Strap buckles

Abstract

A strap buckle (e.g. for cargo tie-down) has a lever arm 3a pivotal on a body (2a, fig 7c) from an open position in which a free end of the strap 1 can be threaded through the buckle to a closed posit

Description

Field of the Invention The present invention relates to buckles for fastening straps and the like, and particularly but not exclusively to over-centre strap buckles. Background Strap buckles are used for many different applications, for example tie-downs for cargo. Existing types of strap buckle include over-centre buckles, cam straps, ratchet straps and quick release tie-downs. In an over-centre buckle, an over-centre mechanism mechanically holds a pivoting structure in a resting position relative to a pivot point. Over-centre buckles comprise a frame or body and a lever arm pivotally attached to the body. To operate the buckle, a strap is threaded through the body and lever arm and the lever arm is then pivoted (i.e. flipped over) against the body to engage and tension the strap. Over-centre buckles prevent overtensioning of the strap, which may damage the item (e.g. cargo) being fastened. Over-centre buckles require a multi-step strap threading process which may be complex and may vary between different designs. If the operator threads the strap incorrectly, the buckle will not secure the cargo, but it can be difficult for the operator to tell if the buckle is operating correctly. Unfastened cargo in transit can cause damage to the cargo, the operator and the general public. After the threading procedure, the operator must remove any slack in the strap, then flip the lever arm over to secure the strap. If not enough slack is pulled through the buckle before the lever arm is flipped over, the buckle will not secure the cargo. Operating the lever arm directly while putting the strap under tension increases the risk of injury to the operator. Cam buckles use a cam lever to secure a strap. To operate a cam buckle, the cam lever is depressed and the strap is threaded through the buckle. The cam clamps down on the strap when the cam lever is released. Cam buckles are simple to operate, but they are unsuitable for heavier loads as they do not provide mechanical advantage when tightening. Ratchet straps have a ratchet, first and second pawls, a handle which allows the operator to exert mechanical advantage when tightening the strap, and a mandrel for spooling the strap. To thread the strap, the free end of the strap is inserted through a gap in the mandrel. The handle is then operated to spool the strap around the mandrel and achieve the desired strap tension. To release the tension in the strap, the first pawl is lifted and the handle is pushed into an open position. This causes a cam to disengage the second pawl, releasing the tension in the strap. The main advantage of ratchet straps is their mechanical advantage that allows them to secure heavy loads. A disadvantage is that that they can be overtightened causing damage to cargo. Ratchet straps can be also difficult to operate and thread correctly. Quick release tie-downs provide a small amount of tension but hold securely when closed, and are often used as boat tie-downs or for securing packages. Quick release tie downs have three components: a frame or body, wire bail and hinge. To thread the strap, one end of the strap is fed through an inner opening of the frame then back out through an outer opening. The slack is then pulled through the frame and the frame is then flipped to engage the over-centre mechanism and tension the strap. The quick release tie down is easy to thread and there is a low risk of causing damage to cargo from over-tensioning the strap. Quick release tie downs are limited to applications where tensioning is not critical. In summary, the main problems with existing strap buckles are: • They are difficult to thread and operate • There is a risk of over tensioning causing damage to cargo • There is a risk of under tensioning leading to unfastened cargo • The tension is variable, depending on how the buckle is fastened Summary Aspects of the present invention are defined in the accompanying claims. One advantage of at least some embodiments is that the strap is fastened with a predetermined tension once sufficient force is applied to the free end of the strap. The predetermined tension may be fixed for a specific buckle or may be adjustable within the buckle. This may allow the correct tension to be set in the strap, thus avoiding damage to the cargo through over-tightening and / or the cargo being insufficiently fastened through under-tightening. Another advantage of at least some embodiments is that the buckle is easier to thread and / or operate compared to conventional buckles, particularly over-centre buckles, which may reduce the risk of the buckle being operated incorrectly. In at least some embodiments, a free end of the strap is inserted along an entrance path or channel in the body of the buckle, is looped back on itself over a lever arm guide on a lever arm of the buckle and threaded out through an exit path or channel in the body of the buckle. Preferably, the exit channel is sufficiently separate from the entrance channel that there is little friction between the incoming and outgoing parts of the strap. Preferably, the strap is guided from the entrance channel, for example along a guide ramp, through a pivot channel adjacent a pivot of the lever arm and then to the lever arm guide. The pivot channel may maintain a constant or nearly constant length of strap between the entrance channel and the lever arm guide as the buckle is closed, this avoiding loosening the strap. At least some specific embodiments comprise an over-centre strap buckle comprising three interlinked mechanisms: a retention mechanism, a strap fastening mechanism and an over-centre mechanism. The over-centre mechanism may comprise a lever arm pivotally connected to the body, and a bail or similar connector connected to the lever arm, preferably by a pivotal connection. The bail may be connected to a captive end of the strap, or may be connected or connectable to an attachment part, such as a hook, for attaching the buckle to the structure to which the load is to be fastened. Hence, the over-centre mechanism may apply additional tension to the strap by pulling the bail towards the body as the lever arm is moved to its closed position. Alternatively, one end of the strap may be connected directly to the lever arm, without the need fora bail. To operate the over-centre mechanism, the free end of the strap is pulled to first take up any slack in the strap and then apply tension to the strap so as to pull the lever arm into a closed position by means of a single action by the user. The retention mechanism retains the lever arm in an open position until the tension in the strap exceeds a predetermined level, at which point the lever arm is pulled into a closed position by the tension in the strap. This, together with the effect of the over-centre mechanism, sets the predetermined tension in the strap when the over-centre mechanism is closed. The strap fastening mechanism can be provided on the lever arm, the body or both. The strap fastening mechanism secures the strap within the buckle. Preferably, the strap fastening mechanism fastens the strap after the retention mechanism is released, and before or during a first stage of the