Key Assembly Diaphragm Switch Actuator With Overtravel And Feel Mechanisms

Abstract

A key assembly for actuating a diaphragm switch, and including a housing, a keystem movably mounted on the housing, and a switch actuator coupled to the keystem through a lost-motion connection arranged so a relatively large stroke of the keystem is required to effect a small actuator motion needed to operate the switch. A coil spring biases the keystem away from the housing to an extended position in which the switch is not actuated, and a second coil spring urges the actuator away from the keystem. The lost-motion connection between the keystem and actuator provides positive retraction of the actuator during the return stroke of the keystem. A leaf spring on the keystem rides a cam surface or ramp on the housing, and the ramp is shaped to modify the effect of the coil springs such that the keystem tends to toggle past the point of switch actuation to prevent switch jitter and to provide a tactile signal that actuation has occurred.

Description

BACKGROUND OF THE INVENTION

Key assemblies are used in manual actuation of electrical switches or other devices in a variety of communications and data-entry applications. Many key assemblies are normally grouped together to form a keyboard for printing or entering alphanumeric characters and other symbols. Familiar examples of keyboards are found in typewriters, teletypes, calculators, and computer data-entry terminals.

The switch or other device operated by a key assembly typically has a short actuation stroke, but a relatively long finger stroke is normally desired for the key to prevent inadvertent actuation when the operator's fingers rest on the keys. One function of a key assembly is thus to provide a lost motion or overtravel connection between the finger and switch. The key also identifies the character or symbol associated with the switch. These functions are performed by a variety of known key assemblies, but these prior-art devices are typically deficient in providing reliability and a satisfactory tactile "feel," in preventing switch jitter, and in permitting simple manufacturing and assembly procedures.

Conventional key assemblies use one or more return springs which urge the key into an extended or non-actuated position. The restoring force of these springs and the device being actuated, and any frictional drag on the key and device, constitute the force which the operator must overbalance to actuate the key. This force gives the key a characteristic "feel," and the key assembly must be designed to provide an easy and comfortable action fo the operator, while still maintaining a crisp return action when the operator's finger is removed.

The key assembly of this invention uses a combination of spring elements to meet these requirements, and also to provide two additional operating features. The springs are selected such that the key restoring force has a discontinuity at the point of device actuation during the downward key stroke. This discontinuity is sensed by the operator through the fingers, and provides unconscious assurance that the key stroke or displacement has been adequate for actuation. The discontinuity also tends to urge the key away from the exact point of actuation to prevent hovering at this stroke position which could cause switch jitter and unintended multiple actuation. The key assembly is further designed for quick and simple assembly, and is made from relatively inexpensive molded plastic parts.

Another important feature of the invention is a coupling arrangement between keystem and actuator which insures positive retraction or lifting of the actuator during the return stroke of the keystem. The actuator is drawn into a predetermined retracted position where no force is exerted on the device to be actuated, thereby substantially eliminating the risk of undesired actuation due to vibration or shock. Binding or irregular motion due to side loads applied during actuation is minimized by providing relatively long bearing or guide surfaces which support the keystem and actuator.

SUMMARY OF THE INVENTION

The key assembly of this invention includes a housing adapted for mounting on or adjacent to a device to be actuated such as a diaphragm switch. A keystem is slidably mounted on the housing to be movable through a stroke between a normally extended position and a depressed position. A resilient means such as a coil spring is positioned between the housing and keystem to urge the keystem toward the extended positon. An actuator is mounted on the housing to be movable between a retracted position and an actuating position in which the actuator contacts the device to effect actuation such as a switch closure.

A coupling means forms a lost-motion connection between the keystem and actuator, permitting a relatively large keystem stroke to produce a smaller movement of the actuator. The coupling means provides a positive connection of the keystem and actuator as the keystem approaches the normally extended position to insure that the actuator is retracted away from the device after actuation. The keystem and actuator are preferably non-rotatively mounted on the housing, and a second resilient means such as a coil spring is disposed between the keystem and actuator.

