Cable Plug

Abstract

A cable plug for electric lines, in particular for use in measuring and monitoring systems with piezoelectric signal generators, comprising a plug casing having an insulation insert provided with bore holes, each bore hole containing a pre-stressed radially resilient twin-sleeve, one end there-of receiving a contact pin provided at the end of the cable conductor, while a connection pin can be inserted into their other end.

Description

The invention relates to a cable plug for the attachment or mutual connection of electric lines, in particular for measuring and monitoring systems with piezoelectric signal generators.

Cable plugs of this type are used in piezoelectric measuring and monitoring technology for connection of the measuring cables to the signal generators. As is well known, they are provided with a plug casing which contains an insulating insert preferably consisting of ceramic material. The insulating insert contains a bore hole for each cable conductor, in which there is a contact element associated with the conductor for resilient mounting of a connection pin. Recently monitoring of the vibrations produced by rotating machinery, by means of piezoelectric oscillators, has come to be an accepted method. By further development of the piezoelectric signal generators, in particular accelerometers, it has become possible to mount the signal generators even on relatively hot parts of machinery, particularly on turbine engines of aeroplanes. Since modern generators can be used without restriction even at temperatures of more than 600.degree.C this did not produce any problems. On the other hand, difficulties are often encountered when transmitting the measuring signals from the generator to the appropriate electronic systems and devices, the cable connections at the generator end and the cable parts close to the generator being particularly critical.

Depending on the requirements in each given case, there are essentially two different designs of generator cables in use. One design is to a large extent conventional and possesses a flexible insulation which consists of plastics resistant to elevated temperatures, such as polytetrafluorethylene (Teflon). These cables can be employed up to a temperature of 300.degree.C. Basically different is the design of the so called stiff, minerally insulated cables in which steel conductors are embedded in an inorganic insulating material, generally magnesium oxide enclosed by a steel tube. These cables tolerate temperatures up to above 1,000.degree.C. However, since the inorganic insulating material is hygroscopic, i.e. it is capable of absorbing moisture from the air, the cable ends must be hermetically sealed, for the absorption of moisture would cause the insulating value of the cable to drop in an undesirable manner. Hence, with such a cable, the cable plug is subject to special requirements, especially in respect of sealing, particularly if it is to be used in aeroplanes, since, owing to the varying flight altitudes, there arise pressure differences which make it easier for air moisture and impurities to penetrate.

If piezoelectric accelerometers are used, which, as is known, supply charged signals that have to be transmitted through a measuring cable to a more remotely located charge amplifier, it is in addition necessary, for both the connecting line and the cable plug to possess a high insulation value which is maintained even at high temperatures. Besides, the production of interference charges in the cable and in the plug must be avoided.

The cable plugs of the initially described design, which have been used so far, satisfy these requirements only in part. It is true, that by the use of ceramics for the insulation insert, a correspondingly high temperature resistance is achieved. However, the design of the known plugs is relatively complicated. Bearing in mind the large variations in thermal expansion between the contact elements which are designed as metallic cylindrical bushes, the ceramic insulation insert and the plug casing, the insulation insert is loosely fitted within the casing, a Collar-type projection being provided for its retention and an annular spring located in an enlarged section of the casing being axially supported by the said projection. For the same reason, also the contact elements within the insulation insert are loosely arranged. In order to enable the plug components to be assembled, the insulation insert must be constructed of several parts. A further difficulty consists in the fact that effective sealing of the cable end is in practice virtually impossible owing to the complicated design of the plug. It was further found that with the known plug design the production of interference charge can hardly be avoided.

It is the object of the invention to avoid these difficulties and to create a simple cable plug which is sufficiently temperature-resistant, enables secure hermetical sealing of the cable end, and avoids the production of interference charges. In this respect, the invention is based on the fact that the interference charges found with the known design are caused by so-called tribo-electric effects, i.e. by frictions and changes in capacity owing to relative motions between the plug components. Hence it is also an object of the invention to avoid, as far as possible, the use of plug components capable of moving in relation to one another.

