Abstract:
A magnetic attachment device comprises a pad, the pad including a base, a backing plate mounting a plurality of magnets, and a top plate. The base is connected to the backing plate and the top plate is connected to the backing plate, the top plate being connected to the backing plate by fasteners, and the pad further comprises flexible sealant situated in a void formed between surfaces of the base and/or the magnets and/or the backing plate.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device for removably attaching to metallic surfaces, and in particular to a magnetic device for removably attaching to metallic surfaces. 
     BACKGROUND OF THE INVENTION 
     Devices using magnets for attaching to metallic surfaces are known. Such devices are used in rope access systems and are of particular use as positioning aids to hold a rope close to a surface. For example, when working on the outside of a ship&#39;s hull, if access is to be gained by rope from the deck level, because of the shape of the hull it is often necessary to provide some means to secure the rope or a person on the rope to the hull, otherwise the person on the rope is situated too far from the hull to be able to work on it. Another example of the use of such devices is where a person is using rope access techniques to work on a metallic wall, for example the leg of an oil rig. By using a magnetic positioning aid, the worker can secure himself and/or his ropes to the wall on which he is working, thereby limiting the scope for movement of himself with respect to the wall which might result from movement of the object of which the wall forms a part or gusts of wind. Another reason to use a positioning aid is to allow a worker to exert force more effectively. For example, if a worker is hanging from a long rope any force he exerts on the wall will result in him moving away from the wall, which may be undesirable for a particular task. By using a positioning aid, the worker can secure himself with respect to the wall so that when he exerts a force on the wall that force is reacted through the positioning aid, so the worker does not move with respect to the wall. 
     For a magnetic attachment device to generate sufficient force to hold the weight of a man in position, a significant magnetic force is required. The problem arising from this is that the attachment device must be capable of being removed from the surface, which therefore requires a force greater than the magnetic force to be exerted on the device. The types of task where positioning aids are required to be used often involve applying surface finishes such as paints or sealants. Hence, there is a need for a positioning aid to be removable from a wall without the surface of such a wall being damaged. 
     One positioning aid that addresses these problems is described in the applicants&#39; United Kingdom patent 2339633. This positioning aid includes a pad mounting magnets, a handle for a user to grip, holes to accommodate ropes or clips and a cam mechanism which is used to lift the magnets off a surface to which they are attached. In order to protect the surface to which the positioning aid is attached, each cam is provided with a flexible protective element, which in use lies between the surface of the cam and the surface to which the positioning aid is attached. Therefore, when the cam is actuated to lift the magnets instead of the cam engaging with for example the paint work of a ship&#39;s hull, the cam engages with the flexible element. In this way the paint is protected from damage. 
     However, the positioning device described in United Kingdom patent 2339633 does suffer from some disadvantages. For example, the flexible elements are relatively flimsy and may become damaged, either through use or even in storage. If the device is then used without flexible element being in the correct position, or not being present, the surface to which the device is attached is likely to be damaged. 
     The device described in United Kingdom patent 2339633 uses a number of rare earth magnets attached to an iron plate, which serves to distribute the magnetic field. The plate and magnets are then installed in a box and the box welded. Rare earth magnets are hydroscopic. It is therefore imperative that the magnets are sealed from moisture. This is of particular importance given the likely fields of use of such devices (ships, oil rigs and the like). In the device described in GB2339633 the magnets are sealed against ingress of moisture by filling the box housing the magnets with epoxy resin. This construction poses two problems. First, it is difficult to service the device if one of the magnets becomes defective. If a magnet becomes defective it is necessary to break the weld between the iron back plate and the steel box, break the epoxy resin and second, the whole assembly of plate and magnets must be replaced because the epoxy resin cannot reliably be removed from the magnets without damaging them. 
     Whilst United Kingdom patent 2339633 describes a positioning aid, the same principle may be used for many different purposes. For example 2339633 identifies that the positioning aid may be used as a plate lifter. Such a device may also be used simply as a means to hang another object from. For example, a mechanic working on a vehicle may wish to hang up a light, or hang up some tools. 
     It would therefore be desirable to provide an improved magnetic attachment device. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided a magnetic attachment device comprising a pad, the pad including a base, a backing plate mounting a plurality of magnets, and a top plate, wherein the base is connected to the backing plate and the top plate is connected to the backing plate, the top plate being connected to the backing plate, and the pad further comprising flexible sealant situated in a void formed between surfaces of the base and/or the magnets and/or the backing plate. 
