PROTECTION DEVICE FOR USE IN CLIMBING

Abstract

 A protection device for use in climbing of the type known as a "spring-loaded camming device" - abbreviated "SLCD" - is disclosed. The camming device comprises a a shaft (10) and a head (14) mounted on the shaft. The head includes a plurality of cam lobes (20) carried on at least one cam axle (16), each cam lobe being capable of rotation about a pivot axis centred on the cam axle, with an increasing lobe angle ω between a retracted position and an expanded position. Each cam lobe has a cam surface (30) that extends about the axis with a working range of contact angles ω_min ≤ ω ≤ ω_max. The cam surface (30) is at a radial distance r from the axis that continually increases with ω. In at least one of the cam lobes (20) the distance r of the cam surface from the axis is a modified logarithmic spiral that has a cam angle α that decreases with an increase in ω; The head also has biasing means that operate to bias the cam lobes towards the expanded position.

Description

[0001] This invention relates to a protection device for use in climbing. It has particular application to a protection device of the type known as a "spring-loaded camming device", abbreviated "SLCD".

[0002] An SLCD includes a stem on which is carried a head that comprises two or more pivotal cam lobes, each cam lobe being carried on one or more cam axle about which it can rotate. The cam lobes, which are sprung to an expanded position, can be drawn to a retracted position by operation of a manual control, typically by drawing the control along the stem. Transition between the expanded and retracted positions is caused by rotation of the cam lobes about the cam axle. The angular position of the cam lobe on the cam axle is referred to as the "lobe angle", will be donated ω, and will be taken to increase as the cam lobes move towards the expanded position.

[0003] For use, the cam lobes are retracted and the head is inserted into a fissure in a rock. The control is released, to allow the springs to cause the cam lobes to rotate about the cam axle(s) towards the expanded position. The cam lobes make contact with and grip opposing walls of the fissure, and thereby retain the head within the fissure. The cam lobes are arranged such that if a force is applied to the stem that would pull the head from the fissure, the cam lobes are urged to rotate towards the expanded position, thereby enhancing the grip of the cam lobes on the rock.

[0004] The stem normally includes a loop to which a flexible sling can be connected, typically during manufacture, and/or a carabiner can be connected by a user as required. An example of a camming device is disclosed in EP-A-2 853 296 of the present applicant, the content of which is incorporated herein by reference.

[0005] Each cam lobe has a body that has a centre plane, a pivot axis that extends through the body normal to the centre plane, and an external cam surface that extends across the body parallel to the pivot axis. It is the cam surface that makes contact with the walls of the fissure. In the assembled camming device, each cam lobe is carried on the cam axle(s) to rotate with respect to the head about its pivot axis. The radial distance of the cam surface from the pivot axis, denoted r, varies in a continuous curve circumferentially about the pivot axis, the curve closely approximating a logarithmic spiral.

[0006] A basic property of a logarithmic spiral is the polar slope angle, which corresponds to an important parameter in the design of the cam lobes known in this context as the "camming angle", commonly denoted as α. The camming angle in the context of a cam lobe is illustrated in Figure 1. If a radius is extended from the pivot axis through the cam surface, α is the angle between that radius and a line normal to the cam surface at the point it is intersected by the radius.

[0007] It is the conventional view in the technical field that α should have a constant value throughout a working surface of the cam. For example, in "Rybansky, D.; Sotola, M.; Marsalek, P.; Poruba, Z.; Fusek, M. Study of Optimal Cam Design of Dual-Axle Spring-Loaded Camming Device. Materials 2021, 14, 1940. https://dx.doi.org/10.3390/ma14081940" it is stated on page 4, "In the optimal case, the angle α should be constant because a constant value represents uniform loading of the cam in any position".

[0008] In US-A-2005/0037023, US-B-US7740223 and US-B-7802770, the concept of using a variable cam angle is disclosed. In these disclosures, it is stated: It may be desirable to have a variable cam angle and thereby deviate from the logarithmic spiral slightly. .... For example, climbing aids in which the cam angle gradually increases from 13.25 degrees to 16.5 degrees over the subtended angle so as to reduce loading at the cam member tips and prevent over-expansion of the climbing aid may be made.

