Abstract:
A cable bushing which is small, simple and cost-effective as possible, having a cable, a housing, though which the cable is led, and which has a radially inwardly extending step at its first end, a first sleeve, which engages around the cable and which has a first section arranged inside the housing. The first sleeve is held by the step in the housing, the first sleeve bearing in a seal-forming fashion against the cable and there being a sealing means between the step in the housing and the fist sleeve. A second sleeve is arranged in the interior of the housing and has a first cylindrical section which engages closely around the cable, the first sleeve having a second section which adjoins an end of the first section which faces the step, said second section bearing on the first section of the first sleeve, and an attachment element by means of which the second sleeve is pressed against the first sleeve.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application relies for priority on a prior filed provisional application, filed Mar. 15, 2001, and assigned Appln. Ser. No. 60/275,711. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a cable bushing for a measuring probe. 
     2. Background Discussion 
     Measuring probes, for example pressure measuring probes, are frequently used by introducing them into a container on a cable for example. The cable is used here to form an electrical connection to the measuring probe and for its mechanical attachment. Particularly suitable for this are cables which have on the outside a metallic shield which is surrounded example, by a plastic. The supply lines andlor signaling lines can then lead in the interior of the shield, protected against external influences to the probe. 
     The cable leads from the probe out of the container to a superordinate unit, for example a power supply and/or further electronics. 
     One application is the filling level measurement using a pressure measuring probe. Here, the pressure measuring probe is introduced on the cable into a container filled with a material. The pressure determined by the pressure measuring probe corresponds to the height of the material above the pressure measuring probe and is therefore a measure of the filling level in the container. 
     In these applications, in order to provide a seal, for example to ensure that no material escapes or penetrates downstream electronics and/or a superordinate unit, a cable bushing is necessary which has a housing through which the cable is led. The housing can be part of the superordinate unit here and/or also part of an attachment device for the probe, by virtue of the fact that the housing is mounted at the measuring location , for example on a container, by means, for example, of a flange or a thread. 
     The cable bushing can equally be arranged on the probe and connected, for example, to the probe or a probe housing, and ensure that no substance penetrates the probe. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a cable bushing for a probe which is as small, simple and cost-effective as possible. 
     For this purpose, the invention consists in a cable bushing for a probe having 
     a cable, 
     a housing, 
     through which the cable is led, and 
     which has, at a first end, a step which extends 
     radially inward, 
     a first sleeve, 
     which engages around the cable and 
     which has a first section arranged inside the housing, the first sleeve bearing against the cable in a seal-forming fashion, and 
     a sealing means being provided between the step in the housing and the first sleeve, 
     a second sleeve, which is arranged inside the housing, 
     which has a first cylindrical section which engages closely around the cable, 
     which has a second section which adjoins an end of the first section which faces the step, which bears on the first section of the first sleeve, and 
     an attachment element, 
     by means of which the second sleeve is pressed against the first sleeve. 
     According to one embodiment, the attachment element is a ring nut screwed into the housing in the direction facing the step. 
     According to another embodiment, the second sleeve is metallic and is pressed with the cable in order to provide strain relief to the cable. 
     According to a further embodiment, the cable is surrounded on the outside by a metallic shield, the second sleeve is metallic and the second sleeve forms an electrically conductive connection between the housing and the metallic shield. 
     According to yet another embodiment, the first sleeve is composed of a plastic, the cable has a coating made of a plastic and the first sleeve is welded to the coating. 
     According to a first preferred embodiment, the step in the housing is a radially inwardly extending shoulder, and the sealing means is a seal which bears on the shoulder, which is clamped in between the shoulder and the first section of the first sleeve. 
     According to a second preferred embodiment, the step in the housing is a radially inwardly extending shoulder, and the sealing means is provided by means of an inner edge of the shoulder onto which the first section of the first sleeve is pressed by means of the attachment element. 
     According to another embodiment, a spring is clamped in by means of the attachment element and exerts on the first sleeve a force in the direction facing the step. 
     According to yet a further embodiment of the development, the spring bears on an annular plate which bears on the first sleeve. 
     According to a third preferred embodiment, the step in the housing has a conical inner casing surface whose diameter decreases in the direction of the first end of the housing, the first sleeve is composed of an elastomer, the first section of the first sleeve has a conical region, and the first sleeve is pressed against the step in the housing by the attachment element in such a way that the conical region of the first sleeve presses against the conical casing surface of the housing and the first housing bears in a seal-forming fashion against the housing and the cable. 
     According to a particular embodiment, 
     the housing has a first part which comprises the first end of the housing, and a second part, 
     the second part is screwed onto the first part in the direction facing the step, 
     the attachment element is a radially inwardly extending step arranged in the interior of the second part, 
     which bears on the second sleeve and 
     which presses the second sleeve in the direction facing the step as a result of the second part being screwed on. 
     One advantage of the invention consists in the fact that the cable bushing has a very small number of small-dimensioned components. 
