Push button mechanism with locking device for two stable positions
FIELD OF THE INVENTION 
     The invention relates to a push-button mechanism with a locking device of the type that from a first stable position (the initial position) when depressed, locks the button to a second stable position and when depressed a second time, returns to the first position. Such locking function of the push-button is, for example, used in telephone instruments for connecting an extra telephone function such as a loudspeaking receiving function or the like. 
     BACKGROUND 
     It is known in push-button mechanisms of the above type to design the push-button or the elements rigidly connected to this, for example, an activating bar or a rigidly arranged planar slide with a heart-shaped cam groove in which a ball or the end of a tap-formed part runs, to obtain the bistable function when the button is depressed. 
     British Pat. No. 877,223, for example, describes a push-button mechanism in a ball pen in the handle of which the push-button part is formed as a sleeve whose inner cylindrical surface in the lower part is formed at a heart-shaped cam groove for cooperation with a movable ball located in the groove. Upon the first depression, the ball will run along an arcuate groove along the heart-shaped cam and assume a certain position, the sleeve being locked in the depressed position by spring action. Upon a second depression of the sleeve, the ball runs along another groove along the heart-shaped cam and the sleeve as well as the ball return to the original position. 
     Another similar construction of a push-button with a heart-shaped cam as a controlling element to obtain the bistable function when the button is depressed, is described in U.S. Pat. No. 3,493,705. In this construction, the heart-shaped cam is arranged on a contact slide with two oppositely located contact elements. Further examples of known push-button constructions of similar kind are represented in U.S. Pat. Nos. 3,766,346 and 3,808,388 showing a heart-shaped cam arranged on the push-button part or a part rigidly arranged to therewith. 
     SUMMARY OF THE INVENTION 
     A common characteristic of the above mentioned push-button mechanisms is that the heart-shaped cam is designed on such parts in the mechanism, which are either integrated with the push-button or with another part that performs quite another function than locking the push-button to the two bistable positions. The push-button mechanism according to the present invention employs, like the known mechanism, a heart-shaped cam and a locking slot or groove with a ball to obtain the bistable function when repeatingly depressing the push-button, but as a difference from the known arrangements, the element in which the heart-shaped cam is constructed, is movably arranged in relation to the other elements in the mechanism. 
     The button with a mounted heart-shaped cam represents an independent unit which is locked by the ball, keeping the heart-shaped cam and the button together which are then mounted in the mechanism. 
     The object of the present invention is thus to obtain a push-button mechanism, for example, included in the push-button set of a telephone instrument to obtain a bistable function of the mechanism upon two subsequent depressions of the push-button and in which the locking element is movably arranged in relation to the other elements. 
     The advantages of the push-button mechanism according to the invention are 
     (a) The mounting is facilitated, since the locking unit is mechanically connected before being placed in the push-button mechanism. 
     (b) The ball in the locking arrangement cannot slip out of the groove when the button is tilted, nor can it jam, since the heart-shaped cam is directly controlled by the button and follows this in all movements. Since the heart-shaped cam also is loosely located between two stopping planes, jamming is prevented by the heart-shaped cam. 
     (c) Small sensibility of tolerance when dimensioning the push-button, the supporting base plate and other parts in the mechanism, and 
     (d) greater flexibility when interchanging component parts as, for example, contact arrangements, push-button etc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, 
     FIG. 1 shows a push-button with a cover as seen from the side with a mounted locking device according to the invention. 
     FIG. 2 shows the button and a washer or plate with a heart-shaped cam included in the locking device according to the invention. 
     FIG. 3 shows the button alone according to FIG. 2 as seen from below. 
