Arrangement for Elastically Clamping Glass Fibre Rods

The invention relates to fixing of sheaths on glass fibre reinforced plastic rods, particularly where the glass fibre rod constitutes the load bearing core of a composite electric insulator. This invention relates to metal sheaths and also relates to sheaths of any other materials besides metal, so long as the coefficient of expansion of the sheath material differs from that of the glass fibre rod.

A problem arises from application of force by metal sheaths or mountings into glass fibre rods. Because of the generally unidirectional fibre reinforcement of such a rod, both its strength and its elastic and plastic deformability in the longitudinal direction and in the transverse direction are markedly different. Glass fibre reinforced plastic rods also differ from the metal or other materials used in their sheathing in other significant properties.

Various affixation methods are known for obtaining pull proof connections between sheaths or mountings and the ends of glass fibre rods. Thus, German Laid-open Patent Specification (DAS) 1,261,358 and Patent of Addition 1,400,003 describe a method of producing a connection between a metal sheath and a plain rod or tube of plastic material, wherein as the sheath is pressed onto a glass fibre rod, the specific pressing forces decrease from the end of the rod toward the point of its emergence from the sheath. Because of the markedly different elastic elongation and the markedly different elasticity limits of the glass fibre rod and the sheath materials, clamping or gripping lengths equal to six times the diameter of the glass fibre rod are necessary in order to make thorough use of the strength of the rod. With high stresses, relative motion between the highly elastic rod and the sheath cannot, however, be avoided.

Another method for affixing of metal sheaths on glass fibre rods is described in German Laid-Open Pending Patent Application (DOS) 1,921,229. It comprises deforming the ends of the plastic impregnated glass fibre rod and the surrounding parts of the sheath in an undulating fashion. A cap is cemented on the end of the insulator. That end of the insulator is split and is given a conical shape by a wedge. However, the connection of the insulator is disturbed by the cleaving operation and the cleaving impairs its mechanical strength. Moreover, the glass fibre rod may be enclosed at its ends by a bush having a central opening that tapers conically toward the centre of the insulator, and an external thread. To connect the glass fibre rod with this bush, the end of the glass fibre rod that is engaged in the bush is fanned out by driving a conical pin into it, so that the glass fibre rod is applied against the wall of the conical opening of the bush.

Finally, it is also known to provide the sheath with at least one inner and outer adhering surface and to force the glass fibre rod against it by means of an external clamping element. With such compression or clamped joints, disadvantages occur under unfavorable circumstances in that as a result of subsequent shrinkage of the plastic or by reason of the different coefficients of thermal expansion of the metal sheath and the plastic rod, the insulator becomes detached from the sheath.

In all of the above prior arrangements, the tensile strength of the glass fibre rod cannot be fully employed. Fundamentally, all of these wedge shaped and conical connections between the sheath and the plastic rod permit only a very limited utilization of the strength of the rod, since with such rigid connections, only the foremost section of the sheathing is able to transmit forces. Those sections of the sheathing which are located further toward the end of the rod are largely relieved of load by the different elastic properties of the glass fibre rod. Furthermore, all of these forms have the inherent disadvantage that the comparatively highly elastic glass fibre rod is connected directly to the rigid material of the sheath.

The present invention seeks to achieve a substantially uniformly distributed introduction of force from the sheath of metal or other material into the glass fibre rod. Moreover, a suitable accommodation of the thermal expansion between the sheath material having one coefficient of expansion and the glass fibre rod having another coefficient of expansion is to be produced, so that the reliability of the affixation of the sheath to the glass fibre rod is improved. Also, a high anchoring force with greater continuous tensile strength is obtained.

According to the invention, the sheath is comprised of a plurality of longitudinally arrayed sheath sections. The glass fibre rod is connected elastically to the sheath at at least two of the sheath sections. The compliance of the elastic connection of each of the sheath sections increases steadily from the end of the rod to the point where the glass fibre rod emerges from the sheath, so that the compliance corresponds locally in each sheath section to the elastic elongation of the glass fibre rod in consequence of the mechanical load. The elastic connection is achieved by interposing elastic, compliant buffer members, or by ensuring parts of the sheath and the rod have a high elastic deformation or by providing a compliant material of appropriately varying compliance between the sheath and the rod. The increase in the compliance from the end of the rod to the point where the glass fibre rod emerges from the sheath is obtained by means of increasing the thickness of the individual buffer members and/or by changing the material of the buffer members or of the sheath or by changing the compliance of the elements that join the rod and the sheath in each sheath section. Consequently, the elongation of the glass fibre rod can increase substantially linearly within the sheath from the end of the rod to the point of emergence. The sheath and rod are compensated at each point of force transmission.

