CORE PRINCIPLES
What to establish first
- Proof load defines an important elastic loading reference.
- Preload must be matched to joint, fastener grade and tightening method.
- Torque is an indirect preload control because friction is variable.
Bolts & Fasteners
Preload, joint strength, thread engagement and bolted-joint design.
MODULE OVERVIEW
Bolts & Fasteners in practical engineering work.
Bolted joints clamp parts together; a bolt's tension is not simply the external load. Joint stiffness, preload method, friction, embedding and fatigue determine whether the joint remains secure.
DESIGN CONCEPTS
Terms worth checking
- Tensile stress area
- Proof strength
- Joint stiffness
Common applications: Flanged joints · Machine covers · Structural and vibration-resistant assemblies
KEY RELATIONSHIP
Proof load and target preload
Fproof = AsSp; Fpreload = Fproof × target fraction
- As — tensile stress area, mm²
- Sp — proof strength, MPa
- F — axial force, N
The result is a preliminary preload target; it does not prescribe an installation torque.
WORKED EXAMPLE
Known values → substitution → interpretation
- Known values
- As = 84.3 mm²; Sp = 600 MPa; target = 70%
- Method
- Fproof = 84.3 × 600; multiply by 0.70
- Interpretation
- The proof load is 50.6 kN and the preliminary preload target is 35.4 kN.
KEY DESIGN CHECKS
Confirm the conditions behind the number
- Define the governing load case, duty cycle and required design life before using a simplified relation.
- Use compatible units and material data for the actual condition, temperature and manufacturing state.
- Compare the result with strength, stiffness, fatigue, safety, serviceability and applicable-code requirements.
COMMON ENGINEERING MISTAKES
Keep the model within its scope
- Applying proof load and target preload outside the assumptions shown on this page.
- Using a nominal condition while a peak, alternating, transient or environmental case governs the design.
- Treating a calculated value as a final component selection without checking interfaces, tolerances and the current governing standard.
AVAILABLE CALCULATORS
Select a calculator
RELATED RESOURCE
Open Bolt Preload Calculator
Review proof-load and target-preload assumptions before selecting a tightening method.
Open related resource →REFERENCES & LIMITATIONS
Use this content as a transparent starting point
ReferenceVDI 2230, Systematic calculation of high duty bolted joints; consult the current licensed guidance for design.ScopeRelationships are presented for the assumptions stated on this page and linked tool.Design reviewCheck material condition, loads, environment and the applicable current code.SafetyResults are educational/preliminary and do not certify safety or compliance.
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