GB2472398A - A climbing chock with a cross member running between two upright members to provide a tripodal cam device - Google Patents

Abstract

A tripodal cam chock device comprising two parallel upright webs UW with cam profiles P and a cross web CW perpendicular to and running between the upright webs UW wherein the cross web CW has a rounded nose N projecting outwardly from it. In use the nose N and cam profiles P act to engage with the sides of a rock to hold the chock in position. The nose N may be rounded with respect to 2 different planes. The flanks F which extend from the nose N to the upright webs UW may also be rounded. The upright webs UW may have a relieved profile L to allow pivoting at the nose N. An additional loading bar B may be provided between the upright webs UW with a sling S attached to it. The device may also be used as a wedge by reversing the direction of engagement.

Description

A DESIGN AND CONSTRUCTION FOR A ROCK-CLIMBING PROTECTION DEVICE

OF THE RIGID BODIED TRIPODAL CAM CHOCK TYPE

This invention relates to the design and construction of a particular type of protection device used in the sport of rock-climbing, and in the broader field of mountaineering. In these activities it has long been the practice to employ all possible means to protect the lead climber of a roped tc&m frnm fh iniiirini i rnnni iPn nf Innri fII Si ith mn hiv ur fh the jamming action and ensuring sling loading is in plane with the pivot point to provide device stability.

The tripodal cam chock combines established principles of contact stability and cam design, and therefore is founded on basic knowledge long within the public domain. However, there remains inventive scope for embodiments providing an advance, relative to the prior art, in implementing said principles, and it is believed that the present invention represents such.

There follows a detailed description of the present invention, but first a review of the prior art is conducted to give background and context to the invention.

It is not known to what extent Abalakov commercialised his invention within the 1970's Soviet Union, but visiting USA exchange' climbers of the period obtained manufactured embodiments and excellent photographic records exist, showing cam chocks of T' section apparently improvised as asymmetric segments cut from aluminium flat belt pulley wheels, the arc section containing the belt groove being the top of the T', and the web of the wheel its upright section.

Segment asymmetry was produced by making two spaced convergent straight cuts of different length from the rim into the web, their junction providing the cam nose and the rim of the contained circular arc providing twin cam profiles relative to the said nose, the cam crack jamming size varying continuously from a minimum at the short cut side to a maximum at the long cut side of said segment. Sling attachment was effected by passing a rope through a hole in the web, and bringing the two rope ends through aligned holes in the rim groove bottom each side of the web near the segment's long cut side, the rope ends running within said groove and being joined by knotting remote from the chock body to form a sling. It seems that a cam device so made could at best only approximate to purpose designed cam constant angle technology, but providing the actual contact angle did not exceed the so-called friction angle at any point on the cam profiles the device would probably be stable in the rock crack. However, apart from the latter reservation, the type of sling and method of attachment appear less than satisfactory from two aspects; firstly, the acute bending of the rope under tension would cause significant weakening as a load bearing protection device, and secondly, the inherent flexural stiffness of the rope sling would resist entry into the cam groove with implications of device placement difficulty.

The Lowe cam chock, commercialised as the "Tricam" in 1981, and still available today in substantially original form, is the most widely used device of the type. Unlike Abalakov's improvisation, the "Tricam" was purpose designed to embody the tripodal concept, comprising a pyramidal spike pivot nose operating in conjunction with twin, spaced, constant angle cam profile lobes. A strong woven textile tape sling loop is pivotally connected from a load bar bridged between the cam lobes near their maximum size end, and this arrangement together with an inter-cam sling guide provision ensures ready compliance of the device to the loading direction and optimised cam leverage. The "Tricam" size range comprises two distinctly different design/construction embodiments of the tripodal cam chock. Smaller sizes are forged in aluminium alloy as a unit, with only the load bar being a separate inserted component, whereas larger sizes are fabricated from pre-.formed, joined, aluminium alloy sheet (presumably because at the latter sizes the forged embodiment is unsuitable, and vice versa). A feature of the "Tricam" (and also of Abalakov's embodiment) is that the nose is reinforced by a widening section above and below the pivot region in the plane of rotation, and such widening can be detrimental to certain placements where the rock face undulates. The spike nose of the "Tricam" provides only a small area of rock contact to transmit the high forces developed by the cam action, resulting in nose wear, which adversely affects the performance characteristics of the device, nose wear being further increased by the nose configuration of those cam chocks constructed from fabricated sheet. uTricams exhibit sharp pointed features at the nose and also at the load bar end of the cam lobes, said features being accentuated in devices made from fabricated sheet. It is undesirable and ironic that in climber protection equipment such features could cause personal injury from carried items, in the event of a fall.

