Patent Publication Number: US-2012031701-A1

Title: Speed Responsive Engagement Device

Description:
This invention relates to a speed responsive engagement device, and in particularly to a speed responsive engagement device for use in fall arrest apparatus. 
     Speed responsive engagement devices for use with rotating parts to selectively engage the rotating parts to other components if the speed of rotation of the parts exceeds a threshold value are well known. 
     There are two main types of such speed responsive engagement devices. A first type of speed responsive engagement device is of the centrifugal clutch type. Such an arrangement is described in FIG. 1 of WO2008/007119. 
     A second type of speed responsive engagement device is of the rocking pawl type. An exemplary embodiment of a ratchet and pawl speed responsive engagement device is disclosed in, for example WO2008/007119. 
     The speed sensitive engagement means of both of these known types are used in fall arrest systems of the type where personnel working at height are attached to a safety line wound around a drum. The drum has an automatic rewinding mechanism and a speed sensitive attachment mechanism of the type disclosed above, which responds to the rotation of the drum at a speed above a predetermined threshold by holding the drum against further rotation relative to the drum support or other fixed structure. 
     In use, fall arrest systems of this type allow personnel to move freely around a working area including moving upwardly and downwardly in the area, with the safety line being automatically paid out from and wound onto the drum under the control of the automatic rewinding mechanism as necessary to allow the personnel movement and keep the safety line taut. When a personnel fall occurs, the safety line is pulled out at a much greater speed then is necessary for normal movement and the speed of rotation of the drum rises to the threshold value of the speed sensitive engagement means, which brakes the drum against further rotation and so arrests the fall. 
     In practice it has been found that there is a problem with fall arrest systems of both of these known types that after a fall has been arrested the speed sensitive engagement means can release the engagement allowing the personnel to again start to fall until the speed of rotation of the drum again reaches the threshold value and the fall is again arrested. 
     It has been found that it is possible for this cycle to be repeated so that personnel drop in a series of short falls until they reach the ground or some other supporting surface. 
     This problem is generally referred to as bounce. The problem of bounce is caused by the fact that when the known speed sensitive engagement means are in the engaged state the pawls are biased into a unengaged condition and are only kept in the engaged condition by the pawls being held against the biasing by the ratchet teeth. When these engagement means are used in a fall arrest system and a fall arrest occurs there is a stretching or tensioning of the safety line followed by a momentary reduction in tension to zero as the arrested person bounces at the end of the safety line. During this momentary reduction in tension the automatic rewinding mechanism causes the drum to rotate slightly in the rewinding direction, releasing the pawls from engagement with the ratchet teeth. The biasing then causes the pawls to move to the unengaged position, releasing the drum and allowing the personnel to start falling again. 
     Bounce is dangerous and presents a serious problem for a number of reasons. Firstly, the personnel may be injured by impact with other objects during the multiple falls. Further, in general fall arrest systems are designed so that users undergoing a fall arrest event are only subject to a safe level of force. However, these safe levels are calculated on the assumption of single fall event. Even when a single application of a fall arrest force is safe, repeated application of the same force to a user can result in injury. This problem is made more severe by the fact that many fall arrest systems include single use energy absorbing or shock limiting devices so that successive falls and arrests resulting from bounce may result in personnel being subject to higher than expected levels of force because the capacity of the single use energy absorbing or shock limiting devices in the system has been used up. Further, the repeated fall and arrest loads on the fall arrest system due to bounce can result in failure or damage of components of the fall arrest system or the supporting structure to which it is attached. Finally, where bounce results in personnel descending all the way to the ground or other supporting structure in a series of short falls the final impact with the ground or other support structure may be at a sufficiently high speed to cause injury. The bounce problem is addressed by the arrangement disclosed in WO2008/007119. 
     Control over the way in which the pawl moves from the engaged position to the disengaged position when the ratchet and pawl rotate relative to one another in the reverse direction is important in controlling the effect of bounce. In particular manufacturing tolerances of components in the arrangement can have an effect upon the circumstances of such operation. The present invention provides an arrangement in which the operation of the pawl to be held in the engagement position and caused to move from the engaged position to the disengaged position is more accurately controlled. 
