GB2211171A

GB2211171A - Safety device for lifting means

Info

Publication number: GB2211171A
Application number: GB8823814A
Authority: GB
Inventors: Alfred George Johnson; Peter Walker
Original assignee: National Nuclear Corp Ltd
Current assignee: National Nuclear Corp Ltd
Priority date: 1987-10-19
Filing date: 1988-10-11
Publication date: 1989-06-28
Anticipated expiration: 2008-10-11
Also published as: GB8823814D0; GB8724424D0; GB2211171B

Abstract

A device for coupling a load 12 to a tension member, such as a cable 16, has a rotatable element, e.g. a sprocket (24), engaging with a fixed, albeit resiliently mounted, elongate member, e.g. a chain (20). In the event of loss of tension in the tension member, downward movement of pawl carries 53 to a lower position arrests the rotation of the rotatable element to lock the load against travel by the engagement of the element, now locked, with the fixed elongate member. <IMAGE>

Description

A device for coupling a load to a tension member This invention relates to a device for coupling a load to a tension member in such a way that the load is restrained against travel in the event of loss of tension in the tension member.

According to the present invention there is provided a device for coupling a load to a tension member, said device comprising a body adapted for connection between the tension member and the load, a rotatable element on the body for engagement, in use, with an elongate stationary member in such a way that, as the load moves along its designed path of travel, the rotatable element is caused to rotate by virtue of its engagement with the elongate member, and means operable in response to loss of tension in the tension member to arrest rotation of the rotatable element whereby, in use, the travel of the body is restrained by the engagement of the arrested rotatable element with the elongate member.

Preferably there is a plurality of such rotatable elements for engagement with the elongate stationary member which may be common for all the rotatable elements or sub-divided into separate members for each element.

The or each rotatable element may comprise a sprocket wheel or a pinion and the or each elongate member may be toothed, there being meshed engagement of the one with the other.

Where more than one elongate member is employed, they are conveniently spaced angularly and symmetrically about the path of travel of the coupling device and may be affixed to the inner surface of a conduit or shaft along which the coupling device and load are to be moved.

The arresting means conveniently comprises a pawl mounted for displacement on the body and one or more stops on the rotatable element, the pawl being movable in response to loss of tension in the tension member from an inoperative position clear of the stop or stops to an operative position in which the pawl blocks rotation of the stop (or one of the stops) and hence arrests rotation of the rotatable element.

Preferably a face of the stop with which the pawl engages lies on a radius extending from the axis of rotation of the rotatable element.

Preferably, the coupling device is provided with biassing means, such as a spring, arranged to urge the pawl into said operative position. The pawl may form part of a structure through which the tension member supports the load in such a way that, when the tension member is in tension, the pawl-carrying structure is in the inoperative position overcoming the biassing means and, when the tension is lost, is free to move by the action of the biassing means into the operative position.

Means may be provided for enhancing the grip of the rotatable member on the stationary member. Such means may employ a number of stationary and rotary element pairs at angular locations around the path of travel such that each pair is restrained from becoming demeshed by the other pairs. For example, there may be three such pairs of stationary rotatable elements located at substantially equidistant positions around the path of travel of the coupling device.

Means may also be provided for reducing the mechanical shock experienced by the or each elongate member when tension in the tension member is lost. For example, the or each elongate member may be suspended from energy absorbing means, such as a spring arrangement.

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic sectional side view of one half of a hoist arrangement incorporating a coupling device in an unlocked position; Figure 2 is a diagrammatic sectional side view of the hoist arrangement of Figure 1 but with the coupling device in a locked position; and Figure 3 is a schematic plan view of the hoist arrangement.

Referring to the drawings, a hoist arrangement 10, for lowering and raising a load 12 within a conduit or shaft 14, includes a hoisting cable 16 and a coupling device 18 for coupling the load 12 to the cable 16. The conduit 14 is disposed with its longitudinal axis vertical and has three chains 20 located on its inner surface 22 such that, as shown in Figure 3, the chains 20 are equi-angularly, and therefore symmetrically, arranged about the longitudinal axis of the conduit 14, as well as being parallel thereto.

The coupling device 18 includes three sprocket wheels 24 which are independently mounted for rotation on a support 26 by three shafts 28, the sprocket wheels 24 being arranged so that they each mesh with a respective chain 20. Each of the shafts 28 is provided with a stop wheel 30 having four equi-angularly spaced stops 32 with radial abutment faces 33. The stops 32 lie within a diameter less than that of the sprocket wheel 24 so as to be clear of the chain 20.

The upper end of the support 26 is provided with a hollow housing 34 having an upper wall 36 and lower wall 38 which have co-axial circular openings 40 and 42 respectively. A rod 44 having a shoulder 46 passes through the housing 34 via the openings 40 and 42 and is encircled by a coiled compression spring 47 acting between the shoulder 46 and the upper wall 36 of the housing 34. The upper end of the rod 44 is connected to a pulley arrangement 48 which has a pulley wheel 49 with a peripheral groove 50 locating the cable 16. Three equi-angularly spaced pawls 52 extend radially from a pawl-carrier 53 fixed to the rod 44. Each of the pawls 52 has a flat upper surface 54 for co-operation as a stop with the underside of the lower wall 38 of the housing 34 and a nose 56 of generally V-shape engageable with the abutment faces 33 of the stops 32 (see Figure 2).The portions 59 of the pawls 52 which overlie the sprockets 24 are cut-away so as to be clear of the sprockets when the noses 56 engage the abutment faces 33. The pawls 52 are arranged for movement in a vertical direction such that the noses are engageable with, and disengageable from, the abutment faces.

