GB2471734A - A method for controlling rigid bodied cam chock rock-climbing protection devices - Google Patents

Abstract

A control method facilitating use of cam chocks employed in rock-climbing. The method utilizes a flexible stringVconnected at one end to cam chockCCadjacent pivotal noseN, and at the opposite end to a collarKslidingly connected to a prepared slingS. In cam chock placement and removal, sliding of collarKon slingScauses stringVto rotate cam chockCCabout load barBso adjusting setting size to suit the placement, whilst squeezing either stringVor pliable collarKagainst slingSlocks the cam chock at present set size. Flexural stiffness of stringVacting through collarKon slingSresists inversion to wedge mode of cam chockCCduring device suspension from snap-linkSLpending use (the wedge configuration being available by overriding resistance).

Description

A METHOD FOR CONTROLLING ROCK-CLIMBING PROTECTION DEVICES

OF THE RIGID BODIED CAM CHOCK TYPE

This invention relates to a method for advantageously controlling 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 team from the injurious consequences of a long fall. Such means have over the years developed into increasingly sophisticated protection devices which exploit natural features of the climbing environment for places of anchorage. Said features are varied and various, those in context of the present invention being openings in the rock face such as cracks, fissures and other recesses into which protection devices can be secured. All such devices essentially consist of two joined parts, namely a part which anchors to the rock and a part which slidably connects to the lead climber's trailing rope. In the event of the lead climber's fall, said devices act as an arresting pulley when the rope is secured by a team member below.

Depending on the length of the lead it is practice to place several such protection devices at suitable intervals, the last placed being the mainly active one in a fall situation. A well known early 20th century example of such a lead climber protection device is the piton/snap-link combination.

As rock-climbing skills developed there was a corresponding raising in severity of the challenges undertaken, and a general need for improved equipment, not least that of fall protection, and the 1960's saw the introduction of the climbing chock -essentially a lightweight metallic artificial chock-stone carried on a strong textile sling equipped with a snap-link. Such early chocks were in the form of wedges for placement in suitably constricting and orientated cracks in the rock face where they tightly jammed under load developed in fall arrest. As a result of experience gained it was realised that by offsetting the attachment point of sling to chock a twisting action could be incorporated into the primary pull which increased the jamming effect.

This discovery led to a branching in chock development, the new branch leading to the cam chock devices of the 1970's.

Although by the 1970's wedging chocks were in widespread use, their application was limited to placements of a constricting nature. Notably they were useless in parallel sided cracks. This deficiency was the driving force for cam chock development, and by adapting established engineering cam design technology it was shown possible for chocks placed in parallel sided cracks to support load developed in a leader fall, purely by frictional resistance resulting from the rotational thrust of the cam device between the opposed crack walls. Furthermore, the spiral wedge' nature of the cam profile enabled a single cam chock device to accommodate a range of crack sizes. During this stage of development the most effective embodiment of the cam chock was found to be that based on the inherent stability of the tripod, with a central pivotal nose of the device bearing against one side of the parallel crack, and spaced twin cam lobes of the device bearing against the opposite side, the actuating sling of strong woven textile (usually tape) being connected to a loading bar bridging between said lobes and channelled to emerge tangentially at that point where the cam lobes contacted the crack side. From the sport literature it is believed that the Soviet climber/inventor Abalakov produced cam chocks of the above described embodiment either separately or in collaboration with the USA climber/inventor Lowe during the early 1970's, the latter marketing a range of devices called Tn-cams' in 1981 and still commercially available today. UK Patent GB2028455B, (by the present Applicant) published in 1982, is a different embodiment based on essentially the same three-point contact tripodal principle, and subsequently developed as published in climbing literature during the 1980's.

The above described cam devices, effective in parallel sided and even slightly flared cracks, are essentially rigid bodied chocks with no moving parts, and followed the contemporary trend of developing devices to accommodate as many placements as possible with a single equipment item. Thus many cam chocks of the period have dual modes, one for cam operation in parallel cracks and one for wedge operation in constriction cracks, mode selection being by manipulation of the chock relative to the sling prior to placement. However, the penalty of this versatility was, and is, awkwardness in use, especially as a cam, firstly in mode selection and secondly in the actual placement (and extraction). The object of the present invention is to overcome this difficulty, as subsequently disclosed, but before entering into the latter, further background is provided for context of the invention.

