Abstract:
An apparatus for monitoring the closed position of a locking gas cap ( 10 ) with a magnet ( 32 ) disposed at or in the locking gas cap and a solenoid ( 42 ) disposed at the tank connection pipe ( 12 ), the locking gas cap ( 10 ) having a torque limiter ( 36 ), and the magnet ( 32 ) being coupled with the torque limiter in such a manner, that it reaches a position, in which the solenoid ( 42 ) is triggered, only when the limiting torque is reached.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an apparatus for monitoring the closed position of a locking gas cap, with a magnet disposed at or in the locking gas cap and a magnetic switch disposed at the tank connection pipe. 
     An apparatus of this type is described in the DE 44 04 014 A1 and serves to monitor the closed position of the locking gas cap of a motor vehicle so that, when the locking gas cap is not closed or closed incompletely, a warning signal appears on the dashboard of the vehicle. 
     In the case of the known apparatus, the locking gas cap has a bayonet catch and the magnet is disposed in such a manner, that it is in the vicinity of a reed switch, when the bayonet catch is locked. Depending on the construction, the reed switch is thus either open or closed when the locking gas cap reaches the locked position. Preferably, the reed switch is constructed so that it is closed in the locked position and that an associated evaluating circuit causes a warning signal to be displayed when the circuit of the reed switch is interrupted. 
     Since the magnetic field of the magnets cannot be localized to a narrow limited space, the position of the locking gas cap can be determined only relatively inaccurately according to this principle. For many tank caps, the locking gas cap is constructed as a screw-in plug. However, even in the case of bayonet-like tank caps, screw pitch surfaces are usually provided, which ensure that the plug, in the closed position, is pressed firmly against an associated seal. The possibility therefore exists that the locking gas cap is not turned completely into the end position, in which the tank opening is sealed reliably and in which a subsequent loosening of the locking gas cap due to vibrations is prevented because of the frictional engagement. However, because of the aforementioned inaccuracy in determining the position, the reed switch would also respond in such a case, so that the correct closing of the locking gas cap cannot be indicated reliably. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to make it possible to monitor the closed position of the locking gas cap with greater reliability. 
     Pursuant to the invention, this objective is accomplished owing to the fact that the locking gas cap has a torque limiter and the magnet is coupled with the torque limiter in such a manner, that it reaches a position, in which the magnetic switch is triggered, only when the limiting torque is attained. 
     By these means, it is ensured that the magnetic switch responds only when, while screwing in the locking gas cap, the limiting torque is actually reached and it is thus ensured that the locking gas cap is effectively closed tightly and completely. 
     A locking gas cap with a screw-in plug and a torque limiter is described already in DE 196 10 471 C2. This locking gas cap has a cap, which is rotatably disposed on the screw-in plug and is provided with a handle. The torque, which is exerted by the user on the cap, is transferred by the torque limiter to the screw-in plug. As soon as the limiting torque is exceeded when the closed position is reached, the cap rotates relative to the screw-in plug. In accordance with an advantageous development of the present invention, this relative movement is used for the purpose of transferring the magnet into the triggering position. Preferably, the magnet is held axially movable in the locking gas cap and, at the cap or at the plug of the locking gas cap, inclined surfaces are provided, which convert the rotation of the cap relative to the plug into an axial motion of the magnet. The magnet is pre-stressed elastically in the position at rest and is converted into the release position only when the limiting torque is reached by the inclined surfaces. At the same time, the inclined surfaces and the associated mating surfaces can be constructed in such a manner, that they can slide off one another when the cap is rotated further after it has reached the limiting torque. In this case, it is possible that the magnet springs back once again after it has exceeded the limiting torque and is removed from the release position. Preferably, this spring-back motion is, however, limited so that the magnetic switch, because of the magnetic remanence of the reed contacts, nevertheless remains closed. Only when the locking gas cap is loosened once again or removed completely, does the distance between the magnets and the magnetic switch become so large that the switch opens up. In this case, therefore, the magnetic switch exhibits some hysteresis behavior. It closes only when the limiting torque is reached or exceeded at least once, but then remains closed when the magnet once again is removed further from the magnetic switch. 
