Patent Publication Number: US-2016244943-A1

Title: Safety lifting system for a mechanical excavator

Description:
The present invention relates to lifting systems, and particularly relates to a safety lifting system for a mechanical excavator. 
     Mechanical excavators, such as diggers, JCBs, mechanical shovels and 360-degree excavators, are commonly used on construction sites and throughout heavy industry, including mining and demolition. In general, such excavators typically share the same basic design, and thus usually comprise a boom arm that is controlled via a hydraulic piston or ram (invariably referred to as the “crowning ram”) which is most often fitted to the top of the boom arm via suitable brackets. 
     The crowning ram is usually connected to a pair of tipping links, which are themselves connected to a quick-hitch (i.e. latching device) that serves to lock a tool or attachment, for example a bucket, claw or drill, to the overall boom arm assembly. Each of these components are typically connected to each other via a series of pins. 
     The tool or attachment is therefore operated by controlled movement of the crowning ram and boom arm, enabling the tool or attachment to be positioned at the exact location at which the digging, lifting, or scraping operation etc. is to be carried out. 
     However, there are certain operations which are typically better performed without a tool or attachment being connected to the boom arm, and in such circumstances it is common for one or more linkages, such as metal chains etc., to be attached to the end of the boom arm in order to allow various types of objects to be lifted. One such example occurs when there is a need to lift paving components (e.g. paving slabs and/or edging) or objects such as wooden or concrete sleepers etc. In such cases, the chains are then attached to the objects (for example via a hook and harness), which enables the objects to be lifted and transported to a desired location via controlled operation of the crowning ram and boom arm. 
     However, a significant drawback of using such chains is that these may potentially cause harm or injury if they are allowed to dangle or freely hang during times when the excavator is not in use and when no attachment is connected to the boom arm. Consequently, operatives are typically required to remove the chains from the boom arm after the lifting operation is complete. Therefore, once the chains have been removed these must then usually be stowed away on the excavator itself (e.g. via in-built storage compartments) or otherwise be transported away to a safe storage location. Difficulty arises in this operation due to the weight of the chains, which are usually in excess of 25 kg and may sometimes even weigh more than 100 kg. The reverse operation of re-installing the chains may therefore be even more problematic for an operative, as the chains need to be supported while manually raising them up to the boom arm whereupon they are reconnected to the arm via a shackle. 
     Hence, not only is this operation potentially hazardous to an operative, but it may require the assistance of several operatives in order to be achieved, which is not only wasteful of time and resources, but may also expose more operatives to possible harm or injury. Moreover, the removal and re-installation of the chains may also need to take place during conditions of inclement weather, which may pose further risk to the operative(s) while also being generally unpleasant for all concerned. 
     Hence, it is an object of the present invention to address some, if not all, of the above problems in the art, by providing an attachable and/or permanent lifting system for an excavator that allows lifting operations to be carried out without harm or injury to an associated operative. 
     According to an aspect of the present invention there is provided a safety lifting system for an excavator, the system comprising: 
     a movable linkage; 
     an actuator adapted to be coupled to the linkage for displacement of the linkage thereof; 
     an abutment member configured to limit the displacement of the linkage; and 
     a retaining assembly arranged to releasably retain a hook attached to the linkage, 
     wherein the actuator is operable to reversibly displace the linkage between a first position in which the hook is free from the assembly and a second position in which the hook is retained by the assembly. 
     The provision of a lifting system comprising, at least, an actuator which is operable to reversibly displace a linkage between a first position in which a hook is free from the retaining assembly and a second position in which the hook is retained by the retaining assembly is found to be particularly advantageous over the prior art. 
     The functionality to reversibly move and latch/lock the hook in a second position thereby provides the capability to effectively ‘stow’ the linkage away or otherwise limit its movement/displacement relative to the boom arm of the excavator. As a result of this action, the linkage is then prevented from dangling or hanging freely from the boom arm, which not only avoids the need to remove and re-install the linkage between excavator operations but also importantly minimises the risk of harm or injury to an operative associated with the excavator as there is no danger of the linkage coming into unforeseen contact with the operative. 
