Abstract:
A clutch for telescopic antenna including a disk shape clutch element and a shallow, cylindrical clutch element facing each other. The cylindrical clutch element is provide with a recess on its inner wall, and the disk shape clutch element is provided with a pair of sliding contacts which are urged in the radial directions and can come into contact with the inner wall surface of the cylindrical clutch element. An engagement ball provided in one of the sliding contacts can engage with the recess of the second element, securing the coupling of the two clutch elements.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a clutch device for an telescopic power antenna mounted on automobiles, etc. 
     2. Prior Art 
     As is well known, conventional clutches of this type are designed as follows: A rotary force is transmitted by causing one side of a main driving clutch plate, which is connected to a motor side, to press against one side of a driven clutch plate which is connected to an antenna side, so that the antenna element (which constitutes the load) is extended and retracted. When the extension or retraction of the antenna element is completed, the slipping action of the clutch plates allows the motor to rotate alone until the power supplied by a motor is switched off by a limit switch. In this way, motor locking is prevented. 
     In the conventional clutches designed as described above, inconsistency in the clutch plate pressing contact force tends to occur. Accordingly, it is difficult to obtain a stable clutch function over a long period of time. Furthermore, the thickness of the clutch in the axial direction of the clutch plates tends to become large, so that the overall size of the clutch is increased. In an attempt to eliminate such drawbacks, devices have been proposed in which the main driving clutch plate and driven clutch plate are positioned to face each other in a radial direction, and a means for generating a pressing contact force between the inner circumferential surface of one clutch plate and the outer circumferential surface of the other clutch plate is installed so that rotary force can be transmitted. 
     FIGS. 3(a) and 3(b) illustrate a conventional device which is constructed as described above. A rotary shaft C, which is installed inside a housing (not shown) of antenna drive mechanism so as to be able to rotate, is shown at the center of each of the FIGS. An electrically driven telescopic antenna clutch mechanism A is mounted on this rotary shaft C. The electrically driven telescopic antenna clutch mechanism A is made up with a main driving clutch plate 1, a driven clutch plate 2 and a pressing contact mechanism 3. The main driving clutch plate 1 is formed of a synthetic resin and has a gear la formed on its outer circumferential surface for receiving a driving force of a motor (not shown). Similarly, the driven clutch plate 2 is formed of a synthetic resin and has a drive gear 2a on its outer circumferential surface for transmitting a driving force to antenna-driving rope. The main driving clutch plate 1 is mounted on the rotary shaft C so that the plate 1 is free to rotate relative to the shaft C. On the other hand, the driven clutch plate 2 is mounted on the rotary shaft C so that it cannot rotate relative to the shaft C. The pressing contact mechanism 3 is designed as follows: an engaging pin 4 which engages with an engaging groove 2c formed in inner surface 2b of the circumferential wall of the driven clutch plate 2; a first pushing block 5a having a tip which pushes the engaging pin 4 against the inner circumferential surface 2b of the circumferential wall; a second pushing block 5b installed on the opposite side of the mechanism so that it forms a pair with the first pushing block 5a, the second pushing block 5b having a tip caused to be in sliding contact with the inner surface 2b of the circumferential wall; and coil springs 6a and 6b, which are elastically compressed between the rear sides of the first and second pushing blocks 5a and 5b, are installed in a housing recess formed in one side of the main driving clutch plate 1. 
     When the main driving clutch plate 1 is rotated by a motor in a conventional clutch for use in electrically driven telescopic antennas designed as described above, the rotary force is transmitted to the driven clutch plate 2 via the pin 4 and engaging groove 2c. Accordingly, the driven clutch plate 2 rotates together with the main driving clutch plate 1, so that the antenna element is extended or retracted. 
     When the extension or retraction of the antenna element is completed, the driven clutch plate 2 stops its rotation. Since the rotation-stopping force of the driven clutch plate 2 naturally exceeds the coupling force between the main driving clutch plate 1 and the driven clutch plate 2, the pin 4 is released from the engaging groove 2c and slides on the inner circumferential surface 2b of the driven clutch plate 2. In other words, the clutch disengages, so that the main driving clutch plate 1 idles by itself. 