     Alternatively, an embodiment is also conceivable, for which the magnet, in the release position, is relatively close to a magnetizable body, so that, upon reaching the release position, it is held magnetically in the release position and springs back once again into the position at rest only when it is screwed out of the locking gas cap. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, examples of the invention are described in greater detail by means of the drawings, in which 
     FIG. 1 shows a section through a locking gas cap when the closed position in a tank connection pipe is reached and 
     FIG. 2 shows a section through the locking gas cap in a position after the limiting torque of a torque limiter is exceeded. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, a locking gas cap  10  is screwed onto a connection pipe  12  of a fuel tank of a vehicle. The locking gas cap has a screw-in plug  14  and a cap  16 , which is connected rotatably with the screw-in plug and forms a handle  18 . In the opening of the tank connection pipe  12 , a threaded insert  20  is fastened, which has engaged the external thread of the screw-in plug  14 . FIG. 1 shows the locking gas cap in the closed position, in which a flange  22  of the screw-in plug, over a seal  24 , is in sealing contact with the edge of the threaded insert  20 . 
     In the interior of the screw-in plug  14 , a pot-shaped inner part  26  and, further to the outside, that is, further towards the top in FIG. 1, a guiding bush  28  is disposed, in which a magnet carrier  30  is guided axially movably. A magnet  32  (permanent magnet) is held with holding claws at the magnet carrier  30  and lies within the pot-shaped inner part  26 . The guiding bush  28  at the same time forms an abutment for the springs  34 , which place the magnet carrier  30  and the magnet  32  elastically under tension in an upwards direction in FIG. 1 in a position at rest. 
     A known torque limiter  36  is effectively disposed between the cap  16  and the screw-in plug  14 . When the locking gas cap is screwed onto the tank connection pipe, the rotational movement of the cap  16  is transferred by this torque limiter  36  to the screw-in plug  14  until a specified limiting torque is attained. This limiting torque is selected so that the screw-in plug  14  is then screwed firmly into the threaded insert  20  and closes off the tank connection pipe tightly. The torque limiter  36  is indicated in FIG. 1 merely by broken lines and is formed by springs, which are held at the cap  16  and engage a ring of notches surrounding the guiding bush  28  (see DE 196 10 471 C2). In the state shown in FIG. 1, the limiting torque has just been exceeded, so that the spring has been displaced from the associated notch. 
     At its upper end protruding into the handle  18 , the magnet carrier  30  has a ring of cams  38 , which are skewed in the peripheral direction and interact with the releasing devices  40 , formed in the handle  18 . When the limiting torque of the torque limiter is exceeded, the cap  16  turns relative to the screw-in plug  14  and, with that, also relative to the magnet carrier  30 . The releasing devices  40  therefore slide on the skewed cams  38  and force the magnet carrier  30  downward, against the force of the springs  34 , into the release position shown in FIG.  1 . In this position, the magnet  32  brings about the closing of the reed contacts of a magnetic switch  42 , which is disposed on the outside at the tank connection pipe  12 . In this manner, a signal is generated, which indicates the complete closing of the locking gas cap. 
     If the cap  16  is turned further, the releasing devices  40  slide from the cam  38 , and the magnet carrier  30 , under the action of the spring  34 , rebounds up once again, so that it assumes the position at rest, shown in FIG.  2 . In this position, the magnet is further removed from the solenoid  42 . If the magnetic switch  42  previously was closed, it remains closed because of the magnetic remanence in the closed state. 
     When the locking gas cap  10  is screwed out of the tank connection pipe  12 , the distance between the magnet  32  and the magnetic switch  42  becomes larger, so that the magnetic switch opens up. Since the torque limiter  36  can act only in one direction of rotation, a torque, unlimited in principle, can be transferred to the screw-in plug  14  as it is being screwed out. 
     Subsequently, if the locking gas cap is screwed back once again onto the tank connection pipe and has reached once again the position shown in FIG. 2, the magnetic switch  42  remains open, since the force of the magnet  32  alone is insufficient for bringing the reed contacts into the closed position. Only when the limiting torque is exceeded once again and, at the same time, the magnet carrier  30  is moved once more into the release position shown in FIG. 1, does the magnetic force become so large that the magnetic switch  42  is closed once again and then remains closed. 
     In a modified embodiment, it is also possible to form the torque limiter directly by the cams  38  and the releasing devices  40 .