     Advantageously, the top plate is connected to the backing plate is by fastening means, which may include screws, pins, rivets, bolts. 
     Preferably, the backing plate includes an opening extending into the void. 
     The magnets may be rare earth magnets. 
     Advantageously, the sealant is selected from the group comprising: Polyurethane, rubber, silicone compounds or such substance as provide similar properties. The material properties of the sealant advantageously allow the sealant to be removed from the magnets without causing damage thereto, providing the advantage that the backing plate and any magnets attached thereto that are not damaged may be re-used, with only damaged magnets requiring replacement. 
     Preferably, the device further comprises a gasket situated between the top plate and the base. More preferably, the device includes a gasket seat. 
     The base may include a planar underside and walls extending therefrom. 
     The backing plate may be attached to the walls of the base, for example by welding the respective corners thereof. 
     The top plate may be welded to the walls of the base. In such a scenario the gasket may be omitted as the weld may provide the water tight seal. Whilst not providing all the advantages in terms of ease of serving, the backing plate and/or the magnets may nevertheless be re-usable to the extent that they are not damaged. 
     Advantageously, at least one of the side walls includes at least one first element protruding therefrom towards an edge of the back plate. The top plate preferably includes at least one second element extending downwardly therefrom and past an edge of the back plate. More preferably, the second element extends towards the first element. Still more preferably, the gasket seat is at least in part formed by the at least one first element. Yet more preferably, the gasket seat is at least in part formed by the first and second elements. 
     The device may further comprise an elastically deformable protective element attached to the base, which element may extend around the walls of the base. 
     The protective element may include at least one portion extending past the upper edge of the wall of the base. The at least one portion may form part of the gasket seat. 
     The device may further include a handle member engaged with the top plate. 
     The top plate may include a wall element. Advantageously, the handle member engages with the wall element. 
     Advantageously, the device further includes at least one lifting means, the lifting means being disposed to lift the device from an attachment configuration to a release configuration. The lifting means may be connected to the top plate. 
     The device may further include locking means to lock the device in one or both of the attachment configuration and the release configuration. 
     According to a second aspect of the invention there is provided a magnetic attachment device comprising a body and a pair of levers pivotally attached to the body, and actuation means adapted to cause relative movement between the body and the levers, the body and the levers each including a surface for engagement with a substrate, the device having an attachment configuration and a release configuration, and wherein relative movement between the body and the levers moves the device between the attachment and release configurations. 
     The body may include the pad of the first aspect of the invention. 
     Advantageously, the levers of the pair are mounted on common axis. The levers of the pair may be mounted on a common shaft that is supported on the body for rotation about its own axis. 
     Preferably, the actuation means includes a cam associated with each lever, wherein each lever includes a cam engagement surface, and wherein the cams are mounted rotatably on the body, and wherein the cams are connected to the actuation means, and wherein actuation of said actuation means rotates said cams and thereby generates said relative movement between the body and the levers. 
     Advantageously, the cam engagement surface is situated between the substrate engagement surface of the lever and the pivot attachment of the lever to the body. 
     Preferably, the cam engagement surface is situated above the substrate engagement surface. 
     The substrate engagement surface may provided with a compressible element. The compressible element may be rubber, which may be vulcanised, or plastics for example. 
     The actuation means may include a shaft supported on the body for rotation about its own axis, wherein the said cams are mounted on the shaft. Preferably, each lever includes a slot, and the shaft may pass through the slot. Preferably, the ends of the slot constitute limits to the relative movement between the body and the levers. 
     Preferably, each cam is mounted on the body by a bush, and the bush may extend through the slot and through a hole in the body. Advantageously, the shaft extends through the bushes, which extend through the slot in the lever and the hole in the body. 
     The actuation means may include a handle. 
     Preferably, the actuation means and the levers are configured such that upon actuation of the actuation means the body moves with respect to the levers, the levers remaining stationary. 
     The magnetic attachment device may further include biasing means, wherein the biasing means are disposed so as to exert a force on the body and the levers and to bias the levers into engagement with the cams. 
     The body may include a slot and an element connected to the lever may extend through the slot, and the biasing means may engage with the body and with the element. 