     In a conventional clutch for use in electrically driven telescopic antennas constructed as above, the following problems arise: when the main driving clutch plate 1 idles upon completion of the extension or retraction of the antenna element, a large impact noise is generated by the engagement and disengagement of the engaging pin 4 with the engaging groove 2c. When the engaging pin 4 is disengaged from the engaging groove 2c, the frictional force generated between the main driving clutch plate 1 and the driven clutch plate 2 consists almost entirely of the frictional force between the tip of the pushing part 5b and the inner surface 2b of the circumferential wall of the driven clutch plate 2. As a result, slipping occurs between the main driving clutch plate 1 and the driven clutch plate 2, creating a situation in which the antenna element might drop by its own weight. 
     SUMMARY OF THE INVENTION 
     Accordingly, the object of the present invention is to provide a clutch for use in electrically driven telescopic antennas in which the impact noise caused by the engagement and disengagement o engaging members when the clutch is disengaged is remarkably reduced and in which there is almost no danger of the antenna element dropping by its own weight. 
     In the present invention, the following means is adopted in order to solve the prior art problems and to achieve the object: a first rotary element formed in a circular plate; and a second rotary element, which is in a cylinder form having a closed end and has an engaging recess in the form of an indentation along the circumferential direction on the inner surface of the circumferential wall, the second rotary element being installed concentrically with the first rotary element and its open end facing one side of the first rotary element. A plurality of sector-shape sliding contacts, which have arc shaped circumferential surfaces coming in a sliding contact with the inner surface of the circumferential wall of the second rotary element, are installed on one side of the first rotary element which faces the open end of the second rotary element, so that the sector-shape sliding contacts can rotate along with the first rotary element. Furthermore, elastic bodies such as coil springs, etc. are installed in an elastically compressed state between the sector-shape sliding contacts so that the respective arc-shaped surface of the sliding contacts is pressed against the inner surface of the circumferential wall of the second rotary element. In addition, an engagement assembly, such as a cylindrical engaging pin and a coil spring, is installed in at least one of the sector-shape sliding contacts, which are pressed against the interior surface of the circumferential wall of the second rotary element by the elastic bodies, so that a portion of the engagement assembly projects beyond the arc-shaped surface of the sector-shape sliding contact and can be press engaged with the engaging recess of the second rotary element. With the above structure, either the first rotary element or the second rotary element is used as a main driving clutch plate which is connected to the motor side, while the other is used as a driven clutch plate, which is connected to the antenna side. 
     As a result of adopting the means described above, the present invention functions as follows: the arc-shaped surfaces of the sector-shape sliding contacts, each of which has a spreading angle of approximately 150 degrees and rotate together with the first rotary element, are pressed against the inner surface of the circumferential wall of the second rotary element; therefore, the size of area of sliding contact between the inner surface of the circumferential wall and the sliding contacts can be made sufficiently large as required. Thus, the hollow space in the second rotary element, which is in a cylinder form having a closed end, is filled with the sliding contacts, and the second rotary element generates almost no vibration. Accordingly, even if an impact noise should be generated by the engaging recess and engagement assembly, when the main driving clutch plate is idling, this impact noise is not acoustically amplified by the second rotary element and therefore quickly attenuates, so that the noise is eliminated. Furthermore, since the pressing contact force between the sector-shape sliding contacts and the interior surface of the circumferential wall of the second rotary element can be increased, even when the engaging pin, etc. is disengaged from the engaging recess, etc., the frictional force between the main driving clutch plate and the driven clutch plate can be maintained at a required level or greater. Thus, the phenomenon of the antenna element dropping by its own weight in the disengaged state can be eliminated. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIGS. 1(a) and 1(b) illustrate one embodiment of the present invention wherein FIG. 1(a) is a cross section of a clutch for use in electrically driven telescopic antennas. FIG. 1(b) is a cross section viewed in the direction of the arrows form line B--B in FIG. 1(a). 
