Patent Publication Number: US-8522919-B2

Title: Two-way locking device for height safety apparatus

Description:
This invention relates to height safety equipment and in particular to a fall arrest device using a mobile anchorage to secure a user to an elongate support such as a cable lifeline. Such fall arrest devices are an important item of safety equipment for maintenance and construction personnel who work in high places, since they enable the risk of falls to be minimised. 
     In general, cable lifelines extend between end anchors or supports and are supported by intermediate brackets spaced along their length as required to maintain the cable lifeline in the desired path. Immediate brackets may also be located to support the cable lifeline in order to avoid excessive unsupported lengths of the lifeline and to prevent wind driven oscillation of the lifeline. 
     A number of fall arrest devices have been developed which are able to automatically traverse intermediate brackets supporting the elongate support element without any user intervention. One such device comprises a pair of rotatable wheels having a series of recesses at spaced locations around their peripheries, the adjacent recesses being separated by a radially projecting part of the wheel. These wheels are commonly referred to as star wheels. A cooperating slipper part is mounted on the wheels by engaging formations which inter-engage with complimentary formations on the radially projecting wheel parts. The space between the slipper part and the wheels is dimensioned to receive the elongate support element such as a cable lifeline so that the device is retained on the support element. When the device moves along the elongate support element and reaches an intermediate support the support passes between the slipper and the centres of the wheels and is received in one of the recesses of one of the wheels, rotation of the wheel then allows the device to move over the intermediate support without user intervention and without the retention of the device on the elongate support element being compromised. 
     Devices of this type are able to function satisfactorily on essentially horizontal cable lifelines. If the user attached to the device through the safety lanyard should fall, the fall can be arrested by the attachment of the safety lanyard to the cable lifeline through the device. The fall arrest load passing along the safety lanyard will be essentially perpendicular to the cable lifeline so that movement of the device along the cable lifeline will not be significant. 
     Where a device is to be used on a vertical or near vertical cable lifeline, it is necessary to provide some locking means so that the device can move along the cable lifeline to follow the user and will automatically grip or lock onto the cable lifeline when a fall occurs in order to stop the fall. 
     One such device here is described in European Patent No. EP 0272782 which discloses a self locking fall arrest device having a locking cam which is spring biassed to a locking condition in which it firmly grips the safety line to lock the device to the safety line. In use, the device is connected to a lanyard of a personnel safety harness so that the loading applied to the locking cam by the lanyard maintains the locking cam in an unlocking condition until such loading is released, for example when a fall occurs, whereupon the locking cam is automatically moved into its locking condition. 
     Devices of this type are suitable for use in vertical or near vertical installations but have only a uni-directional capability. That is, such devices must be installed on a safety line or cable in the correct orientation for safe operation. Accordingly, a device of this type cannot be used to ascend one side of a tall structure and descend the other side on a single safety line because the device will be incorrect ly oriented for the descent. 
     In practice, this limitation is not normally a problem because it is rare for there to be a requirement for a fall arrest device which has bi-directional capability in a vertical or near vertical orientation. This is because it is seldom the case that workers ascend one vertical or near vertical face of a structure and then descend a vertical or near vertical face of the same structure using a common safety line spanning the two faces. 
     However, the situation is different for safety lines inclined at intermediate angles between horizontal and vertical where it is often desirable for personnel to ascend a sloping surface and then descend another sloping surface on a common cable lifeline spanning both surfaces. This arrangement is commonly required where personnel are intended to work on pitched roofs. 
     In principle, it would be possible to use a uni-directional device and to require personnel to detach, reverse and re-attach the device each time they cross the roof apex. In practice, many workers confronted this requirement will simply not bother to use the safety device, and even workers who do use the safety device will on occasion become confused and attach the device to the cable lifeline in the wrong orientation. Under these circumstances the lives of workers are placed unnecessarily at risk. 
     One known device able to operate an inclined cable lifeline in either orientation is the griplatch device produced by Latchways Plc. 
     The essential features of the griplatch device are shown in  FIGS. 1 and 2 . 
     The griplatch device comprises a star wheel type arrangement having a pair of star wheels  1  mounted on a common axle  2  and mounting between them a cooperating slipper  3 . A pair of cam arms  4   a  and  4   b  are arranged between the wheels  1  and are pivotally supported by the axle  2 . Each cam arm  4  defines a cam surface opposed to the slipper  3  and has an elongate arm section extending beyond the outer circumference of the star wheels  1  towards a remote end. The remote ends of the two cam arms are connected by two links  5   a  and  5   b , each link  5   a  and  5   b  having a first end pivotally connected to a remote end of one of the cam arms  4   a  and  4   b  and a second end pivotally connected to the other link  5   a  or  5   b.    
     In use the griplatch device is mounted on a cable safety line which passes through a receiving space defined between the two star wheels  1 , the slipper  3  and the cam arms  4   a  and  4   b . A safety lanyard  6  connected to a user safety harness is connected to a caribineer or similar connecting loop  7  which is passed around one of the links  5   a  and  5   b . The connecting loop  7  is sufficiently large that it can pass from one link  5   a ,  5   b  to the other over their connecting point under the influence of the forces along the safety lanyard  6 . 
