Abstract:
A center shaft (2a) for holding a key top (1), and springs (2b) are provided in a casing (2) of a small-sized electronic equipment such as an inter-call receiver. The springs (2b) are provided so as to extend from the peripheral portions toward the center shaft (2a), and holes are formed in the springs (2b) respectively. The key top (1) is provided with a stopper nail (1a), first protrusions 1b and second protrusions 1c. The protrusions (1c) of the key top (1) for pushing switches (3) are inserted through the holes of the springs (2b) respectively, and the stopper nail (1a) of the key top (1) is engaged with the center shaft (2a) acting as a fulcrum during the seesawing operation of the key top (1). Thus, the key top (1) is held in a stable manner. The existence of the center shaft (2a) prevents simultaneous depression of the two switches (3).

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a structure for holding a key top used in a small-sized electronic equipment such as an inter-call receiver or the like, and particularly relates to a structure for holding a key top, which is simple and with which the key top can be attached to a casing easily and held stably. 
     One or more switches are provided in a small-sized electronic equipment such as an inter-call receiver or the like. A key top good and stable in manipulability of the switches, and low in manufacturing cost has been desired. 
     A conventional key top is arranged such that a key top itself is constituted by a spring structure or an elastic material, or otherwise, a spring, a cushion or the like acting as an elastic member for return operation is interposed between a casing and a key top, as disclosed in JP-U-2-42292, JP-A-63-254629, JP-A-60-175330, and so on. 
     The key top of this type cannot employ a complicated holding structure to stabilize the key operation in a case where it is applied to a small-sized electronic equipment such as an inter-call receiver or the like, since the small-sized electronic equipment generally has switches disposed close to each other and thus no sufficient space is provided for installation of the holding structure. 
     Such a key top is also available, that an elastic material or a spring structure was adopted for the key top per se in order to stabilize the key operation or in order to prevent abnormal sound or looseness from being caused by vibration of a calling vibrator or speaker singing. The key top is attached to the inside of a casing. The key top requires an additional extra holding mechanism, or the use of an elastic member such as a spring, a cushion or the like to possess the returnability of the key top. This results in the increased number of component parts, and the complicated assembly. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to reduce the number of component parts by using a simple structure of a key top with no elastic material or spring structure in the key top per se or with no elastic member such as a spring, a cushion or the like in the key top per se. 
     It is another object of the present invention to stabilize the key operation, and to prevent abnormal sound or looseness from being caused by vibrations of a calling vibrator or speaker singing. 
     It is further another object of the present invention to make it possible to easily perform assembling from the outside of a casing, and to prevent occurrence of detachment due to external impact such as falling or the like. 
     To attain the above-noted objects, the present invention provides a key top holding structure for holding a key top on a casing to operate a switch. The key top holding structure includes a key top and a casing. The key top includes a first protrusion, a second protrusion and an elastic stopper nail. The casing includes a center shaft and a spring extending from a peripheral portion of the casing toward the central shaft. The key top is pivotably held on the casing by engagement between the stopper nail and the central shaft to depress the spring through the first protrusion and the switch through the second protrusion. 
     The key top holding structure can reduce the number of component parts by providing a spring structure in the casing. The operation and posture of the key top can be stabilized. The generation of the abnormal sound and looseness caused due to vibrations of a calling vibrator or loudspeaker can be surely prevented. Assembly can be also performed easily from the outside of the casing. The accidental detachment of the key top by an external impact can also be avoided. 
     The present disclosure relates to the subject matter contained in Japanese patent application No. Hei. 9-274933 (filed on Sep. 24, 1997) which is expressly incorporated herein by reference in its entirety. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is an exploded perspective view of a key top holding structure according to a first embodiment of the present invention; 
     FIG. 2 is a sectional view showing how to incorporate the key top into a casing; 
     FIG. 3 is a plan view of a portion of the casing for receiving the key top; 
     FIG. 4 is a sectional view taken along line 4--4 of FIG. 3, showing a state in which the key top is incorporated in the casing; 
     FIG. 5 is a sectional view showing a state in which the key top is pressed; and 
     FIG. 6 is a sectional view showing a state in which a key top is incorporated in a casing according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, preferred embodiment of the present invention will be described in detail with reference to FIGS. 1 to 6. 