In one form, the keystem includes a means cooperating with the housing for providing a discontinuity in a force-versus-displacement characteristic of the key assembly during the actuation stroke. The second resilient means is selected to match the characteristics of the device being actuated such that device actuation is effected at a stroke position where the discontinuity occurs. A cap is fitted on the keystem, and is made of a soft material which forms detent sockets receiving projections on the keystem.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side sectional elevation of a key assembly according to the invention, the assembly being mounted on a diaphragm switch;

FIG. 1A is a sectional elevation of a diaphragm switch;

FIG. 2 is a sectional elevation of a housing used in the key assembly;

FIG. 3 is a top view of the housing on line 3--3 of FIG. 2;

FIG. 4 is a bottom view of the housing oriented as shown in FIG. 1;

FIG. 5 is a side elevation, partly broken away, of a keystem;

FIG. 6 is a front elevation of the keystem, partly broken away, on line 6--6 of FIG. 5;

FIG. 7 is a top view of the keystem on line 7--7 of FIG. 5;

FIG. 8 is a bottom view of the keystem on line 8--8 of FIG. 5;

FIG. 9 is an elevation of an actuator;

FIG. 10 is a sectional view of the actuator on line 10--10 of FIG. 9;

FIG. 11 is a top view of the actuator on line 11--11 of FIG. 9;

FIG. 12 is a bottom view of a cap on line 12--12 of FIG. 1; and

FIg. 13 is a graph showing a force-versus-displacement characteristic of the key assembly during an actuation stroke.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A key assembly 10 according to the invention is shown in FIG. 1 mounted on an upper chassis plate 11 having a circular opening 12 therethrough. A diaphragm switch assembly 13 is positioned beneath key assembly 10, and the switch assembly is sandwiched between upper chassis plate 11 and a lower chassis plate 14. A clamping plate 15 holds the key assembly against the upper chassis plate. The upper and lower chassis plates, clamping plate and switch assembly are secured together as a sandwich by screws (not shown) or any other convenient fastening means.

A number of different styles of diaphragm switches (see, for example, U.S. Pat. No. 3,594,684) are commercially available, and a typical style is shown in FIG. 1A. This switch includes a rigid baseplate such as a circuit board 18 having a metalized portion on its upper surface forming a first electrical contact 19. A second electrical contact 20 is formed on the undersurface of a thin flexible plastic sheet 21 disposed above circuit board 18. An insulator sheet 22 (shown exaggerated in thickness) having an opening 23 therethrough is sandwiched between the circuit board and insulating sheet with opening 23 in alignment with the first and second contacts. A flexible protective cover sheet 24 rests on the upper surface of sheet 21 to complete the assembly.

When pressure is applied to cover sheet 24 as indicated by arrow 25, sheets 21 and 24 deflect resiliently to move second contact 20 downwardly to make an electrical connection with first contact 19. When the pressure is removed, the elasticity of sheets 21 and 24 restores the switch to the position shown in FIG. 1A to separate the contacts and open the switch. Other styles of diaphragm switches and similar devices are useful with key assembly 10, and the switch shown in FIG. 1A is simply an example of one suitable version.

Key assembly 10 includes an integrally molded keystem guide or housing 30 (FIGS. 1-4). The housing has a circular base 31, and a circular recess 32 is formed in the undersurface of the base. Recess 32 provides a clearance space above the upper surface of switch assembly 13 (FIG. 1) to avoid stresses which could occur in cover sheet 24 if the entire undersurface of the base rested on the cover sheet. A clamping flange 33 extends radially from the top of base 31, and one corner 34 of the flange is beveled to key the housing to clamping plate 15 which has a mating shape to index the key assembly in a desired position.

An intermediate portion 36 of the housing extends upwardly from the top of the clamping flange, and tapers to an inwardly extending shoulder 37. An upper portion 38 of the housing extends upwardly from intermediate portion 36 above shoulder 37. The upper portion is shaped as a truncated cone with flattened sides 39 (FIG. 3).