     In accordance with the present invention, there is provided a speed responsive engagement device comprising:
         a ratchet wheel having a plurality of outwardly projecting spaced apart teeth;   a pawl arranged for pivotal movement about an axis between an engagement orientation in which the pawl is orientated for braking engagement with one or more of said ratchet wheel teeth, and a disengagement orientation in which the pawl is pivoted away from the engagement orientation; the pawl and the ratchet wheel being rotatable relative to one another such that when relative rotation occurs between the ratchet wheel and the pawl in a first direction the pawl contacts the ratchet wheel, generating an oscillating movement of the pawl with an amplitude dependent on the speed of the rotation, and when the speed of rotation reaches a predetermined value the increased oscillating movement causes the pawl to move into the engaged orientation brakingly engage with the ratchet wheel teeth preventing further relative rotation between the ratchet wheel and the pawl in the first direction;   a resilient biasing arrangement arranged to bias the pawl towards the disengaged orientation; and   a capture arrangement arranged to capture the pawl in the engaged orientation when the pawl is brakingly engaged with the ratchet wheel.       

     In a speed sensitive engagement according to the present invention the biasing arrangement (typically a leaf spring) can be set to preferentially bias the pawls constantly to the disengaged state and therefore the tolerances for setting the biasing arrangement are greater. The capture arrangement only acts to detain the pawl in the engagement orientation when the pawl has moved by the rotational speed threshold having been reached. The pawl is only released from the capture arrangement, not by action of any biasing means, but by some other physical means such as the reverse oscillation of the pawl as relative movement occurs between the pawl and the ratchet wheel in the second direction of rotation (the rewinding direction). The pawl is not biased when in the engaged state contacting the circumferential surface and the small rotation of the drum in the rewinding direction will not be sufficient to move the pawls to the end of the circumferential surface and cause movement of the pawl from the engaged state to the unengaged state and release from the capture arrangement. 
     It is preferred that the pawl comprises a heel portion and the capture arrangement is arranged to hold the heel portion of the pawl captive. Beneficially, the heel portion of the pawl contacts a peripheral surface of the ratchet wheel generating the oscillating movement of the pawl as relative rotation occurs between the ratchet wheel and the pawl in a first direction. The underside of the heel portion typically rides over the peripheral surface of the ratchet wheel teeth in order to effect the oscillating motion of the pawl as relative rotation occurs. 
     The pawl is typically spaced from the capture arrangement in normal operation as the ratchet wheel and pawl rotate relative to one another. The pawl preferably remains free from the capture arrangement until the pawl pivots to brakingly engage the ratchet wheel. 
     Beneficially the pawl is arranged to be released from the capture arrangement by relative rotation of the ratchet wheel and the pawl in a second direction (rewinding direction) opposite to the first rotation direction (paying out direction). 
     In a preferred embodiment the pawl includes a toe portion to brakingly engage the ratchet wheel teeth and a heel portion to be held captive by the capture arrangement and wherein upon relative rotation in the second direction (rewinding direction), the toe portion of the pawl rides over the periphery of the ratchet wheel causing the heel portion of the pawl to pivot out of captive engagement with the capture arrangement. 
     As mentioned previously, it is preferred that, in both the engagement orientation in and the disengagement orientation, the resilient biasing arrangement acts to bias the pawl in favour of the disengaged orientation. 
     In a preferred embodiment the capture arrangement comprises a detent. Beneficially the detent comprises a resilient element (such as a resiliently deflectable arm) able to deflect to and from a normal orientation in order to capture the pawl. It is preferred that the resilient element is caused to deflect to capture the pawl as the pawl moves to the engagement position to brakingly engage the ratchet wheel. 
     In a preferred embodiment, the pawl and the detent include complementary engaging formations arranged to engage when the pawl is held captive by the capture arrangement. One of the formations may beneficially comprise a notch in the heel of the pawl. 
     As mentioned previously, the resilient biasing arrangement preferably comprises a spring element. 
     In one embodiment, adjacent ratchet wheel teeth are separated by a circumferential surface of the ratchet wheel having a substantially constant radius. 
     It is preferred that the capture arrangement is arranged to detain the pawl against movement in both opposed pivotal directions away from the engagement orientation. 
     Beneficially, the ratchet wheel teeth have an undercut surface arranged to urge the pawl into the engaged position. 
     It is preferred that the pawl oscillates in contact with the ratchet wheel in normal when the wheel rotates in either or both directions generating an audible sound. 
     Beneficially, the arrangement includes a plurality of pawls. 
