FREE ENGINEERING TOOLS · FORMULAS & REFERENCES
Bolt Joint Preload Loss & Clamp Force Calculator
Estimate preload loss, load sharing, separation threshold and approximate torque from bolt and joint stiffness, embedment and axial load.
Enter parameters
Calculated results
Parameters changed. Recalculate to update results.
| Bolt load fraction φ | 0.25 |
|---|---|
| Theoretical force loss from embedment | 1.5 kN |
| Approximate torque for initial preload | 16 N·m |
| Residual clamp after embedment | 8.5 kN |
| Theoretical separation load Psep | 11.333333 kN |
| Clamp force under load | 4.75 kN |
| Bolt tension under load | 9.75 kN |
Single-bolt linear elastic estimate under concentric separating load with full load introduction; not measured preload or a joint safety certification.
Model and formulas
φ = kᵦ/(kᵦ+kₘ); ΔF = (kᵦkₘ/(kᵦ+kₘ)) · δ; Fᵣ = F₀ − ΔF; Psep = Fᵣ/(1−φ); Fc = Fᵣ − (1−φ)P; Fb = Fᵣ + φP; T ≈ K·F₀·d.
Convert μm to mm for the loss; in the torque equation use F₀ in N and d in m. Load sharing applies only while the joint remains clamped.
How should results be interpreted?
φ is the fraction of the applied load added to bolt tension; the remainder reduces clamp force. It is not the ratio of maximum to minimum preload. Embedment loss uses the equivalent series stiffness of the two springs, assuming embedment occurs before external loading. The separation threshold is where clamp force reaches zero.
Inputs and limitations
Obtain stiffness and embedment from measurement, testing or a suitable joint model. Plasticity, creep and changing contact in busbars, PCB, washers or coatings can alter stiffness and loss over time, temperature and loading. This tool does not predict actual time-dependent relaxation, thermal effects, fatigue, slip, yielding, strength or load distribution among multiple bolts. Actual load introduction points can change the load fraction; eccentricity, bending, shear and prying are excluded.
Torque and preload
T ≈ K F₀ d is a rough empirical estimate using the entered nut factor K; friction scatter, thread details and tool error are not modeled. Friction and lubrication strongly affect preload. Verify critical joints to the applicable design standard and with assembly testing or preload measurement.
Reference
NASA-STD-5020B: Requirements for Threaded Fastening Systems in Spaceflight Hardware