[0009] The present inventor has realised that camming effectiveness may be enhanced by ensuring that the pressure at the contact between the cam surface and the rock be maintained at a value that is close to constant and, in particular, closer to constant than is the case if a logarithmic spiral (i.e., constant camming angle) is approximated.

[0010] To this end, the present invention provides a camming device comprising:

a. a shaft;

b. a head mounted on the shaft, the head including:

i. a plurality of cam lobes carried on at least one cam axle, each cam lobe being capable of rotation about an axis centred on the cam axle with an increasing lobe angle between a retracted position and an expanded position, each cam lobe having a cam surface that extends about the axis with a working range of contact angles ω_min ≤ ω ≤ ω_max, the cam surface being disposed a radial distance from the axis that continually increases with ω; wherein in at least one of the cam lobes the distance of the cam surface from the axis is a modified logarithmic spiral that has a cam angle α that decreases with an increase in ω; and

ii. biasing means that operate to bias the cam lobes towards the expanded position.

[0011] In arriving at this invention, the present inventor has gone against convention and the abovementioned disclosures by providing a cam in which the camming angle α varies at different circumferential locations around the cam surface so as to decrease as the point of contact approaches the tip of the cam lobe - that is, as the cam lobes move in the direction from the retracted to the extended position (that is, as the lobe angle ω increases).

[0012] Amongst embodiments of the invention, within working ranges of values of ω, the value of α may vary between a minimum of 10° and a maximum of 20°. Such a range may not necessarily be found in any one embodiment. For example, embodiments may have values of α that fall within a range of 13.5° ≤ α ≤ 16.5°, while other embodiments may have values of α that fall within a range 15° ≤ α ≤ 20°.

[0013] In embodiments of the invention, α decreases linearly with increasing ω.

[0014] A camming device may be constructed with a minimum of two cam lobes. However, typical embodiments have four or more. In some embodiments of the invention, all of the cam lobes may have a cam angle α that decreases with lobe angle ω, as described above. In other embodiments, one or more cam lobe may have a constant cam angle and/or one or more cam lobe may have an angle that increases with the value of the lobe angle ω.

[0015] From a second aspect, this invention provides a camming device comprising:

a. a shaft;

b. a head mounted on the shaft, the head including:

i. a plurality of cam lobes carried on at least one cam axle, each cam lobe being capable of rotation about an axis centred on the cam axle with an increasing lobe angle between a retracted position and an expanded position, each cam lobe having a cam surface that extends about the axis with a working range of contact angles ω_min ≤ ω ≤ ω_max, the cam surface being disposed a radial distance from the axis that continually increases with ω; wherein in at least one of the cam lobes a transverse dimension t of the cam surface measured parallel to the axis decreases with an increase in ω; and

ii. biasing means that operate to bias the cam lobes towards the expanded position.

[0016] In embodiments of the invention, t decreases linearly with increasing ω.

[0017] The invention also provides camming devices that embody both the first and second aspect in combination.

[0018] Embodiments of the invention will now be described in detail, by way of example, and with reference to the accompanying drawings, in which:

Figure 1 shows a camming device embodying the invention;

Figure 2 shows the camming device of Figure 1 in place in a rock fissure;

Figure 3 shows in detail the contact between a cam lobe of the camming device of Figures 1 and 2 and a rock surface;

Figure 4 is a side view of a cam lobe being a component of an embodiment of the invention;

Figure 5 is a diagram that shows the variation in camming angle provided by an embodiment of the invention; and

Figure 6 is a graph showing the relationship between cam lobe contact position angle and cam angle.

Figure 1 shows a camming device embodying the invention. This is essentially of conventional construction, so will be described only briefly.