The interior of the housing is hollow, and defines a lower chamber 41 of rectangular cross section, and a smaller upper chamber 42 also of rectangular cross section. Two opposed sidewalls 43 extend along the length of upper chamber 42, and two further sidewalls 44 extend through both the upper and lower chambers. A pair of spaced-apart ribs 45 extend inwardly from each of sidewalls 44 in lower chamber 41 to form a smooth extension of sidewalls 43 into the lower chamber. Each rib terminates short of the bottom of housing 30 to form a shoulder or stop 46.

A channel 48 is formed in each sidewall 44, and extends the length of the housing from the top of upper portion 38 to the lower end of sidewall 44 at the top of circular recess 32. The upper part of each channel 48 is slightly tapered as best seen in FIG. 2. One of channels 48 is interrupted between its ends by a detent or projection 49 (FIGS. 1 and 2) which extends outwardly in the channel toward the center of the hollow interior of the housing to a tip 50 which is flush with the plane of sidewall 44. The bottom surface of this channel curves smoothly inwardly toward tip 50 both above and below the tip as shown in FIG. 1 to form a cam surface or ramp.

An integrally molded keystem 53 (FIGS. 5-8) has a circular flange 54 with an inwardly stepped undersurface forming a spring-retaining shoulder 55. Extending upwardly from the top of flange 54 is a cruciform post 56 having sidewalls 57. A button or projection 58 is formed on each face of each of the sidewalls.

A stem 59 of generally rectangular cross section extends downwardly from circular flange 54 of the keystem. A channel 60 is formed along the length of each of the opposed narrower side faces of stem 59, and a shallower channel 61 is formed along the length of the opposed wider faces of the stem. These channels form outwardly extending guide ribs 62 at each of the four corners of the stem.

A spring-retaining post or lug 63 extends downwardly from the bottom of stem 59. An actuator guide channel 64 extends centrally along the lower portion of each of the wider side surfaces of stem 59. The bottom of each channel 64 defines a horizontal shelf or shoulder 65 extending perpendicularly to the longitudinal axis of the stem. A leaf spring 66 is integrally formed with the upper part of the stem, and extends downwardly within one of channels 60 to terminate in a laterally extending button or follower 67. The follower normally projects beyond the plane of the ajdacent side face of stem 59, but associated channel 60 is sufficiently deep that leaf spring 66 may be deflected inwardly until the tip of follower 67 is flush with the side of the stem. A notch 68 in flange 54 above spring 66 provides clearance for mold tooling during manufacture of the stem.

An integrally molded actuator 72 (FIGS. 9-11) has a base 73 with a pair of retainer arms 74 extending upwardly therefrom. An inwardly extending projection 75 is formed at the upper end of each retainer arm. A pair of spring-retaining lugs 76 are formed on the upper surface of base 73 between arms 74. A pair of sidewalls 77 extend upwardly from opposite ends of base 73, and the outer faces of the sidewalls form guiding and bearing surfaces for the actuator when it is installed in the housing as explained below. A downwardly extending dimple or button 78 is formed on the undersurface of base 73.

A cap 82 for key assembly 10 is shown in FIGS. 1 and 12, and includes an inner core 83 and an outer jacket 84. The cap is made by a known "double-shot" molding process in which the inner core is first formed with an alphanumeric character 85 (only a portion of which is shown in cross section in FIG. 1) projecting upwardly from the upper surface thereof. The outer jacket is formed around the core in the second stage of the molding process, and the material forming the jacket flows around the alphanumeric character so the cap has a smooth upper surface. A central boss 86 is integrally formed within core 83, and is supported by stiffening ribs 87. A cruciform socket 88 is formed in boss 86, and the socket is dimensioned to make a snug press fit over post 56 of keystem 53.