U.K. Patent GB2028455B by Walters (the present Applicant), published in 1982, is a multi-functional cam chock based on the tripodal principle, but in different embodiment to the Abalakov and Lowe devices described above. Cam chock GB2028455B resembles in plan a wedge shaped buckle comprising three parallel bars, the bar at the thin end of the wedge being symmetrically rounded (in plan and elevation) on its outermost face, and the bar at the thick end of the wedge being formed on its outermost face with twin cam lobes separated by a central groove (similar to Abalakov and Lowe). To impart maximum leverage when applied in cam mode the actuating woven textile tape sling is pivotally connected to the rounded central bar, with both sides of the sling passing over the rounded top of the cam bar into the cam groove.

Although effective as a jamming device in both cam and wedge modes, the crack size range as a cam is limited by the embodiment design, and, like other cam chocks, the device proved awkward to place in the climbing situation.

The object of the present invention is to provide an improved embodiment for implementing basic principles underlying the tripodal cam chock, and specifically to provide an embodiment design and construction giving more effective jamming in a variety of rock crack shapes, particularly in parallel and near parallel sided cracks, whilst simultaneously offering desirable features of device low weight coupled with inherent rigidity, increased wear resistance, and a shape configuration commonly applicable to a total range of cam chock sizes.

According to the present invention there is provided a design and construction for a rock-climbing protection device of the rigid bodied tripodal cam chock type as defined in the appended claims.

A specific embodiment of the invention will now be described by way of example with reference to the following drawings, in which:-Figure 1 is an isometric view of the inventive cam chock depicting its design and construction features and the method of sling attachment.

Figure 2 is a sectional side elevation of the inventive cam chock operating in cam mode in a parallel sided crack.

Figure 3 is a sectional plan elevation of the inventive cam chock operating in cam mode in inwardly parallel and tapered cracks.

Figure 4 shows the cam chock operating in wedge mode.

With reference to Figure 1, a cam chock according to the invention is based on an H' section profile produced preferably as a solid integral item, for example as a unit piece cut from a long extruded length. However, other methods of fabricating the H' section comply with and are therefore included in the inventive concept. As shown in Figure 1, the tripodal principle of the inventive cam chock is embodied by forming the spaced upright webs UW of the H' as twin cam profiles Pat one end of the unit piece, and forming a centrally located nose Non the connecting cross web CWat the opposite end of the H'. Thus, with the inventive cam chock, the high compressive lateral forces developed in lead climber fall arrest are carried between the nose and cam profile contact points with the rock by shear stress at the upright and cross web junctions of the H' section. To maximise rigidity and minimise bending moments in the inventive cam chock it is preferable that cross web CW be substantially centrally located between the top and bottom of upright webs UWas indicated in Figure 1. By incorporating nose N as an end profile of cross web CWas shown in Figure 1, only the central, active rock contact region of the nose is in plane with the devices pivotal action, caused by a pull 7' on sling S as shown, and this important feature of the inventive cam chock provides empty space above and below the nose (unlike prior art devices) enabling the cam chock to accommodate rock crack irregularities of a bulging nature which would otherwise obstruct and impede the cam action. Furthermore, by incorporating nose N in cross web CI said nose is reinforced orthogonally to the pivotal cam action, enabling a slim nose profile in the pivotal direction, and thus enabling the nose to advantageously fit into any shallow pockets and transverse grooves present in the rock-face crack, so providing additional security to the device placement.

