Free-Flex® Flexural Pivot Engineering Data

The Patented Flexural Pivot Concept

The FREE-FLEX® pivot is a frictionless, stiction-free bearing ideally suited for angular deflection (rotation) of up to 60°. It is a simply packaged, compact, easily installed bearing with high radial stiffness (up to 100,000 lb/in), no backlash, no metal-to-metal contact and predictable, repeatable performance. The pivot is ideal for precise positioning: adjustments in 0.00005-inch increments can be made with no lost motion. The pivot consists of flat springs crossed at 90° supporting cylindrical counter-rotating sleeves. Friction, stiction, fretting corrosion, lubrication, "space welding" and lubricant out-gassing in space vacuum are all avoided because there is no metal-to-metal contact. By matching pivot size and spring rate to the application, cyclic stress on the springs may be maintained at a low enough value to assure infinite cyclic life.

Major Performance Characteristics

• High radial stiffness and high axial stiffness
• Frictionless, stiction-free operation
• Not susceptible to false brinelling
• Low hysteresis and low centershift
• Exceptional repeatability and predictable performance
• Lubrication and maintenance not required
• Electrical continuity across the pivot
• Infinite cycle life (see life curves)
• Ambient temperature range of -400°F to +250°F

Additional Characteristics

• Radiation resistance
• Low thermal drift
• No rubbing surfaces
• Contaminant tolerant
• Operates in vacuum of space
• Self-centering

Configurations & Materials

The FREE-FLEX® pivot is available in two basic mounting configurations: cantilevered to support an overhung load, and double-ended to bridge-support a central load. Stock outside diameter sizes range from 0.125 in to 1.0 in, withstanding radial loads to 1,600 lb in both series. Standard sizes in several torsional spring rates are available off the shelf in both cantilevered (Series 5000) and double-ended (Series 6000) designs. The materials nominally used for standard pivots are AISI 410 or 420 corrosion-resistant steel for the bodies and AISI 420 for the flat spring flexures. Normal assembly is by brazing the springs to the body halves. For application-specific requirements such as elevated ambient temperature, the springs are electron beam welded to the body. Special spring materials include Inconel, beryllium copper and titanium. Covered by U.S. Patents: 3,807,029; 3,811,665; 3,813,089; 3,825,992; 4,327,527.

Glossary of Key Terms

• Axial (Thrust) Load — acts in the direction of the longitudinal axis of the pivot
• Center Shift — the difference between the diameter centers of the moved and fixed portions; a kinematic characteristic occurring as a function of angle deflected
• Cycle — angular motion from null to a particular angle, return to null, motion to the same angle in the opposite direction, and return to null
• Hysteresis — the total variation from null position when a torque produces a complete cycle, expressed in minutes or seconds
• Radial Rate — pounds of radial load to deflect the pivot a specific unit (lb/in)
• Torsional Rate (K) — torque to rotate the pivot per unit of rotation (lb·in/radian or lb·in/degree)
• Vc Load — a radial load producing the highest flexure stress in compression
• Vt Load — a radial load producing the highest flexure stress in tension

Centershift vs. Angular Deflection

Centershift is a predictable kinematic characteristic that occurs as a function of the angle deflected. To determine centershift at any particular angle, multiply the diameter of the pivot by the respective factor. For example, to determine the centershift of PN 5020-800 (0.6250 inch diameter) at 4 degrees, multiply 0.6250 × 0.001 = 0.000625 inch. Factors are tabulated from 1° through 20° of deflection at no-load conditions. This predictable behavior allows designers to compensate for centershift in precision mechanisms.

Hysteresis & Cyclic Life

Hysteresis curves are provided for each standard spring type (-400, -600, -800), plotting deflection against torque to show the minimal lost motion across a complete cycle. Cyclic life curves express load capacity as a percentage of rated Vc load versus angular rotation for 35,000 cycles, 220,000 cycles, and infinite life bands. By operating below the fatigue limit at the required angle and load, infinite cyclic life is achieved — demonstrated by a pivot cycled for over 12 billion cycles over twenty years.