     When the user ascends or descends with the griplatch device mounted on a inclined cable lifeline the forces along the lanyard  6  will pull the connecting loop  7  along the links  5  until the loop  7  is at or close to the pivotal connection between a link  5  and a cam arm  4  where they are connected at the up slope side of the device, as shown in solid lines in  FIG. 1 . The forces acting along the safety lanyard  6  substantially parallel to the cable will tend to act on the quadrilateral link formed by the two cam arms  4  and two links  5  to move the pivot point between the two links  5  towards the axis of rotation of the star wheels  1  and move the cam surfaces of the cam arms  4  away from the slipper  3 . As a result, the griplatch device will be able to move freely along the cable following the user&#39;s movements. 
     If a fall event occurs, the safety lanyard  6  and connecting link  7  will move downwards and away from the cable lifeline on to the down slope one of the links  5 , for example the position as shown in dashed lines in  FIG. 1 . The fall load applied along the safety lanyard  6  will have a large component acting perpendicularly away from the cable lifeline and this will tend to pull the pivotal connection between the two links  5  away from the axis  2  of the star wheels  1 . This will cause the cam arms  4  to rotate about the axle  2  bringing the cam surfaces of the cam arms  4  towards the slipper  3  to grip the cable lifeline between the cam surfaces of the cam arms  4  and the slipper  3 . In practice, the vertical load along the safety lanyard  6  will also produce a couple causing the entire linkage formed by the cam arms  4  and link lanyard to rotate about the axle  2  in a sense so that the down slope one of the cam surfaces will be the only one which will grip the cable safety line against the slipper  3 . 
     The symmetrical arrangement of the griplatch device enables it to operate as a bi-directional device unaffected by the directional of the slope of the cable. 
     The main limitation of the griplatch device is that it can only operate on cable life lines up to a maximum angle from the horizontal. If the angle of the safety line is too great, the down slope links will be close enough to the horizontal that when a fall arrest event occurs the loop  7  could slide along the down slope link away from the pivotal connection between the two links and towards to the pivotal connection between the down slope link and its associated cam arm. Movement of the loop  7  into this position will cause the quadrilateral linkage to move back towards the position shown in  FIG. 1 , releasing the grip of the device on the cable lifeline. 
     This problem is made worse by the fact that in practice the geometry of many falls will be such that after the fall is arrested the user hanging from the safety lanyard  6  will be swinging beneath the device. Such swinging movement can cause sliding of the loop  7  along the link  5  to a position where the device will release the grip on the cable lifeline when the inclination of the cable lifeline would not otherwise be sufficient to cause such release. 
     This problem is also made worse by the fact that when a fall arrest event occurs it is usual for the cable lifeline to extend due to stretching and/or the deployment of in line energy absorbers so that the cable lifeline sags down between the intermediate supports on either side of the device. This sagging can cause the cable inclination at the device location to be higher than the cable inclination before the fall arrest event occurred. 
     The present invention was made in an attempt to overcome these problems and disadvantages of the prior art. 
     This invention provides a fall arrest device for use on an elongate support, said device comprising: chassis means having safety support retaining means to retain an elongate support whilst allowing movement of the device therealong, and including a sliding element for slidably engaging said elongate support; first and second locking cam means for locking the device to the elongate support in a fall arrest situation; first and second link means; and attaching means for attaching personnel safety means to the device and transmitting a load form the personal safety device to said link means; in which said first and second locking cam means comprise respective first and second cam elements each arranged for rotation about a respective first axis relative to the chassis and able to move between a first locking position in which the cam element traps the elongate support between itself and the sliding element and a second released position in which the cam element does not trap the elongate support; the first and second link means each being connected to a respective one of the first and second cam elements for mutual rotation about a respective second axis separated from said first axis, the first and second link means being connected together for mutual rotation about a third axis separated from said first and second axes, and the attaching means being able to move relative to the link means, so that the first and second locking cam means can be moved between their first and second positions by loads applied to the device through the attaching means; 
     in which each of the first and second link means comprises two parts arranged for reversible relative movement in response to an applied load from the attaching means above a predetermined value, the movement being such that a part of the link means intermediate said second and third axes descends relative to said second axis. 
    
    
     
       Preferred embodiments of the invention will now be described by way of example only with reference to the accompanying diagrammatic Figures, in which: 
         FIG. 1  shows a prior art locking device in the unlocked condition; 
         FIG. 2  shows the device of  FIG. 1  in a locked position; 
         FIG. 3  shows a side view of a first embodiment of a locking device according to the invention in an unlocked condition; 
         FIG. 4  shows a partially cut-away view of the device of  FIG. 3 ; 
         FIG. 5  shows a partially cut-away view of the device of  FIG. 3  in a locked condition; 
         FIG. 6   a  a perspective view of the cam arms and boss of the device of  FIG. 3 ; 
         FIG. 6   b  shows an exploded view of the parts of  FIG. 6   a,    
         FIG. 7  shows a partially cut-away side view of the device of  FIG. 3  when subjected to a vertical load above the buckling threshold of the two part links; 
         FIG. 8  shows a cut away side view of the device of  FIG. 7  mounted on a more steeply inclined cable; 
         FIGS. 9   a  to  9   d  shows a side view of a device according to a second embodiment of the invention. 
     
    
    
     A first embodiment of a two way locking device  10  according to the invention suitable for use in height safety apparatus is shown in side view in  FIG. 3 . The same two way locking device  10  is shown in  FIG. 4  in a partial cut away view in order to allow the locking mechanism to be clearly seen. 
     The device  10  is shown mounted on an inclined safety line cable  11  in the Figures. 