Whilst the primary requirement of rock-climbing protection devices is to arrest lead climber fall, there is a second requirement that the device must lend to easy use by the leader, which implies that the device must be capable of quick and secure single-handed placement. With advances in rock-climbing technical severity the requirement for easy protection placement has assumed greater significance, and at the foremost levels of the sport the ability to place protection quickly with minimum energy expenditure can be decisive in determining the climber's success or failure. By the latter criterion rigid bodied cam chocks of the described type were mediocre, and many climbers sought better, welcoming the arrival in the early 1980's of the spring loaded cam device (SLCD).

SLCD's are now well known and widely used in the sport, and have become for many the preferred parallel crack protection device. It is therefore informative to compare their salient features with those of the cam chock from a user perspective: Firstly, in terms of fall arrest, cam chocks and SLCD's are both generally effective in parallel sided cracks. In certain restricted placements cam chocks perform better.

Secondly, SLCD's have moving parts, whereas cam chocks are rigid bodied. (This difference has led to the widely applied but somewhat confusing practice of referring to SLCD's as active' devices and cam chocks as passive'.

Thirdly, SLCD's by nature of their design, provide the user with precise control to assist device placement and extraction, whereas prior art cam chocks have no such provision.

Fourthly, SLCD's are more complex in design and construction than cam chocks and accordingly more prone to mechanical failure and, other things being equal, more costly to manufacture and purchase.

In summary, rigid bodied cam chocks compare favourably with SLCD's in most areas, except that of easy application by the user. However, this latter disadvantage alone has proved decisive in influencing climbers' equipment choice away from cam chocks towards SLCD's. The object of the present invention is to overcome said disadvantage of prior art cam chocks by providing a method of control for facilitating their use.

In the process of inventing such a control method it was first necessary to characterise the nature of usage problems associated with prior art cam chock devices. For this purpose reference is made to Figure 0 which, it is stressed, represents prior art cam chocks, and forms no part of the present invention, except by association with the subsequently described inventive control method. The illustrations of Figure 0 purposely depict only those basic features of the type necessary for explaining above stated usage problems. Furthermore, Figure 0 depicts a cam chock in horizontal crack placements but clearly placement opportunities are not limited to the latter orientation. With reference to Figure 0, Figure 0.1 depicts a cam chock operating in cam mode in a parallel sided crack, a pull on snap-link SL (e.g. from the climbing rope) tensioning sling S pivotally attached to cam bar B causing the device to rotate about rock contact point C of nose N causing twin cam lobes L to be forced against the opposite crack face at C and thus jamming the device tightly in the crack. Whilst Figure 0.1 shows a general mid range crack size placement, clearly smaller and larger cracks than illustrated could be accommodated by the device with other mutually related contact points on nose and lobes.

Figure 0.2 is a side view of Figure 0.1, illustrating the 3 point contact tripodal principle of the device as referred to earlier, and also depicts the sling channel feature between twin cam lobes L, the bottom of said channel being provided by guide member G (Figure 0.1). Figure 0.3 shows the device in wedge mode in a constricting rock crack. The centre of gravity of the device is marked as CC in Figures 0.1 and 0.3 for subsequent reference. Use of a cam chock device as illustrated and described can be divided into three stages; firstly, a selection and presentation stage where the device is selected for use from various other devices carried by the lead climber, and manipulated into a position suitable for placement; secondly, the actual placement stage; thirdly, the extraction stage, which although usually executed by the following climber from the security of the lead rope, may on occasion be done by the leader. Each of the above mentioned stages poses specific problems of use to the climber, especially when the cam chock is employed in its principal application of cam mode: Firstly at the stage of selection and presentation the device will be stored ready for use in free suspension from its snap-link attached to the climber's equipment rack, and will therefore assume an attitude under gravity similar to (but at 900 to) that depicted in Figure 0.3 with centre of gravity CG directly below bar B, and thus the default attitude of the device on selection is wedge mode not cam mode (as Figure 0.1), and to convert to the latter requires the body of the device to be manually re-orientated relative to sling S engaging said sling in the channel between twin cam lobes L: Secondly at the stage of device placement in cam mode; said prior re-orientation of device relative to sling must be manually retained against the natural tendency for the device body to rotate under gravity, whilst single handedly fitting the device into the crack, before finally applying setting force via sling tension: Thirdly at the stage of device extraction in cam mode it is necessary to manually restrain the device during removal from the crack against the natural tendency to re-engage.

