Abstract:
A switch assembly comprising an actuator plunger for actuating a built-in switch, a rotary shaft for rotating in response to an external operating force, and a cam coupled to the rotary shaft and adapted to contact the actuator plunger, the actuator plunger being adapted to be moved upwardly or downwardly in response to a rotation of the rotary shaft through the cam so as to switch the built-in switch, and the cam having a hole to provide a predetermined free angular play between the rotary shaft and the cam as assembled.

Description:
This application is a continuation of application Ser. No. 776,102 filed Sept. 16, 1985, which was a continuation of application Ser. No. 659,018 filed, Oct. 9, 1984, which was a continuation of application Ser. No. 363,526, filed Mar. 30, 1982, all of which are abandoned. 
    
    
     BRIEF SUMMARY OF THE INVENTION 
     This invention relates to a limit switch assembly generally used as a position detector in a machine or the like, and more particularly to a limit switch assembly of a hold-type which keeps its operating position after relief of an external operating force applied to the assembly. 
     There has long been known a limit switch assembly, for instance, as illustrated in FIGS. 1 to 3, which includes a housing body 1 housing a built-in switch 2 therein, an operating head member 3 fixed on a head portion of the housing body 1, a rotary shaft 5 supported in the operating head member 3, and an L-shaped roller lever 4, which is a means adapting for sensing movement of an external object fixed to a pointed end portion of the rotary shaft 5. In an upper position of the housing body 1, there is disposed an actuator plunger 6 for actuating a push button 2a of the built-in switch 2 for upward and downward movement. A cam 7 abutted by the plunger 6 is fixed to the rotary shaft 5. An upper surface of the cam 7 is pushed by a roller 10 supported by a movable plunger 9 biassed downwardly by a spring 8 in such a manner that when the roller lever 4 is rotated either clockwise or counterclockwise to a certain extent, the cam 7 is adapted to be held in the position. A lower surface of the cam 7 is always pushed by the plunger 6 biassed by a spring 6a so as to be followed by the plunger 6. 
     The above-mentioned limit switch is operated in such a manner that when the roller lever 4 of FIG. 1 is rotated about its own axis through an angle of 90° in a clockwise direction, the rotary shaft 5 is rotated together with the cam 7 so as to depress the plunger 6 downwardly and turn on the switch 2, and then, when the roller lever 4 is rotated about its own axis through an angle of 90° in a reverse direction to its original position, the plunger 6 moves in an upward direction to its original position following the cam 7 so as to turn off the switch 2. 
     When the roller lever 4 is rotated in a clockwise direction, viz., in a setting operation, as illustrated in FIG. 4A, the built-in switch 2 is turned on in a position which is a little over the position (hereinafter described as &#34;mechanical center&#34;) where the roller 10 of the plunger 9 contacts a top portion 7a of the cam 7. Meanwhile, in a resetting operation as illustrated in FIG. 4B, the switch 2 is adapted to be turned off in a position which is a little over the mechanical center. Thus, the respective ON and OFF switching positions of the built-in switch 2 are close to the mechanical center. Therefore, as the position of the cam 7 is approaching to themechanical center, a contact pressure applied in the switch 2 is decreased. Moreover, the respective ON and OFF switching positions are liable to get out of order due to the abrasion or dimensional error developed in the contacts by switching load. Thus, the conventional limit switch assembly has the disadvantage that since the respective ON and OFF switching positions are close to the mechanical center, the exact operation of the assembly is not ensured and undesirable contact chattering is liable to be developed. 
     In order to avoid this disadvantage, there may be proposed to provide a limit switch assembly such that the above-mentioned cam 7 is partially removed therefrom as illustrated in dotted lines of FIG. 5A so that the built-in switch 2 is turned on when the cam 7 is rotated completely over the mechanical center. The proposed assembly, however, has the disadvantage that when the cam 7 is rotated in a counterclockwise direction as illustrated in FIG. 5B, the switch 2 is turned off upstreams of the position of the mechanical center. This advantage is fatal in view of the characteristics required in the limit switch assembly in practical use. 
     This invention, therefore, has been developed with a view to eliminating the above-described disadvantages, and has its essential object to provide an improved limit switch assembly wherein a built-in switch is ensured to be switched after a cam is rotated completely over a mechanical center so as to overcome any drop in a contact pressure and the contact chattering for a precise switching operation. 
