GB2351060A

GB2351060A - Shock load absorber for cables

Info

Publication number: GB2351060A
Application number: GB9914208A
Authority: GB
Inventors: John Crisfield
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-06-17
Filing date: 1999-06-17
Publication date: 2000-12-20
Also published as: GB9914208D0

Abstract

A shock absorber device 1 for absorbing longitudinal tensile loads applied to a cable comprises means such as tube or (13, figure 3) to guide a length of cable into a curved path, the device being resilient or including resilient means responsive to resist said tensile shock loads tending to straighten said cable from said curved path. Preferably, said device is threaded on to the cable. In an alternative embodiment (figure 2), the resilient means comprises a plurality of hydraulic ram and cylinder devices (5) with pulleys (6), which constrain the cable to a serpentine path. The cylinders may be pressurised by nitrogen. The device may comprise a metal bellows or brittle plastics designed to deform and fail when subjected to excessive loads to indicate that such have occurred. An electronic load cell(s) may be provided to monitor the loads.

Description

2351060 A shock absorber device This invention relates to a shock absorber

device for absorbing longitudinal tensile loads applied to a cable or similar elongate flexible element.

Cables are used in many structural applications such as marine rigging, architectural rigging, structural steel work such as bridge and suspension cables, cables for transmitting electrical power, and towing cables for towing vehicles such as motor vehicles or ships. In the majority of such applications, the cable has to be much thicker and stronger than is required to simply carry the normal load of the particular application, in order to accommodate shock loads which could be applied to the cable. Typically, in applications where shock loads may be applied to a supporting cable, the safe working limit of the cable is halved. This means that the effective cross-sectional area of the cable would need to be twice as much as would be the case if no shock loads were involved. This inevitably increases the weight, complexity and cost of any application of cables.

The present invention seeks to provide a device which reduces or even eliminates the possibility of shock loads being applied to cables in' applications such as those outlined above.

According to the present invention there is provided a shock absorber device for absorbing longitudinal tensile loads applied to a cable, the device comprising means to guide a length of cable into a curved path, the device being resilient or including resilient means responsive to resist longitudinal tensile loads tending to straighten the cable from said curved path.

In many applications, particularly those involving structural use of cables, it is an essential requirement that transmission of tensile loads through the cable between the load being supported and the cable mounting, should be continuous, that is, there should be no intermediate 2 structural element, which if it fails, would allow the cable ends to be disconnected. Thus, for example, it would not be acceptable for the mounting of a cable to be on resilient means such as rubber blocks since if these failed, the structural integrity of the cable and the load it was designed to support, would be sacrificed.

In the preferred embodiment of the present invention the shock absorber device comprises a member which can be threaded on to a cable prior to its installation which constrains the cable to follow a serpentine path as it passes through the member. When tensile load is applied to the cable this tensile load tends to straighten the cable and the device incorporates resilient means such as rubber, to resist this straightening force.

In an alternative embodiment, the device includes a plurality of hydraulically operable ram and cylinder devices which are extendable or contractible to constrain the cable to a serpentine path, tensile forces tending to straighten the cable being resisted by the hydraulic devices, which are preferably cooperable with a compressed gas spring to resist the straightening forces.

In another embodiment the device may be formed of a resilient material and may be formed in a serpentine shape, as a tube for example, with a corresponding serpentine path extending through its length. The device may be split longitudinally into two or more parts so that it can be placed over an existing cable installation and secured to gether, by bolts for example, thereby forcing the cable into a serpentine path.

Preferred embodiments of the present invention will now be described by way of example with reference to the accompanying informal drawings, in which:- Figures la to ic show a first embodiment of the present invention utilising a resilient material, Figure 2 shows a second embodiment in which hydraulic piston and cylinder devices are adapted to form a curved path for the cable, and 3 Figures 3a and 3b show, respectively, a plan view and a side view of a third embodiment, formed of a resilient material.

Referring now to Figures la to 1c, there is shown a device having a metal casing 1 containing an elastomeric material 2. The device includes a cable accommodating curved path formed by a sleeve or tube 3 extending through the device which may be defined by or lined with a plastics material or kevlar, terminating at its ends in guide bushes 4. Figure la shows a part sectional side view of the device showing the tube 3 and the resilient material 2. The tube or sleeve 3 is formed of a resilient material.

In operation, a cable is fed through the tube 3 prior to being fastened at one end to a supporting mounting and at its other to a load (not shown).

In operation, when a tensile load greater than a predetermined minimum figure is applied to the cable, this tends to straighten the cable, including the part of the cable extending through the tube 3. This action tends to straighten the tube 3 by deflecting the resilient material 2, either compressing or tensioning it. By appropriately selecting the rigidity and strength of the resilient material, shock loads applied in the longitudinal direction to the cable can be at least partially absorbed by the deflection of the resilient material 2. In this way, shock loads applied to the cable are absorbed, which means that the cable can be of smaller diameter than would otherwise be the case. The density of the material 2 and its resistance to forces applied thereto will depend upon the particular application since the higher the forces which must be absorbed in resisting the straining of the cable, the denser and stronger the material 2 would have to be.

In an alternative embodiment, which is particularly suitable for retro fitting to existing structures, the metal casing I and the resilient material 2 are formed in a twopart structure. The structure is preferably divided along the axis of the tube 3 although this is not essential. In 4 practice, the two parts are placed round the cable and fastened together. As the two parts are secured together the cable is constrained to the curved path as shown.