     The biasing means may be a spring, which may be a coil spring, a leaf spring or a torsion spring for example. 
     The body may include a top plate, the cams and the actuation means, and the top plate may mount the shaft to which the levers are pivotally attached. 
     The top plate may mount the biasing means. 
     The slot may be formed in the top plate. 
     Advantageously, the substrate engagement surface of the body includes a compressible element, which may be rubber, or plastics for example, and may be subjected to a process to change its material properties, for example rubber may be vulcanised. 
     Preferably, the body includes an element extending laterally past at least the edge of the lever proximate the body. More preferably, the element extending laterally past at least the edge of the lever proximate the body is formed in the compressible element. More preferably, the element extending laterally extends past the edge of the lever distal from the body. 
     The device may further include locking means to lock the device in one or both of the attachment configuration and the release configuration. 
     The magnetic attachment device described herein provides a number of advantages. First, due to the structural arrangement of the pad, the device is simple to construct, may be serviced, and is well sealed from ingress of moisture. Typically, to service the pad, the backing plate removed from the base by cutting the small welds at each corner of the pad. The base and backing plate carrying the magnets may then be removed for overhaul. The use of a flexible sealant provides two advantages. First, the sealant is easy to remove from the base and from between the magnets and from the backing plate. Second, the sealant being flexible absorbs at least some of the shock in the event that the device is dropped. Third, the sealant can take up changes in the dimension of the device due to thermal cycling for example without cracking, and can thereby maintain an effective seal. 
     By sealing the magnets effectively from ingress of moisture the service life of the magnets can be improved. Also, by providing a device that may be easily serviced, it is possible to replace only those rare earth magnets which have faults, whereas prior art devices required all the rare earth magnets to be replaced even if only one had a fault. 
     The provision of the parts of the device that contact a substrate on which the device is used with compressible elements, such as rubber sheet, or moulded rubber which may be vulcanised, or moulded plastics materials, is advantageous because the substrate is better protected against damage, such as scratching. Also, the noise upon attachment of the device to a substrate is damped. 
     The actuation means of the device, and in particular the arrangement of cams and levers provide a number of advantages. Whereas in the prior art devices the cam exerts a point load through a protective flexible element, in the device described herein the lever includes a surface engagement element which spreads the force generated by the cam over a surface area significantly greater than the surface area of the contact point of the cam. Hence, there is significantly less possibility of damage to the substrate through the lever than there is with the prior art devices. Further, the arrangement can ensure that the lever remains in contact with the substrate and that it is the base of the body that is lifted from the substrate surface. 
     Also, by positioning the cams between the pivotal attachment of the levers to the body and the part of the lever that engages with the substrate surface, the body is caused to move by a greater distance than would be the case if the cams were situated directly over the point where a protective element engages with the substrate surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which illustrate preferred embodiments of the invention, and are by way of example: 
         FIG. 1  is a schematic representation of a magnetic attachment device according to a first aspect of the invention; 
         FIG. 2  is an exploded view of the magnetic attachment device illustrated in  FIG. 1 ; 
         FIG. 3  is schematic representation of the device in the configuration illustrated in  FIG. 2 , in assembled form; 
         FIG. 4  is a schematic representation of the device shown in  FIG. 3  from the underside; 
         FIG. 5  is an end view of the device illustrated in  FIG. 1 ; 
         FIG. 6  is a cross-sectional side elevation on the axis A-A of the magnetic attachment device illustrated in  FIG. 5 ; 
         FIG. 7  is a cross-sectional side elevation on the axis D-D of the magnetic attachment device illustrated in  FIG. 5 ; 
         FIG. 8  is a schematic illustration of an alternative embodiment of the invention; 
         FIG. 9  is a schematic illustration of a further alternative embodiment of the invention showing the device locked in the clamped position; 
         FIG. 10  is a schematic illustration of the device shown in  FIG. 9  locked in the unclamped position; 
         FIG. 11  is a cross-sectional side view of the device as configured and illustrated in  FIG. 9 ; and 
         FIG. 12  is a cross-sectional side view of the device as configured and illustrated in  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1 , there is shown a magnetic attachment device  1  comprising a magnetic element  2 , release levers  3 , a handle  4  and an actuation lever  5 . 
