Bolt Preload Loss Calculator

Estimate bolt preload reduction from embedment, relaxation and temperature effects using a simplified bolted-joint model.

Initial Bolt Preload

Initial Preload

Force Unit

Embedment Loss (%)

Relaxation Loss (%)

Enter the initial preload and the estimated percentage losses associated with embedment and relaxation.

Temperature Effect

Bolt Material

Clamped Material

Bolt CTE (µm/m·°C)

Clamped-Part CTE (µm/m·°C)

Effective Bolt Length

Length Unit

Effective Grip Length

Temperature Change (°C)

Bolt Stiffness (kN/mm)

Joint Stiffness (kN/mm)

The temperature model estimates differential thermal expansion and converts it into a preload change using the entered bolt and joint stiffness values.

Ready to calculate. Press Calculate Preload Loss.

Preload Loss Estimate

Estimated Remaining Preload
—

Initial

—

preload

Embedment Loss

—

force

Relaxation Loss

—

force

Thermal Change

—

force

Total Change

—

force

Preload Loss Breakdown

Mechanical Losses

Initial preload
—

Embedment percentage
—

Embedment loss
—

Relaxation percentage
—

Relaxation loss
—

Temperature Effect

Bolt thermal expansion
—

Joint thermal expansion
—

Differential expansion
—

Thermal preload change
—

Remaining preload
—

Temperature Calculation

The simplified thermal model compares the free thermal expansion of the bolt with the free thermal expansion of the effective clamped length.

Bolt Expansion = α_b × L_b × ΔT

Joint Expansion = α_j × L_j × ΔT

The differential expansion is then used to estimate a joint preload change based on the entered stiffness values.

ΔF = (k_b × k_j ÷ (k_b + k_j)) × ΔL

Here α represents the coefficient of thermal expansion, L represents effective length, ΔT is temperature change and k represents stiffness.

Embedment and Relaxation

Embedment and relaxation are represented as user-defined percentage reductions in this calculator.

Embedment Loss = Initial Preload × Embedment %

Relaxation Loss = Initial Preload × Relaxation %

The simplified calculation applies these losses sequentially before the thermal effect is added.

Preload After Mechanical Losses = Initial Preload × (1 − Embedment %) × (1 − Relaxation %)

Thermal Preload Direction

A positive differential expansion means the clamped parts expand more than the bolt for a positive temperature increase. In a simplified joint model, this tends to increase bolt tension. If the bolt expands more than the clamped length, the calculated thermal effect reduces bolt tension.

ΔL = Joint Expansion − Bolt Expansion

The calculator therefore reports the thermal effect as either a preload increase or a preload reduction rather than automatically treating every temperature change as a loss.

Example Interpretation

If a joint starts with 50 kN preload and the selected embedment and relaxation percentages reduce that preload, the temperature calculation is then applied to the mechanically reduced preload. A positive thermal preload change can partially offset those losses, while a negative thermal change can increase the total preload reduction.

The numerical result is only as reliable as the stiffness, effective lengths, material properties and loss assumptions entered by the user.

Important Limitations

This is a preliminary preload-loss estimation tool.
Embedment and relaxation percentages are user-defined assumptions.
Actual preload loss depends on joint geometry, surface finish, material behavior, temperature, time and loading history.
The thermal model uses simplified linear elastic stiffness assumptions.
It does not calculate detailed bolt or joint stiffness from joint geometry.
It does not model gasket behavior.
It does not model creep or relaxation as a time-dependent material model.
It does not account for external service loads.
It does not calculate fatigue or joint separation.
It does not determine required installation preload.
It does not replace a bolted-joint engineering analysis.

Engineering warning: Use this result for preliminary estimation only. Critical bolted joints should be evaluated using the applicable design standard, actual joint geometry, material properties, installation procedure, operating loads and qualified engineering analysis.

Frequently Asked Questions

What does the Bolt Preload Loss Calculator calculate?
It estimates remaining preload after applying user-defined embedment and relaxation losses plus a simplified temperature-induced preload change.

What is embedment loss?
Embedment loss is associated with local settling or flattening of contacting surfaces, threads and bearing interfaces after preload is applied.

What is relaxation?
Relaxation is a reduction in bolt tension over time caused by material or interface behavior. The calculator represents it using a user-entered percentage.

Can temperature increase preload?
Yes. If the clamped parts have greater thermal expansion than the bolt, the simplified model can produce a positive thermal preload change. The opposite material relationship can reduce preload.

What stiffness values should be used?
Use appropriate effective bolt and joint stiffness values from the applicable bolted-joint analysis. The calculator does not derive those values automatically.

Is this suitable for final bolt design?
No. It is a preliminary estimating tool and should not replace a detailed engineering analysis for safety-critical or highly loaded joints.

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