Mechanical key actuator including a cantilever beam restoring force means

[57] ABSTRACT

An actuating device having a keytop, base, and an actuator assembly affixed to the base, the assembly including a rod and a cantilever beam arrangement for providing a snap action response to key force and for driving the rod to form an electrical make-break contact, the base and assembly constituting a unitary flexible-tough thermoplastic body.

Claims, 6 Drawing Figures

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PATENTEDJUH bur

3.668,356

FIG. 1α FIG. 1b

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FIG. 2α FIG.2b FIG.2c

INVENTOR

DENNIS H. KEKAS

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ATTORNEY

3,668,356

MECHANICAL KEY ACTUATOR INCLUDING A CANTILEVER BEAM RESTORING FORCE MEANS

BACKGROUND OF THE INVENTION

This invention relates to a mechanical actuating device suitable for use with electric switches, and more particularly for use with an elastic diaphragm switch.

DESCRIPTION OF THE PRIOR ART

In the prior art, many examples of mechanical actuating devices for use with electric switches may be found. Typically the device comprises a keytop, a base or reference surface’ and an actuating assembly affixed to the base. The assembly itself frequently comprises a rod engaging the keytop. The rod is movable along a constrained path, responsive to manual pressure on the key. At or towards the end of its travel, the rod drives an electrical make-break contact. Repositioning of the rod after release of manual pressure or stroking is achieved by having the assembly include a spring bias or other form of restoring force working opposite the force driving the rod.

It has been found desirable to incorporate some form of tactile” feedback to the operator of the key to indicate that the key stroke has been completed. This is often accommodated by designing the restoring force means to exhibit a linear force versus rod travel characteristic which at some point suddenly decays. Illustrative examples may be the sudden buckling of a restoring spring in compression or the snap of a flexible and stressed diaphragm being turned inside out.

An elastic diaphragm switch consists of a pair of conductive surfaces spaced apart by an apertured insulator, electrical contact being made by bringing the two surfaces together through the aperture. At least one of the surfaces is moved to engage the other surface by the action of an actuating device.

Keyboards are fabricated from distributing numbers of the actuators usually in matrix array opposite one of the diaphragm surfaces and over corresponding apertures of the insulator. In order to minimize non-uniform flexing of the relevant diaphragm surface, the force and travel of the actuators must be uniform. Furthermore, it is desired to foster a uniform tactile feedback and not one which varies with flexure as is the case with spring biased key stems. Lastly, the number of sepiarate components adds to both cost and decrease of reliability.

SUMMARY OF THE INVENTION

The foregoing disadvantages of the prior art are overcome by forming the base and actuating assembly including the rod and restoring force means as a unitary flexible-tough thermoplastic body. The assembly includes a cantilever beam arrangement secured to the base and connected to the rod by a rigid hinging engagement. As the rod translates, the movement is converted through the hinging engagement into flexure of the beams. An acute angle between the beam and the axis of the hinging engagement to the rod is maintained. Rod is stopped as the angle approaches 90°. The composition of forces becomes substantially normal to the rod at this point such that a sudden decay or decrease in the restoring force is sensed. This constitutes the tactile feedback. ”

The rod is repositioned when manual pressure is removed. This is due to the force component operating on the rod through the hinges becoming more longitudinal as the beams return to their uhflexed position, and the angle becomes more acute.

The cantilever beam arrangement may preferably be embodied as a pair of spaced longitudinal members or as a substantially cylindrically shaped flexing member surrounding a portion of the. rod. In the first embodiment, rigid members may be hinged to the rod and corresponding longitudinal members by very thin hinging segments. Each hinging segment is formed from thinned cross sections of the thermoplastic in which the long chain polymers have an orientation transverse to the axis of the hinge. Similarly, the same hinging segments can also be fashioned for the flexible diaphragm securing the cylindrical shaped flexing member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. la exhibits a partially exploded perspective view of one embodiment of the actuating device including the keytop “d fo^e unitary body forming the base and actuator assembly.

FIGS, lb and 1c are respective perspective and bottom views of another embodiment of the actuating device.

FIGS. 2a, b, and c respectively show the actuating device in its unstroked restored position, its stroked distended position and in an elevation cut-away view. ’

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. la of the drawings, there is shown a partially exploded perspective view of one embodiment of the invention. Keytop 1 is illustrated as a separate moldable part used as a finger strokable surface. It is fashioned sufficiently large to satisfy the dimensional requirements normally imposed on manually operable keytops found for example on typewriters and adding machines.

Base 3 and actuating assembly 5 constitute a unitary body formed from any thermoplastic material or material exhibiting a thermoplastic-elastomeric response. The material should preferably be selected from at least one of the group consist25 ^lng °^f Polypropylene, nylon, delrin, polyphenylene oxide, and an injection moldable polyurethane. One materialfound especially significant was polypropylene 20 percent or less volume filled with antimony oxide. The materials aspect of this invention will be described subsequently in greater detail.

i The base 3 forms a rigid reference surface to which the actuation assembly is secured and the movement of the keytop and assembly parts referenced. In this regard, the base thickness dimension is selected to obtain the desired rigidity.

The actuator assembly 5 comprises an actuating rod 13 freely translatable through an aperture 9 in base 3. Longitudinal members 15 and 17 form a cantilever arrangement with rod 13 through hinging members 19 and 21. One end of the longitudinal members is moldably secured to the base The other end is moldably fastened to the hinging members. In turn, the hinging members are moldably fastened to the rod. Lastly, rod end 23 mechanically engages protrusion 11 of keytop 1 to complete the actuating device.