Thus the overall object of the present invention can be more specifically stated as: (a) providing means for retaining the cam chock ready for use in principal application mode as a cam device, without detriment to the latter function, whilst reserving use in wedge mode as a secondary option; (b) providing means for remotely controlling expansion and contraction of the cam action such that the chock is readily manipulated and held in position during placement without need to manually engage the chock body in the crack; (c) providing means for remotely contracting the cam action to extract the chock from the crack after use without need for manual removal.

According to the present invention there is provided a method for controlling rock-climbing protection devices of the rigid bodied 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, Figure 1 a and Figure lb show mechanics of a cam chock in operation, as a basis for theoretical analysis and conclusions supporting the invention.

Figure 2 shows a cam chock equipped in accordance with the inventive control method, ready for application in cam mode.

Figure 2a shows a cam chock equipped in accordance with the inventive control method, in typical carrying orientation as slung from the climber's gear rack.

Figure 2b shows a cam chock equipped in accordance with the inventive control method, when converted for use in wedge mode.

Figure 3, comprising Figures 3a and 3b, shows connection details for an embodiment of the inventive control method applied to a cam chock device.

Figure 4 shows a placement and extraction procedure for a cam chock equipped in accordance with the inventive control method, when applied in cam mode.

The control method comprising the claimed invention was developed from theoretical consideration of the basic mechanics of the rigid bodied cam chock as described in the following: Figure 1 shows features of such a prior art device represented in one plane. Contact with the parallel sided rock crack occurs at point a on nose N and point b on cam lobe L. An imaginary straight line q linking a and b is inclined to the normal, defined by crack width t4' the angle of inclination being important in maintaining cam chock stability. Characteristically the device comprises a continuous range of paired contact points a and b, each pair defining a line q at constant elevation from the normal, increasing in magnitude throughout the device's operating range from crack width Wmin to Wmax as shown. Therefore, to understand the mechanics of the device it is necessary to consider only one general setting position. Figure 1 a shows such a position where a cam element of length q and inclination 9 is stable in a parallel sided crack of width W under the action of an external force B applied at distance x from point a, and reaction forces from the crack sides P, Fa and Pb, Fb at a and b respectively, where P denotes the thrust reaction acting on the device in a direction normal to the crack side, and F denotes the related frictional reaction force acting on the device in a direction parallel to the crack side. The forces shown in Figure 1 a can be simplified. Considering the lateral stability of the device it is apparent that force Pa must be equal to force Pb and each can therefore be represented by a common force P. Furthermore, it is a reasonable assumption from a design aspect that frictional conditions on each crack side, as defined by a coefficient ji, will be similar, in which case frictional force Pa can be written as p.Pa and Fb as,i.Pb, and since Pa = Pb = P, Fa = Fb 1u.P. With reference to Figure 1 a, considering device stability in the direction of the crack, it is clear that applied force B must be equal to the sum of frictional forces Fa and Fb, and it follows that B = 2y.P. Thus all forces acting on the device can be expressed in terms of force B, with Pa = Pb = E/2p and Fa = Fb E/2. Figure lb shows the simplified force arrangement.