    
    
     Other objects as well as the numerous advantages of the limit switch assembly according to this invention will become apparent from the following detailed description and the acompanying drawings, in which: 
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is an elevational view of a conventional limit switch assembly; 
     FIG. 2 is right-side partial section view of the assembly of FIG. 1; 
     FIG. 3 is a sectional view taken along the line 3--3 of the device of FIG. 2; 
     FIGS. 4A and 4B are views illustrating a setting and a resetting operation principle of a cam, respectively; 
     FIGS. 5A and 5B are views illustrating a setting and a resetting operation principle of an improved cam, respectively; 
     FIG. 6 is a side partial section view of an operating head member employed in a limit switch assembly as a preferred embodiment of this invention; 
     FIG. 7 is a sectional view taken along the line 7--7 of the assembly of FIG. 6; 
     FIG. 8 is a perspective disassembled view showing an inner mechanism of the head member of FIG. 6; 
     FIG. 9 is a front view of a cam employed in the head of FIG. 8; 
     FIG. 10 is a sectional view of the cam which is interposed in the head member; 
     FIGS. 11 and 12 are views illustrating principles of a setting operation and a resetting operation; and 
     FIG. 13 is a side partial section view of an operating head member as a modification of the head member of FIG. 6. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring, now, to FIGS. 6 through 8, there is shown an operating head member or head housing 11 employed in a limit switch assembly and an inner mechanism of the head member in accordance with this invention. Within the head member 11 there is rotatably disposed a rotary shaft 12 to one end of which a lever (not shown, such as the lever 4 of FIG. 1) is fixed in a construction similar to that of a conventional operating head member. The reference numerals 13 and 14 represent a movable plunger and a spring, respectively. The movable plunger 13 at a slope thereof pushes a slope of a triangular portion 12a formed in the rotary shaft 12 so as to bias the shaft 12 toward one direction or the other. Thus, a turning over mechanism A is constructed. 
     A cam 15 is engaged with a shaft portion 12b having a generally crescent or half-moon-shaped section formed in the rotary shaft 12, and is prevented by an E-shaped ring 16 from being slipped out of the portion 12b. As illustrated in FIGS. 9 and 10, a hole 15a of the cam 15 is engaged with the shaft portion 12b providing a predetermined free angular play or dummy angle D in such a manner that the cam 15 is freely rotatably with respect to the shaft 12 within the angular play D. A groove portion 15b 1 or nub means is formed in a bottom end of the cam 15 in an operating limit position, so that when the cam 15 is rotated to the operating limit position, a head portion of an actuator plunger 17 is engaged with the groove portion 15b. A straight-cut surface 15c is formed in each upper side of the cam 15 to come into contact with an inner surface of the head housing 11, whereby a rotatable angle of the rotary shaft 12 is restricted. A recessed portion 15d is formed in each central side of the cam 15 so that both central sides thereof are sandwitched by a forked tool (not shown). 
     To a bottom opening portion of the operating head 11 is fixed a cover member 18 having a central hole 18a through which the actuator plunger 17 is adapted to be slidably inserted for upper and lower movements so as to actuate a push button of a built-in switch (not shown). The actuator plunger 17 is disposed to be biassed upward by a resetting force from said push botton of the built-in switch (not shown) so as to contact the cam 15, but may be biassed upward by a resetting spring as illustrated in FIG. 3 about spring 6a. 
     The above-mentioned head 11 is assembled as follows: 
     The movable plunger 13 carrying the spring 14 is installed in the head housing 11 from downwards. While the plunger 13 is pushed up by a certain tool, the rotary shaft 12 is inserted into the head housing 11 from sidewards. Then, the cam 15 catched by a certain tool is inserted into the head 11 from downwards so that the through hole 15a of the cam 15 is pierced by one end of the rotary shaft 12. Moreover, the pointed end of the rotary shaft 12 is engaged with a shaft receiving portion 11a of the head housing 11, whereby the triangular portion 12a of the rotary shaft 12 is pushed by the movable plunger 13 and the half-moon shaped shaft portion 12b  of the rotary shaft 12 is coupled with the hole 15a of the cam 15. Thereafter, the E-shaped ring 16 is mounted on the shaft 12 so as to prevent the cam 15 from being slip off. Finally, the cover member 18 is fixed to the lower opening portion of the head housing 11, and the whole assemblage of the head member 11 is completed. 
     In order to insert the cam 15 into the head housing 11, the hole 15a of the cam 15 and the shaft portion 12b of the rotary shaft 12 must be arranged in predetermined directions so as to be engaged with each other, and the degree of the insertion of the cam 15 must be adjusted so that an axis of the shaft 12 is aligned with a center of the hole 15a of the cam 15. Especially, due to the miniaturization of the head member 11, the space of its lower hole is small, so that working for inserting the cam 15 is very difficult to be manually performed. Therefore, a pair of recessed portions 15d are formed in the cam 15 so as to be held by a pincette-shaped machine tool, and such insertion work is able to be automatically performed by an automatic machine. 