All the components described thus far are preferably injection molded from commercially available plastic materials. ABS plastics are suitable for housing 30 and cap 82. Actuator 72 is preferably formed from a polysulfone plastic. Stem 59 is molded from a plastic which is harder than the material forming cap 82, and an acetal resin such as sold under the trademark "Delrin" is satisfactory.

To assemble key assembly 10, a main return spring 90 (FIG. 1) is first slipped over stem 59 of keystem 53 to seat against shoulder 55. Keystem 53 is then fitted into housing 30 with stem 59 extending downwardly into the hollow interior of the housing as shown in FIG. 1. The stem is oriented so follower 67 of leaf spring 66 on the keystem fits into that channel 48 which includes projection 49. The bottom of return spring 90 seats against shoulder 37 (not visible in FIG. 1, but shown in FIGS. 2 and 3) of the housing, and the spring acts to urge the keystem upwardly out of the housing.

An actuator overtravel spring 91 (FIGS. 1 and 6) is fitted between retainer arms 74 of actuator 72. The actuator spring seats around lugs 76 (which restrain any lateral movement of the spring) against the upper surface of base 73. The upper end of the spring temporarily seats against the undersurfaces of projections 75 at the top of the retainer arms.

With keystem 53 held fully depressed in housing 30 against the restoring force of return spring 90, actuator 72 and spring 91 are next inserted through the undersurface of the housing into lower chamber 41. As the actuator is pressed upwardly into the housing, the chamfered upper surfaces of projections 75 on the actuator retainer arms contact the bottom of stem 59, and the actuator material is sufficiently resilient that the retainer arms bend outwardly away from each other to slide upwardly over the lower end of the stem. As this upward motion is continued, projections 75 on the retainer arms slide home into actuator guide channels 64. The resilient retainer arms then snap inwardly to their normal position to seat projections 75 within channels 64, preventing withdrawal of the actuator from the housing.

Actuator spring 91 seats on the undersurface of stem 59 and fits around retaining lug 63 which restrains lateral movement of the upper end of the spring. The actuator spring is a compression spring which urges the actuator downwardly away from the stem toward a position in which projections 75 on the retainer arms seat against shoulders 65 in guide channels 64.

An alternative assembly procedure is to install overtravel spring in actuator 72 as described above, and then to insert the actuator into housing 30. The housing is next placed on a flat surface to hold the actuator in place, or a retaining tool or fixture may be used for this purpose. Return spring 90 is then installed over the top of the housing, and stem 59 of keystem 53 is fitted into the housing and forced into captive engagement with the actuator.

The key assembly is completed by forcing post 56 of keystem 53 into socket 88 of cap 82. The cap and keystem are forced together until the undersurface of boss 86 of the cap fits against the top of flange 54 on the keystem as shown in FIG. 1. There are no clearance channels in socket 88 of the cap for projections 58 on post 56 of the keystem, and these parts accordingly must be forced together.

The plastic material forming the cap is relatively soft compared to the keystem material, and the cap material will cold flow in a relatively short time to form permanent sockets which receive projections 58 on the keystem post. The sockets or depressions remain even if the cap is temporarily removed from the keystem, and they form with the post projections a series of detents which retain the cap on the keystem. The cap detent sockets are thus formed automatically during assembly of the components by virtue of the selection of a relatively softer material for the cap, and a significant tooling complication of molding the sockets during cap fabrication is thus eliminated.

An important feature of the invention is the speed and simplicity with which the several components can be assembled. Both springs used in the assembly are cylindrical rather than conical, and thus cannot be installed upsidedown. Conical springs also have the disadvantage of tending to tangle with each other when stored in bulk, and this problem is substantially reduced with cylindrical springs. When the actuator and keystem axe pressed together to seat projection 75 in channel 64, the keystem and actuator capture each other to prevent disassembly of the components, and no snap rings or other separate retainers are needed to complete the assembly.