A study of Figure 1 b indicates that for jamming of the device in the crack to occur the net turning moment of the forces about point a on nose N must be positive in the direction of the moment of applied force E. In other words the net moment Ma must be positive in a clockwise direction as shown in Figure lb. This requirement can be expressed as;

W E

Ma �= E.x -E.---.W.tanB which reduces to 2 2i W tanO Ma�=E[x---(1+ )] The above theoretical expression is useful for predicting behaviour of the cam chock device in operation. It reveals immediately that whilst applied force B directly influences the numerical magnitude of moment Ma, it is the content of the bracket [....] which is decisive in influencing the directional sense of the moment, and within said bracket three independent variables are at play, namely, the position x of applied force B, the angular inclination and coefficient of friction between rock and device p. Analysis of the interaction of these variables leads to the

following conclusions:

(1) For the device to jam in the crack tanG cannot exceed p, because otherwise the term W/2(1+tanG/u) would exceed and since the largest possible value of x is W, moment Ma would become negative (i.e. anti-clockwise in Figure 1 b) and the device expelled from the crack. A related conclusion is that the design angle of the device 9 cannot exceed what is known as the friction angle c-where tan q =u, even under the most favourable torquing condition given by x-W A further related conclusion is that given the pre-condition that p >tanG, the jamming effect for any given value of applied force B will be greatest when x attains a value as near as possible to W(The latter part of this conclusion merely confirms prior art practice as per Figure 0, and could be reached intuitively).

(2) For the device to jam in the crack, as x reduces from a maximum of W the quantity given by term W/2(1+tan9/p) must similarly reduce to satisfy the condition Ma? 0, irrespective of the magnitude of applied force B. This implies that if x is reduced for design or any other considerations, then either friction p must (somehow) be increased or design angle 8 must be reduced.

(3) Following from conclusion (2) is a conclusion of major significance to the present invention, namely, since the lowest conceivable value of tan9/pis zero (occurring when either9is zero or friction p becomes infinitely great, neither of which is practically possible in the device), it follows that moment Ma cannot remain positive for values of x equal to or less than W/2, and with such values the device is rendered dysfunctional as a jamming tool.

(4) Following directly from conclusion (3), a force E applied at x<W/2 will produce a counter jamming moment and tend to expel the cam chock device from its placement, the smaller the value of x the greater the expelling effect.

From the preceding theoretical conclusions, which have been verified as true by experimental tests, arose the basic concept for controlling the action of a prior art cam chock device (reference Figure 0.1) by attaching a flexible string to the device at a point near its nose N, the string being tensioned by a pull in the general direction of that exerted by sling S when loaded, the tensioned string providing means of retracting the device's cam action, and as subsequently disclosed, providing further advantageous means of device manipulation in use.

Although a control string, as above described, could be separate from the cam chock's actuating sling, such an arrangement is not ideal for single handed application by the lead climber, and a preferred arrangement is for said string to be slidingly connected to the sling, such arrangement providing mutual proximity for ready access and use whilst enabling sling and string to be operated independently of each other. The embodiment of Figure 2 illustrates essential features of such a preferred arrangement according to the invention, in which one end of string V is attached at vn to nose Nof prior art cam chock CC the other end of string V being attached at vk to collar K the latter being freely slidable on prior art sling S. Sling S is connected rotatably captive on load bar B by fixing the two sides of said sling together (e.g. by sewing) at a transverse section just below said load bar, as indicated by A in Figure 2. Said sling sides are held together by sewing or other means to prevent separation over that length between sling S connection to load bar B and the extremity of travel of collar K, denoted by R in Figure 2, to ensure free sliding of said collar on said sling.

Figure 2 indicates control functions provided according to the invention when the chock is operated in cam mode: Firstly, drawing collar K in a direction towards snap-link SL relative to sling S as shown causes flexible string V to tehsibn and the cam to retract by rotation about bar B, such retraction facility being useful for cam chock presentation for placement in the rock-face crack and for removal from the crack after use: Secondly, by providing a degree of compressive stiffness tO flexible string V. pushing collar K in a direction towards cam chock body CC relative to sling S as shown causes string fr' acting as a strut, to expand the cam by rotation about bar B, such expansion facility being useful in sizing the device to the crack during placement prior to setting by Sling S Thirdly, by manufacturing collar K in a pliable material, squeezing the collar onto sling S as shown effectively locks the cam chock in the present setting size, a useful facility when manipulating the device into and out of a placement. With reference to Figure 2, it is evident that said cam chock locking functiôh can alternatively be executed by squeezing string V onto sling S anywhere in the region between the cam chock body and collar K as illustrated, such alternative providing for better placement control with larger sizes of cam chocks where, because of the linkage mechanics and greater chock mass, collar K may be too remote from the cam chock to exercise directional control over the latter via the intervening unsupported length of flexible sling. (The pros and cons of rendering the sling less flexible by stiffening means are considered and concluded upon later in this Application).