     FIG. 4a-d show the plate according to the FIGS. 1-2 with heart-shaped cam and associated ball to illustrate the different phases upon depression. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, therein is seen a button included in a push-button 1 mechanism, for example, of, a telephone instrument. A rectangular frame 2 surrounds the button 1 and forms the a guide therefor. The frame 2 forms an integrated part of a cover 3 of the push-button mechanism, the lower part (the base part) of the cover being supplied with fastening elements in the form of two (or more) supporting pins 5a, b. When mounting the push-button mechanism (cover plus button) the pins 5a, b are inserted into the holes of a printed card and attached in a suitable way. A leaf-shaped metallic tongue 6 constitutes an electric connection and forms inside the cover 3 a contact element in a known way (not shown in the Figures). The cover 3 is broken away as shown in FIG. 1 to illustrate the position of a thin plate 9 which rests against a rigid support 4 inside the cover (the base support of the cover). For this purpose the part 4 is supplied with two knobs 4a, b between which two pins 9a, b of the plate 9 are fitted. As more fully appears from FIGS. 2 and 3, the button 1 is provided at the lower part with supporting elements for the plate 9 in the form of grip arms 7a, b which partly grasp the plate 9. The arms 7a, 7b at the same time constitute guiding elements for the push-button 1 when this is depressed. Behind the plate 9 and level with the guiding elements 7a, b a groove 8 is formed on the same part as these elements, which groove extends horizontally and in which a small ball 10 is located. The diameter of the ball 10 is suitably chosen somewhat larger than the width of the groove 8, the ball 10 then touching both the vertical side surfaces and the longitudinal base surface of the groove 8. The plate 9 is provided with a countersunk part 11 along one of the planar main surfaces, the outline of which describes a heart-shaped cam according to the dotted lines in the FIGS. 1 and 2. The plate 9 is inserted between the supporting arms 7a, b so that the countersunk part 11 faces the groove 8. 
     As appears from the FIGS. 2 and 3, a hollow rod 12 is provided inside the button 1, which rod constitutes an integrated part with the two arms 7a, b through a connection part 14. In an inner cylindrical cavity of the rod 12 a spring 13 is located. the spring 13 bears at one end against the inside of the button (the base of the rod) and at the other end against the surface 4 in the cover 3 so that a spring action is obtained when the button 1 is depressed. 
     The locking device includes the plate 9 with the countersunk part 11, formed as a heart-shaped cam, and the ball 10 located in the groove 8. The plate 9 in addition has an elevated part 15 in the center of the countersunk part 11, whose upper planar surface is situated level with the upper surface of the 9. As appears from the arrows in FIG. 2, the plate 9 is inserted between the supporting arms 7a, b when mounted and the ball 10 snaps into the upper part of the countersunk part 11, i.e. the tip of the heart-shaped cam, the plate then assuming the position shown in FIG. 1. The ball 10 is now substantially freely movable within the space as track limited by the edges of the countersunk part 11 and the edges of the grove 8. The button 11 and the heart-shaped cam 9 are now mechanically connected by means of the ball 10 and operate as a bistable unit. 
     FIG. 4a shows the starting position, i.e. the first stable position, when the ball 10 is located on the upper point 111 of the heart-shaped cam. The push-button 1 is then in the top position. By depressing the button 1, the upper edge surface of the groove 8 will push the ball downwards along to the path of the dotted arrow in FIG. 4a and the ball passes the convex edge surface 151 of the elevated part 15 and the arcuate edge surface 112 of the heart-shaped cam. The ball assumes the position according to FIG. 4b when the button is entirely depressed, i.e. at the semicircular edge surface 113. 
     When the depression of the button 1 ceases, the button is forced upwards by the spring 13 and the lower surface of the groove pushes the ball upwards to the concave surface 152 of the part 15, taking the position according to FIG. 4c. The push-button mechanism is then in the second stable position. When the button is depressed again, the ball is pushed downwards by the upper edge surface of the groove along the path of the dotted arrow in FIG. 4c and, when the button is entirely depressed, is in a final position according to FIG. 4d, i.e. at the semicircular edge surface 114. When the depression ceases, the button will be forced upwards by action of the spring 13, the upper edge surface of the groove then pushed the ball upwards to the initial position according to FIG. 4a as is shown by the dotted arrow in FIG. 4d. 
     The plate 9 with the countersunk part 11 in the form of a heart-shaped cam and the elevated part 15 in the form of a concave-convex curve is thus flexibly mounted inside the body part 3 of the push-button mechanism, and is, in principle, limited only by the two knobs 4a, 4b and the supporting arms 7a, 7b. The heart-shaped cam which forms one part of the locking mechanism, is thus not rigidly mounted in a certain fixed position in relation to the ball 10 when this travels in the groove 8, the second part of the locking arrangement. Also the push-button is flexibly mounted in known way whereby the locking device in the present push-button mechanism complies with the advantages mentioned above.