6.2Coefficient of Utilisation

ECSS-E-HB-32-23A, Space Engineering — Threaded Fasteners Handbook

Last Updated 29 Oct 2026

Hover any section to copy its link or text, ask Copilot about it, or bookmark it.

The size and strength class of fasteners is chosen taking into account the coefficient of utilizationγ . This is the maximum pretension stress divided by the yield stress of the fastener material,

\gamma = \frac{\sigma_{v,\max}}{\sigma_{y}} \tag{6.2.1}

where $\sigma_{v,max}$ is the maximum pretension stress, and $\sigma_{y}$ is the yield stress of the fastener material.

The value of γ can be varied in order to fine-tune the performance of the joint however it is important that the value typically lies in the range from 0,5 to 0,8. The recommended value to start the design process is 0,8.

Once the desired coefficient of utilization is selected, the related maximum preload Fv ,max, can then be calculated by,

F_{v,\max} = \gamma \sigma_{y} A_{S} \tag{6.2.2}

In practice, the preload achieved in service varies around this value due to effects such as friction, inaccuracies in applied torque measurement, embedding and thermal loads. The remainder of this section explains effects such as friction, inaccuracies in applied torque measurement, embedding and thermal loads on the preload and provides theory for quantifying the statistical scatter in the preload.