Note: Descriptions are shown in the official language in which they were submitted.
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WO 98/53542 PCT/GB98/01361
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
s 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.
to Such vibrations, if not damped, can become extremely destructive. A known
type of vibration
damper is the DulmisonTM 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
15 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
20 0441, SVD 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
25 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
3o 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
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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
to 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
2o 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 - l5mm
Width of strip 2.5 - 25mm
Gripping section internal diameter 5 - 35mm
Damping section internal diameter 15 - SOmm
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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
to 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 42A 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
2o driving conditions. A span of standard ADSS (All Dielectric Self
Supporting) optical cable
from PirelliTM having a diameter 14.6mm was placed under tension and vibrated
at a range of
frequencies. The amplitude of vibration was measured using a VIBRECTM
vibration recorder
which measured the bending amplitudes over a range of frequencies in SHz
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
3o vibration damper used in the comparative tests:-
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SVD 1432 Current invention
Overall length 1676mm 1675mm
Grip length 330mm 330mm
Thickness of material l9mrn round section 2.Smm
Width of material 9.Smm
Gripping section internal12.9mm 12.9mm
diameter
Damping section internal25mm 26mm
diameter
Weight 0.79kg 0.2kg
Material Polyvinyl chloride Aluminium alloy
To fit conductor diameter14.3 - 19.3 mm 14.3 - 19.3 mm
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 SVD
1432 damper reduces the amplitude of vibration for frequencies from about l
OHz 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.
1o 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 IOHz to 80Hz.
1s 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
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stiifness 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
s 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.
to
The present invention is not limited to any particular metal and indeed
specifically contemplates
the use of steel for the material of the strip.
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