[0019] With reference to Figure 1, a camming device embodying the invention includes a stem 10 that has a pair of eyes 12 at a first end through which sling or a connector can be attached, the stem 10 extending along a stem axis. A head assembly 14 is attached to the opposite end of the stem 10. The head assembly 14 has first and second cam axles 16', 16" that extend along respective cam axes, transverse to the stem axis, parallel to one another and spaced equidistantly from the stem axis.

[0020] The head assembly 14 further includes a plurality of cam lobes 20 that are substantially identical in this embodiment. Each cam lobe 20 is formed from a single piece of metal with a centre plane and a substantially constant cross-section with several through holes. One of the through holes is a pivot hole 22 of circular cross-section centred on a pivot axis of the cam lobe, the pivot axis extending normal to the centre plane. Another larger hole is a clearance hole 24, the function of which will be described below. Other holes serve to minimise the mass of the lobes.

[0021] Each cam lobe 20 has a cam surface 30 that forms part of its periphery. The cam surface 30 extends along a locus that approximates to a logarithmic spiral with an origin that coincides with the pivot axis. A plurality of transverse and circumferential grooves are formed in the cam surface 30.

[0022] Each cam lobe 20 is carried on one of the two cam axles 16', 16", the axle passing through the pivot hole 22. The cam axle 16', 16" is a close sliding fit in the pivot hole 22, such that the cam lobe 20 can pivot on the cam axle 16', 16" about its pivot axis. The other of the cam axles 16', 16" passes through the clearance hole 24, which is shaped and dimensioned to allow pivotal movement of the cam lobe 20 to take place within a working range of lobe angles.

[0023] This embodiment includes four cam lobes 20, two being carried on each cam axle 16', 16". The two cam lobes 20 closest to the stem 10 are carried on the first cam axle 16' and are arranged in a first orientation. The two cam lobes 20 further from the stem 10 are carried on the second cam axle 16" and are arranged in an opposite orientation.

[0024] A biasing spring (not shown) biases each cam lobe 20 about the cam axle 16' upon which it is mounted to one end of its working angular range (which will be referred to as the expanded position) at an angle denoted ω_max. Movement beyond the extended position is prevented by the other cam axle 16" reaching the limit of the clearance hole 24. In the expanded position, which is shown in Figure 1, the distance of the cam surface 30 from the stem 10 in a distance transverse to the stem axis is maximum. A release trigger 32 is carried on the stem 10. The release trigger 32 can be manually drawn along the stem 10, acting through connecting elements (not shown) to cause the cam lobes 20 to rotate away from the expanded position, movement being limited at a position denoted ω_min. The cam lobes 20 on the first cam axle 16' rotate in the opposite direction to those on the second cam axle 16". For each cam lobe, rotation in a positive angular direction will be deemed to be rotation towards the expanded position - that would be anticlockwise in the case of the cam of Figure 3.

[0025] In use, the release trigger 32 is operated to rotate the cam lobes 20 away from the expanded position until the distance between the cam surfaces in 30 a direction transverse to the stem axis is reduced sufficiently to enable the head assembly 14 to be inserted into a fissure 34 in a rock between two rock surfaces 36. The release trigger 32 is then released to allow the cam lobes 20 to rotate towards the expanded position until their cam surfaces 30 make contact with the rock surfaces 36, as shown in Figure 2. This angular position of the cam lobes 20 on the axles 16' 16" will be referred to as the locking angle ω_look. A loading force F_L applied to the stem 10 as shown in Figure 2 tends to urge the four cam lobes 20 in a direction that tends to increase ω towards the expanded position, thereby forcing the cam surfaces 30 into contact with the rock surfaces 36, the contact being approximated to a line that extends across the cam surface 30. This creates a reaction force F_R directed from the line of contact along a radius from the line of contact to the pivot axis, which can be resolved into a component directed transverse to the stem axis F_Rx and a component F_Ry directed parallel to the stem axis, with F_Ry arising from friction between the cam surface 30 and the rock surfaces 36. It will be seen that for the camming device to be in static equilibrium, the total of the reaction forces resolved along the direction of the stem axis must equal F_L. which means that the total frictional force F_Ry generated by the cams must exceed the loading force F_L. Where the coefficient of friction between the cam surface 30 and the rock is denoted µ, and the cam angle is α an approximation for the condition required for the camming device to hold within the fissure is µ > tan α.

