Rugged Iris Mechanism

: Monday, 11 December 2006

Advantages include capability for full obscuration, low friction, and general adaptability of design.

A rugged iris mechanism has been designed to satisfy several special requirements, including a wide aperture in the “open” position, full obscuration in the “closed” position, ability to function in a cryogenic or other harsh environment, and minimization of friction through minimization of the number of components. An important element of the lowfriction aspect of the design is maximization of the flatness of, and provision of small gaps between, adjacent iris blades. The tolerances of the design can be very loose, accommodating thermal expansions and contractions associated with large temperature excursions. The design is generic in that it is adaptable to a wide range of aperture sizes and can be implemented in a variety of materials to suit the thermal, optical, and mechanical requirements of various applications.

The Iris Is Opened or Closed by turning the outer ring with respect to the inner ring.

The mechanism (see figure) includes an inner flat ring, an outer flat ring, and an even number of iris blades. The iris blades shown in front in the figure are denoted as “upper,” and the iris blades shown partly hidden behind the front ones are denoted as “lower.” Each iris blade is attached to the inner ring by a pivot assembly and to the outer ring by a roller/slider assembly. The upper and lower rings are co-centered and are kept in sliding contact. The iris is opened or closed by turning the outer ring around the center while holding the inner ring stationary. The mechanism is enclosed in a housing (not shown in the figure) that comprises an upper and a lower housing shell. The housing provides part of the sliding support for the outer ring and keeps the two rings aligned as described above. The aforementioned pivot assemblies at the inner ring also serve as spacers for the housing. The lower housing shell contains part of the lower sliding surface and features for mounting the overall mechanism and housing assembly. The upper housing shell contains part of the upper sliding surface.
This work was done by Nelson J. Ferragut of Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free online at www.techbriefs.com/tsp under the Mechanics category. GSC-14550

beer can experiment

Beercan Iris Experiment

There's nothing like a quick experiment to see what the constraints on the problem are. Beyold, the quick and dirty (actually quite smelly) beercan iris. I just cut the pieces by eye from an old beer can, so they're fairly thin aluminium sheet. I didn't optimize the shape in any way and, for this purpose, I didn't need to deal with a fancy closing mechanism - fingers work fine.
Conclusions:

It does close completely (I didn't think it would). Or as near as makes no difference.
Whatever you make the petals out of, it had better be flexible. The petals have a noticeable S-shaped bend when the iris is fully closed and the upper cover is on. If they were more rigid it would not close completely.
My iris would have a problem opening up again - the pivot pins (read "wire brads") stick up just enough that the petals catch on them as they open. Perhaps the petals should have been wider at the outside end, so they didn't clear the pivot pin of the next petal when fully closed?

Anyway, hope that helps you. Certainly helped me understand the problem a bit better.