operation of the over-centre mechanism. In this way, the strap can be tightened by the over-centre mechanism once it is fastened. The buckle may also be easy to release. The operator lifts the lever arm, which releases the tension in the over-centre mechanism and releases the strap fastening mechanism. Brief Description of the Drawings Specific embodiments will now be described with reference to the accompanying drawings, in which: Figures 1-5 are schematic cross-sectional diagrams illustrating the routingof a strap in an over-centre buckle according to different outline embodiments, with Figure 1 showing the buckle in a closing position and Figures 2-5 showing the buckle in an open position. Figure 6 is a diagram showing the interlinked operation of a retention mechanism, overcentre mechanism and fastening mechanism in an over-centre buckle according to at least some embodiments. Figures are perspective views of an over-centre buckle in a first variant of a first specific embodiment, showing sequential stages of th reading a strap therethrough. Figures 8a-8c are respective side, side cross-section in the plane A-A and plan views showing the location of the plane A-A showing the location of the over-centre buckle in the first variant, in an open position with the strap threaded therethrough. Figures 9a-9c are respective side, side cross-section and plan views of the over-centre buckle in the first variant, in a closing position with the strap threaded therethrough. Figures 10a-10c are respective side, side cross-section and plan views of the over-centre buckle in the first variant, in a closed position with the strap threaded therethrough. Figures 11 and 12 are side views of the body and lever arm in an alternative of the first variant, respectively in open and closed positions. Figure 13 is a perspective view from aboveof a second variant of the first specific embodiment. Figure 13a is a side view of the second variant of the first specific embodiment; Figures 13b-13c are side cross-sectional views of the second variant in the planes A-A and B-B respectively; and Figure 13d is a plan viewof the second variant, showing the locations of the planes A-A and B-B. Figures 14a-14c are respectively a side view, a first perspective view and a second perspective view of a cam sub-assembly in the second variant, in a closing / opening position. Figures 15a-15c are respectively a side view, a first perspective view and a second perspective view of a cam sub-assembly in the second variant, in a closed position. Figure 16 is a perspective view of an over-centre buckle in a second specific embodiment with a strap threaded therethrough, in a closing position. Figures 17a-17c are respectively a side view, side cross-sectional view in the plane A-A and a plan view showing the location of plane A-A, of an over-centre buckle in the second specific embodiment, in an open position. Figures 18a-18c are respectively a side view, side cross-sectional view in the plane A-A and a plan view showing the location of plane A-A, of an over-centre buckle in the second specific embodiment, in a closing position. Figures 19a-19c are respectively a side view, side cross-sectional view in the plane A-A and a plan view showing the location of plane A-A, of an over-centre buckle in the second specific embodiment, in a closed position. Description of Embodiments In the following description, the same reference numeral is used to indicate similar or identical parts in each of the embodiments. For clarity, some reference numerals may be omitted in some of the drawings. Directional and positional references such as up, down, above, below, over, under, horizontal, vertical, clockwise and anticlockwise are with reference to the orientation shown in the relevant drawing or drawings, with the outer side of the buckle uppermost and the lever arm pivot on the right hand side of the body. Strap routing This section outlines different strap routing arrangements which may be applied to embodiments. Figure 1 shows schematically an embodiment comprising an over-centre buckle having a lever arm 3 that is pivotally mounted on the body 2 about a lever arm pivot 5. The free end 1 a of a strap 1 is threaded through the body 2 (e.g. over a first body guide 7a) and around a lever arm guide 8 at or near the distal end of the lever arm 3 so that when the strap 1 is under tension, it exerts a tension T on the lever arm guide 8 so as to close the lever arm 3 by pivoting it anti-clockwise about the lever arm pivot 5 until the lever arm 3 overlies the body 2, when the over-centre buckle is in a closed position. A bail 4 is pivotally connected to the lever arm 3 about a bail pivot 6, which is located a distance D away from the lever arm pivot 5. The lever arm guide 8 is located at a distance L away from the lever arm pivot 5. Hence the lever arm 3 operates as a second order lever with a mechanical advantage of L / D. The moment M of the tension T about the lever arm pivot 5 is dependent on the angle a that the strap 1 makes with the lever arm 3: M=TxLxsina (1) Therefore it would be advantageous at least in some positions of the lever arm 3 to route the strap 1 away from the lever arm pivot 5, so that the moment M can act to close the lever arm 3. In at least some embodiments, the lever arm 3 is held open with a resistance R by means of a retention mechanism as described in detail below. To pull the lever arm 3 out of its open position, the moment M applied by the strap tension T must exceed the resistance R. The resistance R may comprise a force or resistance acting against the moment M. In the embodiment of Figure 2, the strap 1 is routed over the lever arm pivot 5 before passing around the lever arm guide at or near the distal end of the lever arm 3. This ensures that the tension T in the strap 1 tends to pull the lever arm 3 towards the closed position; otherwise, the moments of the tension T of the parts of the strap 1 extending either side of the lever arm guide 8 would tend to cancel out. One problem with the arrangement of Figures 1 and 2 is that the resistance R causes the moment M to be transmitted to the body 2, which therefore tends to rotate (flip) anticlockwise as the free end 1a of the strap 1 is pulled tight. In the embodiment shown in Figure 3, the strap 1 passes between first and second body guides 7a and 7b before being routed overthe lever arm pivot 5. This braces the taut incoming end of the strap 1 against the body 2 and prevents the body 2 from flipping as the strap 1 is pulled tight. A gap between the first and second body guides 7a, 7b forms an entrance channel that guides the free end 1a of the strap 1 into the body 2. When the lever arm 3 is rotated anticlockwise towards a closed position, the distance between the lever arm guide 8 and the entrance channel shortens, which can create slack in the strap 1 if the strap 1 is allowed to run directly from the entrance channel to the lever arm guide 8. To avoid this, a lever arm pivot guide 9 may be arranged adjacent the lever arm pivot 5 so that the strap 1 is routed through a pivot channel formed between the lever arm pivot 5 and the lever arm pivot guide 9, before passing around the lever arm guide 8, as in a further embodiment shown in Figure 4. In this way, the strap 1 continues to run through the entrance channel, through the pivot channel and around the lever arm guide 8 as the lever rotates, so that