The sheath material is selected to have thermal expansion that is substantially adapted to that of the glass fibre rod and the buffer means and the clamping action thereby remains fully operative over a wide temperature range of -50.degree. to +100.degree. C.

The above described and other features of the invention will be described hereinafter with reference to the drawings.

FIG. 1 is a side, partially cross-sectional view of a first embodiment of the invention;

FIG. 2 is a side, partially cross-sectional view of a second embodiment of the invention;

FIG. 3 is a side, partially cross-sectional view of a third embodiment of the invention;

FIG. 4 is a side, partially cross-sectional view of a fourth embodiment of the invention;

FIG. 5a is a plan view of one of the force transmission discs used in the fourth embodiment and also in other embodiments;

FIG. 5b is a plan view of another of these discs;

FIG. 6a is an enlarged fragment of the fourth embodiment showing one version thereof;

FIG. 6b is the same type of view as FIG. 6a showing another version of the fourth embodiment;

FIG. 7 is a side partially cross-sectional view of a fragment of a fifth embodiment of the invention;

FIG. 8a is a side elevational view of the upper end of an embodiment of fibre rod used with various embodiments of the invention;

FIG. 8b is a plan view of the arrangement shown in FIG. 8a;

FIG. 9 is a side, partially cross-sectional view of a sixth embodiment of the invention;

FIG. 10 is a side, partially cross-sectional view of a seventh embodiment of the invention;

FIG. 11 is a side, partially cross-sectional view of an eighth embodiment of the invention; and

FIG. 12 is a side, partially cross-sectional view of a ninth embodiment of the invention.

In all of the embodiments of the invention, the transmission of force of motion of the glass fibre rod is transmitted to a plurality of sheath sections A, B, C, D, etc. which are arranged in a longitudinally arrayed series on the glass fibre rod 1. The individual sheathing sections respectively transmit approximately the same proportion of the total force from the sheath to the glass fibre rod.

In the first embodiment of FIG. 1, encircling grooves 2 are formed, for example, by being ground into the glass fibre rod 1. In the individual sheath sections A, B, C, etc., that proportion of the total tension in the glass fibre rod 1 which applies to each sheathing section is transmitted to that section. A cement material 3 located in and around grooves 2 and comprised, for example, of synthetic resin, holds a shoulder piece 5 stationary with respect to rod 1 yet movable with respect to the tubular sheath 4 which is around and spaced from rod 1. There is an abutment piece 7 which is fixedly connected to the sheath 4 in each section A-C, yet which is slidable along the glass fibre rod 1. Connection of the abutment pieces 7 with the sheath 4 is achieved by press fitting the abutment pieces 7 into sheath 4 by plastic deformation of the sheath 4. The shoulder piece and abutment piece in each section are longitudinally spaced apart a distance related to the degree of shifting of rod 1 with respect to sheath 4 in a relative expansion of rod 1. In most sheath sections between the shoulder piece 5 and the respective abutment piece 7, there is an interposed elastic buffer member 6. Suitable elements for the buffer members 6 comprise plastics with a variable modulus of elasticity, metallic spring elements and elastic metal alloys.

According to the embodiment of FIG. 1, both the spaces between shoulder pieces 5 and their respective abutment pieces 7 and the elastic deformability of the buffer members 6 in the individual sheath sections A, B, C, etc., increase toward the point of emergence of the rod 1 from the sheath, 4, i.e., upwardly in FIG. 1. At the same time, the buffer member 6 can be dispensed with in the sheath section A (near the bottom of FIG. 1) and the space between the pieces 5 and 7 in that sheath section A can be minimized because of the slight elongation of the rod 1 at this end.

At the upper end of the sheath 4, there is a control funnel 10 which is inserted in the sheath. The funnel 10 includes a connecting tube 9 which is initially likewise of cylindrical form and is pressed onto the end of the umbrella shell 12 inside of and together with the sheath 4, whereby satisfactory sealing against the penetration of moisture is obtained. The control funnel 10 is provided with an annular array of holes 11 near the base of the funnel, so that no water may be able to collect in it. The umbrella shell 12 is comprised of a material, for example silicone rubber, which is so elastic that it does not impede the relative movement between the glass fibre rod 1 and the sheath 4, as this motion may amount to several millimeters.