With reference to Figure 1, the profile of nose N and reinforcing flanks F are a compromise between theoretical ideal and practicality. Ideally nose N would be a small rock contact point of pivotal rotation, being the point origin of design for cam profiles P, and, as a point, providing least imbalance between twin cam loading due to the effects of inward crack taper or flare (more on which later). However, such an idealised nose would be impractical from a wear and strength aspect in aluminium alloys used for climbing equipment manufacture. Moreover, should such an idealised nose be used as a basis of cam chock manufactured design, wear erosion would in short usage time create a nose profile different to that of design, thereby changing the actual cam characteristics from those designed. Clearly, such a situation is untenable in a device used for safety protection, and the practical alternative of basing design on a larger, more wear resistant nose profile is compelling, whilst recognising the theoretical ideal. Thus, the design of nose N of the inventive cam chock is a rounded surface based simultaneously on a circular profile of radius Rn in the plane of rotation, as depicted in Figure 2, and transverse radius Rt in the plane of cross web CWas depicted in Figure 3.

As previously mentioned, the tripodal principle of cam chock design provides basic means for achieving device stability in parallel sided cracks which inwardly taper or flare. Such cracks are represented in elevation and plan by Figures 2 and 3 respectively. The amount of crack inward taper/flare tolerable by a cam chock before expulsion depends on the frictional interface and is deducible from wedge mechanics theory. To maximise cam chock usefulness in such cracks it is advantageous to relieve the transverse aspect of the nose profile at an angle allowing said tolerated accommodation, as illustrated by flanks F in Figures 1 and 3, where flank relief angle a exceeds maximum tolerable crack taper/flare angle fi as shown. Thus rock contact points of cam chock nose N in accommodating the various crack placements of size and shape offered by the device according to the invention are bounded within a rounded surface defined by radii Rn, Rt and angle 2a' as indicated in Figures 2 and 3. To ensure that nose N can pivot freely over the angular pivotal range corresponding to crack size Cinin to crack size Cmax in Figure 2, whilst simultaneously accommodating inward crack taper/flare as described, nose radius Rn is carried around the full length of flanks F as indicated in Figure 1. For similar reason cam lobes L of H' section upright webs UW are relieved at an angle backwards from their root juncture with nose flanks F of I-I' section cross web Cfri' as shown in Figures 1 and 2, to ensure said lobes remain in clearance of the nose-side crack face in all placement attitudes assumed by the inventive cam chock throughout its design operating range as described.

A distinctive feature of the inventive cam chock, arising from above-described design optimisation, is the platform-like nose region of cross web CW extending beyond the body of the H' section. In the mid operating range of twin cam profiles P the thrust of the cam action lies more or less within the plane of cross web CE but at the extremities of said range corresponding to Cmin and Cmax of Figure 2, the resultant force on nose N acts out-of-plane and therefore produces a bending moment in cross web CW causing maximum bending stress at the section where nose flanks F adjoin cam lobes L, as denoted by dimension K in Figure 2.The cross section at this point must be capable of carrying the maximum design value of said stress without significant distortion, and in particular the minimum practical thickness (or depth) of the section, and consequently minimum nose pivotal radius Rn, are governed by such.

Magnitude of nose transverse radius Rt is not affected by the above considerations, but in interest of equalising cam profile thrust is made as small as possible consistent with durability against wear.

With reference to Figure 2, twin cam profiles P are generated on the basis of constant cam angle with respect to pivotal nose N over the angular range represented from Cmin to Cmax, using established cam design technology.