the length of the path taken by the strap 1 through the buckle does not change substantially as the lever arm 3 rotates. To achieve a more consistent tension in the strap 1 when the lever arm 3 is closed, it is preferable that the angle that the strap 1 makes with the lever arm guide 8 does not depend on the angle at which the strap 1 is pulled. To achieve this, the free end 1a of the strap 1 may be routed through an exit channel between the second body guide 7b and a third body guide 7c, as in the embodiment shown in Figure 5. Regardless of the angle at which the user pulls the free end 1 a of the strap 1, the free end 1 a is routed through the exit channel after passing around the lever arm guide 8, thus achieving a consistent angle between the strap 1 and the lever arm guide 8, dependent only on the angle between the lever arm 3 and the body 2. Although it would be possible to dispense with the third body guide 7c and thread the free end 1 a of the strap 1 back between the first and second body guides 7a, 7b of Figures 3 or 4, so that the entrance and exit channel coincide, this would create friction between the incoming and outgoing sections of the strap 1, which may impede correct operation of the buckle. Hence, it is preferable that the entrance and exit channels are separate. In at least some embodiments, the strap 1 is fastened by a clamping force C acting on the strap 1, for example between a clamping member and a portion of the body 2. To reduce friction between the strap 1 and the buckle, preferably at least some of the guides may comprise rollers which rotate freely about pins, or may comprise fixed pins with smooth, low-friction surfaces. Strap fastening This section of the description outlines strap fastening arrangements in embodiments. These may be used independently of, or with any one of the strap routing arrangements described above, although the strap routing and strap fastening arrangements may be used together with synergistic effect. When the lever arm 3 is in its open position, the free end 1a of the strap 1 should pass freely (i.e. with low friction) through the buckle so that the strap 1 can be pulled taut, for example against the load to be fastened by the strap 1. The tension in the strap 1 then applies force to close the lever arm 3. As the lever arm 3 closes, the strap 1 is fastened by the buckle without slipping. Retention mechanism In at least some embodiments, a retention mechanism holds the lever arm 3 in an open position with resistance R, until a predetermined tension is exceeded in the strap 1 so as to overcome the resistance R, whereupon the lever arm 3 is pulled out of its open position towards a closed position. This may result in the strap 1 being fastened with an overall tension approximately equal to the predetermined tension, with the addition of the tension added by operation of the over-centre mechanism. In this way, the strap 1 can be fastened with a predetermined overall tension, substantially independently of how hard the user pulls the free end 1 a of the strap 1. Preferably, the strap 1 is secured by the buckle in the closed position such that the user cannot tighten the strap 1 further by pulling the free end 1 a. This may ensure that a load is securely fastened without excessive tension in the strap 1 that could damage the load. Various different arrangements may be used for the retention mechanism: Mechanical Clasp: a clasp may mechanically hold the lever arm 3 in the open position. The clasp could take the form of a catch, or a ball plunger and track. A disadvantage of this mechanism is that it may need to be manually reset before each use. Magnetic catch: a magnet (or pair of magnets) may be used to hold the lever arm 3 in the open position until a predetermined tension in the strap 1 is exceeded. An advantage is that the magnetic force falls away quickly when contact is broken, so that there is little resistance to subsequent closing of the lever arm 3. Moreover, the magnetic force may reset the catch as the lever arm 3 is opened. A disadvantage is that the strength of the magnet may degrade over time. Cam and spring: a retention cam 15 biased by a retention spring 16 may provide the desired retention effect. The cam and spring could be mounted in many different positions about the body 2 and lever arm 3, giving a degree of design flexibility. Over-centre spring: an over-centre or bistable spring may allow the tension in the strap 1 to build up to the predetermined tension, after which the spring mechanism switches to the opposite stable state. A disadvantage is that the spring may then apply force to push the lever arm 3 into the closed position, which may make it difficult to open the lever arm 3. To overcome this problem, a disengagement mechanism may be used to disengage the bistable spring from the lever arm 3 at the point when the resistance of the bistable spring is overcome. An additional mechanism such as a latch may be used to retain the lever arm 3 in the closed position so as to prevent the lever arm 3 from opening accidentally during use, while allowing the lever arm 3 to be manually released and moved to the open position. Moreover, the geometry of the buckle is preferably arranged so that the overall tension in the strap 1 tends to hold the buckle closed. Interlinking mechanisms As illustrated in Figure 6, the retention mechanism should preferably maintain the lever arm 3, and therefore the over-centre mechanism, in the open position until a predetermined tension in the strap 1 is exceeded. After the over-centre mechanism begins to engage, the fastening mechanism should preferably engage so as to fasten the strap 1 within the buckle. The over-centre mechanism then continues to tighten the strap 1 as it moves to a closed position The operations of the retention, over-centre and fastening mechanisms should preferably be interlinked in order to achieve the desired effect, for example as described in the following sections. The fastening mechanism and over-centre mechanism can be interlinked in different ways, for example as described below. Since the lever arm 3 forms part of the over-centre mechanism, and may comprise a pivotable member in embodiments where an overcentre mechanism is not required, the description below focusses on how the lever arm 3 interacts with different fastening mechanisms. Cam fastening mechanism The lever arm 3 may be connected to a clamping cam 17, forming part of the fastening mechanism. When the lever arm 3 isopen, the clamping cam 17 is held open so that the strap 1 can move freely. When the lever arm 3 begins to close, the clamping cam 17 is released so as to clamp against the strap 1 under the bias of the fastening spring and thereby fasten the strap 1. The lever arm 3 and the clamping cam 17 may be coaxially mounted, and may be connected internally of the body 2 with a keyed axle or externally of the body 2 by the lever arm 3 contacting a clamping cam pin 17a that protrudes through a slot 13 in the body 2, thus avoiding the need for a keyed axle and potentially reducing manufacturing cost. Ladder lock mechanism The fastening mechanism may alternatively comprise a ladder lock mechanism; this can be linked to the position of the lever arm 3 using a routing system that changes the angle of the incoming part of the strap 1 under tension, relative to the outgoing part of the strap 1 being pulled, as the lever arm 3 