When the coupling device is supported by the cable 16 and a load 12 is fastened to the support 26 the weight of the load 12 causes the support to move downwards, by compression of the spring 47, out of the path of the stop wheels 30 until the flat upper surface of the pawl-carrier 53 is stopped against the underside of the lower wall 38 of the housing 34. The load 12 is then supported by the cable 16 through the intermediary of the flat surfaces 54 of the pawl carrier 53 and the lower wall 38. As the cable 16 is lowered the sprockets 24 rotate in the direction of arrow B and hence the load 12 is lowered down the conduit 14 (see Figure 1).

If the tension in the cable 16 is lost, for example if the cable 16 is severed, the spring 46 ceases to be compressed. Instead, the spring expands and in doing so urges the pawls 52 into the paths of the respective stop wheels 30 so that the pawls 52 engage respective stops 32 on each stop wheel, as shown in Figure 2. When this occurs the sprockets 24 are locked and the load 12 is held stationary relative to the chains. The force exerted on the pawl 52 by the stop 32 tends to push that pawl 52 inwardly towards the conduit axis but movement in this direction is prevented by the other pawls 52. Thus, by reaction, the tendency is for the sprocket wheels to be urged outwardly such that meshing between the sprocket wheels and the chains is maintained, and even enhanced, for maintaining support of the load by the chains. A grab can then be used to recover the load 16.

Each of the chains can be suspended from energy absorbing means, such as a spring arrangement, and slidably located within a guide on the wall of the conduit. Thus, when tension in the cable is lost and the weight of the load is transferred from the cable to the chains, the chains yield downwards by sliding in their respective guides and energy is absorbed by the spring arrangements to reduce the mechanical shock experienced by the chains. Other energy absorbing means can be used, for example elastomeric material can be placed between the stop wheel 30 and sprocket wheel 24, or the stops can be formed from a material exhibiting shear characteristics. It may be desirable to pre-tension the chains so that the displacement of the load on loss of tension in the cable is less than when the chains are not pre-tensioned.

The invention is not restricted to the use of three chains, as more than three or less than three can be used in a symmetrical arrangement. For example, two chains located on opposing walls may be used.

In another embodiment of the invention, the sprocket wheels are replaced by wheels having only frictional engagement with the inner surface 22 of the conduit 14.

A greater importance then attaches to the feature of the locking of the wheels being accompanied by a force urging them outwards so as to enhance the grip on the conduit inner surface. Also appropriate in such an embodiment is that the wheels present friction material, for example in the form of a tyre, for engagement with the surface.

Although the invention is described with reference to a hoist arrangement in which the load travels vertically, the invention is also applicable in situations where the load is to be moved in directions other than the vertical.

Claims

1. A device for coupling a load to a tension member comprising a body adapted for connection between the tension member and the load, a rotatable element on said body for engagement, in use, with an elongate stationary member in such a way that, as the load moves along its designed path of travel, the rotatable element is caused to rotate by virtue of its engagement with the elongate member, and means operable in response to loss of tension in the tension member to arrest rotation of the rotatable element whereby, in use, the travel of the body is restrained by the engagement of the arrested rotatable element with the elongate member.

2. A device as claimed in Claim 1 in which the arresting means comprises a pawl mounted for displacement on the body and one or more stops on the rotatable element, the pawl being movable in response to loss of tension in the tension member from an inoperative position clear of the stop or stops to an operative position in which the pawl blocks rotation of the stop (or one of the stops) and hence arrests rotation of the rotatable element.

3. A device as claimed in claim 2 in which a face of the stop with which the pawl engages lies on a radius extending from the axis of rotation of the rotatable element.

4. A device as claimed in Claim 2 or 3 in which is provided a biassing means, such as a spring, arranged to urge the pawl into the operative position, the pawl being part of a structure through which the tension member supports the load in such a way that, when the tension member is in tension, the pawl-carrying structure is in the inoperative position overcoming the biassing means and, when the tension is lost, is free to move by the action of the biassing means into the operative position.

5. A device as claimed in any of the preceding claims having a plurality of the rotatable elements each with a respective arresting means.

6. A device as claimed in Claim 5 in which the rotatable elements are spaced angularly and symmetrically about the path of travel.

7. A device as claimed in Claim 6 in which the rotatable elements and the respective arresting means so cooperate that arresting of the rotation is accompanied by urging of the elements radially of the path of travel to enhance the grip of the elements, when arrested, on the elongate member.

8. A device as claimed in any of the preceding Claims in which the or each rotatable element is a sprocket wheel or pinion for meshed engagement with a toothed elongate member.

9. A hoist arrangement having a device as claimed in any of the preceding claims and having for each rotatable element an elongate stationary member with which the element is in constant engagement over the designed path of travel.

10. A hoist arrangement constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated by the accompanying drawings.