     In a preferred realisation the invention comprises a fall arrest device comprising a speed responsive engagement device as described herein together with an elongate supporting means wound around a drum, the speed responsive engagement device being arranged to respond to rotation of the drum in a direction unwinding (paying out) the elongate support means. 
    
    
     
       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  is a schematic side view of a fall arrest safety system incorporating a speed responsive engagement device according to the invention; 
         FIGS. 2 to 4  are schematic side views of the arrangement of  FIG. 1  in progressive sequential orientations positions during normal unwinding (paying out) rotational orientation; 
         FIG. 5  is a schematic side view similar to the view of  FIGS. 1 to 4  with the pawl of the speed responsive engagement device orientated to the braking engagement position with respect to the ratchet wheel; 
         FIGS. 6 to 8  are schematic side views of the arrangement of the preceding progressive sequential orientations positions during rewinding following the braking engagement situation as shown in  FIG. 5 ; 
         FIG. 9  is an end view of the fall arrest safety system of the preceding figures; 
         FIG. 10  is a close up view of the orientation of  FIG. 8 . 
     
    
    
     Referring initially to  FIGS. 1 ,  2  and  9 , a fall arrest system  1  includes a speed responsive arrangement arranged to control rotation of a drum  40  around which a safety line (not shown) is wound. The speed sensitive arrangement comprises a toothed ratchet wheel  23  (which is mounted on a shaft  21  to rotate with the drum  40 ) and a pair of pawls  25  which are arranged at diametrically opposed positions on either side of the ratchet wheel  23 . Only one of the pawls  25  is shown in the drawings. A pair of pawls  25  is used to improve safety; the system could however function with only one pawl. The pawls  25  are each capable of independently stopping rotation of the ratchet wheel  23  and drum  40 . The pawls  25  are pivotally mounted and do not rotate with the ratchet wheel  23 . 
     The drum  40  is mounted for rotation between a pair of sideplates  42   a  and  42   b.  The speed sensitive arrangement is located between one of the sideplates  42   a  and a further sideplate  42   c  arranged parallel to the sideplate  42   a  and secured to the sideplate  42   a  by a pair of endwalls  42   d.  The pawls  25  are mounted on pivot bosses  52  for pivotal movement between the sideplates  42   a  and  42   c.  The mounting of the pawls  25  between the two sideplates  42   a  and  42   c  helps to stabilise the pawls  25 . 
     Referring to  FIGS. 2 and 3 , the operating parts of the speed responsive engagement arrangement according to the present invention are shown. The arrangement is responsive to the speed of rotation in a clockwise direction of the drum (and hence responsive to the speed of rotation of the shaft  21  and the ratchet wheel  23 ) relative to a fixed support structure of the safety apparatus (i.e. the sideplates  42   a    42   c ). 
     The ratchet wheel  23  mounted on the shaft  21  comprises a circumferential surface  29  and a plurality of identical teeth  24  arranged evenly spaced around and projecting outwardly from the circumferential surface  29 . Each tooth  24  has an undercut inner front sloping surface  24   a  and an outer rear sloping surface  24   b  (see  FIG. 4 ). The teeth  24  are shaped and spaced to leave a section of the circumferential surface  29  of the wheel  23  between adjacent teeth  24 . The outer rear sloping surface  24   b  of each tooth  24  ends in a step  24   c  down to the circumferential surface  29 . The inner front sloping surface  24   a  of each tooth  24  is arranged to define a recess  28  between the front surface  24   a  and the circumferential surface  29  of the wheel  23 . 
     The pawl  25  is mounted for pivotal movement about an axis on pivot boss  52  on the supporting structure adjacent to the ratchet wheel  23 . The pawl  25  can move in a first oscillating regime in a disengaged manner, shown in  FIGS. 1 to 4 , in which the ratchet wheel  23  and shaft  21  are able rotate relative to the fixed structure  22 . As will be described later rotation of the shaft and drum above a predetermined threshold speed causes the pawl to pivot to an extreme pivoted orientation (engagement orientation) in which the toe of the pawl brakingly engages with the ratchet wheel  23  so that rotation of the ratchet wheel  23  and shaft  21  relative to the support structure  22  (and pawl  25 ) in a clockwise direction is prevented. This is the braking engagement orientation as shown in  FIG. 5 . 
