VIBRATION DAMPERS

Abstract: Disclosed is a vibration damper for elongate objects, the damper comprising a strip of metal formed into a first helix to form a gripping section having a diameter selected to grip the elongate object, and a second helix of a wider diameter selected to interfere and damp vibrations of the elongate object. Also disclosed is a method of making such a vibration damper, comprising the use of apparatus capable of making helixes and of selectively varying the diameter of the helix during production of a single article.

Description: VIBRATION DAMPERS This invention relates to vibration dampers for suspended elongate objects such as cables. In the context of this specification the term cables will be used generally and should be interpreted as meaning elongate objects in general and including, among other things, power cables, optical cables, ropes, and wires. The present invention is particularly, though not exclusively, usable for suspended optical cables. Suspended elongate objects such as cables are liable to vibrate under the action of the wind. Such vibrations, if not damped, can become extremely destructive. A known type of vibration damper is the Dulmison™ Spiral Vibration Damper which comprises a polyvinyl chloride helically formed rod of cylindrical section. The vibration dampers are formed by extruding the polyvinyl chloride as a rod, wrapping the rod while still warm and pliable about a mandrel to form the helix and quenching the rod and mandrel in a water bath. The helical diameter of the damper varies along its length to form a narrower helix and a wider helix. The narrower helix acts as a gripping section having a diameter selected to grip the cable to which it is attached by interwinding the cable and the helical rod. The wider helix acts as a damping section, to provide the action/reaction opposed to the cable vibration by mechanical interaction between damper and cable. Such dampers are available under Dulmison Inc. catalog numbers SVD 0441, SND 0635, SVD 0830, SVD 1173, and SVD 1432 and come in lengths ranging from 1.23 metres( 48.5") to 1.68 metres (66") although the length may be varied to suit the vibrational characteristics of the cable concerned. Such vibration dampers were developed for use on metallic conductors and there have been problems in using them on optical cables. Optical cables are frequently suspended from pylons carrying electrical cables. The optical cables are thus subjected to strong electromagnetic fields. As the optical cable is non-conductive this results in different electrical potentials along the length of the optical cable. The dampers exacerbate this problem and the varying electrical potential can result in discharges that damage the dampers or, worse, the optical cable. There have been many efforts to reduce this problem, including adding conductive materials to the plastic materials from which these dampers are formed. For example, GB 2234830 discloses a rod of metal or metal alloy whose overall diameter over a part of its length increases smoothly. However, a potential problem is that the use of a cylindrical rod means that in high amplitude vibration situations the damping section may strike the cable and such contact will be as tangential point contact where the inner diameter of the helix strikes the surface of the cable The applicants have now realised that the problems can be solved by making the dampers of metal strip as this will firstly provide a means to even out the varying potential and will provide line contact between the damping section and the cable in high amplitude vibration situations. WO 96/14176 describes methods and apparatus for manufacturing helical products from metal strip and provides a discussion of earlier methods of making such products. Such apparatus may be used to form the dampers of the present invention by producing a helix that varies in diameter from flat strip material. Further features of the invention will be apparent from the following description and the claims. The invention is illustrated by way of example in the following with reference to the drawings in which: - Fig. 1 is a part schematic view of a device in accordance with the invention; Fig. 2 is a part schematic view of apparatus as claimed in WO 96/14176 Fig. 3 is a graph illustrating the results of comparative damping tests. In Fig 1. region 1 is the gripping section and region 2 is the damping section of a spiral vibration damper. The spiral is formed from metal strip and typical, but not exclusive, ranges of dimensions are:- Typical Range Overall length 1 - 2 metres Grip length 0.2 - 0.4 metres Thickness of strip 1.2 - 15mm Width of strip 2.5 - 25mm Gripping section internal diameter 5 - 35 mm Damping section internal diameter 15 - 50mm It must be appreciated that the gripping section diameter must be chosen to match the diameter of the cable to be damped so as to be a reasonable grip. The damper may be formed by using apparatus capable of making helixes and of selectively varying the diameter of the helix during production of a single article. Suitable apparatus is disclosed in WO 96/14176. The apparatus of WO 96/14176 is versatile and can be programmed as required and, as described in WO 96/14176, variation of the pitch and diameter of a helix being formed can be selectively varied. As shown in Fig 2 a pair of rollers or other forming members 42A and 42B are used to bend and twist incoming strip material 26 to form a helix 46. By varying the angle and spacing of rollers 42 A and 42B the pitch and diameter of the helix can be varied as required even during the formation of a helix so that switching from a helix of one diameter to a helical spiral of increased diameter is straightforward. By such means it is also possible to provide flared ends to the gripping section so that it does not dig into the surface of the cable. This can be important for optical cables which are fragile in nature. Fig. 3 shows the results of comparative damping tests in which the Y-axis gives the bending amplitude in mm and the X-axis the frequency of vibration of a span of cable under various driving conditions. A span of standard ADSS (All Dielectric Self Supporting) optical cable from Pirelli™ having a diameter 14.6mm was placed under tension and vibrated at a range of frequencies. The amplitude of vibration was measured using a VTB RE C™ vibration recorder which measured the bending amplitudes over a range of frequencies in 5Hz steps. The vibration frequency was automatically swept over the entire range with a uniform amount of power applied to the span. Comparative tests were performed using an undamped cable, a cable damped with a Dulmison Inc SVD 1432 spiral damper, and a cable damped with a spiral damper in accordance with the present invention. Below is a comparison of the SVD 1432 spiral vibration damper and the above mentioned vibration damper used in the comparative tests :- In Fig .3 the symbol ■ indicates the results found for an undamped span of cable The symbol O shows the results for a cable damped with the SVD 1432 spiral vibration damper (made, as stated above, of polyvinyl chloride) It can be seen that the known SND 1432 damper reduces the amplitude of vibration for frequencies from about 10Hz to about 40Hz. From 40Hz to 80Hz it only provides a small amount of damping and, effectively, is not doing its job of reducing vibration in this range The symbol • shows the results of using a vibration damper according to the present invention and it can be seen that a considerably higher damping effect is achieved, and across a wider range of frequencies, than the conventional SVD 1432 spiral vibration damper The present vibration damper gives effective damping from 10Hz to 80Hz Such a large difference in damping efficiency would not be expected given the broadly similar dimensions of the dampers. However the damper according to the present invention is less stiff than the conventional damper and this may provide the explanation for the improved efficiency of damping. Under vibration the damper according to the present invention appears to "come alive" and impact the cable along most of the length of the damping section. In contrast, the stiffness of conventional spiral dampers means that they only impact the cable in their last third of the damping section. Its reduced stiffness, in combination with its lower weight, makes the damper of the present invention easier to install than a conventional spiral damper of similar overall dimensions. The damper used in the above mentioned tests was made from a high tensile strength 6000 series aluminium alloy. The low weight of the resulting damper means that the damper is less likely than conventional dampers to migrate along the cable under vibration and so permits a less rigid gripping section to the damper. The present invention is not limited to any particular metal and indeed specifically contemplates the use of steel for the material of the strip.