     A further advantage consists in the fact that in the cable bushing according to the invention, not only is there a strain relief for the cable but also at the same time the shield is connected to ground or to a reference potential via the housing. 
     The invention and its advantages will now be explained in more detail with reference to the figures in the drawing in which four exemplary embodiments are illustrated; identical elements are provided with the same reference symbols in the figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic view of a cable bushing having a first sleeve which is welded to a cable in which a seal is arranged between the housing and the first sleeve; 
     FIG. 2 shows a schematic view of a cable bushing having a first sleeve which is welded to a cable, in which the sleeve bears in a seal-forming fashion on an inner edge of the housing; 
     FIG. 3 shows a schematic view of a cable bushing having a first sleeve made of an elastomer, which first sleeve bears in a seal-forming fashion against the cable and against the housing; and 
     FIG. 4 shows a schematic view of a cable bushing in which the housing has two parts screwed to one another. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a schematic view of a cable bushing according to the invention for a probe. It has an essentially cylindrical housing  1  through which a cable  3  is guided. 
     The cable  3  is used for the mechanical attachment and electrical connection of a probe (not illustrated in FIG.  1 ), for example a pressure measuring probe. Lines (not illustrated in FIG. 1) run in the interior of the cable  3  and are used to make the electrical connection. The lines are surrounded by a metallic shield  5 , for example a wire mesh. The shield  5  is covered with a cable jacket  7  made of a plastic, for example made of polyethylene (PE). 
     The housing  1  has, at a first end, a radially inwardly extending step. In the exemplary embodiment illustrated in FIG. 1, the step is a shoulder  9  which extends radially inward. 
     A first sleeve  11 , which engages around the cable  3 , is provided. The sleeve  11  is composed of a plastic. It is preferably composed of the same plastic as the cable jacket  7  of the cable  3 , that is to say for example of polyethylene (PE). 
     The sleeve  11  has a first section  13  which is arranged inside the housing  1  and a second section  14  which is arranged outside the housing. 
     The first sleeve  11  bears in a seal-forming fashion against the cable  3 . In the exemplary embodiment illustrated in FIG. 1, this is achieved by the sleeve  11  being welded to the cable jacket  7  of the cable  3 . 
     Furthermore, a sealing means is present between the step in the housing  1  and the first sleeve  11 . In the exemplary embodiment illustrated in FIG. 1, this sealing means is a seal  15 , for example an O ring, which bears on the shoulder  9  in the housing  1  and is clamped in between the shoulder  9  and the first section  13  of the first sleeve  11 . 
     A second sleeve  17  is arranged on the side of the first sleeve  11  facing away from the step, in the interior of the housing  1 . The second sleeve  17  is composed of a metal, for example a stainless steel. It has a first cylindrical section  19  which engages closely around the cable  3 . Furthermore, it has a second section  21  which adjoins an end of the first section  19  which faces the step and which bears on the first section  13  of the first sleeve  11 . 
     The cable jacket  7  of the cable  3  is removed on a side of the first sleeve  11  facing away from the step, and the second sleeve  17  is fitted onto the exposed metallic shield  5 . In order to provide strain relief to the cable  3 , the second sleeve  17  is pressed with the cable  3 . 
     The second sleeve  17  preferably forms an electrically conductive connection between the housing  1  and the metallic shield  5 . For this purpose, the second section  21  of the second sleeve  17  is shaped in such a way that it is in contact with the housing  1  and as a result produces an electrically conductive connection. In the exemplary embodiment illustrated in FIG. 1, the second section  21  of the second sleeve  17  has an external diameter which is equal to an internal diameter of the housing  1 . The conductive connection to the cable  3  is provided by pressing the metallic second sleeve  17  onto the metallic shield  5 . This electrical connection ensures that the metallic shield  5  is at the same potential as the housing  1  to protect the lines running in it against electromagnetic interference. This can be, for example, ground or a fixed reference potential. 
     In the housing  1 , an attachment element is provided by means of which the second sleeve  17  is pressed against the first sleeve  11 . In the exemplary embodiment illustrated in FIG. 1, the attachment element is a ring nut  23  which is screwed into the housing  1  in the direction facing the step. The ring nut  23  engages around the first section  19  of the second sleeve  17  and bears with a ring-washer-shaped end face on the second section  21  of the first sleeve  17 . The second sleeve  17  is pressed against the first sleeve  13  by the ring nut  23 , As a result, the first sleeve  13  is in turn pressed against the shoulder  9  of the housing  1  and thus causes the seal  15  to be clamped in such a way that it exerts its sealing effect. FIG. 2 shows a further exemplary embodiment of a cable bushing according to the invention. Owing to the large degree of correspondence with the previously described exemplary embodiment, only the differences are explained in more detail below. The step in the housing  1  is also a radially inwardly extending shoulder  9  in the exemplary embodiment illustrated in FIG.  2 . However, the sealing means between the housing  1  and the first sleeve  11  is not provided by means of a seal clamped in between the shoulder  9  and the first section  13  of the first sleeve  11  but rather by means of an inner edge  25  of the shoulder  9  onto which the first section  13  of the first sleeve  11  is pressed by the ring nut  23 . A spring  27 , which exerts a force on the first sleeve  17  in the direction facing the step is preferably clamped in by the attachment element. In the exemplary embodiment illustrated in FIG. 2, the spring  27  is clamped in between the ring nut  23  and the second sleeve  17 . The spring  27  brings about that a force which acts on the first sleeve  11  through the ring nut  23  and is necessary for the sealing means, is relatively constant and sufficiently large even if the cable bushing is exposed to large temperature fluctuations which lead to different degrees of thermal expansion of the individual components of the cable bushing. 
