Annular Pocketed Flange


Bolted flanged interfaces are commonly used in spacecraft and other construction. The flanged joint provides assembly convenience, but as they are currently designed, flanges contribute flexibility to the structure. Increasing the stiffness of a flanged joint without increasing its weight is a worthwhile design objective. A typical spacecraft structural flange is shown below in Figure 1:

Figure 1: Titanium flanged interface for Astrolink receive antenna deployable subsystem.

The taped dots shown in Figure 1 are photogrammetry targets and will be removed before the installation of the mating aluminum hinge flange.

A drawing of a typical round flange (such as in Figure 1, above) is shown below in Figure 2:

Figure 2: Drawing of a typical round flange.

Figure 3, below, shows the tensile load path in a typical flanged joint:

Figure 3: Cross section of a typical flanged joint showing tensile load path.

As tensile load is applied to the joint, the flange deflects in bending and the cylindrical sections of the structure come apart, leaving the bolt to carry the load. It is the bending of the flange under tensile load that contributes flexibility to the assembly. When a flanged interface is put into bending (of the longitudinal axis), one side of the flange goes into tension and the other side goes into compression so the net result is an increase in linear flexibility of the bolted flanged assembly (over what it would be if there were no flange as in, for example, a welded assembly).

In space structures, stiffer is generally better. Where flexibility is desirable, as, for example, when tuning a structure to a specific vibrational frequency, it is fairly easy to add flexural elements. But when a structure is too flexible, making it stiffer without adding weight is usually difficult.

Key Features

My invention adds an annular pocket (or recess) to the flange to create a preload at the innermost edge of the flange. As long as the tensile load does not exceed this preload, the inner edges of the flange will not separate, and the load path will be through the shell of the structure, not through the flange, eliminating that flexibility contribution. A drawing of the annular pocketed flange is shown below in figure 4:

Figure 4: Drawing of the annular pocketed flange.

Figure 5, below, shows a cross section of the annular pocketed flange assembly with the tensile load path:

Figure 5: Cross section of an annular pocketed flanged joint showing tensile load path.

The resulting joint is significantly (on the order of a factor of two, depending on flange thickness) stiffer (less flexible) than typical flanges as currently designed. A finite element analysis (FEA) was performed to verify stiffness improvements. Another key feature of this invention is that only one side of the flanged joint needs to be pocketed to benefit from increased stiffness.

For applications that require shear pins to be used for flange alignment, radial lands between pairs of screws may be used without significant loss of stiffness.

Prior Art

As noted above, the conventional practice is to use a plain flange with its attendant flexibility contribution to the bolted structure.


The annular pocketed flange has application in a variety of spacecraft structures and anywhere increased stiffness without increased weight is desired, such as in aircraft, building construction, etc. Note that although a round section of flange has been shown above for illustrative purposes, any shape structure can benefit from this treatment. For example, Figure 6, below, shows the polygonal shaped flange from the Astrolink transmitt antenna deployable subsystem interface:

Figure 6: Polygonally shaped flanged interface for Astrolink transmit antenna deployable subsystem.

Email Richard dot J dot Wagner at gmail dot com

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Last updated December 25, 2011, by Rick Wagner. Copyright © 2001-2011, all rights reserved.