     Returning to FIGS. 11 and 12, the operations of the limit switch assembly according to this invention are described hereinafter. 
     In an initial position where a lever (not shown) fixed to the pointed end of the rotary shaft is not actuated yet (lever angle R=O°), the rotary shaft 12 is biased in a counterclockwise direction by the movable plunger 13 wherein the actuator plunger 17 is not depressed yet. As the rotary shaft 12 is rotated clockwise by moving the lever, the triangular portion 12a lifts up the plunger 13 but the cam 15 is not yet rotated because of a provision of the free angular play (D=35°) between cam 15 and shaft portion 12b. As the lever angle R goes beyond 35°, the cam 15 is rotated in cooperation with the roatary shaft 12 and the actuator plunger 17 is gradually depressed. When the lever angle R becomes 45°, the switch assembly reaches a mechanical center, wherein a top end of the triangular portion 12a of the rotary shaft 12 is in the highest position in cooperation with the movable plunger 13. If the shaft 12 is further rotated, it quickly rotates by the plunger 13 downwardly biassed by spring 14 until the lever angle R becomes 90°. When the lever angle R further reaches 60°, the built-in switch is turned on by the plunger 17 depressed by the cam 15. When the lever angle R becomes 90°, the head portion of the plunger 17 is engaged with the recessed portion 15b beyond a projecting portion of the cam 15, wherein the lever, the rotary shaft 12 and the cam 15 keep their positions. 
     As the lever is rotated in a reverse direction, the rotary shaft 12 is rotated together therewith but the cam 15 is not rotated until the lever angle R returns 55°. Further, when the lever angle R returns to 30° beyond the mechanical center (the lever angle R=45°), the built-in switch is turned off, and, thereafter, the rotary shaft 12 quickly returns to its original position where the lever angle R is 0°, thanks to a biassing force by the movable plunger 13. 
     Thus, in the above-described setting operations (see FIG. 11) the cam 15 is rotated just within 45°-35°=10° up to the mechanical center (lever angle R=45°), where the actuator plunger 17 is little depressed, because of the free angular Play (D=35°) between rotary shaft 12 and cam 15. Therefore, it does not happen that the contact pressure of the built-in switch is descreased as the mechanical center is approached. The built-in switch is switched in the position (lever angle R=60°) sufficiently beyond the mechanical center, so that the switching position is shifted from the mechanical center through 60°-45°=15°. Therefore, even if contacts are worn out or a scale error exists, the limit switch assembly is free from unstable operations and contact chatterings. Thus, it can be seen that the switch exhibits a hysteresis in its operation. In the resetting operations (see FIG. 12), the cam 15 is rotated through 10° to reach the mechanical center, and only where it is sufficiently beyond the mechanical center, the built-in switch is actuated. 
     In order to avoid the disadvantage that the cam 15 in the above-described resetting operations is rotated together with the rotary shaft 12 by a friction force developed between the shaft portion 12b and the hole 15a and the built-in switch is turned OFF upstreams of the mechanical center, the groove portion 15b is formed on a contact surface of the cam 15 in the operating limit position so as to be engaged with the head portion of the actuator plunger 17 and stop the movement of the cam 15 by such a friction force. 
     Returning to FIG. 13, there is shown a modified operating head member 11 of the embodiment of FIG. 6. The operating head member 11 has a turning over mechanism A1 wherein a steel ball 19 is rotatably installed into a recessed portion 131a formed in one end of a movable plunger 131 and is adapted by a biassing forced of spring 14 to push a slope of triangular portion 12a of rotary shaft 12. Other components are the same as those of the assembly of FIG. 6. According to this modified embodiment of this invention, the movable plunger (131) goes over a top end of the triangular portion (12a) of the rotary shaft (12) through the steel ball (19) in its mechanical center. Therefore, a free rotation of the steel ball decreases the waste of said top end of the triangular portion, whereby the position of the mechanical center is not shifted in even long time period. Thanks to the steel ball (19), the rotary shaft (12) smoothly turns over prividing a quick tutrning operation. 
     From the foregoing description of this invention, it has now become clear that by having a free angular play between a rotary shaft and a cam, the switching position of a built-in switch is sufficiently delayed after the mechanical center is reached, and any decrease of contact pressure or the chattering of contacts is overcome, thus ensuring stable operation characteristics. 
     It should be understood that the above description is merely illustrative of this invention and that many charges and modifications may be made by those skilled in the art without departing from the scope of the appended claims.