closes. When the lever arm 3 is in the open position the incoming part of the strap 1 under tension does not contact the outgoing part of the strap 1. When the lever arm 3 begins to rotate, the incoming part of the strap 1 under tension pinches the outgoing part against a part of the ladder lock mechanism, which clamps the strap 1 in place. Adjustable resistance The resistance or retention force of the retention mechanism may be adjustable so as to adjust the pre-tension in the strap 1 before the over-centre mechanism engages. This can be used to adjust the overall strap tension when the strap 1 is secured. A weaker retention force will cause the lever arm 3 to rotate at a lower pre-tension in the strap 1, which will result in a lower overall tension in the strap 1. A stronger retention force will cause the lever arm 3 to rotate at a higher pre-tension in the strap 1, which will result in a higher overall tension in the strap 1. The method of adjusting the retention force that retains the lever arm 3 may vary depending on the retention mechanism selected: Magnetic catch: adjusting the distance of the magnet from its coupling (e.g. another magnetic or ferromagnetic material) in the open position of the lever arm 3 may adjust the retention force. Cam and spring: changing the profile of the retention cam 15 or the strength of the retention spring 16 may adjust the force of retention. Where the retention spring 16 is a leaf spring or resilient rod, the strength of the retention spring 16 may be adjusted by changing the effective length of the spring, for example by moving a support along its length. Where the retention spring 16 is a compression or extension spring, the strength of the retention spring 16 may be adjusted by changing the pre-tension of the retention spring 16. Over-centre or bistable spring: changing the strength of the spring or the geometry of the over-centre mechanism may adjust the retention force. Mechanical clasp: changing the profile, engagement or spring force of the clasp may adjust the retention force. The tension in the strap 1 required to overcome the retention force may also be adjusted by changing the angle that the strap 1 makes with the lever arm guide 8, when the lever arm 3 is in its open position; this may be achieved for example with a moveable guide around which the strap 1 is routed before being routed around the lever arm guide 8. Alternatively or additionally, the tension T required to overcome the retention force may be adjusted by adjusting the distance of the lever arm guide 8 from the lever arm pivot 5 and thereby adjusting the moment M that that the tension T exerts. The predetermined overall tension applied to the strap 1 may also be adjusted by changing the geometry of the over-centre mechanism, such as the position of the bail pivot 6 on the lever arm 3. If the bail pivot 6 is mounted closer to the lever arm pivot 5, the over-centre mechanism will pull the strap 1 a shorter overall distance resulting in a lower overall strap tension. If the bail pivot 6 is mounted further from the lever arm pivot 5, the over-centre mechanism will pull the strap 1 a longer overall distance resulting in a higher overall strap tension. Hence, the position of the bail pivot 6 along the lever arm 3 may be adjustable in order to adjust the tension applied to the strap 1. First and second specific embodiments incorporating at least some of the above features will now be described. The first and second specific embodiments have been selected to illustrate different implementations of features of the buckle. It will be understood that implementations of these features could be combined as between the different embodiments, or could be replaced by alternative implementations, for example as described above. First Specific Embodiment Two variants of the first specific embodiment will now be described. The first variant has an interlinking mechanism between the lever arm 3 and the fastening mechanism, which is located externally of the body 2. The second variant has an interlinking mechanism which is housed internally within the body 2. First Variant In the first variant, the body 2 is an inverted U-shape in cross-section with parallel and preferably identical sides 2a interconnected by a base 2b that extends perpendicularly between the upper ends of the sides 2a. Preferably the sides 2a and base 2b of the body 2 are integral, and may be formed of a single piece of bent sheet metal. The lever arm 3 is U-shaped in cross-section with parallel sides 3a interconnected by a base 3b that extends perpendicularly between the sides. Preferably the sides 3a and base 3b are integral, and may be formed of a single piece of bent sheet metal. The lever arm guide 8 is mounted between holes in the opposite sides 3a of the lever arm 3 and may provide structural support to the lever arm 3. The lever arm 3 is pivotally connected to the body 2 by a pivot pin 5a forming the lever arm pivot 5. The pivot pin 5a runs through two opposite holes in the sides 2a of the body 2 and two opposite holes in the sides 3a of the lever arm 3. The bail 4 comprises a piece of metal that is U-shaped in plan and which connects the lever arm 3 to the captive end 1 b of the strap 1. The bail 4 is connected pivotally to each side of the lever arm 3 by means of the bail pivot 6. The U-shape of the bail 4 allows the lever arm 3 to pass through the bail 4. A ramp guide 14, the leading end of which serves as the first body guide 7a, is connected between the sides 2a of the body 2 and is held in place on each side, for example by a screw or rivet and a protrusion that sits in a notch in the side 2a of the body 2. The purpose of the ramp guide 14 is to guide the strap 1 from the entry channel of the body 2 to a pivot channel between the lever arm pivot 5 and the underside of the base 2a of the body 2. Hence the base 2a functions as the lever arm pivot guide 9 in the routing examples of Figures 4 and 5. The ramp guide 14 may also provide structural support to the body 2. The ramp guide 14 may be formed of moulded plastic or bent sheet metal, for example. The second and third body guides 7b, 7c are mounted between mounting points, such as holes, in the opposite sides 2a of the body 2 and may provide structural support to the body 2. The retention mechanism of the first variant comprises a retention spring 16 that is connected directly or indirectly to the body 2. In this variant, the retention spring 16 comprises a leaf spring or resilient sheet mounted at one end to the underside of the ramp guide 14 and / or to part of the body 2; the other end projects through cut-outs in the sides 2a of the body 2 and is biased against a cam profile 10 of the lever arm 3, at the proximal end of the lever arm 3. The bias of the retention spring 16 against the cam profile 10 biases the lever arm 3 into the open position. The fastening mechanism in the first variant comprises a clamping cam 17 pivotally mounted about the lever arm pivot 5. The clamping cam 17 is biased towards a closed position to clamp the strap 1 against the underside of the base 2b of the body 2. The bias may be provided by a clamping spring 18, in this case a torsion spring mounted about the lever arm pivot 5, with one end of the torsion spring abutting the underside of the ramp guide 14 and the other end abutting the clamping cam 17. The clamping cam 17 may have a toothed surface 17c to