GB2441140A

GB2441140A - Rope grab

Info

Publication number: GB2441140A
Application number: GB0616528A
Authority: GB
Inventors: Hugh Irving Banner
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-08-21
Filing date: 2006-08-21
Publication date: 2008-02-27
Anticipated expiration: 2026-08-21
Also published as: GB2441140B; GB0616528D0

Abstract

A rope grab is disclosed that has particular, but not exclusive, application to providing a fall arrest safety system for rope access climbers. The rope grab comprises a rope path that extends between a cam 14 and an abutment 16 located between two metal plates 10, 12. Holes 36 in the plates serve as connection means, by which the grab can be attached to a connecting line by a connector. When the device is supported from the connection means, the cam is urged by the rope 32 to a position in which it has a point of minimum radius proximal to the abutment such that the rope can slide between the cam and the abutment. When the connection means is pulled downwards, the cam is urged to a position in which a point of greater radius is proximal to the abutment. The rope is thus gripped between the cam and the abutment thereby resisting it movement through the rope grab.

Description

Rope grab This invention relates to a rope grab. Particularly, but not

exclusively, it relates to a fall-arresting rope grab for use in rope access climbing or as a belay brake in leisure climbing.

it is common to use rope access to perform work on a structure where the extent of the work cannot justify a full installation of scaffolding. Rope access techniques typically involve use of a main rope upon which a worker can ascend and descend using ascenders. An additional safety rope is also normally provided. The safety rope does not play an active role unless the main rope or other equipment should fail. Upon failure of the main rope, the safety rope prevents the worker from falling, and allows him or her to make a controlled descent to safety.

Ideally, the safety rope should not impede the worker's progress up and down the main rope.

Its presence should be apparent only upon failure of the main rope. To this end, the worker is connected to the safety rope using a lanyard or sling and a rope grab. The rope grab is intended to slide freely on the safety rope during normal ascent and descent, but grab the safety rope on application of a large, sudden force as would occur following failure of the main rope or associated apparatus.

There are also specific constraints regarding the maximum forces to which a climber can be subject when their fall is caught by the safety system. The force can be limited in two main ways: first, by ensuring that the rope grab will ultimately slip upon the rope when a threshold force is applied to it. The second is to provide shock absorbency in a lanyard between the climber and the grab that can stretch when subject to load. The first of these measures requires that the grab be carefully designed and used only with suitable ropes. The second is simple to implement, but the length of lanyard can be an inconvenience to the climber.

EP-A-1525903 discloses a rope grab that has a rolling element which engages with the surface of a rope. A centrifugal coupling means is arranged between a drive member of the roller and the support arm so as to occupy a disengaged position or an engaged position. The centrifugal coupling means comprises at least one flyweight movable along a ramp of the drive member against a compression spring, and a connecting interface engaged inside a cylindrical rim of the support arm to transmit the elastic force of the spring to the flyweight.

This is a device that is complex to manufacture. It also locks upon a rope with no slip, so it must be used in conjunction with a force-limiting device.

An aim of the invention is to provide a rope grab that is effective, economical to manufacture, and which is convenient to use.

The invention provides a rope grab comprising a rope path that extends between a cam and an abutment, and connection means by which the grab can be attached to a connecting line, in which, when the device is supported from the connection means, the cam is urged by the rope to a position in which it has a point of minimum radius proximal to the abutment such that the rope can slide between the cam and the abutment, and in which, when the connection means is pulled downwards, the cam is urged to a position in which a point of greater radius is proximal to a convex profile of the abutment, and the rope is gripped between the cam and the abutment thereby resisting it movement through the rope grab.

It has been found that this arrangement can provide an effective grab with a small number of components that can be manufactured economically. The frictional engagement with the rope ensures that the grab does not lock fast upon the rope, so limiting the force that it can apply.

The shape of the profile ensures that the gripping force is enhanced by additional loading.

applied to the rope grip.

For maximum effectiveness, the abutment preferably has a convex profile facing towards the cam. For example, the abutment may be cylindrical; a shape that is particularly easy to manufacture.