     The engagement between the pawl  25  and ratchet wheel  23  only prevents rotation of the shaft  21  in one direction, clockwise in the figures. Similarly to the prior art devices rotation of the shaft  21  in the opposite direction, anticlockwise in the illustrated embodiment, releases the engagement between pawl  25  and ratchet wheel  23 . The speed responsive engagement device according to the present invention could be made opposite handed to be responsive to rotation in an anticlockwise direction. 
     The pawl  25  (as most clearly shown in  FIG. 10 ) is arranged for pivoting movement around the axis of pivot formation  52  and has a toe end  25   a  and a heel end  25   b  arranged on opposite sides of the pivot axis. The toe end  25   a  of the pawl  25  is shaped to be able to engage with a tooth  24  of the ratchet wheel  23  when the pawl  25  is in the braking engaged position, as shown in  FIG. 5 . The lower side of the pawl between the toe and the heel  25   b  has a smoothly curved concave inner surface  25   c  so that when the pawl  25  is in the disengaged position shown in  FIGS. 1 to 4  and the ratchet wheel  23  rotates in a clockwise direction underside of the pawl is contacted by a tip of each tooth  24  of the ratchet wheel  23  so that as the ratchet wheel  23  rotates the second end  25   b  of the pawl  25  is urged outwardly. 
     Also as shown most clearly in  FIG. 10 , a leafspring  27  connects a point  27   a  on the fixed structure  22  to a point  25   f  on the pawl  25 . The leafspring  27  is held in compression so that it permanently preferentially urges the pawl  25  to rotate clockwise towards the disengaged position shown in  FIG. 10 . The clockwise rotation of the pawl  25  driven by the leafspring  27  is limited by the heel  25   b  of the pawl  25  contacting a tooth  24  of the ratchet wheel  23 . The heel  25   b  is provided with a v shaped notch  55 , the purpose of which is explained later. 
     Accordingly, when the shaft  21  and the attached ratchet wheel  23  rotates clockwise under normal operation, each tooth  24  of the ratchet wheel  23  in turn contacts the heel  25   b  of the pawl  25  and urges the heel  25   b  of the pawl  25  outward against the bias of the leaf spring  27 . As a result, the pawl  25  follows an oscillating movement as shown in  FIGS. 1 to 4 . 
     The higher the speed of relative rotation of the shaft  21  and ratchet wheel  23 , the greater the amplitude of the oscillation of the pawl  25  will be. When the speed of clockwise rotation of the shaft  21  and ratchet wheel  23  rises to a threshold speed the amplitude of the oscillation of the pawl  25  will be sufficient to pivot the pawl  25  to its extreme position, to bring the toe end  25   a  of the pawl  25  into contact with a tooth  24  of the ratchet wheel  23 . The braking engagement position as shown in  FIG. 5 . 
     When the pawl  25  is in the disengaged position and the ratchet wheel  23  rotates anticlockwise (rewinding in the line on the drum  40 ) the heel  25   b  of the pawl  25  is contacted by the outer surface of each tooth  24  of the ratchet wheel  23  so that as the ratchet wheel  23  rotates the heel  25   b  of the pawl  25  is urged outward against the bias of the leaf spring  27 . As a result, the pawl  25  follows an oscillating movement out of the unengaged position shown in  FIG. 3  towards the engaged position shown in  FIG. 4  and then back to the unengaged position shown in  FIG. 3 . The toe end  25   a  of the pawl  25  has a smoothly curved concave inner surface  25   g.    
     As explained above, rotation of the shaft  21  and ratchet wheel  23  in either direction causes contact of each tooth  24  in turn with the pawl  25 . These contacts produce a clicking sound which provides an audible indication of proper operation of the engagement device  20  to a user. 
     The toe end  25   a  of the pawl  25  has an outer end surface  25   d  shaped to cooperate with the front surface  24   a  of the tooth  24  so that when the end outer end surface  25   d  contacts front surface  24   a  of a tooth  24  the first end  25   a  of the pawl  24  is urged into the recess  28 . As a result, when the speed of rotation of the shaft  21  and ratchet wheel  23  rises to the threshold value the pawl  25  will be pivoted into the engaged position shown in  FIG. 5  where a tip  25   e  of the first end  25   a  of the pawl  25  is inserted as far as possible into the recess  28  and contacts the front surface  24   a  of the tooth  24  and the circumferential surface  29  of the wheel  23 , which extends between the teeth  24 . This engagement will brake the ratchet wheel  23  against the pawl  25  and stop further clockwise rotation of the shaft  21  and ratchet wheel  23  relative to the fixed structure  22 . 