Claims: CLAIMS 1. A vibration damper for elongate objects, the damper comprising a strip of metal formed into a first helix to form a gripping section having a diameter selected to grip the elongate object, and a second helix of a wider diameter selected to interfere and damp vibrations of the elongate object. 2. A vibration damper as claimed in claim 1 in which the gripping section has a length of 0.2-0.4 metres. 3. A vibration damper as claimed in claim 1 or claim 2 in which the overall length of the damper is 1-2 metres. 4. A vibration damper as claimed in any preceding claim in which the strip of metal has a thickness in the range 1.2mm - 15mm and a width in the range 2.5mm - 25mm. 5. A vibration damper as claimed in any preceding claim in which the strip of metal is an aluminium alloy. 6. A vibration damper as claimed in any preceding claim which gives effective damping over the range 10Hz to 80Hz. 7. A method of making a vibration damper as claimed in any preceding claim comprising the use of apparatus capable of making helixes and of selectively varying the diameter of the helix during production of a single article.

Inventors: MURPHY MAURICE WILLIAM, MURPHY, MAURICE, WILLIAM
Applicants: MORGAN CRUCIBLE CO, MURPHY MAURICE WILLIAM, MURPHY, MAURICE, WILLIAM, THE MORGAN CRUCIBLE COMPANY PLC
Application year: 1998
Main classification: H02G7
IPC codes: H02G7