     By ‘excavator’ we mean any mechanical excavator, whether that be mobile or static, and/or hydraulically, electrically and/or pneumatically powered, and is particularly intended to include the non-exhaustive list of diggers, JCBs, mechanical shovels and 360-degree excavators, without limitation. 
     Moreover, it is to be appreciated that any references herein to a ‘linkage’ is intended to include all forms of chain, cables, tethers and any other segmented mechanical couplings in the form of a chain, cable or tether, which incorporate a mechanical hook or hook-like component. Most preferably, the linkages associated with this invention will be heavy gauge metal chains. 
     The lifting system of the present invention may be retro-fitted to any existing type of excavator or alternatively may be installed during initial fabrication of the excavator. Where the lifting system is retro-fitted, the system may be scaled to suit or match the size and/or configuration of the physical dimensions of the particular boom arm and excavator itself. Therefore, it should be appreciated that the present system is inherently scalable and may be used in conjunction with most, if not all, models of excavator, subject to any modifications as prescribed herein. 
     The actuator is adapted to be coupled to the linkage, such as a chain, for displacement of the linkage. The actuator is preferably attached to the boom arm of the excavator, either permanently or detachably, via appropriate couplings, such as rigid brackets and pins etc. 
     In particularly preferred embodiments, the actuator is in the form of a hydraulic piston or ram that is operable to reversibly extend and retract under the action of hydraulic pressure. In other embodiments, the actuator may be electrically or pneumatically driven, or possibly driven via a combination of all three. 
     Where the actuator is a hydraulic piston or ram, the piston/ram may be coupled directly to the existing hydraulic system of the excavator. Moreover, the control system of the excavator (usually provided within the cabin or seated area of the excavator) may also be modified to incorporate a switching gear (e.g. switch, button, level, joystick etc.) to operably control the actuator via the existing control system. Therefore, in preferred embodiments an electrical switch may be used to activate the hydraulic system to thereby extend or retract the hydraulic ram. 
     Preferably, the lifting system further comprises an engaging member coupled to an end portion of the actuator, the engaging member being configured to engage with the abutment member when the hook is free from the retaining assembly, namely when the linkage has been displaced to the first position. 
     The engaging member is most preferably in the form of a piston ram block, that is connected or otherwise coupled to the extendible end of the hydraulic ram. 
     The piston ram block may comprise a solid body, preferably including coupling means to enable the linkage to be coupled to the actuator. The body may be shaped to include a neck portion of relatively smaller dimension to that of the remainder of the body. The neck portion being that part which is preferably connected to the hydraulic ram. 
     The coupling means may be in the form of a pair of projections, that preferably project along the direction of movement of the hydraulic ram. The function of the projections is to allow the chain to be coupled to the piston ram block via a suitable retaining means, such as a pin or a bolt etc., that preferably passes through a link of the chain and through the projections. 
     The abutment member is configured to limit the displacement of the linkage. In preferred embodiments, the abutment member is in the form of a piston block housing, which preferably comprises a bore to enable the linkage to pass therethrough. In alternative embodiments, the bore may be replaced by an open topped channel or recess etc. 
     The piston block housing may comprise a solid body having a substantially inverted ‘U-shape’ or horseshoe shape. However, any other suitable shape may be used depending on the particular application and/or implementation of the lifting system. 
     The piston block housing preferably comprises an abutment surface to inhibit movement of the piston ram block, which may be extended into and retracted from the piston block housing under the action of the actuator. The abutment surface may correspond to an internal wall or other surface of the piston block housing. 
     In preferred embodiments, the lifting system may further comprise at least one guide rail for guiding the displacement of the engaging member. Most preferably, the lifting system incorporates a pair of guide rails that extend along at least part of the boom arm of the excavator substantially co-axial with the direction of movement of the actuator. 