With the cable plug in accordance with the invention this problem is solved by inserting as contact elements pre-stressed radially resilient twin-sleeves directly and without play into the bore holes of the insulation insert, one end of the said twin-sleeves receiving a contact pin provided at the conductor end while a connection pin can be inserted into their other end. Preferably, at least one prestressed, radially resilient spring sleeve is provided also between the insulation insert and the plug casing. These radially resilient intermediate components retain the contact element in the insulation insert and also the latter within the plug casing in a largely immovable manner, so that no relative motion causing tribo-electric effects can occur between the plug components during the measurement of vibrations. Nevertheless, the elastic retention of the plug components enables even major variations in thermal expansion to be compensated. In addition cable assembly can be made simple since it is sufficient for insertion pins to be crimped or welded at the cable ends, the said insertion pins being introduced into the cable plug from the rear. The insertion pins engage within the rear half of the radially resilient twin-sleeves and clamp them fast directly within the bore holes of the insulation insert, the pressure being relatively high. As a result, the twin-sleeves in the insulation insert are safely anchored and a good electrical contact is achieved without additional fastening means. There is no need for cylindrical contact bushes of the type used hitherto, which contain resilient contact elements by way of inserts and must be separately anchored. Furthermore, the plug design in accordance with the invention is also advantageous for connecting two cable ends.

It is true, that twin-sleeves as contact elements for plug devices are as such already known from the Swiss patent No. 373.439. These consist of a tubular support element inside which there are needle-type contact springs which receive a contact pin. These twin-sleeves are, however, not resilient in the radial outward direction. Just as the known simple contact bushes they are rounded with an insulating sheath consisting of rubber, molded material or plastic, in order to protect them against contact, or they inserted into metallic contact bushes, which, in turn, are anchored within a plug. However, special retaining devices are necessary for this purpose, and appropriate play must be provided for in order to compensate any variations in thermal expansion which may occur.

On the other hand, with the cable plug design in accordance with the invention, whereby the twin-sleeves are radially resilient and are clamped fast directly within the insulation insert itself without intermediate metal bushings, no projections or shoulders are required in order to anchor the insulation insert within the plug casing. Hence, the casing can be designed in a compact manner with a relatively small diameter while maintaining the specified contact spacings. It is, therefore, possible, if the design embodies a union nut, this being normally provided for attachment of the plug to a signal generator, to design in accordance with a further characteristic of the invention the insulation insert as an integral cylindrical body with a substantially uniform outer diameter, while designing the plug casing where it adjoins the union nut with a smaller outer diameter than the inner diameter of the union nut, the union nut being so designed that it can be pushed back over the plug casing in order to mount or replace a sealing ring. Thus, provided that the cable plug has been designed in accordance with the invention, it is for the first time possible to achieve secure and at the same time simple sealing between the cable plug and the signal generator.

The drawing illustrates embodiments of the cable plug in accordance with the invention.

FIGS. 1 and 2 each show a specimen embodiment of the plug in axial section along the centre line,

FIG. 3 shows a detail in side view and

FIG. 4 shows a section along the line IV--IV in FIG. 3.

In both embodiments the cable plug consists of a substantially cylindrical plug casing 1, which for production reasons consists of two parts welded together and is provided with a union nut 2 for attachment of the cable plug to a signal generator which is to be connected. A sealing ring 3 is mounted on plug casing 1 in order to provide the required sealing action, the said sealing ring being pressed by the union nut 2 against a collar of the casing and thus ensuring secure sealing of the cable plug. After pushing the union nut 2 back over casing 1, the sealing ring 3 can be easily mounted and replaced. Within the plug casing 1 there is an insulation insert 4 which consists of an integral cylindrical body produced from ceramic material and having two axial bore holes 5 and 6. In each of the bore holes 5 and 6 there is a twin-sleeve 7 which serves as a contact element.

FIGS. 3 and 4 show such a twin-sleeve 7. This consists of a roughly hollow cylindrical bushing with three spaced annular sections 8, narrow spring laminations 9 curving radially inwards being provided between the said annular sections. Twin-sleeve 7 is provided with a longitudinal slot 10 passing also through the annular sections 8, as a result of which the said twin-sleeve is radially resilient. In the unstressed condition the twin-sleeves 7 have a larger diameter than the bore holes 5 and 6 of insulation insert 4 and they are inserted into the latter in a prestressed condition so that the annular sections 8 are firmly pressed against the bore holes as can be seen from FIG. 1 and 2. Hence they are retained securely and without play in the bore holes 5 and 6, and are yet capable, owing to their elasticity, to compensate, even with large changes of temperature, differences in thermal expansion with regard to insulation insert 4. Between the insulation insert 4 and plug casing 1 there is a resilient sleeve 11 designed in a similar manner as twin-sleeves 7, the said resilient sleeve being likewise provided with resilient laminations and a longitudinal slot so that it retains the insulation insert 4 securely within the plug casing and is also capable of compensating variations in thermal expansion.

