GB2068081A

GB2068081A - Vibration dampers of the stockbridge type

Info

Publication number: GB2068081A
Application number: GB8101072A
Authority: GB
Original assignee: Brush Power Equipment Ltd
Current assignee: Brush Power Equipment Ltd
Priority date: 1980-01-16
Filing date: 1981-01-14
Publication date: 1981-08-05

Abstract

Aeolian vibrations of a suspended cable 10 are damped by a conventional Stockbridge-type vibration damper in which weights 16 are secured in spaced relation to and on opposite sides of a clamp 11, 12 by a messenger cable 15. In order to limit sympathetic vibrations set up in the messenger cable 15 and thereby improve the effectiveness of the damper at higher vibrational frequencies, weights 17 are suspended from the messenger cable respectively on either side of the clamp, each weight being capable of limited sliding movement along a rod 20 between stops 21 and 22. <IMAGE>

Description

SPECIFICATION Vibration damper This invention relates to vibration damper for a suspended cable or the like, such as an overhead electrical conductor.

Aeolian vibration is set up by the effect of wind on a suspended cable. Static strain existing in the cable added to the dynamic strains due to this vibration can lead to fracture. There have been proposed various forms of apparatus for damping these vibrations, the most common being known as a Stockbridge damper. This comprises a clamp for attachment to the cable and carrying a pair of weights. The weights are attached to the clamp by means of a length of high tensile steel wire cable called a messenger cable. The device, which is placed at a distance from the suspension point of the cable, is calculated to control the vibration set up in the cable by sympathetic vibration.

Whilst this type of vibration damper is satisfactory over certain frequency ranges of vibration, it is not very effective in other ranges, notably the higher frequency aeolian vibrations, for example in the region 15Hz and above. It is an object of the present invention to obviate or mitigate this problem.

According to the present invention, there is provided a vibration damper for a suspended cable or the like, comprising a clamp for attachment to said cable, and a weight attached to the clamp for limited movement relative thereto.

A pair of such weights can be provided, in which case they are disposed on opposite sides of the clamp in spaced relation thereto.

In one particular arrangement, the invention is applied to a Stockbridge damper. More particularly, a further pair of weights are secured to the clamp in fixed relation thereto, the further weights being spaced from the clamp on opposite sides thereof and being connected to the clamp by means of messenger cables, and the first-mentioned weight or weights is or are attached to the messenger cables or the further weights.

The present invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic side view of a vibration damper according to the present invention; Figure 2 is a graph of vibration amplitude vs.

frequencyforthe damper of the invention and also a conventional Stockbridge damper; and Figure 3 is a similar graph of mean vibration amplitude vs. total damper means.

Figure 1 shows a Stockbridge vibration damper attached to a cable 10 to be controlled. The damper is usually fitted near one end of a suspended length of the cable and the position is calculated in accordance with a formula related to the weight, tension and diameter of the cable. The Stockbridge vibration damper comprises a cable clamp in the form of two separableelements 11, 12. These define between them an opening through which the cable 10 can pass and they are secured together by means of a clamping bolt which engages in a hole 13 in the part 12. The clamp element 11 has at its lower end a through bore 14 in which is secured a high tensile steel messenger cable 15. Provision (not illustrated) is made for securing the messenger cable within the bore 14 so that lengths of the messenger cable 15 extend from opposite ends thereof, respectively.

Each end of the messenger cable 15 carries a respective weight 16, which is typically of cylindrical shape. Each weight 16 is partly hollow and is secured concentrically onto the respective outer end of the messenger cable 15.

The Stockbridge vibration damper is attached to the cable 10 with the messenger cable 15 generally in the same plane as the cable 10 so that the two weights 16 lie below the cable 10.

Secured to the messenger cable 15 between the clamp 11, 12 and each weight 16 is a respective device which provides additional control by limiting the sympathetic vibration set up in the messenger cable 15. Each such device comprises a substantially spherical weight 17 suspended below the messenger cable 15 by an attachment member 18, the weight 17 being free to slide up and down along a rod 20 between fixed straps 21 and 22. The weight 17 has flattened ends which confront the straps 21 and 22 respectively, there being a gap 19 between the upper end of the weight and the upper strap 21 when the device is in its rest position.

Critical to the damping of vibrations in chosen ranges are the dimension g of the gap 19 and the distance d of the attachment member 18 from the centreline of the Stockbridge vibration damper (identified by the position of the centre of the holes 13). In one example where the mass of the weight is 0.75 kg, the optimum value of the dimensions g and dhave been calculated at 1.5mm and 45mm respectively.

Figure 2 shows in graphical form the vibration amplitude as a function of frequency obtained with an unmodified Stockbridge vibration damper (curve A) and a similar damper to which devices as described above are attached (curve B), from which it will be seen that the latter damper is much more effective. Although some of this increased effectiveness is due to the increased weight of the damper as a whole, the present invention still offers a significant improvement over conventional Stockbridge dampers of similar weight. This is illustrated by Figure 3 which shows in graphical form the effect of total damper mass on the mean amplitude for a range of unmodified Stockbridge dampers (curve C) and a range of similar dampers fitted with timed devices as described above. From Figure 3 it will be appreciated that, for a given total damper mass, a small Stockbridge damper fitted with the abovedescribed devices is more effective than a large unmodified Stockbridge damper.

Various shapes of weight other than spherical may be incorporated and provision for adjustment of the gap 19 and of the position of the device on the messenger cable 15 may be adopted, for example by varying the dimensions of the weight 17 orthe stop 22. In addition, the described device may be secured to the clamp 11, 12 or the weight 16 or the cable 15 at any suitable location, even within the weight 16. Finally, although the weight 17 is described as being suspended below the messenger cable 15 it could be located above if space permits.

Claims

1. A vibration damper for a suspended cable or the like, comprising a clamp for attachment to said cable, and a weight attached to the clamp for limited movement relative thereto.

2. A vibration damper as claimed in claim 1, wherein a pair of such weights are provided and are disposed on opposite sides of the clamp in spaced relation thereto.

3. A vibration damper as claimed in claim 1 or 2, wherein a further pair of weights are secured to the clamp in fixed relation thereto, the further weights being spaced from the clamp on opposite sides thereof and being connected to the clamp by means of messenger cables, and the first mentioned weights or weights is or are attached to the messenger cables or the further weights.

4. A vibration damper for a suspended cable or the like, substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.