       FIG. 2  shows the parts of the device  1  in greater detail. The magnet element  2  comprises a ferrous metal back plate  10  upon which are mounted six rare earth magnets  11 . The back plate  10  includes a number of threaded bores  12 ,  12 ′ the purpose of which is described below. The plate  10  and attached magnets  11  fit into a tray  14  which in the illustrated example is formed of stainless steel. Attachment of the plate  10  to the tray  14  is achieved by welding at the respective corners  10 ′,  14 ′. With the plate  10  secured to the tray  14 , a flexible sealant is introduced into the space between the underside of the plate  10  and the bottom  14   a  of the tray  14  through the hole  13 . The sealant occupies the spaces  11 ′ between adjacent magnets  11 . The free surface of the magnets  11  may or may not be in contact with the bottom  14   a  of the tray  14 . Where the free surface of the magnets  11  is not in contact with the bottom  14   a  of tray  14 , the sealant also fills this space so that each magnet  11  is sealed completely against moisture. 
     The sealant also serves two other useful purposes. First, because the sealant is flexible it dissipates energy in the event of the device being dropped, or repeatedly being brought into contact with a metal surface in and being subjected to shock loads. It should be noted that when the magnets  11  come within a certain proximity of a metal object the magnetic force attracting the device to the object is very strong, which can exert a significant shock load on the device. Second, again because the sealant is a flexible sealant it is possible to remove the sealant from between and around the magnets  11  without damaging them. It has been found that where electrical potting compounds are used, which set hard, attempts to remove the potting compound often results in the magnets  11  being damaged, for example by cracking or the edges thereof breaking away. When this occurs the magnet is no longer of use. Hence, in servicing the positioning aid of the prior art the customary practice is to replace the whole assembly of the back plate and the magnets. Due to the expense of rare earth magnets it is desirable to be able to replace as few as possible. By using a flexible sealant in many cases it is believed that it will be possible to remove the sealant without damaging the magnets  11 . Any individually defective magnets can then be replaced, the plate  10  re-attached to the tray  14  and the space between the plate  10 , magnets  11  and tray  14  re-filled with flexible sealant. 
     With the plate  10  attached to the tray  14  and the space therebetween filled with sealant, the top plate  20  may be attached. The plate  20  is provided with upstanding walls around the perimeter thereof, namely front and back walls  21   a ,  21   b  and side walls  21   c ,  21   d . The plate is also provided with apertures  22 ,  22 ′ which are located so as to align with the threaded bores  12 ,  12 ′ in the back plate  10 . As can be seen from  FIG. 2 , a fastener  24  in the form of a screw in the present example is provided for each hole  22 , a seal in the example in the form of an O ring  23  also being provided to ensure a seal between the head of the screw  24  and the surface of the plate  20 . 
     The handle  4  comprises a bracket  30  which includes flange portions  31  each including an aperture  32  sized to receive a fastener  24 . The fasteners  24  pass through the respective holes  32  and  22  to attach he bracket  30  to the plate  10 . As can be seen from  FIG. 2 , an O ring  23 ′ is provided between the underside of the flange  32  and the upper surface of plate  20 . The bracket  30  also includes slots  33 , which are located so as to align with corresponding slots  25  in the plate  20 . The arrangement of slots  25  and  33  provide lateral strength to the assembled device as can be understood from  FIG. 1 . The handle  4  is provided with a grip portion  34 , which in the example includes a plastics or rubber moulding, but may equally be formed of other suitable materials. For example, the grip portion could be fabricated from metal, which may be wrapped with a flexible material such as rubber. The bracket  30  also includes apertures  35  which a user may pass a rope or other attachment element through. 