Most typically, the cam is pivotable about a pivot axis. In such embodiments, the cam may have a curved region extending from the point of minimum radius. In such cases, the radius of the curved region may increase with angular distance form the point of minimum radius. It has been found to be particularly effective to form the curved region as a part of a logarithmic spiral. For example, the spiral may have the formula Re = &eZIO where: R0 is the radius at point M; R8 is the radius at angle e from the line XM; 0.2 = b = 0.6; and 9 is the angle between the radius XM and the radius P9 in radians..

The cam may also have a lead-in region extending substantially or partially tangentially from the point of minimum radius away from the curved region. The lead-in region, when in use, abuts a rope such that the cam is turned when a sudden load is applied to the connecting means.

In a preferred embodiment, a line intersecting the centre of curvature of the abutment and the centre of the connection means makes an angle.p with a line that passes through the centre of curvature of the abutment and the pivot axis of the cam, where 80 = q =1200. Embodiments may have an approximately right-angled arrangement with.p 90 .

In a convenient constriction of a rope grab embodying the invention, it may comprise two plates (for example, formed from steel or aluminium by pressing or stamping) between which the abutment and the cam are retained. In such embodiments, the connection means may conveniently comprises a respective aperture in each plate through which an attachment device (e.g., a karabiner or connector) can be passed. In these embodiments, the rope path may pass between the plates. Optionally, the plates may be interconnected by a fastening means that allows them to be separated to allow a rope to be placed within or removed from the rope path.

To enhance the grip applied to a rope by the grab, at least one of the cam and the abutment may have rope-engaging surfaces, the rope engaging surfaces are formed with roughening formations. The roughening formations may include grooves, indentations or raised features.

An embodiment of the invention will now be described in detail, by way of example, and with reference to the accompanying drawings, in which: Figure 1 is a partly assembled rope grab being a first embodiment of the invention; Figures 2 and 3 are top and bottom views of the embodiment of Figure 1; Figure 4 is a detailed view of a cam being a component of the embodiment of Figure 1; Figure 5 is a further view of the cam component on which some important dimensions are marked; and Figure 6 is a scrap section on A-A' in Figure 5.

The embodiment comprises five principal components: a front plate 10, a rear plate 12, a cam 14, an abutment 16, and an axle 26.

The front plate 10 and rear plate 12 have a similar, approximately triangular outline with rounded corners. Each is formed from a plate of metal. The greater part of each plate is flat, with an angled region 20, 22 of each plate towards a first of the corners deviating from the plane at an angle of approximately 10 . When the front and rear plates 10, 12 are placed to overlie one another, the angled regions 20, 22 are deflected towards one another. An aperture 34, 36 is formed through each of the angled regions 20, 22.

The cam 14, the shape of which will be described in detail below, comprises a body having parallel opposite faces. An aperture is formed through each of the plates 10, 12 remote from the angled regions. The apertures receive end portions of an axle 26. The end portions of the axle 26 are externally threaded. A nut 28 is applied to each end portion, whereby the plates are secured to the axle 26, with the axle being firmly connected to the bottom plate 12 and rotatably connected to the top plate 10. The spacing of the flat regions is just greater than the diameter of the rope with which the grab is intended for use. The plates can therefore serve to guide a rope through the grab. The space between the flat regions is such that the remote extremity of the angled regions 20, 22 just touch one another. The cam 14 has an aperture passing through it between its parallel faces, through which the axle 26 passes. The cam 14 is therefore retained for rotation between the plates 10, 12.

The abutment 16 is an cylindrical metal body. A screw 30 projects from the abutment where it is received in a threaded hole in the rear plate 12 where it is secured with a nut. A slot if formed in the front plate 10, within which a machined slot in the abutment can be received.

The top plate 10 can pivot about the axle 26. This allows a rope 32 to be passed between the plates 10, 12. When in position, the axis of the screw 30 and the axis of the axle 26 are parallel and close to second and third corners of the plates. Although the abutment 16 is cylindrical in this embodiment, this is a matter of manufacturing convenience. The part of the abutment 16 that faces the cam 14 and which may make contact with the rope must be convex, but the shape of the rest of the abutment 16 is arbitrary, since it is not involved in the function of the rope grab.