increase the clamping friction on the strap 1. First Variant Strap Threading Figures are perspective views of successive strap threading stages in the first variant: Figure 7a: The free end 1 a of the strap 1 is inserted through the entrance channel between the second body guide 7b and the ramp guide 14. The ramp guide 14 guides the free end 1 a of the strap 1 between the clamping cam 17 and the underside of the base 2b of the body 2. Figure 7b: The free end 1a of the strap 1 then passes under the lever arm guides and is looped back over to return on itself. Figure 7c: The free end 1a of the strap 1 then passes over the base 2b of the body 2 and is inserted through the exit channel between the second and third body guides 7b, 7c, in the opposite direction to the incoming portion of the strap 1 passing through the entrance channel below it. The free end 1 a of the strap 1 is then pulled so that all the slack is taken up. The above threading operation is simple to perform, as it involves threading the free end 1a of the strap 1 in one direction, then around the lever arm guide 8 and back in the other direction; this can easily be performed with one hand while holding the buckle with the other hand. Operation of the first variant is shown in Figures 8-10, with suffixes a-c indicating respectively a side view, a cross-sectional side view through the plane A-A and a plan view showing the plane A-A through which the cross-section is taken. This operation demonstrates how the over-centre mechanism, retention mechanism and the fastening mechanism are interlinked. The lever arm 3 in its open position, as shown in Figures 8a-c, holds the clamping cam 17 in its open position by means of a notch 11 in the lever arm 3 that engages a pin 17a connected to or forming part of the clamping cam 17, and mounted away from the lever arm pivot 5. The pin 17a can move along an arcuate slot 13 in the side of the body 2; preferably, there is a pin 17a on both sides, each of which moves along a corresponding arcuate slot 13 in the respective side 2a. The retention spring 16 provides enough bias on the lever arm 3 in the open position to hold the clamping cam 17 in the open position against the bias of the clamping spring 18. After the slack in the strap 1 is taken up, additional tension in the strap 1 causes a perpendicular force or moment M on the lever arm guide 8 about the pivot 5. This causes the cam profile 10 on the lever arm 3 to push against the retention spring 16, which retains the lever arm 3 in the open position allowing pre-tension to be built up in the strap 1 until the retention force on the lever arm 3 is exceeded. The lever arm 3 then begins to rotate to a closing position, as shown in Figures 9a-c, in which the retention spring 16 abuts a circumferentially extending part of the lever arm cam profile 10, so that little or no retention force is applied which would otherwise bias the lever arm 3 into the open position. The notch 11 in the lever arm 3 pivots upwardly and allows the pin 17a to move upwardly along the arcuate slot 13, such that the clamping cam 17 rotates anticlockwise under the bias of the clamping spring 18 and clamps the strap 1 against the inner surface of the base 2a of the body 2. Pulling the free end 1a of the strap 1 further causes the lever arm 3 and therefore the overcentre mechanism to close, as shown in Figures 10a-10c. The lever arm 3 pulls the bail 4 towards the body 2 such that the bail pivot 6 is positioned towards the body 2 relative to the lever arm pivot 5, thereby applying additional tension to the strap 1. Preferably the bail pivot 6 is positioned below the lever arm pivot 5 in the closed position, such that the tension in the strap 1 tends to hold the over-centre mechanism in the closed position. In the closed position of the lever arm 3, the retention spring 16 may abut a part of the lever arm cam profile 10 that is angled so as to bias the lever arm 3 into the closed position. The user can then release the free end 1 a of the strap 1, as the over-centre mechanism now holds the tension in the strap 1. Any additional force on the free end 1a of the strap 1 will increase the closing force on the over-centre mechanism, ensuring it remains closed, but without increasing the tension in the strap 1. As shown in Figure 10b, the strap 1 passes under the base 2b of the body 2 and turns 180° around the edge of the base. This arrangement creates friction between the strap 1 and the base, which may prevent further tension being applied to the strap 1 by pulling the free end 1a and may prevent the strap 1 from slipping under the clamping cam 17. The clamping cam 17 also provides friction which prevents further tension being applied. To release the strap 1, the user manually lifts the lever arm 3 so as to open the over-centre mechanism. The lever arm 3 then pushes on the clamping cam pin 17a that projects through the arcuate slot 13 and is connected to the clamping cam 17. This releases the grip of the clamping cam 17 on the strap 1. which allows it to run freely. The retention spring 16 pushes on the cam profile 10 on the lever arm 3, causing it to open completely. The strap 1 can then be pulled completely from the body 2 with little or no resistance. Advantages of the first variant include one or more of the following: can be manufactured using sheet metal no specialised axles and grub screws required easy to assemble. Disadvantages of the first variant may include: risk of damage or injury during use due to the mechanism being exposed a less pleasing overall aesthetic due to parts of the mechanism being visible. First Variant - Pin Blocking In the above-described version of the first variant, if the pin 17a were struck in a downwards direction with enough force when the lever arm 3 is in the closed position, this could cause the clamping cam 17 to open, releasing its grip on the strap 1 and allowing the strap 1 to become loose. To counter this, the lever arm 3 may have a pin-blocking profile 12 that holds the pin 17a in the clamping position when the lever arm 3 is in the closed position, as shown in Figures 11 and 12. This may also apply additional force on the clamping cam 17 to grip the strap 1 in the closed position, by virtue of the geometry of the over-centre mechanism, so as to make the buckle more secure in its closed position. An additional advantage of the lever arm 3 applying additional force to the clamping cam 17 in the closed position is that the minimum force required in the clamping spring 18 may be reduced. Since the clamping spring 18 acts against the lever arm retention spring 16 in the open position, the strength of the lever arm retention spring 16 may also be reduced, which may in turn reduce the minimum strength required of components on which the lever arm retention spring 16 acts. Second Variant The second variant is shown in a closed position in Figures 13a-d, where the suffixes a-d indicate respectively a side view, side view cross-sections in planes A-A and B-B, and a plan view showing the locations of those planes. Parts of the retention and clamping mechanisms are shown in the opening or closing position in Figures 14a-c, and in the closed position in Figure 15a-c. The body 2, lever arm 3 and bail 4 of the second variant are similar in form to those of the first variant apart from adaptations for the retention and clamping mechanisms as described below. In the second