     The mechanism for lifting the magnetic attachment device from a metal surface is mounted on the top plate  20 , and will now be described in greater detail. The lifting mechanism includes levers  3  which are mounted on the top plate  20  by means of shafts  40  and  41 . Each lever  3  is mounted on one end of the shaft  40  and is free to rotate, the levers being held in position by clips  42 . The position of each lever  3  with respect to the top plate  20  is controlled by a cam  55 , which includes a bush  51  that is configured to slide in a slot  52  in the lever  3  and to pass through a corresponding hole  21   f  in the side wall  21   d  of the top plate. A collar  53  on the bush  51  secures the bush  51  with respect to the lever  3 . The collar  53  may take the form of a circlip, and the bush  51  may be provided with a groove for such a circlip to sit in. A cam  55  is attached to one end of the bush  51 . The cam  55  includes two aligned bores  56 ,  57  extending laterally through the cam. The cam also includes an aperture  58  having two flat sides. The aperture is configured to receive the end  63  of shaft  60 , which includes flat sides  65  and has external dimensions substantially similar to the internal dimensions of the aperture  58 . The end  63  of shaft  60  also includes a bore  64  which is positioned such that when the said end  63  is located in the aperture  58  the bore  64  is aligned with the bores  56 ,  57 . The cam  55  may then be secured to the shaft  60  by inserting a roll pin  57 ′ or the like through the aligned bores  56 ,  57  and  64 . 
     The configuration that the device  1  adopts when attached to a metallic surface is illustrated in  FIG. 1 . The smaller part of the cam  55 ′ is in engagement with lever  3 , and the bush  51  is situated to the bottom of the slot  52 . To lift the magnet device from a metallic surface to which it is attached sufficient force to overcome the attraction between the device and the metallic surface must be generated. This is achieved by grasping the grip portion  62  of handle  61  and rotating the lever clockwise. The handle  61  is fixed with respect to the cam  55  and hence rotation of the shaft  60  changes the part of the cam that is in contact with the lever, that part of the cam being part  55 ″ where the cam surface is situated further from the axis of rotation of the cam  55 . The position of the levers  3  and handle  61  are illustrated in  FIGS. 2 and 3 . 
     In the illustrated arrangement, during rotation of the shaft  60 , the levers remain stationary and the rest of the device moves with respect to the levers, the bushes  51  sliding from the bottom of the slot  52  to the top thereof. 
     When attaching the device  1  to a metallic substrate, such as the wall of a ship, it is desirable that the magnetic force of attraction be taken up gradually. This may be achieved by presenting the device  1  to the substrate with the cams  55  in the “lifting off” position. In such a configuration only one end of the base of the device is brought into contact with the substrate, with the remainder of the base being positioned at an angle with respect to the surface. There is still a significant magnetic force of attraction between the device and the substrate, but that force is reduced because of air gap between most of the device and the substrate. The speed at which the magnetic force of attraction between the device and the substrate is taken up can be controlled by controlling the speed at which the handle  61  and hence shaft  60  and cams  55  are moved from the configuration shown in  FIGS. 2 and 3  to the configuration shown in  FIG. 1 . The magnetic force of attraction between the device  1  and the substrate results in a significant frictional force between the cam  55  and the surface  3 ′ of lever  3 , which prevents the shaft  60  from rotating unless a force is exerted on the handle  61 . Therefore, when an operator presents the device up to a metal substrate, he then pushes or pulls the handle  61  from the position shown in  FIGS. 2 and 3  to the position shown in  FIG. 1 . 
     The cams  55  run on surfaces  3 ′ formed in the levers  3 . As can be seen from  FIGS. 1 to 3 , the surface  3 ′ is spaced apart from the underside of foot  3 ″ which in the illustrated embodiment is protected by a rubber boot  70 , which may be vulcanised rubber to increase wear resistance. Whereas in the positioning aid of the prior art, the cam acted on one side of a flexible element, the other side of which rested on the surface of the metal substrate, in the illustrated embodiment of the present invention, the cam  55  acts on a surface  3 ′ the other side of which is not in contact with the substrate. 
     In the illustrated example, the underside (or sole) of the tray  14  is protected by a rubber element  71 , which may be formed by a moulding process. The rubber may be vulcanised to increase its hardness and resistance to wear. However, it should be noted that the device may be used without a protective rubber moulding, or an alternative protective material may be applied to the underside of tray  14 . For example, rubber sheet material could be attached to the underside of tray  14  by a suitable adhesive. 
     The moulding  71  includes corner elements  71   a  which extend outward beyond the outer wall  3   a  of the lever  3 . The corner elements  71   a  protect the levers  3 . 