With reference to Figure 2 the minimum length of string V is important and decided from considerations of cam chock device use. If it is decided to limit device use to cam mode only, the main safety concern is that under no circumstances of application should string V become tensioned, because, as theoretically proven, a direct result of the latter would be to retract and expel the device from its placement, Such string tensioning will occur if collar K is prevented from riding freely on sling S towards cam chock body CCas demanded by the placement, the most probable cause being said collar coming into contact with and obstructed by the device body. It is a straight forward design exercise to ensure the latter cannot occur within the design range of application, but there are additional environmental influences which need taking into account when allocating string length, such as the effect of sling oscillation caused by natural disturbances transmitted from the climbing rope as the climber progresses, and the effect of the sling being flexed into conformity with the rock-face contour in certain placements when loaded.

However, if it is decided to retain dual cam/wedge modes as in the prior art already reviewed, experience of use indicates that the extra length of string V necessary to enable transition from cam to wedge mode (as subsequently described) is sufficient to accommodate all operational conditions described above.

Foregoing analysis identified the need to pre-orientate prior art cam chocks from wedge to cam mode as a usage difficulty, to be overcome as an object of the present invention. The device control method shown in Figure 2 inherently provides such a solution as follows: In the normal cam working range of the device, as per Figure 2, the linkage provided by string fr collar K and sling Sallows collar K to slide freely on sling S However, when the device is inverted in carrying position prior to use as typified by Figure 2a, said free sliding of collar K due to rotation of chock body CC about bar B under gravity is arrested when string V commences to flex on contacting chock body guide C as shown, said flexing increasing to a point where pressure exerted between slide collar K and sling S is sufficient to tension string V and support the device against further rotation.(The method of connecting string V to collar K at vk influences said pressure, and is described subsequently). Although the device would typically be beyond normal cam operating range in carrying position Figure 2a, it is primed' in cam mode and readily brought into use by drawing on collar K relative to sling S. With reference to Figure 2a, a readily applied means to achieving this end is to hold the inverted device by collar K and shake the cam chock downwards, such action automatically causing sling S to wrap around the cam profile groove and the device to assume the fully retracted cam position corresponding to Wmin in Figure 1. Should it be desired to convert the device to wedge mode from the position of Figure 2a, such can be achieved by pushing on slide collar K whilst holding sling S to overcome flexural resistance of string V and attain the configuration shown in Figure 2b. With reference to Figure 2a, a readily applied means to achieving this end is to hold the inverted device by sling S at a point above collar K and shake the cam chock downwards. (Resulting wedge mode for the inventive cam chock as depicted by Figure 2b is comparable with prior art drawing Figure 0.3).

It is possible to revert to cam mode by reversing the latter action, though for ready use as a cam the primed' cam mode configuration of Figure 2a is a preferable carrying arrangement.

In describing essential features of the inventive cam chock control method with reference to Figures 2, 2a, 2b, embodiment details were omitted in interest of simplification. Such details are now provided in the following examples, which are not intended to be exclusive of other possible means of embodying the inventive concept: To fulfil its duty, string V requires properties of flexural pliability, resilience, partial stiffness, toughness and abrasion resistance. A material meeting these requirements is a monofilament polymer such as nylon, the cross-sectional area of the filament increasing pro rata with the cam chock size.

Slide collar K requires properties of shape resilience, pliability and abrasion resistance.