[0026] Insofar as it has been described above, the embodiment is conventional in its construction, with the value of α being substantially constant throughout the working angular range to keep the frictional force constant throughout the working angular range. A consequence of this is that the pressure between the cam surfaces 30 and the rock surface 36 is not constant for a given loading force F_L - it is dependent upon the locking angle of the cam lobes 20, which is in turn dependent upon the width of the fissure 34. In practice, when the approximation that contact between each cam surface 30 and the rock surface 36 is a line is abandoned, and the actual contact is examined more closely, it is seen that the area of contact between each cam surface 30 and the rock surface 36 increases as the locking angle increases, so a greater locking angle results in a lower pressure if α and F_L are constant.

[0027] In this embodiment, the cam lobes 20 are shaped such that α decreases as the locking angle increases, so the pressure between the cam surface 30 and the rock surface 36 is more nearly constant with a variation in locking angle that is the case with a cam in which the cam angle α is constant. That is, the peripheral shape of the cam surface 30 deviates from a logarithmic spiral such that the distance r from the cam axis to the locking surface is progressively less than would be the case for a true logarithmic spiral as ω increases. As shown in Figures 5 and 6, as the value of ω increases as ω₁, ω₂ and ω₃, the corresponding values of r (r₁, r₂ and r₃) also increase, the value of the cam angle α decreases linearly (α ₃, α ₂ and α ₁) with an increase in ω.

[0028] A decrease in value of α causes an increase in the locking force applied by the cam surface 30 to the rock surface 36 for a given loading force F_L, which compensates for the increase in contact area so as to maintain the contact pressure more nearly constant.

[0029] Alternatively or additionally, one or more cam lobes in embodiments of the invention may have a non-constant thickness, as shown in Figure 4. In this example of a cam lobe 120, the thickness t decreases with an increase in lω. This reduces the width of the contact between the cam surface 30 to the rock surface 36, so reducing the contact area and for a given reaction force F_R increasing the contact pressure.

Claims

1. A camming device comprising:

a. a shaft (10);

b. a head (14) mounted on the shaft (10), the head including:

i. a plurality of cam lobes (20) carried on at least one cam axle (16), each cam lobe being capable of rotation about a pivot axis centred on the cam axle with an increasing lobe angle ω between a retracted position and an expanded position, each cam lobe (20) having a cam surface (30) that extends about the pivot axis with a working range of contact angles ω_min ≤ ω ≤ ω_max, the cam surface (30) being disposed a radial distance r from the pivot axis that continually increases with ω; and

ii. biasing means that operate to bias the cam lobes towards the expanded position

characterised in that

c. in at least one of the cam lobes (30) the distance r of the cam surface from the pivot axis is a modified logarithmic spiral that has a cam angle α that decreases with an increase in ω.

2. A camming device according to claim 1 in which the camming angle α is in a range of 10° ≤ α ≤ 20°.

3. A camming device according to claim 2 in which the camming angle α varies between 13.5° ≤ α ≤ 16.5° over the working range of ω.

4. A camming device according to claim 2 in which the camming angle α varies between 15° ≤ α ≤ 20° over the working range of ω.

5. A camming device according to any preceding claim in which α decreases linearly with increasing ω.

6. A camming device according to any preceding claim having four cam lobes (20).

7. A camming device according to any preceding claim in which each cam lobe (20) has a cam angle α that decreases with angular position ω.

8. A camming device according to any one of claims 1 to 5 in which one or more cam lobe (20) has a constant cam angle α.

9. A camming device according to any one of claims 1 to 5 in which one or more cam lobe (20) has a cam angle α that increases with angular position ω.

10. A camming device according to any preceding claim wherein in at least one of the cam lobes (20) a transverse dimension t of the cam surface measured parallel to the pivot axis decreases with an increase in ω.

11. A camming device according claim 11 in which t decreases linearly with increasing ω.

Drawing