ADVANCED FLEXURAL PIVOT


Advanced Design Group
Philadelphia, Pa


ABSTRACT

Flexural pivots are devices that permit mechanical members to pivot about a common axis relative to each other through a limited angle. Inasmuch as this angular motion is effected through the flexing of elastic flexural elements rather than contact surface displacement, the flexural pivot operates without friction. Because they are generally too expensive to be considered for mass-market products the use of such pivots is confined to specialized high-technology products such as control linkages for aircraft and gimbals for scientific instruments. This is unfortunately because alternative pivots such as rolling-element or knife-edge bearings used in mass-market products constitute over-design or involve inherent bearing instabilities arising from oscillating motion.


BACKGROUND

Conventional flexural pivots comprise many complex elements involving complicated assembly techniques with tight tolerances; processes that do not lend themselves to automated production. The individual flexural elements of the pivots must be sequentially and precisely brazed to sleeve elements while retained by special fixtures. The high unit costs contribute to the low production volume of flexural pivots, and the complexity of the pivot precludes the high production volume required for a mass market.

Figure 1. Conventional Flexural Pivot

The end sleeves are complicated components requiring precision turning, milling and finishing operations. In contrast, the Advanced Flexural (AF) pivot disclosed herein eliminates these complex machining operations and substitutes simple cylindrical end housings and sheet metal stampings.


ADVANCED FLEXURAL PIVOT

Configuration

As illustrated in Figure 2 the AF pivot resembles the conventional flexural pivot.

Figure 2a. Cantilevered Flexural Pivot

However the AF pivot comprises two identical sleeves and two identical stamped sheet-metal flexure elements..

Figure 2b. Components of AF Pivot

 

The AF pivot is assembled from these four components as shown in Figure 2c.

Figure 2c. AF Pivot Assembly

The sheet-metal flexure elements combine both the flexure support, flexure beams and the beam supports in a single stamping. Two identical sleeves and stampings are required for each flexure pivot.

The flexure support must be sturdy enough to support the flexure beam without deforming. However the flexure beam must to thin enough to accommodate the required elastic flexure without permanent deformation. Hence the flexure beam must be significantly thinner than the its flexure support. Such changes in thickness are commonly achieved by electroforming as shown in Figure 3a.

Figure 3a. Electroforming Arrangement

Electroforming is a rapid process wherein the metal component is masked and then subject to electrolytic dissolution. The process is precise and most important for this applications the parts are free of machining marks that can originate fatigue cracking.

Prior to stamping the pivot elements are electroformed in production quantities as shown in Figure 3b.

Figure 3b. Production Electroforming

Assembly

The stampings are relatively simple and comprise two operations. After elecroforming the next step is stamping and deburring followed by configuring. The braze preforms are placed on the stampings as shown in Figure 3.

 

Figure 4. AF Pivot Stampings

Once clamped as shown schematically in Figure 4, the assembly is ready for brazing, resulting in a complete AF pivot.

 

Figure 4. Fixturing for Brazing

Of course using the same technique the double-ended pivot can be constructed.

Figure 5. Double-Ended AF Flexural Pivot

Metallurgical Treatment

Their are several viable heat treatment schedules. If the flexure elements comprise age hardened alloys these would be in sequence:

If the fusion temperature of the brazing alloy is correctly chosen then annealing and solution heat-treating can be done simultaneously with brazing. Probably vacuum heat-treating would be advantageous. The flexural elements can then be hardening to the proper strength.

Integrity

Fatigue fracture is the predominant mode of failure of oscillating structures such as flexural pivots. Failure of the flexure beams of the AF pivot would occur at the flexure beam of the stamping. However the smooth transition from the thicker support to the flexure beam would effectively eliminate this problem.


CONCLUSION

The advantage of the AF pivot is its simplicity and amenability to mass production. Four elements comprise the entire cantilever structure: two tubes and two stampings. Hence ease of manufacture should bring the cost of flexural pivots down to the point where unit prices would permit their utilization in hitherto unexpected applications. However the commercial problem remains: mass production requires a mass market, a mass market requires mass production. Product promotion is the usual solution, but one that requires a substantial investment.


Moishe Garfinkle PhD
Advanced Design Group
Philadelphia PA
(215) 235-5042


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