     The device  10  comprises a pair of spaced apart star wheels  12  mounted for rotation about a common axis  17  on an axle  31  and supporting between them a slipper  13  mounted on star wheels  12  by means of formations which inter-engage with cooperating formations on the radially projecting points of the star wheels  12 . 
     As explained in the introductory section of this application, star wheel type devices have been in use for many years so that their general function and operation will not be described in detail herein. 
     A pair of cam arms  14  and  15  are mounted between the star wheels  12  so that a receiving space is defined between the star wheels  12 , slipper  13  and cam arms  14  and  15 . The cable  11  passes through the receiving space so that the two way locking device  10  is retained on the cable  11 . The cam arms  14  and  15  are mounted for mutual pivotal movement about an axis  16  parallel to but offset from the axis of rotation  17  of the star wheels  12 . The axis  16  is located so that the axis  17  lies between the receiving space and the axis  16 . Each of the cam arms  14  and  15  have a respective engaging portion  14   a ,  15   a  which can be brought into engagement with the cable  11  by rotation of the respective cam arm  14 ,  15  about the axis  16  so that the cable  11  can be gripped between either or both of the engaging portions  14   a  and  15   a  and the slipper element  13  to lock the device  10  to the cable  11 . In  FIG. 4 , part of the cam arm  14  lies in front of the cam arm  15 . Each cam arm  14  and  15  has an arm portion extending away from the pivot axis  16  and ending in a respective end section  14   b ,  15   b . Each of the cam arms  14 ,  15  is connected at it&#39;s respective end section  14   b ,  15   b  to a first end of a respective two part link  18  and  19  for mutual pivotal movement about a respective axis  14   c ,  15   c . The two two part links  18  and  19  are connected together at their respective second ends remote from the first ends for mutual pivotal movement about an axis  20 . 
     Each two part link  18  or  19  is made of two arms  18   a ,  18   b  and  19   a ,  19   b . Each of the arms  18   a ,  18   b  and  19   a ,  19   b  is substantially straight having first and second ends. The two arm sections  18   a ,  18   b , and  19   a ,  19   b  respectively making up each two part link  18  and  19  are pivotally connected together for mutual rotation about an axis  18   c ,  19   c.    
     The cam arms  14 ,  15  and two part links  18 , 19  form a quadrilateral, or four arm, linkage. 
     The pivotal connection between the first and second arms  18   a ,  18   b ,  19   a ,  19   c  of each two part linkage  18 , 19  allows rotation about a respective axis  18   c ,  19   c  limited by a stop  18   f ,  19   f , formed by opposed engaging surfaces arranged radially to the respective axis  18   c ,  19   c  on the arms  18   a ,  18   b ,  19   a ,  19   b . The effect of the stops  18   f ,  19   f  is to limit relative pivotal movement of the arms  18   a ,  18   b ,  19   a ,  19   b  of each two part link  18 ,  19  in a direction moving the respective axis  18   c ,  19   c  inwardly towards the pivotal axis  16  of the cam arms  14  and  15 . In the illustrated embodiments this stopping occurs when the pivoting axes  14   c ,  18   c  and  20  and  15   c  and  19   c  and  20  respectively of each two part link  18 ,  19  are arranged in a straight line. This straight line arrangement of the axes at the stopping position is convenient, but is not essential. 
     A torsion spring  21  passes around the axle of the pivot axis  20  and is arranged to bias the two part arms  18  and  19  about the pivoting axis  20 . The biassing acts in a sense which will rotate the cam arms  14  and  15  about their axis of mutual rotation  16  into gripping engagement with the cable  11 . This biassing also urges the axes  18   c ,  19   c  between the respective two arms  18   a ,  18   b ,  19   a ,  19   b  of each of the two part links  18  and  19  inwardly towards and against their respective stop mechanisms  18   f ,  19   f.    
     As a result, when no external loading is applied to the device  10 , the device  10  automatically moves as a result of the action of the biassing spring  21  into the position shown in  FIG. 5  where the cable  11  is gripped between the slipper  13  and the engaging portions  14   a ,  11   a  of both of the cam arms  14  and  15  so that the device  10  is locked in place on the cable  11 . 
     In use, the user wears a fall safety harness attached by a safety lanyard to a connecting loop  22 . The connecting loop  22  is sized to slip freely under an applied load over the two part links  18  and  19 . When the applied load is applied to the device  10  along the safety lanyard substantially parallel to the cable  11 , as shown in  FIGS. 3 and 4 , the applied load counteracts the biassing by the spring  21  and moves the cam arms  14  and  15  into an open position where their engaging portions  14   a  and  15   a  do not grip the cable  11 . As a result, the device  10  can move freely along the cable  11 . 
     This is the situation which will apply when the user is moving up or down alongside the inclined cable  11 . When the user is ascending, the device  10  will be dragged up the cable  11  by the safety lanyard. When the user is descending, the device  10  will be lowered down the cable  11  hanging from the safety lanyard. In order for the device  10  to be able to automatically descend along an inclined cable  11 , the biassing force of the spring  21  must be selected such that the device will remain in the non-gripping state when its weight is supported from the safety lanyard. 
     The first arm  18   a ,  19   a  of each two part link  18  and  19  includes an extension portion  18   e ,  19   e  extending to the opposite side of the respective axis  14   c ,  15   c  as the remainder of the two part link  18 ,  19 . These extension sections  18   e ,  19   e  are arranged and shaped so that when the device  10  is in the gripping or locked position as shown in  FIG. 5 , the extension sections  18   e ,  19   e  project further into the interior of the four arm linkage formed by the cam arm  14 ,  15  and two part links  18 ,  19  than the respective end sections  14   b ,  15   b  of the cam arms  14 ,  15 , but are substantially coplanar with the inner surfaces of the respective end sections  14   b ,  15   b  when the device  10  is in the unlocked position as shown in  FIG. 4 . 
