = AsSp; Fpreload = target × FproofVariables and units
- As = tensile stress area (mm²)
- Sp = proof strength (MPa)
- target = chosen fraction of proof load
Worked example
For As = 84.3 mm², Sp = 600 MPa and target = 70%, Fproof = 50.58 kN and target preload = 35.41 kN.
Calculation method
- Use the tensile stress area for the actual thread, not the shank area.
- Use a source-controlled proof-strength class.
- Select preload with the joint, tightening method and service load in view.
Practical applications
- Machine joints
- Flanged connections
- Preloaded covers and brackets
Review boundary and reference
- Torque does not uniquely determine preload because friction varies.
- Joint stiffness, embedment, fatigue and tightening process need a dedicated analysis.
Formula and displayed units independently checked for the stated conditions. This is not a code-compliance design method.
Source: VDI 2230 bolted-joint principles and J. E. Shigley et al., Mechanical Engineering Design.