     Hence, in a particularly preferred embodiment, the guide rails extend from the abutment member along an upper surface of the boom arm, which permits the engaging member to be located and guided along the rails, thereby enabling reliable alignment of the engaging member with the abutment member during extension and retraction of the actuator. 
     The engaging member may comprise at least one laterally extending pin or lug adapted to engage with the guide rail. Most preferably, the engaging member will comprise a pair of pins, oppositely opposed on each side of the engaging member. The function of the pins is to ensure proper engagement and alignment with the rails, which themselves preferably comprise a respective recessed channel or groove to receive a pin. However, as will be appreciated, any suitable slot and pin arrangement may be used to guide the movement and alignment of the engaging member. 
     The guide rails may be covered via any suitable means to prevent debris and contaminants (e.g. dirt, stones, dust, mud etc.) from entering into the region substantially between the rails. The covering means of the guide rails may also include a plastic liner comprising downwardly-directed fibres, fingers or stipples etc., which are intended to reduce clattering or clunking of the linkage as it passes therethrough. 
     The retaining assembly is arranged to releasably retain the hook attached to the linkage. In particularly preferred embodiments, the retaining assembly comprises an automatic gripping mechanism for retaining the hook. 
     The gripping mechanism preferably comprises a pair of pivotable surfaces biased towards each other in order to grip the hook. In an exemplary embodiment, the pivotable surfaces are in the form of a pair of spring-loaded paddles that may be deflected and which automatically press against the hook to thereby releasably grip the hook therebetween. 
     The retaining member may be in the form of a substantially flat rectangular box through which the linkage is able to pass through one end and out the other. An aperture or opening at one end is dimensioned to allow a hook to enter into, and emerge from, the box. Preferably, the spring-loaded paddles are disposed adjacent to the opening to thereby releasably grip the hook when the hook enters the box. 
     The surfaces of the paddles that contact the hook may include a respective recess to facilitate easier gripping of the hook. 
     The retaining assembly preferably further comprises a hook ejection mechanism. The hook ejection mechanism may comprise a biased plate including an aperture to enable the linkage to pass therethrough. The biased plate is preferably spring-loaded, and most preferably comprises a pair of compressible springs. 
     When the actuator retracts the engaging member from the abutment member, the linkage is drawn through the retaining assembly until the hook is engaged with the paddles in the retaining assembly. The hook preferably presses against the biased plate, which due to the compressible springs provides an ejecting force to the hook when the actuator is again extended. In this way, the biased plate effectively provides a push or kick to the hook enabling the hook to be ejected from the retaining assembly. While the hook is retained, the linkage is thereby essentially stowed, or otherwise inhibited/prevented from moving, which avoids the linkage from dangling or hanging freely with respect to the boom arm. As a result, there is then little or no risk of the linkage coming into unforeseen contact with an operative associated with the excavator, while the act of stowing the linkage avoids the need to remove and re-install the linkage between use of the excavator. 
     It is to be appreciated that by ‘stow’ or ‘stowing’ we mean that the linkage is safely held against, adjacent to, or otherwise in relation to the boom arm of the excavator and as such remains out of harms way when the attachment to the excavator has been removed. Hence, when the linkage is stowed, the linkage is displaced to the second position, in which the hook is retained by the assembly, or conversely, when the hook is retained by the assembly, the linkage is then safely stowed away. 
     The retaining assembly may be covered by any suitable means to prevent debris and contaminants (e.g. dirt, stones, dust, mud etc.) from entering into the assembly. For box-like arrangements, the cover may correspond to a simple lid or cap etc. 
     In preferred embodiments, the aperture in the biased plate of the retaining assembly may have a tapered edge to receive a reciprocally shaped neck of the hook. This is to ensure that the hook fully engages with the plate, so that the ejection mechanism provides the maximum ejection force when the actuator is extended. 