     As a result, when the connecting loop  22  moves over the two part links  18  and  19  in response to a load applied substantially parallel to the cable  11 , the connecting loop  22  will bear on the inner surface of one of the extending sections  18   e ,  19   e  at a position between the respective axis  14   c ,  15   c  and the cable  11 . As a result, the load applied through the end loop  22  will have a considerable mechanical advantage due to leverage assisting it in overcoming and reversing the biassing of the device  10  into the closed or gripping position due to the spring  21  and the weight of the device  10 . 
     Each cam arm  14  and  15  has a respective outwardly projecting shoulder portion  14   f ,  15   f . The shoulder portions  14   f  and  15   f  are sized so that the connecting loop  22  cannot pass along the cam alms  14  and  15  past the respective shoulders  14   f ,  15   f . This arrangement is preferred in order to prevent connecting loop  22  passing too far along the cam arms  14  and  15 . In the preferred embodiment of the device  10 , even if the safety lanyard becomes looped over the cable  11  or around the device  10 , for example by passing over the top of the slipper  13 , when a fall arrest load is applied the device  10  will rotate around the cable  11  into an alignment allowing good and reliable gripping of the cable  11 . Such automatic rotation might be prevented or rendered unreliable if the connecting loop  22  was able to pass too far along the cam arms  14  and  15 . This possibility is prevented by the shoulders  14   f  and  15   f  which limit the movement of a detached loop  22  along the cam arms  14  and  15 . Other arrangements for controlling movement of the connecting loop  22  along the cam arms  14  and  15  would be possible, or for some designs of device may not be required. However, use of the shoulders  14   f ,  14   f  is preferred. 
     In practice, it is possible that the device  10  could be damaged by torsional loads transmitted along the safety lanyard to the device  10 . In order to eliminate this possibility, it is preferred for the detached loop  22  to be linked to the safety lanyard by an arrangement allowing torsional loads to be eliminated without transmission to the device  10 . A preferred arrangement is shown in  FIG. 3  where the connecting loop  22  is linked to the safety lanyard through a twistable connector  24  able to freely rotate relative to the connection loop  22  about an axis linking the connection loop  22  to the safety lanyard loads, an axis lying in the plane of the paper in  FIG. 3 . 
     As explained above, the axis  16  of the mutual pivotal movement of the cam arms  14  and  15  is offset from the axis of rotation  17  of the star wheels  12 . The mechanism to do this is shown in more detail in the perspective view  6   a  showing the cam arms  14 ,  15  in detail and the corresponding exploded perspective view  6   b.    
     The cam arms  14  and  15  are arranged to be able to rotate about a cylindrical boss  23 . The cylindrical boss  23  is itself arranged for rotation about the start wheel axis  17 , such that the axis  17  is offset from the axis  16  at the centre of the boss  23 , about which the cam arms  14  and  15  rotate. 
     As can be seen in  FIG. 6 , the overlapping parts of the cam arms  14  and  15  are arranged between the star wheels, each having a thickness of about half of the separation between the star wheels  12  while the respective engagement portions  14   a  and  15   a  of the cam arms  14  and  15  extend across the full separation between the two star wheels  12  in order to ensure good gripping of the cable  11 . Bach of the engagement portions  14   a  and  15   a  has a recessed part  14   d ,  15   d  having a substantially cylindrical concave face matching the external surface profile of the cable  11 . Inclusion of the recesses  14   d ,  15   d  is preferred to improve the grip on the cable  11 , but this is not essential. 
     It should be understood that if no restraint is placed on the relative pivotal movement of the cam alms  14  and  15  about the boss  23  and of the boss  23  about the axis  17 , it would be possible for the cam arms  14  and  15  and boss  23  to move into positions which could cause problems. For example, when the device  10  was not mounted upon the cable  11 , it might be possible for the cam arms  14  and  15  and boss  23  to be moved into a position in which a cable  11  could not be passed through the device  10  in order to install the device  10  on the cable  11  and the cam arms  14  and  15  could not easily be moved to a position allowing cable  11  to pass through the device  10 , causing frustration and inconvenience. 
     The boss  23  has a radially extending pin  23   a  located midway along the boss  23  so that the pin  23   a  extends between the cam arms  14  and  15 . Each of the cam arms  14  and  15  has a respective control slot  14   e  and  15   e  arranged so that the pin  23   a  is received within the control slots  14   e ,  15   e . In this arrangement, relative movement of each of the cam arms  14   a ,  15   a  relative to the boss  23  is controlled by the length of the respective control slot  14   e  and  15   e . When the pin  23   a  contacts the end of the control slot  14   e ,  15   e , movement of the respective cam arm  14 ,  15  is stopped. Thus, the pin  23   a  and the control slots  14   e ,  15   e  set the available range of pivotal movement of the cam arms  14  and  15  relative to one another and to the boss  23 . Although this does not directly limit rotation of the boss  23  about the axis  17 , it will be understood that available range of movement of the cam arms  14  and  15  about the axis  17  is limited by contact of the cam arms  14  and  15  with the cable  11  or slipper  13  so that the pin  23   a  and control slots  14   e ,  15   e  also limit the possible range of rotation of the boss  23  about the axis  17 . 