For use, the rope grab is opened by pivoting the top plate 10 as described in the last-preceding paragraph, and the rope 32 is placed between the cam 14 and the abutment 16. The top plate is then closed to retain the rope 32 within the grab. A connector is then passed through the apertures 34, 36. Attached to the connector is a lanyard that attaches it to a climber's harness.

It will be noted that the connector prevents the grab from opening.

In alternative embodiments, the axle 26 and the abutment 16 are fixed permanently to the plates 10, 12. In such cases, the end of the rope must be inserted into the rope grab.

Important considerations concerning the shape and dimensions of the components will now be discussed.

As seen in Figure 5, the cam 14 has an operative surface in the range A to B in the figure.

The shape of the cam is defined with reference to the point on the cam at which its radius from the pivot axis X is minimum. This point is indicated at M in Figure 4. A lead-in portion of the cam extends from point M towards the point A, substantially tangentially, in a direction that is upwards with respect to a safety rope 32. The lead-in portion is substantially straight. A locking portion of the cam extends from the point M towards the point B. In the locking portion, the cam surface is of progressively increasing radius away from the point M. In this embodiment, the radius of the surface is defined as a logarithmic spiral having the general formula: Where: R0 is the radius at point M & is the radius at angle 0 from the line Xlvi 0.2 = b = 0.6; and 0 is the angle between the radius XM and the radius R. in radians.

The value b is a function of the development angle a of the cam, and has the formula: b 1 -tan (rn 2-a) with a in radians. The angle a is the angle between the spiral and its tangent. A spiral with a=24 (b=0.445) has found to be effective. However, experimentation may show that other angles are effective and possibly more effective. For example, a range of 23 = a = 25 , 22 = a = 26 or 21 = a = 27 may be effective. The corresponding values of b can be calculated easily from the above formula. Figure 5 is a specification example of a cam for an embodiment of the invention. The dimensions shown in the drawing are as follows: Position mm R, (n debrees from R0) _______________ R0 10.0 11.0 RM 11.88 R 12.85 __________________ 13.89 15.01 R74 16.22 __________________ 17.53 R 18.95 20.48 R114 22.14 R1 23.92 R1 25.86 Alternative shapes for the cam and the abutment can be contemplated. An important (and possible essential) feature of these shapes is that the shortest distance between the cam surface and the abutment decreases as the cam is rotated from the position in which the shortest distance between the cam surface is minimal.

The cam surface bears a pattern of surface roughening, such as grooves (shown in Figure 6) or other features to enhance the friction that will occur between the surface and a rope. This also ensures that the contact area between the cam surface and the rope is approximately constant for moderate variations in the diameter of the rope. The shape of those parts of the cam 14 that do not make contact with the rope (in the region ACB) largely are arbitraiy. The shape is chosen to ensure that the cam has no sharp edges that could snag on a fabric rope and that there is sufficient material around the aperture through which the axle 26 passes to locate the cam with sufficient strength.

The centre of the apertures 34, 36, the centre of curvature of the abutment 16, and the pivot axis of the cam 14 define a triangle. The vertex at the centre of the abutment 16 defines an angle p. It has been found that the grab operates particularly effectively where 80 = p =120 . In this embodiment, the value of q is close to 90 .

In normal use, the grab adopts approximately the orientation shown in Figure 1. This is a result of the weight of the cam 14 and the abutment 16 being disposed towards one end of the rope grab, which hangs from a connector (not shown) passing through the attachment apertures 36. The attachment apertures 36 and the abutment 16 are on the same side of the rope 32 and the cam 14 is on the opposite side. As can be seen, the rope 32 can pass freely between the cam 14 and the abutment 16. Thus, the rope grab can slide easily up and down the rope 32, and does not interfere with the normal activities of the climber.

if the climber falls, the connector is pulled downwards sharply. This causes the rope grab to pivot in a direction that is generally clockwise in the arrangement of Figure 1, partly shown in dashed lines. The lead-in portion of the cam 14 bears against the rope 32, and this causes the cam to rotate anti-clockwise with respect to the plates 10, 12. The part of the cam 14 that most closely approaches the abutment 16 is no longer the part of minimum radius M; rather, it is somewhere between the point M and the point B on the curved surface of the cam.