     The use of a hook with a substantially conical shaped neck is most preferable, as not only will such a hook engage fully with the biased plate, but this will also ensure that the hook lies essentially flat when it enters the retaining assembly. Preferably, the conical shaped neck is truncated such that a flat surface is present adjacent to where the moveable linkage is connected to the hook. The flat surface advantageously may facilitate full engagement with the biased plate. 
     Preferably, the conical shaped neck is narrower adjacent the linkage than adjacent the hook. Advantageously, this facilitates guiding into place of the hook within the retaining assembly. 
     Preferably, the conical shaped neck is not circular in cross-section and has a greater extension in one dimension than the other. For example, the conical shaped neck may be elliptical in cross-section or may be Obround in cross-section. 
     The retaining assembly may also include a wear plate that serves as an inclined surface (e.g. ramp) for guiding the hook into the biased plate, while also facilitating ease of displacement of the linkage through the assembly. In addition, the wear plate may also reduce any noise associated with the linkage ‘clattering’ or ‘clunking’ through the biased plate and the assembly itself. 
     As mentioned previously, the lifting system of the present invention may be retro-fitted to an existing excavator or installed as part of the fabrication of a new excavator. In some cases, it may be necessary to modify the position and arrangement of the crowning ram. Hence, it is envisaged that for at least some types of excavator, the brackets of the crowning ram may need to be increased by around 10-15 mm or more to create space for the actuator. Moreover, it may also be the case that a recess or channel be cut or drilled in the upper surface of the boom arm, again to accommodate the axial movement of the actuator. 
     Where any structural changes are made to the boom arm, one or more strengthening plates may be fixed to the arm to provide additional support. However, in most case, this would not be necessary. 
     Further modifications may also include the need to relocate/re-position one or more hydraulic feed lines and/or the pipes associated with the quick-hitch. However, all of the modifications would be well within the capabilities of a skilled person in this field. 
     To prevent wear to the boom arm, and also reduce noise, plastic wear plates or shims may be fitted to the upper surface of the boom arm to avoid the linkage from rubbing or clattering against the arm. Therefore, a shim may be fitted between the abutment member and the retaining assembly. 
     In addition, a metal wear plate may be fixed to the end of the boom arm, which preferably has upturned edges to serve as a guide for the linkage and to prevent the linkage from entering into the gap where the quick-hitch is fixed to the boom arm. 
     As will be appreciated the lifting system as described in the foregoing embodiments advantageously has few moving parts and in exemplary embodiments the retaining assembly itself requires no electronics or hydraulics for operation. Therefore, the lifting system is not only reliable but is also easy to service and maintain—thereby promoting longevity of use. 
     Moreover, the whole system typically weights around 100 kg or less, which does not affect the performance of the excavator, nor does it hinder any of its capabilities in any way. 
     The present invention also provides an excavator comprising a lifting system according to any of the preceding embodiments. 
     It is to be understood that none of the embodiments described in relation to the present invention are mutually exclusive, and therefore the features and functionality of one embodiment may be used interchangeably or additionally with the features and functionality of any other embodiment without limitation. 
    
    
     
       Embodiments of the present invention will now be described in detail by way of example and with reference to the accompanying drawings in which: 
         FIG. 1 —shows a side view of an example boom arm having associated therewith a particularly preferred embodiment of the present invention; 
         FIG. 2 —shows a side close-up view of the embodiment of  FIG. 1 ; 
         FIG. 3 —shows a top plan view of a particularly preferred embodiment of a lifting system according to the present invention; 
         FIGS. 4 a    &amp;  4   b —show respective top plan and side end views of an abutment member and engaging member according to a particularly preferred embodiment of the present invention; 
         FIG. 5 —shows a top plan view of a retaining assembly according to a particularly preferred embodiment of the present invention; and 
         FIGS. 6 a  and 6 b   —show respective side and angled views of a hook and linkage according to a particularly preferred embodiment. 