       FIGS. 5 to 7  illustrate certain parts of the device shown in  FIGS. 1 to 4  in greater detail. In particular, the tray  14  is shown in greater detail. It can be seen that the side walls  14   b ,  14   c ,  14   d  and  14   e  include a return,  14   b ′,  14   c ′,  14   d ′ and  14   e ′. The returns  14   b ′ to  14   e ′ are so shaped and dimensioned that either they abut or are situated in very close proximity to the vertical edges  10   a  of plate  10 . Also, the plate  20  includes portions  27  extending downwardly from the underside of the plate  20  and which engage with the upper surface of the returns  14   b ′ to  14   e ′ respectively. A channel is formed between the outer face of the portions  27 , the upper surface of the returns  14   b ′ to  14   e ′ and the inner surface of the overmoulding  71 . A gasket  28  is inserted into the so formed channel. In  FIG. 6 , the gasket is shown in its uncompressed form with the gasket extending through the plate  20 . This is in order that the nature of the gasket in the illustrated example may be appreciated. When the top plate  20  is presented up to the back plate  10  the gasket  28  is compressed between the underside of the said plate  20  and the other walls of the channel. The gasket  28  is pressed into the interfaces between the portions  27  and the returns  14   b ′ to  14   e ′, thereby sealing the interfaces against ingress of moisture. It will be appreciated that the gasket may provide an effective seal with the overmoulding  71  omitted. In such an embodiment, the gasket is simply compressed between the underside of plate  20 , the outer surface of portions  27  and the returns  14   b ′ to  14   e′.    
       FIG. 7  is a cross-section through the lever  3  and cam  55  and illustrates the cam  55  in engagement with the surface  3 ′ of the lever  3 . Referring also to  FIG. 3 , bolts  15  pass through slots  16  in the walls  21   c ,  21   d  and pass through holes  18  in the levers  3 . The bolts  15  are secured in place by nuts  19 , or alternatively, the bolts  15  could be self tapping, and tap into the hole  18 . Referring in particular to  FIG. 3 , a spring  43  is mounted on the shaft  40 . One leg  44  of the spring  43  engages on the underside of bolt  15 , with the other leg  45  engaging on the upper surface of the top plate  20 . The springs  43  therefore exert a force on the levers  3  which are free to move on shaft  40  so that the surfaces  3 ′ of levers  3  are at all times brought up against the surface of cam  55 .  FIG. 3  also illustrates the assembly of the shaft  40  in a bush  46  which extends through the aperture  19   a  in the lever  3 , and is secured in place by clip  42 . 
       FIG. 8  illustrates an embodiment including a safety catch for preventing uncontrolled movement of the lever  5  with the levers  3  raised. The lever  5  comprises a first shaft member  61   a  and a second shaft member  61   b  arranged to slide on the first shaft member  61   a . The first shaft member  61   a  includes protrusion  61   c  which slides in a slot  61   d . The length of the slot  61   d  determines the extent to which the second shaft member  61   b  may slide with respect to the first shaft member  61   a . A coil spring  61   e  is mounted on the first shaft member  61   a  between the end of the second shaft member  61   b  and the shaft  60 . A locking arrangement  80  comprises a first locking member  81  mounted on the second shaft member  61   b  and a second locking member  82  comprising an opening in the back wall  21   b . The first locking member  81  includes hooks  81   a  which are is configured to co-operate with the opening of the second locking member  82 , to lock the lever  5  such that the levers  3  are in their raised configuration. In order to release the lever  5  from its locked configuration the grip portion  62  is grasped and pushed in the axial direction of the first and second shaft members  61   a ,  61   b . The second shaft member  61   b  slides with respect to the first shaft member  61   a  against the restoring force of the spring  61   e , until the end of the slot  61   d  proximate the grip portion  62  engages the protrusion  61   c . In this configuration the first locking member  81  is released from and clear of the second locking member  82 . In use the operator would hold on to the grip portion  62  and mover the lever  5  through an arc, rotating the shaft  60  about its own axis and thereby raising the levers  3  until the device is in the configuration illustrated in  FIGS. 1 and 7 . When the user releases the force acting against the spring  61   e  the second shaft member  61   b  slides along the first shaft member  61   a  away from the shaft  60  until the protrusion  61   c  reaches the end of the slot  61   d . As can be seen from  FIG. 8  the hooks  81  are provided with curved faces  81   b . When the operator wishes to release the device, the lever  5  is moved through an arc. As the curved faces  81   b  engage the upper ends of back wall  21   b , they act to push the second shaft member  61   b  towards the shaft  60  when a continued downward pressure is exerted on the lever  5 . When the face  81   b  has passed the upper edge  82   a  of the opening in the back wall  21   b , the second shaft member  61   b  is pushed away from the first shaft member  61   a  and the first locking member  81  locks with respect to the second locking member  82 , the flat surface  81   c  engaging with the wall  21   b.    