     The described structure of the cam arms  14  and  15  in which the respective engagement portions  14   a  and  15   a  extend across the full separation between the two star wheels  12  will automatically limit the amount of possible relative pivotal movement of the cam arms  14  and  15  about the boss  23  by contact of the engagement portions  14   a  and  15   a  with one another and the other parts of the cam arms  14  and  15 . 
     However, it is preferred to have the pin  23   a  and control slots  14   e ,  15   e  limit the relative movement of the cam arms  14  and  15  as well as their movement about the boss  23  so that it is not necessary to select the shape and materials of the cam alms  14  and  15  to support the loads which will occur at the points of contact between the cam arms  14  and  15  at the limits of their movement. However, it would be possible to have the pin  23   a  stop only the rotation of the cam arms  14  and  15  about the boss  23  while the relative movement of the cam arms  14  and  15  was limited by some other stopping mechanism such as contact between parts of the cam arms  14  and  15 . 
     When a fall occurs, the load applied through the safety lanyard will drop to substantially nothing, the safety lanyard will go slack and the connecting loop  22  will tend to drop towards the connection point between the two two-part links  18  and  19  and will come to rest on the downslope two part link, the two part link  19  in the Figures. 
     The release of the load applied through the connecting loop  22  will allow the device to move back towards the gripping position as shown in  FIG. 5  under the influence of the bias from spring  21 . When a fall occurs, the connecting loop  22 , after moving over the two part links  18  and  19 , will apply a vertically downward load passed along the safety lanyard to the down slope two part link, the two part link  19  in the Figures. Usually, this vertically downward load will be applied to the arm of the downslope two part link closest to the axis  20 , the arm  19   b  of the two part link  19  as shown in the Figures. The component of the fall arrest load acting away from the cable  11  tends to move the axis  20  between the two two part links  18  and  19  away from the mutual pivoting axis  16  of the cam arms  14  and  15  and this component of the load, together with the biassing force from the spring  21 , urges the device  10  towards the gripping position shown in  FIG. 5  in which the cam arms  14  and  15  grip the cable lifeline  11  against the slipper  13 . 
     Further, this vertical load applied through the connecting loop  22  generates a couple on the entire linkage formed by the cam arms  14  and  15  and two part links  18  and  19  which tends to rotate the linkage around the axis of rotation  16  of the cam arms  14  and  15  about the boss  23 . This couple tends to rotate the downslope engagement portion  14   a  of the cam arm  14  towards the cable  11  and the slipper  13 . 
     Finally, the vertical fall arrest load applied through the connecting loop  22  also produces a couple about the axle  17  of the star wheels  12 . Because the centre of the boss  23  is offset from the axle  17 , this produces a rotation of the boss  23  and the entire linkage supported on the boss  23  about the axle  17  in a sense, again, tending to bring the downslope gripping portion  14   a  towards the cable  11  and the slipper  13 . 
     When a fall arrest event occurs, the combination of these three movements produced by the vertical load transmitted through the safety lanyard and connecting loop  22  causes the cam arms  14  and  15  to move so that the downslope engaging portion  14   a  of the cam arm  14  moves quickly and positively to grip the cable lifeline  11  against the slipper  13 . 
     As a result, the use of an arrangement in which the axis  16  of the pivotal movement of the cam arms  14  and  15  is offset from the axis of rotation of the star wheels  12  allows an improved gripping action. 
     The rotation of the cam arms  14  and  15  and attached parts about two parallel spaced apart axes  16  and  17  allows the geometry of the cam arms  14  and  15  relative to the cable  11  to change in response to the applied load. When the device  10  is being locked to the cable  11  by a vertical fall arrest load, or other applied vertical, or non-horizontal load, that is, the device  10  is moving from the unlocked position to a locked position, the applied load will tend to move the cam arms  14  and  15  about the axis  16  and also the boss  23  about the axis  17  in the same sense, clockwise in  FIG. 4 . These combined movements will change the geometry of the cam arms  14  and  15  relative to the cable  11  so that the point of contact of the downslope engagement portion, the engagement portion  14   a  of the cam arm  14  in the figures, will move up slope along the cable  11  closer to the centre of the slipper  13  compared to the position at which it would contact the cable  11  if no rotation of the boss  23  about the axis  17  took place. 
     Once the downslope engagement portion is in contact with the cable  11 , the applied load will cause further relative rotation of the cam arms  14  and  15  until the upstream engagement portion is also in contact with the cable  11  and the device  10  is in the locked position, as shown in  FIG. 5 . While this further movement is taking place, the rotation of the boss  23  about the axis  17  will be reversed, returning the device  10  to a symmetrical position where the axes  16 ,  17  and  20  are all coplanar along the centreline of the device  10 . This is also the position into which the device  10  is urged by the torsion spring  21 . 
     As a result of this change in geometry, when the device  10  is closed or locked by a large vertical load such as a fall arrest load, the geometry of the cam arm  14  having the downslope gripping portion  14   a  is made more like a self closing cam or cleat geometry. This results in an improved gripping action and makes the device more resistant to incorrect releasing of the grip of the cable  11  due to bouncing or rebounding of the user following a fall arrest event. Such bouncing or rebounding can result in the load applied along the safety lanyard dropping momentarily or for short periods to a low level or in extreme cases to zero. In previously known devices, such temporary reductions in the applied vertical load can result in the device temporarily unlocking itself from the cable and then re-locking again when the load is re-applied. Such locking and re-locking is uncomfortable and alarming for the user and can be dangerous. 