     FIGS. 2(a) and 3(b) are graphs which shown a comparison of the clutch force characteristics of the embodiment and those of a conventional example. 
     FIGS. 3(a) and 3(b) illustrate the structure of a conventional example. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1(a) and 1(b) illustrate one embodiment of the present invention. FIG. 1(a) is a cross section of a clutch for use in electrically driven telescopic antennas, and FIG. 1(b) is a cross-sectional view taken along the arrows B--B in FIG. 1(a). 
     Reference numeral 11 is a first rotary element which is in the form of circular plate. In the present embodiment, this first rotary element is a main driving clutch plate. More specifically, a gear 11a, which receives the rotary force of a motor (not shown), is formed on the outermost circumferential portion of the first rotary element 11 which is mounted on a rotary shaft C so that the first rotary element is free to rotate around the shaft C. Reference numeral 12 is a second rotary element which is in a cylinder shape having a closed end. In this embodiment, this second rotary element 12 is the driven clutch plate. More specifically, a drive gear 12a, which is for driving antenna driving rope, is formed on the outer surface of the circumferential wall of the second rotary element 12, and an engaging groove 12c is formed in the inner surface 12b of the circumferential wall as an engaging recess which is in a form of indentation in the circumferential direction. The open end of the second rotary element 12 faces one side of the first rotary element 11 and is mounted on the rotary shaft C so that the second rotary element does not rotate relative to the shaft C. 
     Two projections 13a and 13b are formed, in symmetrical positions with respect to the rotary shaft C, on one side of the first rotary element 11, i.e. on one side of the first rotary element 11 which faces the open end of the second rotary element 12. Two sector-shape sliding contacts 14a and 14b are installed so that they sandwich the projections 13a and 13b between their respective rear ends. Each of these sector-shape sliding contacts 14a and 14b spreads outward with an angle of approximately 150 degrees centered on the rotary shaft C. The inner circumferential surfaces of the sliding contacts 14a and 14b are in sliding contact with the outer circumference of the rotary shaft C, and the outer circumferential surfaces of the sliding contacts 14a and 14b are shaped in arc-shaped surfaces capable of sliding contact with the inner surface 12b of the circumferential wall of the second rotary element 12. Thus, the sector-shape sliding contacts 14a and 14b are provided on the first rotary element 11 so that they can rotate with the first rotating element 11. Elastic bodies 15a and 15b such as coil springs, etc. are mounted in an elastically compressed state between the facing rear ends of the sector-shape sliding contacts 14a and 14b so that each arc-shaped surfaces of the sliding contacts 14a and 14b are pressed against the inner surface of the circumferential wall of the second rotary element 12. In order to keep the elastic bodies 15a and 15b in an elastically compressed state, spring-housing recesses 16a and 16b are formed in the sliding contacts 14a, and other spring-housing recesses 17a and 17b are similarly formed in the sliding contact 14b. These recess 16a, 16b, 17a and 17b are formed perpendicular to a line which connects the projection 13a, rotary shaft C and projection 13b. An engagement assembly 18, which consists of a cylindrical engaging pin 18a and a coil spring 18b, is provided in one of the sector-shape sliding contacts 14a. This engaging member 18 is designed so that a portion of the engaging pin 18a projects beyond the arc-shaped surface of the sliding contact 14a and is thus able to engage, via pressing contact, with an engaging groove 12c formed on the inner surface of the circumferential wall of the second rotary element 12. A housing hole 19 is formed in the arc-shaped surface of the sliding contact 14a so that the engaging pin 18a and coil spring 18b are kept stable. The coil spring 18b is independently mounted in an elastically compressed state in the innermost portion of the housing hole 19. 