Whilst the ability to return to shape after deformation is necessary to ensuring free sliding of the collar on sling 5, pliability is also required to provide the cam chock squeeze-locking control function and to enable the collar to conform to any flexing of the sling under load due to the nature of the placement. The abrasion resistance requirement arises from unavoidable contact with rough rock surfaces in normal use. A further requirement is that slide collar K be readily connectable to string V The above-stated requirements can be satisfied by several slide collar design and construction methods and it is not the intention of the present invention to limit the field in that respect. For example, slide collar K could be moulded in a suitable plastic or rubber, or could be fabricated by wrapping and sewing suitable strong textile webbing.

The connection of string V to cam chock nose N, designated vn in Figures 2, 2a, 2b, requires to be of low profile, robust and simple, because of its proximity to device contact with the rock-face. Furthermore, it is preferable that connection vn be pivotal to enable the cam chock to be freely rotated into expansion or contraction by slide collar K via string [ Figure 3a illustrates a means of embodying such a connection, where string V is looped through an eye d(either anchored to or an integral part of cam chock body CC) located centrally in the vicinity of nose N, the two sides of the emergent loop being clamped together by a ductile swaged-on ring eat a point near eye d thereby positionally captivating string V relative to nose N whilst allowing free pivotal movement at the connection.

The connection of string V to slide collar Ac designated vk in Figures 2, 2a, 2b, requires to be robust and generally unobtrusive both from aspects of manually accessing slide collar K and avoiding environmental snagging. Furthermore, whilst, for reasons already explained, it is preferable for string to cam chock nose connection vn to be pivotal (as Figure 3a), it is preferable for connection vk to be essentially of fixed nature, because the latter type of connection promotes local resistance to flexing of string such resistance being advantageous in providing the device's cam mode retention state as previously described and illustrated in Figure 2a. Figure 3b shows one embodiment of fixed connection vk where the slightly spaced sides of open ended looped string V are firstly clamped near their ends in swaged-on fitting c leaving protruding tails of length just less than that of slide collar K Secondly, said tail ends of string V are engaged in pockets provided in slide collar K(the latter being of the wrappedlsewn webbing type in Figure 3b) such that fitting c abuts the collar end as shown, and fitting c being attached to the collar by sewing or other means.

With reference to Figures 2 and 3, the length of looped string V between connections to cam chock nose vii and collar v-k may optionally be fitted with a loose flexible sleeve, such as woven tubular flat webbing, to prevent mid-length separation of said loop, thus eliminating any functional problems which may arise therefrom, and facilitating cam chock locking when the latter action is applied directly to string V (rather than collar K) for reasons as previously described with reference to Figure 2.

After completing above described string connections to cam chock nose and collar, the free looped end of sling S(minus snap-link) is threaded through slide collar K to complete assembly of the control apparatus on the cam chock according to the invention as depicted in Figure 2, or alternatively sling S may be threaded through collar K in the opposite direction and load bar B fitted to the cam chock and sling as a final assembly step.

In earlier description of the essential features of the invention, reference was made to the possibility of stiffening sling S over all or part of that length between load bar B and slide collar K as designated by dimension R in Figure 2, with the object of facilitating placement and extraction of the cam chock. Whilst such stiffening is achievable by various means, and enables the already described lock off function to be effectively applied at greater distance from the chock body, any placement advantage thereby gained must be weighed against increased risk of the device being loosened or dislodged from placement due to increased leverage effects of sling stiffening arising from ever present, unavoidable disturbances transmitted from the climbing rope via snap-link SL. Within the inventive concept of the control method the cam chock may be rigged for maximum sling flexibility to minimise risk of accidental dislodgement, or rigged for maximum sling stiffness to assist facilitating placement, or rigged with any intermediate combination of these extremes. Since essential features of the invention as previously described with reference to Figures 2, 2a, and 2b are substantially unaffected by such rigging, the latter is a matter of user preference.