     (First Embodiment) 
     FIG. 1 is an exploded perspective view of a key top and a casing according to a first embodiment of the present invention. FIG. 2 is a sectional view showing how to incorporate the key top into the casing. FIG. 3 is a plan view of a portion of the casing where the key top is to be incorporated. FIG. 4 is a sectional view showing a state in which the key top is incorporated in the casing. FIG. 5 is a sectional view showing a state in which the key top is pushed. The key top and the casing are, for instance, molded from resin. 
     The method of assembling the key top will be described with reference to FIG. 2. First, a stopper nail 1a of a key top 1 is forced vertically onto a center shaft 2a of a casing 2 in the direction of the arrow B. The stopper nail 1a of the key top 1 is spread in the direction of the arrow A because of its own elasticity. As the stopper nails 1a of the key top 1 are fitted on the center shaft 2a, protrusion portions 1b for pushing springs 2b of the casing 2 abut against heads 2d (see FIG. 3) of the springs 2b, respectively. Each of the protrusion portions 1b acts as a point of application of the spring. On the other hand, protrusion portions 1c for pushing switches 3 are respectively inserted into holes 2c (see FIG. 3) of the springs 2b of the casing 2, so that the key top 1 can be prevented from rotating when the key top is inserted into the casing 2. When the stopper nail 1a is entirely fitted onto the center shaft 2a, the assembling is completed. Preferably, the upper surface of the center shaft 2a is formed into a spherical or tapered shape, so that the stopper nail 1a can be fitted onto the center shaft 2a smoothly. As described above, the key top 1 can be assembled easily from the outside of the casing 2. 
     Next, the shape of each of the springs 2b of the casing 2 will be described with reference to FIG. 3. Each spring 2b of the casing 2 extends toward the center shaft 2a from the periphery of a recess portion of the casing 2 in which the key top 1 is to be set. The spring 2b is formed at its central portion with a hole 2c into which the protrusion portion 1c of the key top 1 is to be inserted. The spring 2b is strong in structure because of its U-shape. Therefore, the spring 2b is substantially free from the adverse deformation due to a force when the casing 2 is removed from a molding die during resin-molding. In addition, a gradient for improving mold releasability during the resin-molding is given to the outer circumference of the spring 2b to suppress the adverse deformation of the spring. If the spring is not U-shaped and has a structure constituted by two thin spring members, the possibility of deformation of the spring during molding is high so that there is a fear that the quality lacks stability. 
     FIG. 4 shows the state in which the key top 1 is incorporated into the casing 2. Since the stopper nail 1a of the key top 1 is entirely fitted onto the center shaft 2a of the casing 2, the key top 1 cannot be removed from the casing 2 unless the stopper nail 1a of the key top 1 is forcibly spread in the direction F. That is, since the key top 1 cannot be taken off from the outside of the casing 2, it is possible to prevent the key top 1 from being detached by an external impact such as falling or the like. 
     In the state shown in FIG. 4, each spring 2b of the casing 2 is given downward tension by the corresponding protrusion 1b of the key top 1, whereas the key top 1 is always pushed in the direction E (upward) by the elastic force of the springs 2b. That is, the balanced elastic force of the two springs holds the key top 1 such that the protrusions 1c are spaced slightly from respective switches 3 on printed boards 4. Since the key top 1 always receives the tension vertically, it is possible to prevent abnormal sound or looseness from being caused by vibrations of a calling vibrator or speaker singing. 