Figure 2 shows an alternative embodiment of the present invention which may also be suitable for retro fitting to existing structures.

In this embodiment, the device includes five hydraulically operable ram and cylinder devices 5 which are mounted in a supporting metal framework 1. Three of the ram and cylinder devices 5 are located on one side of the cable passage 3 at spaced intervals whilst the other two ram and cylinder devices are placed diametrically opposite to the first three and are disposed equidistantly between adjacent ones of the three ram and cylinder devices first mentioned. The ram and cylinder devices carry on their output end pulleys 6 which engage the surface of the tube 3. Each of the ram and cylinder devices 5 is connected to a source 7 of pressurised hydraulic fluid. Pressure to the hydraulic fluid is applied through a nitrogen accumulator 8. A control system is provided to selectively pressurise and energise each of the ram and cylinder devices 5 to constrain the tube 3 to a serpentine path by extending or retracting the ram and cylinder devices 5 as appropriate. Longitudinal tensile forces tending to straighten the cable are absorbed in a resilient manner by the hydraulic fluid and the nitrogen gas accumulator. The use of five ram and cylinder devices enables the passage of the cable to be adjusted to a serpentine path and the degree of reflection of the path 3 can be chosen to provide the appropriate degree of resilient movement and shock absorption. Although five ram cylinder devices are shown, any suitable number can be used. This embodiment is particularly suitable for retro fitting to existing structures. The characteristics of the material used in the devices described, can be selected to provide the necessary damping, shock absorbing or vibration absorption characteristics required for the particular application. By the provision of such a device intermediate the length of the cable, the integrity of the cable's capacity to absorb the design load is not compromised but shock loads can be accommodated.

In another embodiment in accordance with the present invention, the device may be provided with means to indicate that a predetermined limit tensile force has been applied to the cable. In a modification of the embodiment illustrated in Figure 1, the outer casing could, for example, be formed of a brittle material such as a plastic which is designed to fail at a predetermined tensile load applied to the cable. The device could thus indicate that the cable has reached or exceeded its designed load and should be replaced.

In another embodiment, instead of the resilient material as illustrated in Figure 1 or the hydraulic cylinders illustrated in Figure 2, the straightening forces could be absorbed by a flexible metal bellows or similar construction which is designed to fail in a visually obvious manner when more than a certain force has been applied to the cable.

Figures 3a and 3b show a further embodiment in which the device itself is formed of a resilient material such as a plastics or composite material which may be reinforced with carbon fibres or glass fibres. The device consists of an elongate body 11 of generally tubular cross-section which is formed, moulded or shaped into a serpentine shape along its length and is split or formed into two mating parts 13, 14 along a dividing line 12 extending along the longitudinal axis of the device. Each part has a semicircular, in crosssection, channel extending along its length which together form a serpentine bore adapted to receive a cable.

The two parts are clamped together by four sets of bolts 15 which pass through cooperating pairs of flanges 16, 17 spaced along the length on opposite sides of each part.

In operation, this embodiment can be fitted on an existing installation by placing the two parts round the cable and clamping them together by means of the bolts 15. As the parts are drawn together the cable is forced into a 6 serpentine path. When tensile loads are applied to the cable, the device tends to straighten out against the inherent resilience of the device material thus absorbing shock loadings.

It is also possible that the device may include electronic load cells to enable the load on the cable and fluctuations in the load in various conditions to be monitored by providing a continual readout of the transverse load applied to the deflected part of the cable. Apart from absorbing shock loads and providing a ready means for establishing the tensile loads in a cable, the device could also be utilised to compensate for thermal expansion in cables. It is particularly envisaged that the device may be used to absorb shock tensile loads which may be provided more or less continually, such as in the rigging of sailing ships, or it may be used for a one-of f emergency use to indicate that a safe load of the cable or its mountings has been exceeded.

7

Claims

A shock absorber device for absorbing longitudinal tensile loads applied to a cable, the device comprising means to guide a length of cable into a curved path, the device being resilient or including resilient means responsive to resist longitudinal tensile loads tending to straighten the cable from said curved path.
2. A device according to claim 1, including a member threadable on to a cable prior to its installation which constrains the cable to follow a serpentine path as it passes through the member, so that when tensile load is applied to the cable this tensile load tends to straighten the cable.
3. A device according to claim 1, wherein the device includes a plurality of hydraulically operable ram and cylinder devices which are extendable or contractible to constrain the cable to a serpentine path, tensile forces tending to straighten the cable being resisted by the ram and cylinder devices.
4. A device according to claim 3 wherein the ram and cylinder devices comprise hydraulically operable devices and/or compressed gas springs to resist the straightening forces.
5. A device according to claim 1 in which the device is formed of a resilient material and is formed in a serpentine shape with a corresponding serpentine path extending through its length.
6. A device according to claim 5, wherein the device is divided along a longitudinal axis in two or more parts so that the parts can be placed over an existing cable 8 installation and secured together, thereby forcing the cable into a serpentine path.
7. A device according to any one of the preceding claims, including electronic load cells to enable the load on the cable and/or fluctuations in the load in various conditions to be monitored by providing a measure of the transverse load applied to the deflected part of the cable.
8. A device according to any one of the preceding claims adapted to compensate for thermal expansion in cables.
9. A device according to any one of claims 1 to 8, including means to indicate that a safe load of the cable or its mountings has been exceeded.
10. A shock absorber device for absorbing longitudinal tensile loads applied to a cable, substantially as described herein with reference to, and as illustrated in, the accompanying drawings.