     Referring now to  FIGS. 9 to 12 , the embodiment illustrated provides for the device to be locked in both the clamped and the unclamped configurations. This is achieved by an arrangement which prevents movement of the handle in one of two locking positions, yet permits movement of the handle in when the arrangement is not in either of the two locking positions. 
       FIGS. 9 to 12  illustrate an embodiment including a safety catch for preventing uncontrolled movement of the lever  5  with the levers  3  raised or lowered. The lever  5  comprises a first shaft member  61   a  and a second shaft member  61   b  arranged to slide on the first shaft member  61   a . The first shaft member  61   a  includes slot  61   d . A pin  61   c  is attached to the second shaft member  61   b  and extends through the slot.  61   d . The pin  61   c  extends beyond the outer surface of the second shaft member  61   b  to engage with tracks  91  formed in the walls  90   a  of a bracket  90 . The bracket  90  is formed by walls  90   a  joined by a base  90   b , the bracket  90  being substantially U-shaped in cross-section. The bracket  90  is attached to the top plate  20  of the device. The tracks  91  include end portions  91   a ,  91   b . The slot  61   d  is of a dimension that allows the pin to move into the end portion  91   a  or  91   b , when the handle is moved so that the pin  61   c  is aligned with one of the end portions. A coil spring  61   e  is mounted within the second shaft member  61   b  between the end of the second shaft member  61   a  and the base of the grip portion  62  of actuation lever  5 . 
     In order to release the lever  5  from one of its locked configurations the grip portion  62  is grasped and pushed in the axial direction of the first and second shaft members  61   a ,  61   b . The second shaft member  61   b  slides with respect to the first shaft member  61   a  against the restoring force of the spring  61   e , until the end of the slot  61   d  distal from the grip portion  62  engages the pin  61   c . In this configuration the pin is moved out of the end portion  91   a ,  91   b  into the axis of the central section of the track  91 . In use, once the pin  61   c  is in the central section of the track  91  the operator may push or pull the actuation lever  5  to operate the levers  3  without holding the grip portion  62  in any particular position. The action of the pin  61   c  in the track  91  determines the position of the grip portion  62 . To move the device from the clamped configuration shown in  FIGS. 9 and 11  to the unclamped configuration shown in  FIGS. 10 and 12 , the operator pushes the grip portion  62  towards the device and then rotates the lever  5  clockwise. When the operator is prevented from further rotating the lever  5 , because the pin  61   c  has come to abut against the end of the track  91 , the first and second shaft members  61   a ,  61   b  are forced apart and the pin  61   c  situates itself in the track end portion  91   b . The lever  5  is then locked against rotation and hence the device locked in the unclamped configuration. To move the device to its clamped configuration the same procedure is followed but in reverse. 
     The locking arrangement follows the same principles as that illustrated in  FIG. 8 , but allows the device to be locked in both the clamped and unclamped configurations, thereby reducing the risk of inadvertent change in state of the device. The operator must actively let the device down from its unclamped to its clamped configuration, and must actively engage the lever to lift the device from its clamped to its unclamped configuration. 
     The magnetic attachment device of the invention provides a number of advantages over the prior art. The device is constructed in such a manner that it may be serviced easily. Given the susceptibility of rare earth magnets to decay resulting from moisture ingress and shock loads, this is particularly advantageous, especially when considering the environment in which the devices may well be used, and that many of their applications involve securing people safely to structures. Not only does the construction of the device provide for ease of servicing, but also, the magnets are well sealed against ingress of moisture and they are protected against shock loads. Further, the cam and lever assemblies are robust and ensure that shear forces arising from movement of the handle and hence the cams are transmitted to a substrate to which the device is attached. The provision of compressible protective elements in the form of a rubber overmoulding covering the base of the device, and rubber boots covering the feet of the levers assists not only in preventing the substrate from being damaged, but also assists in dissipating shock loads on the device, and also reduces noise when the device attaches to a metal substrate.