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Seek and Geek 3: Flexbar

While hunting for design inspiration, I came across the Flexbar, a reconfigurable positioning mechanism. The flexbar universal holder is pictured below.

The flexbar mechanism allows a user to tune the stiffness of system using friction between sequential ball and socket joints. The friction force is controlled by a cable running down the center line of the structure. When the cable is pulled tight by a cam (red handle), the normal force between ball and socket members increases and the device locks into a single rigid position.

The cam engagement mechanism is demonstrated in the video below.

In order to provide variable levels of stiffness and also account for manufacturing defects in either the ball-socket elements or the cable dimensions, there is a screw at the top of the mechanism that allows the effective length of the cable to be modified by the user. Unscrewing the mechanism stretches the cable length and as a result creates greater stiffness when the cam is turned.

The friction acting on each ball joint in a linear configuration is governed by the equations below.

The holding friction force, or F Link, decreases in angled configurations. The angled holding force is approximately proportional to the cosine of the ball joint angle.