     As this reorientation of the pawl  25  to the engaged position occurs, the heel  25   b  of the pawl is captured by a capture arrangement  60  comprising a resiliently flexible detent arm  61 . The capture arrangement is fixed to the structure plate  42   d  by means of fasteners  62 , and the detent arm  61  extends from a fixed end to a free end  61   a  which is provided with a male formation for engagement in the v shaped notch formed in the heel  25   b  of the pawl  25 . The detent arm flexes as the heel  25   b  passes by the free end of the arm. Then the heel is captured by the capture arrangement as shown in  FIG. 5 , the leaf spring  27  still acts to promote reorientation of the pawl  25  to the disengaged position but movement of the pawl to revert to the disengaged position is prevented by engagement with the tooth recess  28  and also the location of the detent arm  61  of the capture arrangement  60 . 
     If bounce occurs (or when the line is deliberately re wound) causing movement of the ratchet wheel in the anticlockwise direction, the tip  25   e  of the pawl toe end  25   a  rides along the constant radius surface until the step formation  24   c  is reached. Up until this point the pawl remains in the engagement orientation even though the tip  25   e  of the toe end  25   a  of the pawl  25  is no longer located fast in the tooth recess  28 . This is because the heel end  25   b  of the pawl  25  remains held captive by the detent of the capture device. During bounce, this degree of anticlockwise rotation is not surpassed and therefore during a fall event, once the pawl is pivoted to the engagement orientation to engage the teeth, it is not subsequently moved back to the disengaged position. 
     In other situations where for example the pawl  25  is pivoted to the engaged situation, then allowing the line to be rewound (in the anticlockwise direction of rotation of the ratchet wheel) causes the tip  25   g  of the toe end  25   a  of the pawl  25  to reach the step  25   c  ( FIG. 6 ) and then pass up along the outer surface  24   b  of a tooth  24  ( FIG. 7 ). In so doing the pawl is caused to pivot about the pivot formation  45  and the heel  25   b  of the pawl is released from being held captive by the detent arm  61  of the capture arrangement. The pawl is then urged back to the disengagement orientation by the leaf spring  27  ( FIG. 8 ). 
     The surface profile of the ratchet wheel between adjacent teeth  24  sets a threshold amount of counter rotation (anticlockwise in the embodiment) required to disengage the pawl  23  from a tooth  24 . Counter rotation by less than this threshold amount will not disengage the pawl  23  from a tooth  24 . The counter rotation causes the pawl to be pivoted by reaction against the ratchet wheel to be released from the capture arrangement. The constant biasing to restore the pawl to the disengaged position is free to act to move the pawl  25  once the pawl has been released from the capture arrangement  60 . 
     As a result, when the speed responsive engagement device  20  of the present invention is used in a fall arrest system, if a fall causes a safety line to be unwound from a drum at or above the threshold speed the pawl  25  will engage with a tooth  24  of the wheel  23 , stopping the rotation of the drum and arresting the fall. If the tension in the safety line then temporarily drops to a low value or zero because of the arrested person bouncing on the end of the safety line, or other transient effects, the resulting small anticlockwise rotation of the wheel  23  produced by the automatic rewinding mechanism will not disengage the pawl  23  from the tooth  24  and allow the person to resume their fall. Accordingly, the problem of bounce is overcome. 
     In the descriptions of the preferred embodiment set out above the use of a safety line wound around the drum is referred to. This is not essential and other forms of elongate support such as a cable or a webbing strap could be used instead. 
     In the embodiment described, the ratchet wheel is rotatable with respect to the support surface  22  and side plates  42   a,    42   c.  In an alternative embodiment the ratchet wheel could be fixed and the pawls  25  mounted for rotation with the drum  40 . 
     The above description refers to fall arrest systems for arresting a fall by a user. This is the most common application of a fall arrest system. However, the present invention can also be used in a height safety system to arrest falls by objects, for example, equipment being used or moved at height. 
     The embodiments discussed above are examples only and are not exhaustive. The skilled person will be able to envisage further alternatives within the scope of the present invention as defined by the attached claims.