     The change in geometry of the device  10  under the load allowed by the use of two offset axes of rotation  16 ,  17  to move the contact point of the downslope engagement portion nearer to the centre of the device  10  improves the initial grip on the cable  11  by the device  10 . This both ensures quicker and more definite working and locking of the device  10  to the cable  11  under an applied fall arrest load when a fall arrest event occurs and also increases the grip of the device  10  due to its own weight and the bias of the spring  21  if the load applied to the device along the safety lanyard is temporarily reduced or removed during the locking process so that the device  10  is more resistant to unwanted unlocking and re-blocking when the user bounces, rebounds or oscillates during a fall arrest event. 
     In addition to the actions described above, the vertically downward fall arrest load applied to the two part link  19  through the connecting loop  22  will cause the two part link to buckle or yield, moving the pivotal axis  19   c  between the two arms  19   a  and  19   b  of the two part link  19  downward against the bias applied by the spring  21 . Downward movement of the pivotal axis  19   c  will require rotation of the arms  19   a  and  19   b  of the two part link  19  away from their stopped position. This change in the geometry of the two part link  19  will move the pivotal axes  15   c  and  20  connecting the two part link  19  to the cam arm  15  and the two part link  18  respectively towards one another, towards and then into the position shown in  FIG. 7 . 
     This buckling or yielding of the two part link  19  will occur mostly after the downstream engaging portion  14   a  of the cam arm  14  has been brought in contact with the cable  11  and begun gripping it against the slipper  13 . Until this contact is made, the linkage will tend to respond to the applied load by rotation of the cam arms  14  and  15  and the boss  23  about the axes  16  and  17 . However, under the suddenly applied fall arrest loading some buckling of the two part link  19  may occur before this contact is made. 
     The buckling of the two part link  19  while the device is moving from the unlocked position to the locked position will tend to close the cam arms so that the upstream gripping portion is brought towards the cable  11 . 
     As can be seen in  FIG. 7 , the yielding of the two part link  19  results in the connecting loop  22  being suspended from the two part link  19  close to or at the pivoting axis  19   c  and below the axes  15   c  and  20 . As a result, sliding of the connecting loop  22  along the two part link  19  towards the cam arm  14  is suppressed or prevented by the upward slope formed by the interior face of the arm  19   a  between the axes  15   c  and  19   c . As a result, the device  10  according to the present invention can safely and reliably operate on a cable  11  inclined at larger angles to the horizontal than previously known devices. 
     The lowering of the centre of the two part link  19  relative to its ends due to yielding will inhibit or prevent movement of the connecting loop  22  into a position where it will tend to release the grip of the device  10  on the cable  11  both due to the static geometry of the device mounted on a cable lifeline inclined at a large angle to the horizontal and also due to the dynamic loads encountered when a user is swinging below the device  10  after a fall arrest event has occurred. 
     It should be understood that the symmetrical arrangement of the device  10  allows it to operate with equal effectiveness on a cable inclined in either direction, without any user action being required when the sense of the inclination is reversed. 
     As can be understood from the above, the buckling of the two part link  19  tends to close up the cam arms  14  and  15  bringing the respective pivot axes  14   c  and  15   c , connecting the cam arms  14  and  15  to the two part links  18  and  19  towards one another. As explained above, the downslope engagement portion  14   a  of the cam arm  14  is brought first into gripping contact with the cable  11  by the applied fall arrest load and as a result the relative movement of the cam arms  14  and  15  due to the buckling or yielding of the two part link  19  before the upstream engagement portion  15   a  is brought into grippiong contact is accommodated by moving the other parts of the linkage around the contact point between the engagement portion  14   a  and the cable  11 . This movement tends to centralise the rotation of the boss  23  about the star wheel axis  17  and the rotation of the linkage comprising the cam arms  14 , 15  and the two part links  18 ,  19  about the axis  16  of the boss  23 . Thus the buckling of the link and the closing of the cam arms  14  and  15  both tend to bring the engagement portion  15   a  of the cam arm  15  towards the cable  11 . 
     When the device  10  is attached to a cable  11  having a relatively low angle of inclination to the horizontal both of the engagement portions  14   a  and  15   a  of both of the cam arms  14  and  15  will be brought into gripping engagement with the cable  11  and the rotation of the boss  23  will be substantially reversed to a central, symmetrical, position. An example of this is shown in  FIG. 7  where the device  10  is shown mounted on a cable  11  inclined at 47 degrees to the horizontal. 
     When the device  10  is attached to a cable  11  having a larger angle of inclination to the horizontal the engagement portion  15   a  of the cam arm  15  will still move towards the cable  11 , but not sufficiently far to make contact with the cable  11  so that only the downslope engagement portion  14   a  of the cam arm  14  will be gripping the cable  11  against the slipper  13 . An example of this is shown in  FIG. 8  where the device  10  is shown mounted on a cable  11  inclined at 75 degrees to the horizontal. 
     In order to unlock the device  10  from the cable  11  it is necessary to apply a load along the safety lanyard to move the connecting loop  22  along the two part links  18  and  19  towards the axis  14   c  between the two part link  18  and the cam arm  14 . 
