Patent Description:
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.

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.

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 <CIT> of the present applicant.

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.

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 <FIG>. 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.

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 "<NPL>" it is stated on page <NUM>, "In the optimal case, the angle α should be constant because a constant value represents uniform loading of the cam in any position".

In <CIT>, <CIT> and <CIT>, 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 <NUM> degrees to <NUM> 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.

<CIT> discloses in Figures <NUM> and <NUM> several separate cam lobes mounted on the same axis, fitting inside each other. Each cam lobe is specified as having a different angle between the tangent and radial line at the point (camming angle) than the other ones.

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.

To this end, the present invention provides a camming device comprising:.

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).

Amongst embodiments of the invention, within working ranges of values of ω, the value of α may vary between a minimum of <NUM>° and a maximum of <NUM>°. 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 <NUM>° ≤ α ≤ <NUM>°, while other embodiments may have values of α that fall within a range <NUM>° ≤ α ≤ <NUM>°.

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

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 camming angle α that decreases with lobe angle ω, as described above. In other embodiments, one or more cam lobe may have a constant camming angle and/or one or more cam lobe may have an angle that increases with the value of the lobe angle ω.

From a second aspect, this invention provides a camming device comprising:.

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

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

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

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

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

The head assembly <NUM> further includes a plurality of cam lobes <NUM> that are substantially identical in this embodiment. Each cam lobe <NUM> 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 <NUM> 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 <NUM>, the function of which will be described below. Other holes serve to minimise the mass of the lobes.

Each cam lobe <NUM> has a cam surface <NUM> that forms part of its periphery. The cam surface <NUM> 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 <NUM>.

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

This embodiment includes four cam lobes <NUM>, two being carried on each cam axle <NUM>', <NUM>". The two cam lobes <NUM> closest to the stem <NUM> are carried on the first cam axle <NUM>' and are arranged in a first orientation. The two cam lobes <NUM> further from the stem <NUM> are carried on the second cam axle <NUM>" and are arranged in an opposite orientation.

A biasing spring (not shown) biases each cam lobe <NUM> about the cam axle <NUM>' 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 ωmax. Movement beyond the extended position is prevented by the other cam axle <NUM>" reaching the limit of the clearance hole <NUM>. In the expanded position, which is shown in <FIG>, the distance of the cam surface <NUM> from the stem <NUM> in a distance transverse to the stem axis is maximum. A release trigger <NUM> is carried on the stem <NUM>. The release trigger <NUM> can be manually drawn along the stem <NUM>, acting through connecting elements (not shown) to cause the cam lobes <NUM> to rotate away from the expanded position, movement being limited at a position denoted ωmin. The cam lobes <NUM> on the first cam axle <NUM>' rotate in the opposite direction to those on the second cam axle <NUM>". 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 <FIG>.

In use, the release trigger <NUM> is operated to rotate the cam lobes <NUM> away from the expanded position until the distance between the cam surfaces in <NUM> a direction transverse to the stem axis is reduced sufficiently to enable the head assembly <NUM> to be inserted into a fissure <NUM> in a rock between two rock surfaces <NUM>. The release trigger <NUM> is then released to allow the cam lobes <NUM> to rotate towards the expanded position until their cam surfaces <NUM> make contact with the rock surfaces <NUM>, as shown in <FIG>. This angular position of the cam lobes <NUM> on the axles <NUM>' <NUM>" will be referred to as the locking angle ωlock. A loading force FL applied to the stem <NUM> as shown in <FIG> tends to urge the four cam lobes <NUM> in a direction that tends to increase ω towards the expanded position, thereby forcing the cam surfaces <NUM> into contact with the rock surfaces <NUM>, the contact being approximated to a line that extends across the cam surface <NUM>. This creates a reaction force FR 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 FRx and a component FRy directed parallel to the stem axis, with FRy arising from friction between the cam surface <NUM> and the rock surfaces <NUM>. 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 FL. which means that the total frictional force FRy generated by the cams must exceed the loading force FL. Where the coefficient of friction between the cam surface <NUM> 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.

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 <NUM> and the rock surface <NUM> is not constant for a given loading force FL - it is dependent upon the locking angle of the cam lobes <NUM>, which is in turn dependent upon the width of the fissure <NUM>. In practice, when the approximation that contact between each cam surface <NUM> and the rock surface <NUM> 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 <NUM> and the rock surface <NUM> increases as the locking angle increases, so a greater locking angle results in a lower pressure if α and FL are constant.

In this embodiment, the cam lobes <NUM> are shaped such that α decreases as the locking angle increases, so the pressure between the cam surface <NUM> and the rock surface <NUM> 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 <NUM> 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 <FIG>, as the value of ω increases as ω<NUM>, ω<NUM> and ω<NUM>, the corresponding values of r (r<NUM>, r<NUM> and r<NUM>) also increase, the value of the cam angle α decreases linearly (α<NUM>, α<NUM> and α<NUM>) with an increase in ω.

A decrease in value of α causes an increase in the locking force applied by the cam surface <NUM> to the rock surface <NUM> for a given loading force FL, which compensates for the increase in contact area so as to maintain the contact pressure more nearly constant.

Claim 1:
A camming device comprising:
a. a shaft (<NUM>);
b. a head (<NUM>) mounted on the shaft (<NUM>), the head including:
i. a plurality of cam lobes (<NUM>) carried on at least one cam axle (<NUM>), 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 (<NUM>) having a cam surface (<NUM>) that extends about the pivot axis with a working range of contact angles ωmin ≤ ω ≤ ωmax, the cam surface (<NUM>) 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 (<NUM>) the distance r of the cam surface from the pivot axis is a modified logarithmic spiral that has a camming angle α that decreases as the radial distance, r, increases with an increase in ω.