With reference to Figures 1, 2 and 3, sling S of strong woven textile tape is pivotally attached to the cam chock by means of a load bar B orthogonally bridged between twin cam lobes L near the maximum setting size of the device Cmax as shown, the width of sling S fitting that of cross web CW to provide uniform load distribution on bar B and sling guidance between said cam lobes, tangential tracking of sling relative to cam profile being provided by the recessed profiled rear V of cross web CW together with one or more (depending on cam chock size) inter-lobe bars G. The above-described method of sling attachment and guidance is in essence similar to

that of the prior art.

With reference to Figures 1 and 2, sling bar B and guide bar(s) C are laterally secured to cam lobes L by end-face riveting or other suitable means, an effect of which is to tie said lobes together and thereby increase cam chock rigidity, the latter being further increased on larger sizes by providing sling bar and guide bar sleeves (or bushes) as indicated in Figure 2, sleeve length being equal to the width of H' section cross web CW In addition to providing the climber with advanced fall protection as described in the foregoing, the design of the inventive cam chock provides means of withstanding the forces developed in fall arrest within an inherently low weight structure, the latter feature being clearly illustrated in Figure 1 and of practical importance to the lead climber given the amount of various equipment items carried. Moreover, it is clear that the inventive design as shown in Figures 1, 2 and 3 is capable of extrapolation over a wide range of cam chock sizes without need for significant change to the basic design concept.

Whilst the principal use of the above-described inventive cam chock is as a cam device for providing protection placements in rock cracks where simpler wedge type devices are ineffective, in common with prior art tripodal cam chocks the inventive device is readily converted from cam to wedge mode, if required, by reversing the direction of sling action as shown in Figure 4.

Claims (10)

1. Claims 1. A design and construction of a tripodal cam chock device, the body of said device being of "H" section wherein twin cam profiles are provided on the spaced upright web end faces of the "H' section at one end of said body and a rounded pivotal nose, acting in conjunction with said cam profiles, is provided centrally on a relieved profiled projection of the cross-web end face of the "H" section at the opposite end of said body.
2. 2. A tripodal cam chock device according to claim 1, in which said pivotal nose of said cross-web is specifically rounded both in the plane of cam pivotal action (i.e. within the thickness of said cross-web) and in the orthogonal plane (i.e. within the plane of said cross-web).
3. 3. A tripodal cam chock device according to claims 1 and 2, in which the extent of orthogonal rounding of said cross-web central nose is tangentially bounded by symmetrical flanks carried to the full width of said "H" section, thus providing lateral nose relief.
4. 4. A tripodal cam chock device according to claims 1, 2 and 3, in which said cross-web relieving flanks are additionally relieved by rounding within the thickness of said cross web.
5. 5. A tripodal cam chock device according to claims 1, 2, 3 and 4, in which the innermost faces of said spaced upright webs of the "H" section (forming the cam lobes) are relieved from points of juncture with said cross-web flanks, providing said device with pivotal freedom over the designed cam operating range including any lateral tilting of the cam chock to accommodate flared placements.
6. 6. A tripodal cam chock device according to claims 1, 2, 3, 4, and 5, in which a loading bar carrying a pivotally captive sling is secured to and orthogonally bridged between the profiled spaced upright webs of said "H" section (forming the cam lobes) at a position adjacent the maximum setting size of the cam chock.
7. 7. A tripodal cam chock device according to claims 1, 2, 3, 4, 5, and 6, in which that part of the "H" section cross-web lying between the cam profiles is recessed inward of said profiles to provide passage and tangential guidance for the emergent sling.
8. 8. A tripodal cam chock device according to the preceding claims, in which additional sling guidance and lateral chock body stiffening is provided as necessary depending on cam chock size.
9. 9. A tripodal cam chock device according to the preceding claims, in which said device is converted from cam to wedge mode by reversing direction of the emergent sling.
10. 10. A tripodal cam chock device substantially as herein described above and illustrated in the accompanying drawings.