Having described in detail features of the inventive method for controlling cam chock devices when applied in cam mode, it remains to describe how the invention is most advantageously used by the climber in placing and removing such devices, noting that it has already been described with reference to Figure 2a how the invention advances the prior art by automatically retaining the device in ready-for-use cam mode. In the following a procedure of use is described with reference to Figure 4, but especially noting that in said procedure and drawing, for simplicity of presentation, the cam chock locking function is described and depicted as applied via slide collar K, whereas in practice this function may, with some device arrangements, be equivalently but more readily applied via string V at a position between chock body and collar, as previously described with reference to Figure 2, and therefore reference to device locking via collar K in the following should be equally construed as applying, where practically advantageous, to device locking via string V: Having selected the appropriate size of cam chock for the intended placement, the device, typically as depicted in Figure 2a, is firstly retracted to its minimum setting size, by either drawing on slide collar K relative to sling S or alternatively holding collar K with the cam chock body downwards and shaking sling S through. Secondly, the device is locked at said minimum size by squeezing pliable collar K into tight contact with sling 5, and entered into the placement (shown as a parallel sided crack in Figure 4). Thirdly, whilst retained in locked status, the cam chock is sized to the crack by rotating the device about collar K in direction 1 to position I of Figure 4. During the latter stage it is important that both cam lobes make contact b with the rock, and that the active plane of the cam chock is aligned with the anticipated direction of loading in the event of the climber's fall, both said requirements being satisfied by the precise positional control afforded by the invention. Fourthly, whilst in position 1, the lock via collar K is released, and sling S rotated under holding tension in direction 2 to loading alignment at position 2 of Figure 4, where a pull on connected snap-link SL sets the device firmly in place: Removal of the device after use is carried out (after first loosening the device if necessary) by rotating sling S in direction 1 from position 2 to position 1, drawing on collar K to remove any slackness in string V then squeeze locking the linkage as described, and finally rotating the locked device by collar K from position 1 in direction 2, which action tensions string V and retracts the device for removal.

Claims (7)

1. Claims 1. A method of control facilitating use of a cam chock device comprising a rigid bodied cam chock and a connected actuating sling, said method pre-configuring the cam chock device ready for primary use in cam mode thereby simplifying device handling prior to placement and providing means of precise manipulation of the cam chock body during placement and subsequent removal, said method utilising a flexible string connected at one end to the cam chock body adjacent the pivotal nose of the latter and at the opposite end slidingly connected to the device's sling, sliding motion of said string along the device's sling imparting rotation of the cam chock body about the device's body-to-sling connecting bar thereby adjusting the effective size of the cam chock body to suit the placement and squeeze clamping of said string against the device's sling providing means of temporary locking said device at any in-range setting size during placement/removal, said string being of sufficient length to accommodate the range of movement occurring in use and said string being of flexural stiffness sufficient to retain the cam chock device in cam operating configuration whilst carried in suspension pending use.
2. 2. A method of control facilitating use of a cam chock device according to claim 1, in which the sliding connection between said flexible string and the device's sling is made by an intermediate collar, the collar being attached to said string end and being free sliding on the device's sling.
3. 3. A method of control facilitating use of a cam chock device according to claims 1 and 2, in which said collar is preferably made in a pliable material enabling as/when convenient the device locking function of claim 1 to be alternatively executed via the collar, and enabling said collar to conform to flexing of the device's sling in use.
4. 4. A method of control facilitating use of a cam chock device according to claims I and 2, in which said string end connection adjacent the cam chock nose is preferably pivotal enabling said connection to freely accommodate string movement without detriment to the security of the device placement, and in which said connection of string end to collar is preferably of a fixed "built in" nature thereby locally increasing flexural resistance of said string and supplementing the retained cam configuration feature of the invention.
5. 5. A method of control facilitating use of a cam chock device according to claims I *". and 2, in which the device's sling is rendered pivotally captive on the cam chock body *1, connecting bar, and facing sides of said sling are preferably attached together over that :: length of the device's sling traversed by said slide collar in use.: ..
6. 6. A method of control facilitating use of a cam chock device as described in preceding claims, in which said device can optionally be converted from primary use as a cam to secondary use as a wedge by manually overriding the flexural resistance of said string. * * *I*.*
7. 7. A method of control facilitating use of a cam chock device, said method being :: substantially as herein described above and illustrated in the accompanying drawings.