     FIG. 3 shows a further exemplary embodiment of a cable bushing according to the invention. Owing to the large degree of correspondence with the exemplary embodiments described above, here too only the differences are explained in more detail below. 
     In the exemplary embodiment illustrated in FIG. 3, the step in the housing  1  has a conical inner casing surface  29  whose diameter decreases in the direction of the first end of the housing  1 . The first sleeve  11  is composed of an elastomer, for example of a rubber, and the first section  13  of the first sleeve  11  has a conical region  21  which is conforming with the conical inner casing surface  29 . The first sleeve  11  is pressed by the ring nut  23  against the step in the housing  1  in such a way that the conical region  31  of the step  11  is forced into the conical casing surface  29  of the housing  1 , and the first sleeve  11  bears in a seal-forming fashion against the housing  1  and against the cable  3 . 
     Here too, a spring  27  is provided between the ring nut  23  and the second sleeve  17 , said spring  27  ensuring that, even when there are different degrees of thermal expansion of the individual components of the cable bushing, a sufficient force is always exerted on the first sleeve  11  in the direction facing the step, in order to ensure the seals between the sleeve  11  and the cable  3  and between the sleeve  11  and the housing  1 . 
     FIG. 4 shows a further exemplary embodiment of a cable bushing according to the invention. Owing to the large degree of correspondence with the previously described exemplary embodiment, here too, only the differences are explained in more detail below. 
     In the exemplary embodiment illustrated in FIG. 4, the housing  1  has a first part  33  surrounding the first end, and a second part  35 . The first part  33  has a cylindrical section  37  and an adjoining conical section  39  which tapers in the direction of the first end of the housing  1  to the external diameter of the cable  3 . The conical section  39  forms the step in the housing  1  by means of which the first sleeve  11  is held in the interior of the housing  1 . The first sleeve  11  is arranged completely within the conical section  39 , Here too it is composed of an elastomer, for example rubber, and bears in a seal-forming fashion against the housing  1  and the cable  3 . For this purpose, the first sleeve  11  engages closely around the cable  3  and has an externally cylindrical region facing away from the step and an externally conical region which faces the step and tapers in the direction facing the step to the external diameter of the cable  3 . 
     The second part  35  of the housing  1  is screwed onto the first part  33  in the direction facing the step. For this purpose, an external thread is provided on the cylindrical section  37 . In the direction facing the step, underneath the external thread, a radially outwardly extending stop  41  is provided, against which the second part  35  is screwed. 
     In the direction facing away from the step, above the stop  41 , a radially outwardly extending shoulder  43  is provided inside the second part  35 , which shoulder  43  forms, together with the stop  41 , a groove for receiving a seal  45 . The seal  45  ensures that an annular-cylindrical gap between the two housing parts is sealed. 
     In the interior of the housing  1 , a spring  27  is arranged between the first and second sleeves  11 ,  17 . The spring  27  bears on an annular plate  47  which bears on the first sleeve  11 . The annular plate  47  provides the advantage that a force exerted by the clamped-in spring  27  is transmitted in the direction facing the step uniformly to the sleeve  11 . 
     The force which is necessary for clamping in the spring  27  is applied by means of the attachment element. In the exemplary embodiment illustrated in FIG. 4, the attachment element is a radially inwardly extending step  49  which is arranged in the interior of the second part  35  and which bears on the second sleeve  17 . As a result of the second part  35  being screwed onto the first part  33 , the step  49  presses the second sleeve  17  in the direction facing the step. 
     In the exemplary embodiment illustrated in FIG. 4, the metallic shield  5  is not a stable metallic wire mesh but rather a thin metal foil. The second sleeve  17  is fitted onto the outside of the cable jacket  7  of the cable  3  and pressed with the cable  3 . On a side of the second sleeve  17  facing away from the step, the cable jacket  7  is removed and the lines are exposed. A filler stranded conductor  51  which is electrically conductively connected to the metallic foil has a blank line element which is clamped in between the cable jacket  7  of the cable  3  and the second sleeve  17 . There is thus, as in the previously described exemplary embodiments, an electrically conductive connection between the housing  1  and the metallic shield  5  by means of the second sleeve  17 . Laying a filler stranded conductor between the cable jacket  7  and the second sleeve  17  can, of course, also be used to form the desired electrical connection in the previously described exemplary embodiments.