A contact pin 12 is inserted into the inner end of each twin-sleeve 7, the said contact pin being attached in an electrically conducting manner to one end of a conductor 13 of the cable 14 which is fitted within the plug. The resilient laminations 9 of twin-sleeve 7 also ensure good electrical contact between the contact pin 12 and the twin-sleeve 7. At the same time, insertion of contact pin 12 causes the twin-sleeve 7 which is resilient in a radial, outward direction, to be pressed under relatively high pressure against the wall of bore hole 5 or 6 and thus to be securely fixed within insulation insert 4. The front end of twin-sleeve 7 is meant to receive connection pins, e.g. the connection pins of a signal generator. However, it is also possible to insert the contact pins of another cable into the front section of the twin-sleeves, so that the cable plug in accordance with the invention can also be used for mutual connection of electrical cable lines without any change in its basic design.

The cable plug shown in FIG. 1 is connected to a flexible cable 14 designed in a conventional fashion. The cable is mounted within the plug in a such known manner by means of a union nut 15 serving as a pressure screw and being screwed onto the plug casing 1, with the aid of a pressure ring 16 acting on an elastic element 17. In addition, an earth connection ring 18 is provided, which presses the end of screen 19 of the cable in an electrically conducting manner against the plug casing 1. Two union nuts 2 and 15 are provided with milled recesses 20 and 21 which are located opposite one another. In order to secure the union nuts 2, 15, the threads of which are handed in opposite directions, a spring engaging in the milled recesses 20 and 21 can be placed about plug casing 1.

The embodiment shown in FIG. 2 is intended for a stiff, minerally insulated cable having a steel sheath. In order to achieve hermetic sealing, the steel sheath of cable 14 is brazed into a casing 22 which consists of several parts. The two conductors 13 are taken out of casing 22 through tubular conduits 23 lined with insulating material whereby the ends of the said conductors are located in tubular contact pins 12. After baking out and evacuation of casing 22 through the contact pins 12, which are still open, the cable is dried and highly insulated, whereupon the exposed terminations of the conductor ends are hermetically welded to the contact pins 12. As shown in FIG. 2 casing 22 which has thus been hermetically sealed is inserted into the plug casing 1, its rear end being welded to plug casing 1 by means of a peripheral welding seam 24. This causes the cable end to be hermetically sealed so that no foreign substances, which might reduce the insulation value of the mineral cable insulation, can enter into it. The contact pins 12 are located in the rear parts of the twin-sleeves 7, as in the embodiment shown in FIG. 1, so that good electrical contact is ensured by the resilient laminations 9. It is extremely simple to fit the cable plug to the cable end.

Claims

We claim:

1. A cable plug for attachment or mutual connection of electrical cable lines and adapted particularly for use in measuring and monitoring with piezoelectric signal generators, comprising:

a plug casing having an opening therethrough;

an insulative insert received within the plug casing opening of external dimensions slightly less than those of said casing opening, said insert including a bore hole for each cable conductor;

a prestressed, radially resilient sleeve received onto said insulative insert and resiliently contacting both said insert and the plug casing walls defining the plug casing; and

a generally cylindrical contact element having a longitudinal slit received in prestressed condition within each bore hole, snugly conforming to the bore hole walls, said element having first and second longitudinally spaced lamination means which are radially resilient, one end of the contact element receiving a contact pin provided at the end of the cable conductor and the other contact element end for receiving a further pin.

2. A cable plug as in claim 1, in which said sleeve includes a generally cylindrical body member of spring metal having a longitudinal slit therein, and laminations at each end extending radially outwardly, the ends of which are received against shoulders formed in the plug casing opening walls.

3. A cable plug as claimed in claim 1, wherein a union nut is provided over the casing in order to attach the plug to a signal generator, the insulative insert being formed into an integral cylindrical body with a substantially uniform outer diameter and the plug casing in its section adjoining the union nut having an outer diameter which is smaller than the inner diameter of the union nut, whereby the union nut can be pushed back over the plug casing for mounting and replacing a sealing ring.