The figure illustrates a device that flexes to allow rotation about a single axis through a total range of ±12°. This device was designed to offer the following advantages over commercial flexural pivots:

  • Greater ratios of radial to rotational and axial to rotational stiffness for a given load capability;
  • Higher load capabilities for a given rotational stiffness;
  • No shift in the center of rotation assuming flexures are uniform in thickness;
  • Theoretical unlimited fatigue life at ±10° excursion;
  • Monolithic construction for higher reliability and greater likelihood of attaining the theoretical fatigue life; and
  • No global buckling modes.

The device is called a "trefoil rotary flexure" because its flexible members are three radial, equally spaced thin plates that extend from an outer cylinder to the inner tri-lobed support. The distance from the inner terminus of the flexures to the rotational axis is made as small as possible to minimize rotational stiffness. The three lobes of the inner support are joined at the rotation axis to provide an extremely rigid attachment for the flexure elements, allowing high radial and axial stiffnesses. The tri-lobed support rotates relative to the outer cylinder on the flexures to create the flexural pivot motion. The total rotational range of ±12° is defined by hard stops in the lobes and the outer cylinder.

This Trefoil Rotary Flexure is a monolithic device, machined from a solid metal rod. It is designed for high radial and axial stiffness, low rotational stiffness, and long fatigue life.

The lobes, fins, and outer cylinder are integral parts of the monolithic device, which was fabricated by electrical-discharge machining of a solid metal rod. To reduce concentrations of stresses and thereby ensure long fatigue life, generous fillet radii were incorporated at the inner and outer ends of the fins.

This work was done by Robert J. Calvet of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at under the Mechanics category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Technology Reporting Office
Mail Stop 122-116
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-20228, volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).
Trefoil rotary flexure

(reference NPO20228) is currently available for download from the TSP library.

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