     
    
    
     Referring to  FIG. 1 , there is shown a particularly preferred embodiment of a lifting system  10  according to the present invention. The lifting system  10  is shown attached to an example excavator  30  having a boom arm  31  and a hydraulic (crowning) ram  32  fitted on top of the boom arm  31  via brackets  33 . The crowning ram  32  is connected to tipping links  34 , which are in turn connected to a quick-hitch  35  that acts to secure the tool or attachment  36  (shown in ghost lining) to the boom arm  31 , as conventionally known in the prior art. 
     The lifting system  10  comprises a movable linkage  14 , an actuator  11  adapted to be coupled to the linkage  14 , an abutment member  12  configured to limit the displacement of the linkage  14  and a retaining assembly  13  arranged to releasably retain a hook  15  attached to the linkage  14 . The skilled person will appreciate that the term “hook” encompasses any suitable mechanism for connecting loads to the moveable linkage  14 . Such suitable mechanisms may include open hooks, closed hooks (which may be referred to as an eyelet), or the like. 
     In the example of  FIG. 1 , the linkage is a metal chain  14  of heavy gauge. In other embodiments, the linkage may be other forms of flexible member. 
     The actuator  11  is in the form of a hydraulic piston or ram, that is operable to reversibly extend and retract under the action of hydraulic pressure. The hydraulic ram  11  is attached to the boom arm  31  of the excavator  30  via a rigid bracket and pin  16 . 
     The hydraulic ram  11  is coupled directly to the existing hydraulic system  37  of the excavator  30 . It is possible that some modification to the hydraulic feed lines/pipes may be required, depending on the particular implementation and/or type of excavator be used. However, the modification would not be onerous and would be well within the knowledge and expertise of the skilled person. Therefore, the lifting system of the present invention may be retro-fitted to most, if not all, excavators. 
     Moreover, the control system of the excavator  30  (usually provided within the cabin or seated area of the excavator—not shown in the figures) may also be modified to incorporate an appropriate switching gear (e.g. switch, button, level, joystick etc.) to operably control the hydraulic ram  11  via the existing control system. Again, such a modification would be a routine exercise for a skilled person. 
     Therefore, in practice an electrical switch (not shown) would be used to activate the hydraulic system  37  to thereby extend or retract the hydraulic ram  11 . 
     Referring now to  FIGS. 1 and 2 , the lifting system  10  further comprises an engaging member  11   a  coupled to an end portion of the hydraulic ram  11 , the engaging member  11   a  is configured to engage with the abutment member  12  when the hook  15  is free from the retaining assembly  13 , namely when the chain  14  has been displaced to the first position (as described in further detail below). 
     The engaging member  11   a  is in the form of a piston ram block, that is welded to the extendible end of the hydraulic ram  11 . 
     The piston ram block  11   a  comprises a solid body including coupling means  11   b  to enable the chain  14  to be coupled to the hydraulic ram  11 . As best shown in  FIGS. 3 and 4   a , the body is shaped to include a neck portion of relatively smaller dimension to that of the remainder of the body. The neck portion being that part which is welded to the hydraulic ram  11 , as shown in  FIG. 3 . 
     Referring to  FIG. 4 a   , the coupling means  11   b  are in the form of a pair of projections, that project along the direction of movement of the hydraulic ram  11  (cf.  FIG. 3 ). The function of the projections is to allow the chain  14  to be coupled to the piston ram block  11   a  via a suitable retaining means, such as a pin or a bolt etc., that passes through a link of the chain  14  and through the projections. 
     The abutment member  12  is configured to limit the displacement of the chain  14  and is in the form of a piston block housing. The piston block housing  12  comprises a bore to enable the chain  14  to pass therethrough (cf.  FIGS. 2, 3 and 4   a ). 
     As shown in  FIGS. 3 and 4   a , the piston block housing  12  comprises a solid body having a substantially inverted ‘U-shape’ or horseshoe shape. The solid body is fabricated from heavy cast steel and would be welded to the upper surface of the boom arm  31 , as shown in  FIGS. 1 and 2 . 