The embodiment of FIG. 2 is similar to that of FIG. 1, but with the difference that the cross-section of the glass fibre rod 1 is not reduced by annular grooves. The full strength of rod 1 is now utilized. Moreover, the shoulder pieces 5, which are movable with respect to the sheath 4, are pressed, shrunk or cast rigidly onto the glass fibre rod 1. The abutment pieces 7, which are movable with respect to the rod 1, and the buffer members 6 between pieces 5 and 7 may be made in the form of split rings, so that they may be mounted directly before their introduction into the sheath 4.

In the embodiment of FIG. 3, the full cross-section of the glass fibre rod 1 is likewise utilized. This embodiment makes it simple to assemble. To this end, the shoulder piece 5 is not cemented onto the glass fibre rod. It is instead held on the rod 1 through friction. The shoulder piece 5 is a two part clamping sleeve 11a and 11b with the conical outer surface of inner sleeve 11a and the mating conical inner surface of outer sleeve 11b both having a very shallow cone angle in the range of about 0.5.degree. to 10.degree. . The inner sleeve tapers narrower toward the top end of the rod 1. The inner clamping sleeve 11a presses in against rod 1 and is feathered in order to achieve a uniform distribution of the pressure force on the rod, while the clamping sleeve 11b is preferably of solid form. The shoulder piece 5 is comprised of a metallic material, preferably with a modulus of elasticity 400,000 N/mm.sup.2. It is pressed against the glass fibre rod 1 with a force which increases as the tension of the glass fibre rod 1 increases.

So that the connection of the rod and the sheath caused by the shoulder pieces 5 cannot become loose on removal of the load on the rod, a spring 13, preferably a cup spring, is inserted between the lower, captured end of the glass fibre rod 1 and the closed end of the sheath 4. The spring 13 is depressed by the glass fibre rod 1. If the depression of the spring 13 is effected by means of a threaded pin screwed through the bottom of the sheath, a releasable connection can be created if the cone angle of the parts 11a and 11b is suitably designed.

In this and at least some of the other embodiments, the abutment pieces 7 may be fitted into the sheath 4 not by pressing on, but instead by being screwed into the sheath 4 with the abutment pieces and the sheath being provided with corresponding respective engaging external and internal threads.

In the fourth embodiment of FIG. 4, the glass fibre rod 1, is provided with a chamfer at its bottom end. The rod 1 is pushed into the sheath 4 until the cup spring 13 on the bottom of the sheath is deformed. On removal of the load on the rod 1, self locking of the rod occurs and the rod 1 can no longer be moved back or up in FIG. 4. For this purpose, in FIG. 4, the shoulder pieces 5, which are movable with respect to the sheath, are in the form of unidirectionally self locking, internally feathered, toothed discs 15, and the abutment pieces 7, which are fixed with respect to the sheath but are movable with respect to the glass fibre rod 1, are in the form of externally feathered, self locking, toothed discs 16. Moreover, exact matching of the toothed discs to one another is important.

Full details of the discs 15, 16 are apparent from FIGS. 5a, 5b, 6a and 6b. The externally toothed discs 16 brace themselves with respect to the internal bore of the sheath 4 either through friction against the plain internal bore 17 (FIG. 6a) or positively by means of a rigid, one way motion toothing 18 on the inner face of the sheath 4 (FIG. 6b). The toothing only permits descent of discs 16. The solution according to FIG. 6a has the advantage in comparison with that of FIG. 6b that it can be optimized, an equalization of load being obtained by slipping of some of the toothed discs 15 in the event of local overloading of the sheath and in consequence of inaccuracies of assembly. So that the stack consisting of the toothed discs 15, buffer members 6 and toothed discs 16 may be compressed as far as the outermost abutment piece on assembly, it is useful to employ one or more additional internally feathered, self locking, toothed discs 15.

Returning to FIGS. 6a and 6b, the buffer members 6 are positioned between the toothed discs 5, 15 and the abutment pieces 7, 16 to provide elongation compensation between the rod and the sheath. Intermediate pieces, in particular toothed discs 15, buffer members 6 and abutment pieces 7 may follow directly on one another in each case in this sequence. It is more suitable, however, with a view to a simplified design, especially of the buffer members 6, for a plurality of intermediate pieces to be stacked directly on one another from time to time, then to have a suitably dimensioned buffer member 6 follow and, finally, a larger number of abutment pieces 7.