The actuator serves as a retainer which limits upward travel of the keystem away from the housing. The upper surface of base 73 of the actuator abuts stops 46 (FIG. 1) on the undersurface of ribs 45 of the housing to limit upward motion of the actuator within the housing. Projections 75 of the actuator retainer arms in turn seat against shoulders 65 of the actuator guide channels 64, and thus limit upward travel of the keystem with respect to the housing. The components of the key assembly are shown in this fully extended position in FIG. 1, with main return spring 90 urging the keystem to its uppermost position with projections 75 seated against shoulders 65, and the top of the actuator base seated against stops 46. In an alternative arrangement, the housing and actuator can be dimensioned such that the "roof" surfaces (marked 41 in FIG. 4) of lower chamber 41 serve as shoulders or stops which abut the tops of retainer arms 74 when the actuator is fully retracted.

Keystem 53 makes a slip fit within the housing, and guide ribs 62 on stem 59 ride smoothly on the sidewalls of upper chamber 42, and on the sidewall extensions formed by ribs 45. Close lateral guidance is thus provided for the keystem over its full range of travel, and side loads on the cap do not result in any misalignment or binding of the key action. Similarly, actuator 72 makes a slip fit in lower chamber 41 with base 73 and sidewalls 77 sliding against and being guided by sidewalls 43 and 44 of the housing.

The keystem and actuator are thus closely guided over their full ranges of travel by the inner surfaces of the housing, and binding or irregular motion of these components due to cap side loads is avoided. The rectangular configuration of the actuator also prevents rotation of this part within the housing. This is desirable from the standpoint of quality control as any burrs or defects in the molded parts which might cause binding will be revealed immediately by operational testing after assembly. If rotation was permitted, a burr might be in a non-binding position during inspection, and not cause faulty operation until the actuator eventually rotated to a position in which binding occurred.

The coupling of keystem 53 and actuator 72 arising from the engagement of retainer-arm projections 75 in guide channels 64 is a lost-motion connection which is desired when the key assembly is used to actuate a short-travel device such as a diaphragm switch. In this style of switch, a very slight depression of switch cover sheet 24 (FIG. 1A), typically in the range of 0.007 inch, is sufficient to deflect movable contact 20 against stationary contact 19. On the other hand, keyboard operators normally prefer a key action with a relatively long stroke (typically in the range of 0.1 to 0.2 inch) to actuate the switch. The keystem must thus have a considerable range of overtravel with respect to the movement of the actuator, and the lost-motion connection or coupling of these components satisfies this requirement.

In use, key assembly 10 is mounted on a diaphragm switch as shown in FIG. 1. To actuate the switch, cap 82 is depressed by the operator to drive keystem 53 downwardly in housing 30. This motion of the keystem compresses actuator spring 91, urging the actuator 92 downwardly on the upper surface of the diaphragm switch against the natural restoring force of the switch. As the downstroke of the cap and keystem continues, the downward force on the actuator eventually overcomes the restoring force of the diaphragm switch and contact 20 moves against contact 19 to close the switch.

As the keystem moves downwardly during the actuation stroke, follower 67 on leaf spring 66 rides downwardly in channel 48 into contact with projection 49. During continued downward movement of the keystem, leaf spring 66 is deflected inwardly until follower 67 rides over tip 50 on projection 49. The leaf-spring follower thereafter rides "downhill" on the lower part of projection 49. An additional force on the cap is thus required to deflect the leaf spring as the follower rides over projection 49, and a discontinuity in the force-versus-displacement characteristic of the switch is introduced in the mid-travel range of the key stroke.

This discontinuity is illustrated in FIG. 13 which is a graph showing total downstroke restoring force of the key assembly (which force must be overcome by the operator's finger to effect keystem motion) on the ordinate, and displacement of the cap and keystem on the abcissa. During the initial part of the downstroke, the restoring force increases essentially linearly, but a distinct increase in spring rate occurs as leaf-spring follower 67 contacts and rides up projection 49. When the follower passes over tip 50 of projection 49, the curve changes slope, reflecting the release of energy stored in the leaf spring as the follower rides "downhill" on the projection. As the downstroke continues, follower 67 passes below projection 49, and the force again increases in a relatively linear manner.