Therefore, the distance between the cam 14 and the abutment 16 is reduced, and this causes the rope to be pinched between the cam 14 and the abutment. The rope is therefore gripped by the rope grab.

The greater the force that is applied to the connector, the more the cam 14 is caused to rotate with respect to the plates 10, 12, so further reducing the gap between the cam 14 and the abutment 16, and thereby further increasing the force with which the rope is gripped. The the centre of the abutment 16 and the nearest point on the cam 14 become closer together and the rope, applying friction to the cam causes it to rotate and present a progressively larger radius towards the abutment thus pinching the rope. The rope acts on the curved part of the cam at this time. The lead-in portion of the cam has minimal or no effect on the forces acting on the cam. The nature of a logarithmic spiral is that the angle of contact between the rope and the cam is constant; that is the development angle a, described above.

The convex shapes of the surfaces of the cam 14 and the abutment 16 that make contact with the rope ensure that the path of the rope 32 through the grab is as close to straight as possible.

If its path deviates from straight, its effective length is reduced. Therefore, if the ends of the rope 32 are fi.xed, the tension in the rope does not increase significantly when the rope is locked. If this were not the case (for example, as is the case with some prior art devices) tension in the rope may prevent the grab from locking. It also allows the grab to operate on an inflexible steel rope or a metal rod, although the optimal shapes, angles and dimensions of the cam and the abutment may differ from those specified above.

It will also be seen that the region at which the rope is gripped between the cam 14 and the abutment 16 tapers in an upward direction, effectively creating a region within which the rope 32 is wedged. This enhances the grip upon the rope. It also means that the grip upon the rope is self-reinforcing, loading upon the rope grab causing enhancement of the grip.

Claims

Claims 1. A rope grab comprising a rope path that extends between a cam

and an abutment, and connection means by which the grab can be attached to a connecting line, in which, when the device is supported from the connection means, the cam is urged by the rope to a position in which it has a point of minimum radius proximal to the abutment such that the rope can slide between the cam and the abutment, and in which, when the connection means is pulled downwards, the cam is urged to a position in which a point of greater radius is proximal to a convex profile of the abutment, and the rope is gripped between the cam and the abutment thereby resisting its movement through the rope grab.

2. A rope grab according to claim I in which the rope is gripped in an upwardly tapering region between the cam and the abutment.

3. A rope grab according to claim 2 in which the abutment is cylindrical.

4. A rope grab according to any preceding claim in which the cam is pivotable about a pivot axis.

5. A rope grab according to claim 4 in which the cam has a convex curved region extending from the point of minimum radius.

6. A rope grab according to claim 5 in which the radius of the curved region increases with angular distance form the point of minimum radius.

7. A rope grab according to claim 6 in which the curved region is a part of a logarithmic spiral.

8. A rope grab according to claim 7 in which the logarithmic spiral has the formula = Roebe where: R0 is the radius at point M; R8 is the radius at angle 0 from the line XM; 0.2 = b = 0.6; and 0 is the angle between the radius XM and the radius R8 in radians.

9. A rope grab according to any one of claims 5 to 8 in which the cam has a lead-in region extending substantially tangentially from the point of minimum radius away from the curved region.

10. A rope grab according to any preceding claim in which a line intersecting the centre of curvature of the abutment and the centre of the connection means makes an angle 4) with a line that passes through the centre of curvature of the abutment and the pivot axis of the cam, where 80 = 4) =120 .

11. A rope grab according to claim 10 where 12. A rope grab according to any preceding claim that comprises two plates, between which the abutment and the cam are retained.

13. A rope grab according to claim 12 in which the connection means comprises a respective aperture in each plate through which an attachment device can be passed.

14. A rope grab according to claim 12 or claim 13 in which the rope path passes between the plates.

15. A rope grab according to any preceding claim in which the cam and the abutment have rope-engaging surfaces, the rope engaging surfaces are formed with roughening formations.

16. A rope grab according to claim 15 in which the roughening formations include grooves, indentations or raised features.

17. A rope grab substantially as described herein with reference to the accompanying drawings.