variant, the pivot pin 5a forming the lever arm pivot 5 is rotationally fixed relative to the lever arm 3 such that it pivots with the lever arm 3. The retention mechanism of the second variant comprises a retention spring 16 mounted to the underside of the ramp guide 14 and body 2, and extending through the underside of the body 2. The retention spring 16 may comprise a leaf spring or resilient rod. The retention spring 16 acts on a retention cam 15 which is mounted on the pivot pin 5a and rotates with the pivot pin 5a. For example, the retention cam 15 may be fixed to the pivot pin 5a by a grub screw 21 or may be integrally formed with the pivot pin 5a. The retention cam 15 is profiled so that the retention spring 16 biases the retention cam 15 into the open position with a predetermined retention force or resistance R. The retention cam 15 may be profiled so that the retention spring 16 applies little or no bias to the rotation of the lever arm 3 in the closing position and / or applies bias towards the closed position when the lever arm 3 is at or near the closed position. Hence, the retention cam 15 of the second variant operates in a similar way to the cam profile 10 of the lever arm 3 in the first variant. The fastening mechanism of the second variant is similar to that of the first variant, except that the clamping cam 17 has an opening, as shown in Figure 14c, in which the retention cam 15 is located so that the clamping cam 17 and the retention cam 15 can pivot about the same axis. The clamping cam 17 is mounted so as to be free to rotate about the lever arm pivot 5. The over-centre mechanism, retention mechanism and the fastening mechanism are interlinked in the second variant by connection between the retention cam 15 and the clamping cam 17. When the lever arm 3 is in the open position, the retention cam 15 engages a latch 17b on the clamping cam 17 which holds the clamping cam 17 in the open position. As the lever arm 3 moves into the closing position, the retention cam 15 releases the latch 17b allowing the clamping cam 17 to clamp against the strap 1 under the bias of the clamping spring 18. Hence, the connection between the retention cam 15 and the clamping cam 17 in the second variant is analogous to the connection between the lever arm cam profile Wand the clamping cam 17 via the pin 17a moving in the arcuate slot 13 in the first variant. Second Variant - Retention Force Adjustment The second variant includes a retention spring force adjustment mechanism, as described below. A similar adjustment mechanism may be applied to other embodiments and variants. The two parallel sides 2a of the body 2 include respective internal parallel grooves, which function as an internal track for a slider 19. The retention spring 16 behaves as a fixed cantilever beam, supported at one end by the body / ramp guide 14 and the unsupported other end abutting the retention cam 15. The slider 19 acts as a support such that the retention spring 16 behaves as a propped cantilever beam. Changing the position of the slider 19 (support) changes the force applied by the retention spring 16 (supported cantilever beam) to the retention cam 15. The position of the slider 19 may be adjusted as follows. The slider 19 includes a threaded section that engages with a screw 20 that runs through the centre of the body 2. The head of the screw 20 is held captive by a block 24 that is fixed to the body 2. The block 24 has a hole in the end so that the head of the screw 20 can be turned, for example using a screwdriver or an Allen key, so as to move the position of the slider 19 backwards or forwards dependent on the direction of turning. This changes the position of the support on the spring, so making the spring stiffer or weaker and thereby adjusting the retention force on the lever arm 3 so as to adjust the pre-tension in the strap 1 before the lever arm 3 begins to rotate. This adjustment mechanism can be used to change the final overall tension in the strap 1 after the over-centre mechanism has engaged. This tension may be calibrated with the position of the slider 19 so that an accurate final tension in the strap 1 can be calculated based on the position of the slider 19. Markings may be provided to indicate the final tensions corresponding to different positions of the slider 19. Second Variant Strap Threading The free end 1 a of the strap 1 may be threaded through the over-centre buckle of the second variant in a similar way to the first variant as follows, in sequence: 1. The free end 1a of the strap 1 is inserted through the entrance channel between the second body guide 7b and the ramp guide 14. 2. The ramp guide 14 guides the strap free end 1 a of the 1 into the opening between the clamping cam 17 and the base 2b of the body 2. 3. The free end 1a of the strap 1 passes through the opening and over the top of a fourth body guide 7d. 4. The free end 1a of the strap 1 is inserted under the lever arm guide 8 and looped back over to return on itself. 5. The free end 1a of the strap 1 is inserted through the exit channel between the second and third body guides 7b, 7c in the opposite direction to the incoming portion of the strap 1 passing between the ramp guide 14 and the second body guide 7b. 6. The free end 1 a of the strap 1 is then pulled taut. After the free end 1 a of the strap 1 is pulled taut, the tension T in the strap 1 acts on the lever arm 3. This will cause the retention cam 15, which is axially fixed to the pivot pin 5a and therefore to the lever arm 3, to push against the retention spring 16. The retention spring 16 retains the lever arm 3 in the open position allowing pre-tension to be built up in the strap 1 until the retention force on the lever arm 3 is exceeded. The lever arm 3 then begins to rotate, causing the over-centre mechanism to begin to engage. At the same time, the retention cam 15 that is fixed on the pivot pin 5a releases the clamping cam 17 to close on the strap 1. Pulling the free end 1a of the strap 1 further causes the lever arm 3 and therefore the overcentre mechanism to close. The user can then release the strap 1 as the over-centre mechanism now holds the tension in the strap 1. Any additional force on the free end 1 a of the strap 1 will increase the closing force on the over-centre mechanism, ensuring it remains closed. To open the over-centre mechanism, the lever arm 3 is manually lifted. The lever arm 3 is axially fixed to the pivot pin 5a which is axially fixed to the retention cam 15, so that a catch 15a on the retention cam 15 engages with the clamping cam 17 and pushes it open, releasing the grip of the clamping cam 17 on the strap I.The retention spring 16 pushes on the retention cam 15, fixed axially to the pivot pin 5a, causing the retention cam 15 to open completely. The strap 1 can then be pulled completely from the body 2 with little or no resistance. Advantages of the second variant relative to the first variant may include: the mechanism is better protected during use, reducing risk of damage or injury a more pleasing overall aesthetic due to parts of the mechanism being hidden within the body 2, as shown in Figure 13. Second Specific Embodiment In a second specific embodiment as shown in Figures 16-19, the fastening mechanism comprises a ladder lock mechanism in which a change of angle of the lever arm 3 causes the incoming portion of the strap 1 to pinch on the outgoing