Actuator spring 91 is chosen to match the characteristics of diaphragm switch 13 such that switch closure is effected just as follower 67 passes over tip 50 of projection 49. When the switch is actuated, the operator thus senses a distinct change in the "feel" of the key, and recognizes subconsciously that an adequate key stroke has been made. Some overtravel range of the keystem is provided beyond the point of actuation, and the keystem bottoms when projections 75 on the actuator reach the top of guide channels 64.

In addition to providing a tactile sense of switch actuation, the leaf spring and follower have the further advantage of urging the keystem away from the exact position in which actuation occurs. This is desirable to prevent switch jitter which could occur if the operator's finger is rested on the cap to depress the keystem partially toward an actuated position. That is, the discontinuity in the force-versus-displacement characteristic of the key assembly tends to make it difficult to hold the keystem at the exact point of switch actuation, and the possiblity of undesired multiple switch closures is minimized.

Although the tactile sensing of switch actuation is normally desired, this feature may not be wanted in some applications for the key assembly. In these cases, keystem 53 is simply rotated 180.degree. from the position shown in FIG. 1 before being inserted in housing 30 during assembly. In this position, follower 67 on leaf spring 66 rides in an empty channel 48 which does not have a projection 49. The force discontinuity shown in FIG. 13 is thus eliminated, but only a single keystem need be maintained in inventory for either style of key assembly.

When the key downstroke is completed, the operator's finger is lifted from the cap, and the cap and keystem are driven upwardly by main return spring 90. Closure of the diaphragm switch is terminated when the restoring force of the switch itself overcomes the decreasing force exerted on the actuator by spring 91 during the upstroke of the key. An important feature of the invention is that the actuator is positively retracted into housing 30 during the final portion of the return stroke of the cap and keystem. This occurs because projections 75 on retainer arms 74 of the actuator bottom on shoulders 65 of the keystem before the return stroke is completed, and the actuator is lifted by the keystem and return spring 90 to a fully retracted position with the top of base 73 seated against stops 46 as shown in FIG. 1.

If desired, stops 46 may be positioned such that button 78 (which bears against the diaphragm switch during switch actuation) on the actuator is lifted away from the top of the diaphragm switch at the end of the return stroke. This positioning of the parts is desirable when the key assembly is used in conditions of vibration and shock which might result in inadvertent switch closure if the actuator rested against the top of the diaphragm switch at the end of the key return stroke.

In a typical configuration, the components of the key assembly are selected to provide an actuation stroke of about 0.11 inch, and a total stroke of about 0.17 inch. Return spring 90 is selected to provide a total key actuating force of about 3 ounces at the point of switch closure. As mentioned above, actuator spring 91 is selected to have a restoring force equal to the switch restoring force at contact closure when the keystem is in a downstroke position placing follower 67 on tip 50.

Inadvertent disassembly of the key assembly is virtually eliminated because snap rings or other separate fasteners susceptible to unintended disengagement are not used. The assembly can be easily dismantled, however, for inspection or other purposes. The housing is removed from the diaphragm switch or other device to provide clearance below lower chamber 41, and the cap and keystem are fully depressed to drive actuator 72 partly out of the bottom of the housing. In this position, retainer arms 74 are accessible and can be urged apart to release projections 75 from captive engagement in guide channels 64. The actuator is thereby released from the keystem, and all components of the assembly can be separated and inspected.

In a typical application, a number of keystem assemblies are arrayed together in a framework to form a multiple-character keyboard. It may sometimes be desirable to provide extra diaphragm switches in the array to permit future addition of new symbols or characters which expand the capability of the keyboard. In such cases, the housing, actuator and overtravel spring are installed on the frame over the inactive diaphragm switch, and installation of the keystem is deferred. When the switch is to be activated, a keystem, cap and return spring are simply plugged into the housing to make automatic captive engagement with the actuator, and the new key assembly is ready for use.