     As can be seen in the figures, the arms  18   b ,  19   b  of the two part links  18 ,  19  connected at the axis  20  have inner surfaces  18   d ,  19   d  which are curved to present a concave profile. When no load is applied along the safety lanyard the connecting loop  22  will fall under its own weight, and the weight of the safety lanyard, into the bottom corner of the linkage, that is adjacent the axis  20  at the pivotal connection between the two two part links  18  and  19 . Further, if the safety lanyard is moved in orientation with a load continuously applied between a substantially vertical load direction locking the device  10  into engagement with the cable  11  towards a load direction substantially parallel to the cable, the applied load must move through a position urging the connecting loop  22  into this bottom corner of the linkage. 
     As a result of the concave profile of the inner surfaces  18   d ,  19   d  of the arms  18   b ,  19   b , if the connecting loop  22  is pulled from the bottom corner location adjacent the axis  20  by a continuous force along the safety lanyard acting substantially parallel to the cable, the connecting loop will become trapped in the concave surface  18   d  of arm  18   b  and will not be able to pass off the arm  18   b  over the joint between the arms  18   a  and  18   b  to reach a position where it can unlock the grip of the device  10  on the cable  11 . In order to pass the connecting loop  22  off the concave surface  18   d  and off the arm  18  it is necessary for the user to jerk or crack the safety lanyard. 
     The use of concave inner surfaces on arms  18   b  and  19   b  is preferred because this requirement for a positive user action to unlock the device  10  from a gripping state can be a useful safety feature, but this is not essential. 
     The inner surfaces  18   e  and  19   e  of the arms  18   a  and  19   a  of the two part links  18  and  19  also have a concave profile. When the two part link  18  or  19  is buckled by an applied fall arrest load, for example as shown in  FIGS. 7 and 8 , this concave profile increases the steepness of slope of the inner surface  18   e ,  19   e  presented to the connecting loop  22 . This increase in steepness makes it less likely that the connecting loop  22  will be able to move along the arm  18   a ,  19   a  ( 19   a  in the figures) to a position adjacent the axis  15   c  and incorrectly unlock the grip of the device  10  on the cable  11 . The use of concave inner surfaces on arms  18   a  and  19   a  is preferred to give an increased margin of safety, but this is not essential. 
     As the linkage moves from the engaged or gripping position shown in  FIG. 5  to the free or released position shown in  FIG. 4 , the engagement portions  14   a  and  15   a  of the cam arms  14  and  15  are withdrawn from gripping contact with the cable  11 , by mutual rotation of the cam arms  14  and  15  about the axis  16  of the boss  23 . The axis  16  is displaced from the axis of rotation  17  of the star wheels  12  and this results in a smoother and improved release of the grip on the cable  11 . This smoother release of the grip is partially due to the lateral component of movement of the engagement portions  14   a  and  15   a , that is the component of movement parallel to the cable  11 , produced by the offset axes of rotation  16  and  17  of the cam arms  14  and  15  and the star wheels  12 . The movement of the contact point of the downslope engagement portion allowed by the use of two parallel spaced apart axis of rotation  16  and  17  results in the geometry and movement of the cam arms  14 ,  15  relative to the cable  11  being different during gripping of the cable  11  by the device  10  in response to an applied vertical load and release of this grip under an applied load substantially parallel to the cable  11 . This difference in the geometry of the gripping and ungripping actions allows both actions to be improved. Further, the offset between the axes  16  and  17  increases the amount of movement of the engagement portions  14   a ,  15   a  of the cam arms  14 , 15  away from the cable  11  for a given angular movement of the cam arms  14 ,  15 . This also improves the release of the grip. Further, this increased travel of the engagement portions  14   a ,  15   a  increase the clearance between the cable  11  and the cam arms  14 , 15  in the unlocked position, making it simpler for the device  10  to traverse intermediate supports. These improvements could otherwise only be achieved by undesirable increases in the size or extent of allowed pivotal movement of the device  10 . 
     The smooth release of the grip is further improved by the recesses  14   d ,  15   d  in the engagement portions  14   a  and  15   a  which reduce the point loads between the engagement portions  14   a ,  15   a  and the cable  11 . However, the use of such recesses is not essential. 
     As explained above, the device  10  according to the invention is intended to be operated by a safety lanyard attached to a personal user safety harness, so that the device  10  can be automatically locked to or released from a safety line cable  11  by the load applied along the safety lanyard. In practice, there may be some height safety system arrangements in which the device  10  cannot properly function. For example, if the cable  11  is above the user&#39;s work or travel area so that the cable  11  is overhead the user, it may be difficult or impossible for the user to apply a load to the device  10  along the safety lanyard at an angle which will unlock the device  10  from the cable  11 . It is advantageous to be able to use the device  10  in such a height safety system geometry in order to allow the device  10  to be used in as wide as possible arrange of height safety systems. This allows the device  10  to be used throughout a height safety system in which some parts have such a geometry and other parts do not. Further, extending the range of possible height safety systems in which device  10  can be used may avoid the requirement to employ multiple types of fall arrest device, so making it easier to maintain the devices and provide the necessary range of spares. 
     A device  30  according to the second embodiment of the invention is shown in  FIG. 9 . The device  30  is similar to the device  10  but has an additional control member  25 . The control member  25  is mounted for rotation about the axis  17  of the star wheels  12  and passes through the four part linkage formed by the cam arms  14 ,  15  and two part links  18  and  19 . The control member  25  is substantially C-shaped. A manual control tether  26  is connected to the control member  25 . By pulling on the control tether  26  the control member  25  can be rotated about the axis  17  bringing the control member  25  into contact with a respective one of the extended sections  18   e ,  19   e  of the two part links  18 ,  19 . Thus, by pulling on the control tether  26  a load can be applied to a respective one of the extension sections  18   e ,  19   e  to move the device  30  from a locking condition to an unlocking condition in a similar way to a load applied along the safety lanyard parallel to the cable  11  through the connecting loop  22  as discussed above. 