     When the motor (not shown) rotates in the forward or reverse direction to extend or retract the antenna element, the rotary force of the motor is transmitted to the gear 11a of the first rotary element 11. As a result, the first rotary element 11 rotates in the forward or reverse direction. At this time, the pin 18a of the engagement assembly 18 is engaged with the engaging groove 12c of the second rotary element 12; accordingly, the second rotary element 12 can rotate together with the first rotary element 11. Thus, the antenna element, which is driven by a rope coupled to the drive gear 12a, is extended or retracted. When the extension or retraction of the antenna element is completed, the rotation of the second rotary element 12 is stopped, and the pin 18a is disengaged from the engaging groove 12c. Thus, the pin 18a is released from the engaging groove 12c and slides along the inner surface 12b of the circumferential wall of the second rotary element 12. At this time, since the arc-shaped surfaces of the sector-shape sliding contacts 14a and 14b are pressed against the inner surface 12b of the circumferential wall of the second rotary element 12, the first rotary element 11 rotates while generating a relatively large frictional force at the positions pressed against the second rotating element 12. Then, the first rotary element 11 alone idles for several rotations, with the pin 18a repeatedly engaging with and disengaging from the engaging groove 12c. Afterward, motor power supply is switched off by the action of a limit switch so that the rotation of the main driving clutch plate 11 also stops. 
     FIG. 2(a) is a graph which shows the characteristics of the clutch force (frictionally transmitted force) of the present embodiment of the clutch for use in electrically driven telescopic antennas provided by the present invention. FIG. 2(b) is a graph of the characteristics of a conventional example shown for comparison. 
     First, FIG. 2(a) will be described. It is assumed here that the main driving clutch plate 11 is caused to idle with the driven clutch plate 12, which are shown in FIGS. 1(a) and 1(b), locked. In this case, each time the main driving clutch plate 11 completes one rotation, the pin 18a engages with the engaging groove 12c and then disengages from the engaging groove 12c. P1, P2, ... in FIGS. 2(a) and 2(b) indicate the points the pin 18a engages with the engaging groove 12c. The peak clutch force FPa at the engagement points P1, P2,... is approximately 6.5 kg. The basic clutch force FBa in regions other than the engagement points P1, P2,... is attributed mainly to the frictional force between the arc-shaped surfaces of the sector-shape sliding contacts 14a and 14b and the inner surface of the circumferential wall of the second rotary element 12 and is approximately 4.0 kg. The basic clutch force FBa is sufficiently greater than the force required to maintain the antenna element in a stable extended state (i. e., approximately 2.5 kg); accordingly, as described above, there is no danger that the antenna element will drop by its own weight. 
     Meanwhile, in FIG. 2(b), the peak clutch force FPb is also approximately 6.5 kg and is therefore roughly the same as FPa in FIG. 2(a). However, the basic clutch force FBp depends only on the frictional force between the tip of the pushing block 5b and the interior surface of the circumferential wall of the driven clutch plate, and such force FPs is therefore approximately 1.5 to 2.0 kg. Thus, the antenna element cannot be maintained in a stable extended state and may drop by its own weight. 
     As the basic clutch force FBa shown in FIG. 2(a) increases, the load on the main driving clutch plate during the idling gradually increases. Thus, an excessive current corresponding to the amount of the increase flows through the motor wires. Accordingly, there is, naturally, a limitation on the amount by which the clutch force can be increased. However, the time span between the point of completion of the extension or retraction of the antenna element and the point at which the motor power supply is cut off is only about 20 ms. Accordingly, even if the basic clutch force FBa increases for certain amount, this causes no practical problems. The inventors of the present application have confirmed through tests that there is no burning of motor wires even if the basic clutch force FBa is increased to approximately 4 kg. 
     The present invention is not limited to the embodiment described above. It goes without saying that various modifications are possible as long as there is no departure from the essence of the present invention. 
     In the present invention, the arc-shaped surfaces of the sector-shape sliding contacts which rotate together with a first rotary element are installed so that they press against the inner surface of the circumferential wall of a second rotary element. Accordingly, the second rotary element, which is in a form of a drum, does not generate any vibration which causes impact noise of the engagement assembly and engaging recess. Thus, the clutch force (frictionally transmitted force) between the first and second rotary elements can be increased. Accordingly, a clutch for use in electrically driven telescopic antennas, in which the impact noise caused by repeated engagement and disengagement of the engagement assembly with engaging recess of the clutch can be remarkably decreased and in which there is virtually no danger that the antenna element might drop by its own weight, can be obtained.