Referring now to FIG. 2a, there is shown the actuating device in its rest or non-finger stroked position. The keytop 1 is fixedly mounted on the rod end so that its bottom edge 22 extends a distance d above the base surface 25. Longitudinal members 15 and 17 form a cantilever beam arrangement secured to base 3 at one of their respective ends. Members 19 and 21 of length I hinge the beams to rod 13. They thereby define an acute angle Θ between member 19 and beam 15 and a member 21 and beam 17. The requirement of acuteness of angle 0 may be restated as Θ should be in the range defined by

Θ &lt; COS^-1 y- &lt; I — I

I 2

Referring now to FIG. 2b taken together with FIG. 2a, there is shown the actuating device in its fully actuated position. The actuating device is ordinarily used to distend, for example an elastic diaphragm switch 25, 27, 29. Such a switch has a pair of deformable conductive surfaces 25 and 29 spaced apart by an apertured insulating layer 27. ³

Keytop 1, in FIG. 2b is manually depressed such that rod 13 translates downward through distance d. This translation is converted into an equivalent bending moment about beams 15 and 17 through corresponding hinge members 19 and 21. The deflective loading and beam shaping and displacement of the beams for any constitutive material may be determined by reference to any standard treatise in strength of materials such as, Strength of Materials” by Timoshenko, Vol. 1 and 2, A Van Nostrand - Reinhold Co., 450 33rd St., N. Y. 10001, copyright 1955. The bending moment restores the rod 13 to the position shown in FIG. 2a upon the release of keytop 1

In order to form a unitary body of the base and actuator assembly, the material selected and its preparation should

3,668,356 receive careful consideration. It is evident referencing FIGS. 2a and b that the material must be rigid at base 3 and the base attached portions of beams 15 and 17 and rod 13. Hinge members 19 and 21 must be sufficiently rigid to transmit the bending force to displace beam ends 15 and 17. Notably, the beams must be susceptible of flexure. Concomitantly, material forming the actual hinging engagements 33 and 35 must also be substantially elastomeric. In order to fabricate both a unitary body and a reliable “hinge” it was found that a material formable by a molding process would be advantageous.

It was unexpectedly discovered that flexible-tough thermoplastics could be molded, i.e., injection molded into a unitary body and that flexible-tough thermoplastics also had long oriented polymer chains which constituted a good hinge.

Some thermoplastics such as plexiglas are rigid and brittle materials. Normally, such thermoplastics do not have the stretch or elastic recovery for hinge flexing purposes. However, polypropylene, nylon, delrin, polyphenylene oxide, and moldable polyurethanes exhibited the appropriate mechanical and fabrication characteristics. Accordingly, thermoplastics having the flexure-elastic properties of interest are denominated flexible-tough thermoplastics. In contrast, thermosetting materials except for hard rubbers do not exhibit the desired rigidity and furthermore do not have orientable · polymer chains for the desired hinge action.

Referring now to FIG. 2c, there is shown a cut-away or sectional view of the actuator in its undistended position. Keytop 1 is set forth as a molded piece separated from rod 13 and base 3. Protrusion 11 of the keytop constitutes a stem having a 30 keyway 37 into which key 23 of the rod may be snugly fitted. The keytop also has a hollowed out cavity 45 formed interior which generates a distinctive acoustic snap when the cavity is brought into sharp contact with base 3.

In FIG. 2c, the base, rod, and translating means such as rigid 35 members 19 and 21 and their hinging attachments 33 and 35 are formed from the same material. In this regard, it was found that an injection molding process could be used. The process comprised the steps of gating material into the mold positioned as in FIGS. 2a, b, and c along the outer base and flowing 40 the material down the beam sections 15 and 17, across the outer hinge 33 to inner hinge 35, and finally into rod 13. This manner of flowing the material under suitable pressure and temperature serves to align the polymer chains in a plane transverse to the hinge axes. This polymer chain orientation will produce a hinge capable of sustained flexing and reflexing. Note also, that lip 7 may be molded as part of base 3. Lip 7 is designed to engage apertured retaining plate 43. Any rigid plate can be used and against whose apertured sides lip 7 is flexed.

Referring now to FIG. lb, there is shown another embodiment of the invention. In this embodiment, a longitudinally slotted cylindrical shell 47 replaces beams 15 and 17 shown in FIG. la. One end of the cylindrical shell is moldably secured to base 3. The shell surrounds at least a portion of the rod extent. The shell is longitudinally slotted 49 to permit flexure of the other end.

In FIG. 1c, a bottom view of the assembly illustrated in FIG. lb is set forth. A rigid diaphragm 51 is hingedly attached 33 to the cylindrical end along the diaphragm outer periphery. It is likewise hingedly attached 35 to rod 13 towards the nose end 31. Flexure is achieved in the same manner as shown in FIGS. 2α, b, and c.

It is contemplated that the embodiments will be considered 65 for use only in a suitable temperature environment. That is, recognition must be accorded both the limits as well as the advantages of thermoplastic materials. Furthermore, some variation in material flexibility and toughness may occur as a function of the tacticity of the thermoplastic.

It should be noted that flexure and hinge performance varies inversely with hinge thickness. Relatedly, hinge thicknesses in the range between 0.010 to 0.001 are adequate.

This description of the present invention has been given as an example and it will be understood that various changes in 75 form and details may be made therein without departing from the spirit and scope of the invention.