There has been described a key assembly particularly suitable for devices having a short actuation stroke such as a diaphragm switch. The key assembly is made of inexpensive molded parts which are easily and rapidly assembled without use of separate fasteners or adhesives. Close lateral guidance for the moving parts of the key assembly is provided by the housing, and positive retraction of the non-rotating actuator assures reliable operation.

Claims

What is claimed is:

1. A key assembly for actuating a device, comprising:

a housing;

a keystem mounted on the housing to be movable through a stroke between a normally extended position and a depressed position, the keystem having a pair of elongated channels therein, each channel having a lower end defining a stop shoulder;

an actuator mounted on the housing to be movable through a stroke between a normally retracted position and an extended position, the actuator having a pair of arms extending therefrom over opposite sides of the keystem, each arm having an inwardly extending portion fitting into a respective channel to form a lost-motion connection which captively retains the actuator on the keystem;

first resilient means disposed between the keystem and actuator for urging the actuator away from the keystem; and

second resilient means disposed between the housing and keystem for urging the keystem toward the normally extended position;

the keystem and first resilient means being operative to move the actuator toward the extended position when the keystem is moved toward the depressed position, the arm portions being movable in the keystem channels to permit the keystem stroke to exceed the actuator stroke, the arm portions abutting the channel stop shoulders to drive the actuator to the retracted position when the keystem is returned to the normally extended position.

2. The assembly defined in claim 1 in which the actuator and keystem are non-rotatively mounted on the housing.

3. The assembly defined in claim 2 and further comprising a third resilient means arranged between the keystem and housing to be operative during an intermediate portion of the keystem stroke to impose on the keystem a force resisting keystem motion followed by an opposite force aiding further keystem motion as the keystem is moved through the intermediate stroke portion toward the depressed position.

4. A key assembly for actuating a diaphragm switch, comprising:

a hollow housing having a lower end adapted for mounting on the switch;

a keystem slidably mounted in and extending from an upper end of the housing, the keystem being movable between a normally extended position and a depressed position;

a first spring disposed between the keystem and housing for urging the keystem toward the extended position, the first spring exerting an increasing restoring force opposing an actuating force exerted on the keystem to move the keystem from the normally extended position toward the depressed position;

a resilient means interposed between the keystem and housing for modifying the total force opposing the actuating force by first increasing the total force and then decreasing the total force as the keystem is moved through an intermediate portion of full downstroke travel toward the depressed position;

an actuator slidably mounted in the housing and having a lower end movable into operative contact with the switch when the housing is mounted on the switch, the actuator having a coupling means extending therefrom in captive engagement with the keystem, the coupling means providing a lost-motion connection which forces the actuator away from the switch into a retracted position when the keystem is in the normally extended position while permitting limited downstroke motion of the keystem toward the depressed position without requiring equal motion of the actuator; and

a second spring disposed between the keystem and actuator to urge the keystem and actuator apart, the second spring providing a resilient connection between the keystem and actuator such that an increasing restoring force exerted by the second spring during the keystem downstroke is effective to urge the actuator away from the keystem into actuating contact with the switch, the second spring permitting overtravel movement of the keystem beyond a position adequate to obtain switch actuation, while maintaining a restoring force against the actuator to hold the switch in an actuated position during overtravel movement and until the keystem is permitted to move back toward the extended position.

5. The assembly defined in claim 4 in which the keystem and actuator are non-rotatively mounted in the housing whereby rotation is prevented as the keystem and actuator are moved linearly with respect to the housing.

6. The assembly defined in claim 5 in which the housing defines a stop surface abutting the actuator in the retracted position to prevent further motion of the actuator toward the upper end of the housing, and to limit upward travel of the keystem with respect to the housing.