     As shown in FIG. 5, the key top 1 is operated like a seesaw. When the key top 1 is pushed in the direction of the arrow D, the key top 1 rotates about the center shaft 2a of the casing 2 as a fulcrum, so that the switch 3 (left in the drawing) is pushed. In this condition, the tension by the right spring 2a in the direction of the arrow G about the central shaft 2a is released. When the load to press the key top 1 is released, the key top 1 is returned to its original state by the balanced elastic force of the springs 2b. The action of the springs 2b can keep the stable posture and operation of the key top 1. Even if the key top 1 is erroneously pushed at its central portion, the key top 1 does not push both the switches 3 simultaneously because the center shaft 2a acts as a stopper. 
     As described above, the key top holding structure according to the first embodiment of the present invention has at least the following advantages: The stopper nail is engaged with the center shaft of the casing. The key top is held to push the springs through the respective first protrusions and the switches through the respective second protrusions. The posture and operation of the key top is stabilized by the spring action, thereby preventing the occurrence of abnormal sound and looseness. The center shaft acts as a stopper, thereby preventing the two switches from being pushed simultaneously. The number of component parts can be reduced, and the assembly can be made easy. 
     (Second Embodiment) 
     A key top holding structure according to a second embodiment of the present invention will be described with reference to FIG. 6. The second embodiment of the present invention is different from the first embodiment in that the key top holding structure of the present invention is applied to a rocking piano-key type arrangement for pushing a single switch. 
     A key top 1 has a stopper nail 1a, a protrusion portion 1b and a protrusion 1c. A casing 2 has a center shaft 2a and a spring 2b. The spring 2b of the casing 2 extends toward the center shaft 2a from the periphery of a recess portion of the casing 2 in which the key top 1 is to be set. The spring 2b is formed at its central portion with a hole 2c through which the protrusion 1c of the key top 1 is to be inserted. A switch 3 is provided on a printed board 4. 
     To incorporate the key top 1 in the casing 2, the stopper nail 1a of the key top 1 is forced onto the center shaft 2a of the casing 2 vertically. As the stopper nail 1a of the key top 1 is fitted onto the center shaft 2a, the protrusion portion 1b of the key top 1 abuts against a head 2d of the spring 2b. This protrusion portion 1b acts as a point of application of the spring 2b. The protrusion portion 1c for pushing the switch 3 is inserted through the hole 2c of the spring 2b of the casing 2, so that the key top 1 can be prevented from rotating when the key top 1 is assembled. The key top 1 can be assembled easily from the outside of the casing 2 in this manner. 
     When the stopper nail 1a of the key top 1 is entirely fitted onto the center shaft 2a of the casing 2, the key top 1 cannot be removed from the casing 2 unless the stopper nail 1a of the key top 1 is forcibly spread in the direction F. That is, the key top 1 cannot be taken off from the outside of the casing 2. Therefore, it is possible to prevent the key top 1 from being detached by an external impact such as dropping of the small-sized electronic equipment, or the like. 
     The spring 2b of the casing 2 is given downward tension by the protrusion 1b of the key top 1, while the key top 1 is always pushed in the direction E (upward) by the elastic force of the spring 2b. Since there is no spring on the opposite side with respect to the center shaft 2a, the head of the key top 1 abuts against the casing 2 so as to bear against the force of the spring 2b, thereby holding the top key in a stable manner. Since the key top 1 always receives the vertical tension in this manner, there is no fear that abnormal sound or looseness is produced in the key top 1 by vibrations of a calling vibrator or speaker singing. 
     When the key top 1 is pushed in the direction D for key operation, the key top 1 rotates about the center shaft 2a as a fulcrum, so that the switch 3 is pushed. When the load to press the key top 1 is released, the key top 1 is returned to its original state by the action of the spring 2b. The key top 1 is kept in a stable state by the action of the spring 2b. 
     As described above, the key top holding structure according to the second embodiment of the present invention has at least the following advantages: The stopper nail of the key top of a rocking piano-key type is engaged with the center shaft of the casing. The key top is held to press the spring through the first protrusion and the switch through the second protrusion. The center shaft of the casing acts as a fulcrum during the operation of the key top. Accordingly, the operation and posture of the key top is made stable by the spring action. It is possible to prevent abnormal sound or looseness from occurring. The number of component parts can be reduced, and the assembly can be made easy.