portion. An advantage of the second specific embodiment is that it is less mechanically complex than the first specific embodiment. A disadvantage is that the second specific embodiment requires a more complex strap threading method, which may increase the risk of the strap 1 being threaded incorrectly and therefore not being properly fastened. In the second specific embodiment, the body 2 consists of two opposite sides 2a interconnected by body guides 7a-7e. The sides 2a have holes for mounting the body guides 7a-7e, one hole used for mounting a pivot pin 5a, and a sprung latch 22 which forms a first part of a mechanical latch that engages with the lever arm 3 to form the retention mechanism. The body guides comprise first, second and third body guides 7a-7c at the entrance / exit side of the body 2, as in the embodiment of Figure 5, together with fourth and fifth body guides 7d, 7e adjacent the lever arm pivot 5. The pivot channel is formed between the lever arm pivot 5 and the fourth body guide 7d. The fourth and fifth body guides 7d, 7e control the angle that the strap 1 makes with the ladder lock mechanism at different positions of the lever arm 3. The fastening mechanism comprises three lever arm guides forming a ladder lock mechanism, as explained below. These will be referred to as the first, second and third lever arm guides 8a, 8b, 8c, in order of distance from the lever arm pivot 5. The lever arm 3 consists of two sides 3a interconnected by the lever arm guides 8a-8c and is pivotally connected to the body 2 by the pivot pin 5a. Each of the sides 3a has one hole for mounting the pivot pin 5a, three holes for mounting the respective lever arm guides 8a-8c and a latch cut-out 23 that forms the second part of the mechanical latch. The bail 4 is a U-shaped piece (e.g. of metal) which pivotally connects to the sides 3a of the lever arm 3 at the bail pivot 6. The retention mechanism comprises the mechanical latch formed by the sprung latch 22 on the body 2 engaging the cut-out 23 in the side 3a of the lever arm 3. Other types of mechanical latch between the lever arm 3 and the body 2 may be used. To thread the free end 1 a of the strap 1 through the over-centre buckle of the second specific embodiment, the following steps are performed in sequence: 1. The free end 1a of the strap 1 is inserted through the entrance channel between the first and second body guides 7a, 7b attheentrance / exitsideof the body 2. 2. The free end 1a of the strap 1 is passed over the top of the pivot pin 5a and beneath the fourth body guide 7d adjacent the lever arm pivot 5. 3. The free end 1a of the strap 1 is passed over the top of the second lever arm guide 8b. 4. The free end 1a of the strap 1 is looped back on itself and over the top of the first lever arm guide 8a. 5. The free end 1 a of the strap 1 is looped back on itself under the first lever arm guide 8a, then underneath the third lever arm guide 8c. 6. The free end 1 a of the strap 1 is looped back on itself over the top of the third lever arm guide 8c and over the fifth body guide 7e. 7. The free end 1a of the strap 1 is inserted through the exit channel between the second and third body guides 7b, 7c. The free end 1 a of the strap 1 is then pulled taut so that the slack is taken up and tension T is applied to the lever arm 3. The mechanical latch resists the tension on the lever arm 3, retaining the lever arm 3 in the open position and allowing pre-tension to be built up in the strap 1 until the retention force on the lever arm 3 is exceeded. The lever arm 3 then begins to rotate, causing the over-centre mechanism to begin to operate. When the lever arm 3 is in the open position, the strap 1 does not contact itself so can run freely around the three lever arm guides 8a, 8b and 8c forming the ladder lock mechanism as shown in Figure 17b. When the lever arm 3 begins to close, the change in geometry of the strap 1 entering the ladder lock mechanism causes the strap 1 to grip against itself, particularly around the first and / or second lever arm guides 8a, 8b as shown in Figure 18b, so clamping the strap 1. Pulling the free end 1a of the strap 1 further causes the lever arm 3 and therefore the overcentre mechanism to close, as shown in Figures 19a-19c. The user can then release the free end 1 a of the strap 1 as the over-centre mechanism now holds the tension in the strap 1. Any additional force on free end 1 a of the strap 1 will increase the closing force on the over-centre mechanism, ensuring it remains closed. To release the over-centre mechanism, the lever arm 3 is manually lifted. As the lever arm 3 is opened, the section of the strap 1 under tension moves out of contact with the slack end of the strap 1, causing the ladder lock mechanism to disengage. The strap 1 can then be pulled from the body 2 with little or no resistance. Hence in the second specific embodiment, the lever arm retention mechanism and the fastening mechanism are interlinked by the position of the lever arm 3: as the lever arm 3 is released from the open position, the ladder lock fastening mechanism begins to engage. Design for Manufacture To reduce the cost of manufacture, the lever arm 3 and body 2 may be constructed from stamped and bent sheet metal, for example stainless steel (e.g. 304 / 316). If a cam and spring retention mechanism is selected, the retention spring can also be manufactured from stamped sheet metal, for example stainless steel (e.g. 304 / 316); this part may also be doubled back on itself to from the ramp guide 14. The bail 4 could also be formed from bent sheet metal, for example stainless steel (e.g. 304 / 316). Depending on the mechanisms selected, standard pins and rivets could be used for the guides and assembly in embodiments, which would further reduce cost. The retention and / or clamping cams 15,17 could either be stamped or die cast for a more robust design. Alternative Embodiments Specific features of the above embodiments and variants may be combined. For example, the strap routing variants of Figure 1-5 may each be applied to the first and / or second specific embodiments and their variants. 5 In alternative embodiments in which an over-centre mechanism is not required, the captive end of the strap or the attachment part may be attached to the body rather than to the lever arm, in which case the lever arm comprises a pivotable member which actuates the fastening mechanism when the predetermined tension is reached, without applying additional tension using an over-centre mechanism. 10 Alternative embodiments, for example which may become apparent to the skilled person on reading the above description, may nevertheless fall within the scope of the present invention as defined by the accompanying claims. References Strap 1 Free end 1a Captive end 1b Body 2 Side 2a Base 2b Lever arm 3 Side 3a Base 3b Bail 4 Lever arm pivot 5 Pivot pin 5a Bail pivot 6 First body guide 7a Second body guide 7b Third body guide 7c Fourth body guide 7d Fifth body guide 7e Lever arm guide 8 First lever arm guide 8a Second lever arm guide 8b Third lever arm guide 8c Lever arm pivot guide 9 Cam profile 10 Notch 11 Pin-blocking profile 12 Slot 13 Ramp guide 14 Retention cam 15 Retention cam latch 15a Retention spring 16 Clamping cam 17 Clamping cam pin 17a Clamping cam latch 17b Toothed surface 17c Clamping spring 18 Slider 19 Screw 20 Grub screw 21 Sprung latch 22 Latch cutout 23 Block 24