In the fifth embodiment shown in FIG. 7, the plurality of abutment pieces 7 are replaced by a single solid abutment piece onto which the sheath 4 is pressed. Satisfactory operation is obtained through the correct matching both of the springiness and of the spreading properties of the toothed discs 15 and the abutment pieces 7. Great importance also attaches to the correct choice of the angle of inclination of the toothed discs 15.

FIG. 9 shows a sixth embodiment of the invention. If it is desired to utilize the maximum gripping or retaining force, i.e., clamping with the minimum length is required, it is advantageous to introduce the partial forces in accordance with the sheath sections A, B, C not into the full rod cross-section A.sub.D, but only into the partial cross-sections corresponding to the partial forces. To this end, the rod is so stepped that the differences in cross-section from sheath section to sheath section are in proportion to the forces absorbed by the sheath sections. With a uniform distribution of the total force of the glass fibre rod 1 over the sheath sections, this means that the cross-sections of the rod vary by equal steps at the individual sheath sections. As in other embodiments, buffer pieces 6 compensate for the stresses between the rod and the sheath. In the embodiment of FIG. 9, the introduction of force for the sheath section A.sub.D is solved in accordance with the embodiment of FIG. 7. As shown, abutment piece 7 is press fit in sheath 4.

FIG. 10 shows a seventh embodiment of the invention. The shoulder pieces 5 and the abutment pieces 7 do not appear. The function of the shoulder pieces 5 on the rod 1 is performed by the conical surfaces 18a ground into the rod. The function of the abutment pieces 7 on sheath 4 is performed by the internal conical surfaces 18b formed in the bore of the sheath 4. The buffer means are all formed by one mass of an elastic cement material 19. The increasing compliance of the buffer means from the lower end of the rod 1 to the point where the glass fibre rod 1 emerges from the sheath is achieved by means of suitably adapting the thickness of the elastic cement material 19. In contrast to the embodiments of FIGS. 1 to 9, the buffer means 19 is subjected in this case not only to pressure stresses, but also to shearing stresses.

In the eighth embodiment of FIG. 11, elastic suspension of the glass fibre rod 1 in the sheath 4 is achieved through the individual spring elements 20 being elastically deformable. To this end, individual spring elements 20 are let into the sheath and are so constructed that they increase in their compliance from the end of the rod to the point where the glass fibre rod 1 emerges from the sheath 4. The glass fibre rod 1 is either united with the spring elements 20 via a layer of cement or it is screwed directly into the stack of spring elements. In such case, an external thread must be ground on the rod 1 and the stack of spring elements 20 must have a corresponding internal thread.

In the embodiment of FIG. 12, separate buffer members are absent. Because of this, the entire block of shoulder pieces 15 and/or abutment pieces 16 must be elastic. In this case, the internally toothed discs 15 (as in FIG. 5a) must be made larger toward the end of the assembly from which the glass fibre rod 1 emerges. In practice, it may also be imagined that the shoulder 5 and/or abutment pieces 7 are combined to form one piece and are deformable in themselves.

To complete the description, reference is also made to FIGS. 8a and 8b. Here, in contrast to the other examples, the suspension fork 10a is used in place of the funnel 10 at the end of rod 1. Fork 10a is arranged with its bores 22 at the sides of the sheath 4. In this way, a smaller overall length of the insulator can be achieved. This solution can always be used when the sheath 4 is not pressed onto the abutment pieces 7, but, for example, is instead screwed on or holds by friction at the inner face of the sheath.

The advantages of the various embodiments have already been described. In contrast to press on sheaths and conical connections, optimum values are obtained concerning the holding power and the long term tensile strength. This can be attributed to the fact that the increasing elastic deformation of the glass fibre rod toward its end that emerges from the sheath is compensated by increasing compliance of the buffer members. Moreover, the design measures according to the invention ensure a reliable positive or frictional connection over the entire length of the sheath with the use of simple means. Moreover, the advantages of conical and pressure connections are combined with one another by simple means. In addition, the mechanical strength of the insulator is considerably increased by the arrangement.

Although the present invention has been described in connection with a plurality of embodiments thereof, many variations and modifications will now become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.