7. The assembly defined in claim 4 in which the housing defines a channel extending in the direction of keystem travel, the channel having a projection intermediate its ends extending toward the keystem, and in which the resilient means includes a resilient arm on the keystem and having a follower which fits in and rides along the channel as the keystem is moved in the housing, the follower riding over the channel projection to deflect and then release the resilient arm as the keystem is moved through the intermediate position.

8. The assembly defined in claim 7 in which the keystem has a pair of side surfaces defining channels which are elongated in the direction of keystem motion, each channel having a stop shoulder at a lower end thereof, and in which said coupling means comprises a pair of arms secured to and extending from the actuator over the side surfaces of the keystem, the arms having inwardly extended ends slidably fitted into the channels and abutting the stop shoulders to limit separation of the keystem and actuator.

9. The assembly defined in claim 8 in which the actuator and arms are integrally formed from a material having sufficient resiliency that the arms may be spread apart to slide over the keystem and then resume a normal spacing with the inwardly extending ends of the arms captively engaged in the keystem channels.

10. The assembly defined in claim 4 in which the keystem includes a cap post, the post having projections extending therefrom, and further comprising a cap having a socket and formed of a plastic material which will cold flow, the post making a force fit in the socket, the cap being formed of a softer material than a material forming the keystem post whereby the cap will cold flow around the post projections to form detent sockets receiving the projections and retaining the cap on the post.

11. A key assembly comprising:

a diaphragm switch having an actuating surface which is movable to effect switch actuation;

a hollow housing having an undersurface disposed adjacent the switch actuating surface;

a clamping means securing the switch and housing together;

a keystem slidably mounted in and extending from an upper end of the housing, the keystem being movable toward and away from the switch actuating surface between a depressed position and a normally extended position, the keystem having a pair of side surfaces defining channels which are elongated in the direction of keystem motion, each channel having a stop shoulder at a lower end thereof;

a first coil spring disposed between the keystem and housing for urging the keystem toward the extended position;

an actuator slidably mounted in the housing between the keystem and switch actuating surface, the actuator having a pair of arms secured to and extending therefrom over the keystem side surfaces in captive engagement of the keystem, the arms having inwardly extending ends slidably fitted in the keystem channels and abutting the stop shoulders to limit separation of the keystem and actuator, the arms thereby forming a lost-motion connection which pulls the actuator in a direction away from the switch actuating surface into a retracted position when the keystem is in the normally extended position while permitting limited downstroke keystem motion toward the depressed position without requiring equal motion of the actuator; and

a second spring disposed between the keystem and actuator to urge the keystem and actuator apart, the second spring providing a resilient connection between the keystem and actuator such that an increasing restoring force exerted by the second spring during a keystem downstroke toward the depressed position is effective to urge the actuator away from the keystem against the switch actuating surface to effect switch actuation, the second spring permitting overtravel movement of the keystem beyond a position adequate to obtain switch operation, while maintaining a restoring force against the actuator to hold the switch in an actuated position during overtravel movement and until the keystem is permitted to move back toward the extended position.

12. The assembly defined in claim 11 in which the actuator and arms are integrally formed from a material having sufficient resiliency that the arms may be spread apart to slide over the keystem and then resume a normal spacing with the inwardly extending ends of the arms captively engaged in the keystem channels.

13. The assembly defined in claim 12 in which the keystem and actuator are non-rotatively mounted in the housing, and in which the housing defines a sto surface abutting the actuator in the retracted position to prevent further motion of the actuator toward the upper end of the housing, and to limit upward travel of the keystem with respect to the housing.

14. The assembly defined in claim 13 in which the housing defines a channel extending in the direction of keystem travel, the channel having a projection intermediate its ends and extending toward the keystem, and in which the keystem includes an integrally formed spring arm having a follower which fits in and rides along the channel as the keystem is moved in the housing, the follower riding over the channel projection to deflect and then release the spring arm as the keystem is moved through an intermediate downstroke position to effect switch operation.