7.8Joint Separation

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

Last Updated 29 Oct 2026

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7.8.1 Introduction

Under an externally applied tensile load it is generally required that no gapping occurs at any interstice between clamped parts in a joint (i.e. the clamping force does not reach zero). Many joints have additional requirements for a positive clamping force for sealing and other operational functions. Thus, the failure mode of joint separation is defined to occur when the clamping force is reduced to its critical level, FK,req (which is greater than or equal to zero).

The joint diagram at the critical condition for separation failure is depicted in Figure 7-25. It can be seen that, under the action of FA , the compression in the clamped parts is reduced from the initial preload of FV,min to the minimum specified level of FK,req . Any increase in FA results in insufficient compression in the clamped parts, thereby constituting failure by separation.

Influence of the load introduction factor n on bolt and clamping force components Force-Deformation

Figure description:

Influence of the load introduction factor n on bolt and clamping force components

Force-Deformation Diagram of a Bolted Joint

Legend

n = 1 — color: black; symbol: Dashed line
n < 1 — color: black; symbol: Dashed line
Stiffness lines — color: black; symbol: Solid line

Annotations

$F_{V,min}$ (vertical_line - position: Vertical span on the left axis representing initial preload)
$F_A$ (vertical_line - position: Vertical span representing total external axial force)
$\Delta F_{b,A}$ (vertical_line - position: Top segment of $F_A$ corresponding to bolt force increase)
$\Delta F_{c,A}$ (vertical_line - position: Bottom segment of $F_A$ corresponding to reduction in compression)
$F_{K,req}$ (vertical_line - position: Remaining force at the lower clamping limit)

Variable	Description
$F_{V,min}$	Minimum preload force
$F_A$	External axial force applied to the joint
$\Delta F_{b,A}$	Increase in bolt force due to the external load $F_A$
$\Delta F_{c,A}$	Decrease in clamping (compression force due to the external load $F_A$ )
$F_{K,req}$	Required clamping force remaining in the joint
$n$	Load introduction factor, where $n=1$ is application at the ends and $n<1$ is application inside the parts

Notes: The diagram illustrates the mechanical relationship between preload, external loading, and the resulting forces in the bolt and clamped parts. It specifically highlights how the load introduction factor 'n' modifies the slope of the force distribution lines.

Figure 7-25 - Joint Diagram at Onset of Separation Failure

7.8.2 The Margin of Safety for Joint Separation

The MoS for joint separation is not intended to cover the case of slipping of a friction grip joint. The appropriate MoS for slipping is defined in Section 9.2.1.

The Margin of Safety for joint separation is given by,

MoS_{sep} = \frac{F_{v,\min} - F_{K,req}}{(1 - \Phi_{n}) F_{A} sf_{sep}} - 1 \tag*{[7.8.1]}

where sfsep is the safety factor defined for separation as defined in Table 5-4,

and Φn is the force ratio defined in equation [7.4.6].