Claims

1. A strap buckle for fastening a strap, comprising:a) a pivotable member pivotally connected to a body and pivotable between an open position in which a free end of the strap can be threaded through the buckle, and a closed position in which the strap is fastened by the buckle; andb) a strap fastening mechanism arrange to fasten the strap as the pivotable member moves out of its open position;wherein the pivotable member is arranged to engage the free end of the strap such that pulling the free end of the strap under tension causes the pivotable member to move from its open position and towards its closed position so as to fasten the strap.

2. The strap buckle of claim 1, wherein the pivotable member comprises a lever arm forming part of an over-centre mechanism that increases tension in the strap as the lever arm is moved towards the closed position.

3. The strap buckle of claim 2, wherein the over-centre mechanism includes an attachment part or bail connected to the lever arm.

4. The strap buckle of any preceding claim, including a retention mechanism arranged to retain the pivotable member in its open position until the tension in the free end of the strap reaches a predetermined tension.

5. The strap buckle of claim 4, wherein the retention mechanism is arranged to retain the pivotable member in the open position with a predetermined resistance, such that the pivotable member is caused to the move from its open position when the tension in the strap overcomes the resistance.

6. The strap buckle of claim 5, wherein the retention mechanism comprises a retention spring that acts on a retention cam surface so as to retain the pivotable member in its open position with the predetermined resistance and to reduce or remove the resistance when the pivotable member moves out of its open position.

7. The strap buckle of claim 6, wherein the retention spring is arranged to act on the retention cam surface so as to retain the pivotable member in its closed position.

8. The strap buckle of claim 6 or claim 7, wherein the retention cam surface comprises an edge surface of the pivotable member, located externally of the body.

9. The strap buckle of claim 6 or claim 7, wherein the retention cam surface is provided on a retention cam housed within the body and pivotable with the pivotable member.

10. The strap buckle of any one of claims 5 to 9, wherein the predetermined resistance is adjustable.

11. The strap buckle of claim 10 when dependent directly or indirectly on claim 6, wherein a force applied by the retention spring is adjustable.

12. The strap buckle of claim 11, wherein the retention spring comprises a cantilevered spring, the force applied by the retention spring being adjustable by means of a moveable support.

13. The strap buckle of claim 4, wherein the retention mechanism comprises a latch between the pivotable member and the body.

14. The buckle of any preceding claim, wherein operation of the strap fastening mechanism is interlinked with the pivotable member moving out of its open position.

15. The buckle of claim 14, wherein the strap fastening mechanism comprises a clamping member that is biased towards a closed position in which the clamping member clamps against the strap, the clamping member being held in an open position, in which the clamping member does not clamp against the strap, by the pivotable member in its open position.

16. The buckle of claim 15, wherein the clamping member is pivotable and coaxially mounted with the pivotable member.

17. The buckle of claim 15 or claim 16, wherein the clamping member has or is connected to a pin that passes through and is moveable along a slot in the body, wherein the pin is engaged by a first part of the pivotable member in its openposition so as to hold the clamping member in its open position and is released by the part of the pivotable member when the pivotable member moves out of its open position so as to allow the clamping member to move to its closed position.

18. The buckle of claim 17, wherein the pin is engaged by a second part of the pivotable member in its closed position so as to prevent the clamping member from moving to its open position.

19. The buckle of claim 15 or claim 16, each when dependentdirectly or indirectly on claim 9, wherein the clamping member is engaged by the retention cam when the pivotable member is in its open position so as to hold the clamping member in its open position and is released by the retention cam when the pivotable member moves out of its open position so as to allow the clamping member to move to its closed position.

20. The strap buckle of any preceding claim, wherein the body includes an entrance channel for guiding the free end of the strap entering the body.

21. The strap buckle of claim 20, wherein the body includes an exit channel for guiding the free end of the strap exiting the body, the entrance and exit channels being mutually separate.

22. The strap buckle of claim 20 or claim 21, wherein the body includes a pivot channel for guiding the free end of the strap from the entrance channel past the pivotable connection of the pivotable member to the body.

23. The strap buckle of claim 22, wherein the body includes a ramp guide for guiding the free end of the strap from the entrance channel to the pivot channel.

24. The strap buckle of any preceding claim, wherein the pivotable member includes at least one pivotable member guide around which the free end of the strap may be threaded such that the pivotable member engages the strap.

25. The strap buckle of claim 24, wherein the pivotable member includes a plurality of pivotable member guides around which the strap may be threaded so as to form the strap fastening mechanism.

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