     As a safety precaution, the shape of the control member  25  and the profile of the extension sections  18   e ,  19   e  are preferably arranged so that when the device  30  is subjected to a vertical load sufficiently large to buckle one of the two part links  18 ,  19  the resulting change in the geometry of the device  30  will move the downslope extension section  18   e ,  19   e  into a position where the contact geometry between the extension sections  18   e ,  19   e  and the control member  25  is such that loads applied along the control tether to the control member  25  cannot unlock the device  20  from the cable  11 . Such an arrangement is shown in  FIG. 9   d  where it can be seen that the control member  25  cannot act on the section  19   e  of the downslope two part link  19  in such a way as to unlock the device  20  from the cable  11 . Such an arrangement is not essential, but it is preferred so that after a fall arrest event, so long as a vertical load greater than the threshold value required to buckle the two part links  18  and  19  is applied, pulling on the control header  26  will not unlock the device  20  from the cable  11 . Clearly, unlocking the device after a fall arrest event while the user is still suspended from the device  20  could be highly dangerous. 
     When using the device  30  which can be locked or unlocked from the cable  11  using a remote tether, it may be desirable to limit the range of movement of the connecting loop  22  so that the device  30  can only be released from gripping the cable  21  by the control element  25  and the control tether  26  and not by loads applied along the safety lanyard. This may also be desirable where it is possible for a user to fall substantially parallel to the cable  11 , in order to prevent the device  10  being unlocked by the fall loads. 
     One method of controlling the movement of the connecting loop  22  in this way is shown in  FIG. 9  where a control tag  27  is attached for rotation about the axis  20  between the two two part links  18  and  19 . The control tag  27  is a substantially oval loop and the connecting loop  22  passes through the control tag  27 . The control tag  27  is sized so that it limits the movement of the connecting loop  22  to be such that it can only bear against the arm sections  18   b ,  19   b  of the respective two part links  18  and  19  and cannot pass over the axis  18   c ,  19   c  to bear on the arm sections  18   a ,  19   a . As a result, loads applied through the safety lanyard to the connecting loop  22  can only cause the device  30  to lock onto the cable  11 . The control tag  27  is free to rotate around the axis  20  so that connecting loop  22  is free to apply loads to the arm sections  18   b ,  19   b  even when the two part links  18  or  19  are buckled, as shown in  FIG. 9   d.    
     The operation of the device  30  is otherwise substantially the same as the operation of the device  10  of the first embodiment. However, there are further minor differences. In the device  30 , a boss  28  is enclosed between the cam arms  14  and  15 . Arcuate slots  29  are provided through each of the cam arms  14  and  15  through which the axle  31  passes. In this arrangement the movement of the cam arms  14  and  15  relative to one another and the boss  28  is limited by the star wheel axle  31  contacting the ends of the arcuate slots  29 . Accordingly, in this embodiment the pin  23   a  and cooperating slots  14   e ,  15   e  are not required. 
     An alternative arrangement to provide offset axes of rotation without requiring the use of a boss would be to connect the two cam arms for mutual pivoting about an axis and to provide an arcuate slot through each cam arm extending circumferentially about the axis. If the arcuate slots overlie one another and the axle on which the star wheels rotate passes through the arcuate slots, this arrangement will allow the cam arms to pivot about an axis offset from and able to rotate about the axis of rotation of the star wheels. 
     The biassing of the device  10  or  30  as a whole to a gripping position and the biassing of the two part links  18  and  19  into their stopped position in which they act as substantially rigid elements is preferably carried out by a single torsion spring acting between the two links  18  and  19  about the axis  20  as shown in the embodiments. Other forms of biassing instead of a torsion spring could be used. Further, the device could be biassed into the gripping position by some other biassing arrangement such as biassing means acting directly between the two cam arms about the axis  16 . However, if such biassing means is used it would be necessary to provide some further biassing means to maintain the two arm links  18 ,  19  in their substantially rigid orientation until a load exceeding the desired threshold was applied. 
     As a result, the use of passing means acting around the axis  20  between the two part links  18  and  19  is preferred because this is the only location at which a single biassing means is efficient. If biassing means is arranged elsewhere, multiple biassing means will be required. 
     Star wheel type devices allowing a fall arrest device to be selectively attached to or removed from a cable or other elongate support are known. The present invention could be combined with such a removable device, but for clarity, such a combination is not described herein. 
     It is preferred to provide for the cam arms to rotate about a common axis offset from the axis of rotation of the star wheels for the reasons set out above. However, this is not essential and the use of yielding or buckling two part links will provide the advantages set out above, even when used in a device where the cam arms and star wheels rotate about a common axis or where the cam arms rotate about different axes. Further, the use of yielding or buckling two part links can provide the advantages as set out above, even when used in a device using other known mechanisms to negotiate intermediate supports in place of a star wheel system. Finally, it is believed that an arrangement in which the cam arms rotate about a common axis offset from the axis of rotation of the star wheels will be useful in its own right for star wheel type devices even when used without the yielding or buckling two part links. 
     In describing the preferred embodiments the attachment of the device to a cable is referred to. The device could instead be attached to another form of elongate support such as a safety track.