Calculate preload from torque, diameter, yield strength, friction. Review utilization, clamp force, and assembly checks. Plan reliable bolted joints for demanding structural work today.
| Nominal Diameter | Pitch | Proof Strength | Preload Target | Bolts | External Load | Preload per Bolt | Model Torque | Residual Clamp per Bolt |
|---|---|---|---|---|---|---|---|---|
| M20 | 2.5 mm | 600 MPa | 70% | 4 | 120 kN | 102.81 kN | 426.32 N·m | 80.31 kN |
This example uses common construction assumptions for a preloaded M20 joint. Replace every field with project data before using the result for procurement, method statements, or inspection limits.
1) Tensile stress area
As = (π / 4) × (d − 0.9382p)²
2) Proof load
Proof Load = As × Sp
3) Target preload
Preload = Proof Load × Preload Percentage
4) Torque by nut factor
T = K × F × d
5) Torque by friction model
Ttotal = Tthread + Tbearing
6) Service load check
Final Bolt Load = Preload + C × External Load per Bolt
Residual Clamp = Preload − (1 − C) × External Load per Bolt
The calculator uses metric units. Stress is in MPa. Force is in kN. Torque is in N·m. These equations are useful for quick design reviews, bolt tightening plans, and field verification checks.
Preloaded bolts help joints resist slip, vibration, and repeated service loading. Good preload improves stiffness and keeps plates in firm contact. Low preload can reduce clamp force and increase joint movement. Excess preload can overstress the fastener or damage threads.
Torque alone does not guarantee the same preload every time. Friction at the threads and under the nut or bolt head strongly changes the final clamp force. Lubrication, coating, washer condition, and surface finish all matter. That is why this calculator reports both a nut factor estimate and a friction model estimate.
Construction teams often need quick checks before installation starts. This page helps estimate preload force, clamp retention, torque demand, and service utilization. It also shows a simple separation check under external load. These outputs support method statements, inspection plans, and workshop reviews. Always compare the results with governing codes, manufacturer tables, and project specifications.
Bolt preload is the tension created in a bolt after tightening. That tension clamps connected parts together and helps the joint resist slip, opening, and vibration during service.
Most tightening torque is lost to friction. Small changes in lubrication, coating, or washer condition can produce large preload changes. That is why friction assumptions must be realistic.
Many preloaded structural joints target about 70 percent of proof load. Exact values depend on the bolt standard, installation method, washer details, and project requirements.
Joint constant is the share of external load added to the bolt. The remaining share reduces clamp force. Lower values mean the joint members carry more of the external load.
You can use it for quick checks on many standard bolts, but not as a substitute for product data. Always verify the stress area, grade, and tightening method.
One value uses the common nut factor equation. The other splits torque into thread and bearing components. Comparing both helps you review whether your friction assumptions are reasonable.
For a single bolt before service loading, preload is effectively the initial clamp force. Under service load, some clamp force is lost while bolt tension can increase.
Use the result as a planning aid. Final tightening procedures should follow the approved specification, calibrated tools, installation method, and inspection requirements for the project.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.