Helix Coupling

FIG. 1 illustrates a helix coupling utilizing U-shaped units in accordance with the invention; and

FIG. 2 illustrates a couplng unit-forming jig.

The present invention is directed to a coupling for helical members, and is particularly adapted for use with helix memory alloy members utilized in lowtemperature heat engines. Such memory alloys may be Nitinol (nickel-titanium), CuAlNi, CuZn, CuAlZn, FePt, AuCd, or any other alloy that exhibits shape-memory properties. The coupling basically consists of a pair of U-shaped units, each unit having one longer leg. The legs of the U-shaped units are provided with end loops or openings through which an associated helix is threaded, with the "U" sections of the units being interlocked to couple the helix members together. To prevent unthreading of the units, the end of each helix is provided with a collar, or the like, secured such as by crimping.

Referring now to FIG. 1, the coupling embodiment illustrated consists of a pair of identical coupling halves, each half consisting of a U-shaped unit 10, only one unit fully shown. Each coupling half or unit 10 consists of a pair of legs 11 and 12 interconnected at a U-section 13, leg 11 being of greater length than leg 12. Legs 11 and 12 are provided with end loops or openings 14 and 15, respectively. A helix 16 is threaded through the end loops 14 and 15 of unit 10, and a collar 17 (constructed of stainless steel or nickel, for example) is crimped or otherwise secured on the end of helix 16 to prevent the helix from threading back through the end loops. As shown, the U-sections 13 of units 10 are interlocked, it being understood that a similar helix is secured in the same manner to the other of the pair of U-shaped units. Also, while not shown, a series of helixes may be connected by utilizing pairs of U-shaped units for coupling each adjacent pair of helixes.

By way of example, the U-shaped units, when utilized to couple helixes of memory alloys such as Nitinol (NiTi), may be formed of stainless steel, inconel, nickel, or similar corrosion-resistant wire. Also, some embodiments of the U-shaped units may be made of material that also is spot-weldable, as well as being corrosion resistant. For coupling Nitinol close-wound helixes of 0.105 inch diameter made from 0.020 inch diameter wire, the U-shaped units are formed from 0.010 inch diameter wire with the end loops or openings having diameters slightly larger than 0.020 inch diameter to allow some freedom of the helix when threaded therethrough. The length of the legs of the U-shaped units are determined by the distance between the individual turns of the helix to which it is to be coupled so that the helix is readily threaded through the end loops in the legs of the U-shaped units. This provides a twopoint contact between the helix and the coupling half or U-shaped unit while allowing some degree of rotational freedom of the helix. For example, in the abovedescribed embodiment, the legs 11 and 12 have a length of 0.142 inches and 0.106 inches, respectively, from the center of the U-section 13 to the end of the legs.

The U-shaped units are easily formed, as described with reference to the forming jig of FIG. 2 which consists of a jig plate 20 and three spaced pins 21, 22 and 23 secured to the jig plate. The jig pins, in this embodiment, are constructed of 0.020 inch diameter spring steel with the jig plate being constructed of aluminum. The spacing of pins 21, 22 and 23 is dependent upon the helix pitch and diameter, and the diameter of the pins depends on the diameter of the helix material. It is readily seen that an increase or decrease in the diameter of pins 21-23 will change the curvature of the "U" section and the diameter of the end openings or loops. After the U-shaped unit is formed by wrapping appropriate diameter wire 24 around the jig pins 21-23, as illustrated in FIG. 2, the thus formed unit is removed from the jig and the lap joints forming the leg end loops are spot welded, for example. The completed U-shaped units are interlocked at the "U" sections thereof and the leg and loops threaded onto an associated helix, whereafter the collar is secured to the free end of the helix, as by crimping. It is recognized that the leg and loops or opening can be formed by other methods, such as by flattening the wire and drilling holes therein. Also, the U-shaped units can be formed by stamping the unit from a sheet of corrosion-resistant metal and forming holes in the end of the legs.

The coupling described above, and exemplified by the embodiment illustrated in FIG. 1 for use with Nitinol helical elements, provides the following advantages:

1. Helix ends are allowed some freedom to rotate about the helix axis.

2. No rigid metallic attachments to the helix.

3. Helix ends maintained in alignment.

4. Helix coupling region can pass over a small radius roller or pulley.

5. Minimum number of helix turns inactive.

6. Strain extent in the coupling region during cycling no greater than elsewhere in the helix element.

7. Corrosion resistant, low cost, easily fabricated, and reusable.

8. Long life, greater than 2.4.times.10.sup.5 cycles.

9. Will not unscrew from helix.

The helix coupling of this invention, while having specific application for connecting helical Nitinol elements of low-temperature heat engines, can be used to join the ends of all types of helical members where flexibility and relative motion of the helix ends are important. In addition to the advantages of the coupling set forth above, it should also be noted that: (1) each coupling half is attached to the helix at two points, thereby providing axial loading; (2) coupling halves are formed from readily available wire rather than precision-drawn or specially designed material; and (3) the couplings are easily removed from the helix.

It has thus been shown that the present invention provides a coupling for helical members which overcomes the problems of the prior known coupling, particularly in the field of low-temperature heat engines utilizing series-connected memory alloy elements of helical configuration.

While a particular embodiment of the invention has been illustrated and described, modifications and changes will become apparent to those skilled in the art, and it is intended to cover in the appended claims all such modifications and changes as come within the spirit and scope of the invention.