     The piston block housing  12  comprises an abutment surface  12   a  (as shown in  FIG. 4 a   ) to inhibit movement of the piston ram block  11   a,  which may be extended into and retracted from the piston block housing  12  under the action of the hydraulic ram  11  (cf.  FIGS. 3 and 4   a ). The abutment surface  12   a  corresponds to an internal wall of the piston block housing  12  and may be created via the casting process or else later machined after the body has been cast. 
     The lifting system  10  further comprises a pair of guide rails  17  that extend along at least part of the boom arm  31  of the excavator  30  substantially co-axial with the direction of movement of the hydraulic ram  11  (see  FIGS. 3 and 4   a ). 
     The guide rails  17  extend from the piston block housing  12  along the upper surface of the boom arm  31 , which permits the piston ram block  11   a  to be located and guided along the rails  17 , as shown in  FIG. 3 . In this way, a reliable alignment of the piston ram block  11   a  with the piston block housing  12  can be achieved during extension and retraction of the hydraulic ram  11 . 
     Referring to  FIGS. 4 a  and 4 b   , the piston ram block  11   a  comprises a pair of pins or lugs  11   c  (shown in ghost lining), oppositely opposed on each side of the piston ram block  11   a . The function of the pins  11   c  is to ensure proper engagement and alignment with the rails  17 , which themselves comprise a respective recessed channel or groove to receive a pin  11   c  (see ghost lining in  FIG. 4 b   ). 
     Although not shown in  FIGS. 3 and 4   a , the guide rails  17  may be covered by a removable access cover, such as made from a metal sheet (e.g. light steel), that is screwed or bolted onto the rails  17  to thereby avoid any debris and contaminants (e.g. dirt, stones, dust, mud etc.) from entering into the region between the rails. The covering means of the guide rails  17  may also include a plastic liner comprising downwardly-directed fibres, fingers or stipples etc., which are intended to reduce clattering or clunking of the chain  14  as it passes therethrough. 
     The retaining assembly  13  is arranged to releasably retain the hook  15  attached to the chain  14  via an automatic gripping mechanism. 
     Referring to  FIGS. 3 and 5 , the gripping mechanism comprises a pair of pivotable surfaces  13   a  biased towards each other in order to grip the hook  15 . The pivotable surfaces  13   a  are in the form of a pair of spring-loaded paddles that may be deflected and which automatically press against the hook  15  to thereby releasably grip the hook therebetween. 
     The retaining member  13  is in the form of a substantially flat rectangular box through which the chain  14  is able to pass through one end and out the other (cf.  FIG. 3 ). The box is made from steel plate, typically 4-12 mm in thickness. An aperture or opening  13   b  at one end is dimensioned to allow the hook  15  to enter into, and emerge from, the box. The spring-loaded paddles  13   a  are disposed adjacent to the opening  13   b  to thereby releasably grip the hook  15  when the hook enters the box, under the action of compressible springs  13   c.    
     The surfaces of the paddles  13   a  that contact the hook  15  may include a respective recess to facilitate easier gripping of the hook. The paddles  13   a  would be made from tempered steel. 
     The retaining assembly  13  further comprises a hook ejection mechanism. The hook ejection mechanism includes a biased plate  13   d  including an aperture to enable the chain  14  to pass therethrough (cf.  FIG. 3 ). The biased plate  13   d  is spring-loaded and comprises a pair of compressible springs  13   e.    
     During use, when the hydraulic ram  11  retracts the piston ram block  11   a  from the piston block housing  12 , the chain  14  is drawn through the retaining assembly  13  until the hook  15  is engaged with the paddles  13   a  in the retaining assembly  13  (cf.  FIG. 3 ). The hook  15  then presses against the biased plate  13   d,  which due to the compressible springs  13   e  provides an ejecting force to the hook  15  when the hydraulic ram  11  is again extended. 
     In this way, the biased plate  13   d  effectively provides a push or kick to the hook  15  enabling the hook to be ejected from the retaining assembly  13 . While the hook  15  is retained, the chain  14  is thereby essentially stowed, or otherwise inhibited/prevented from moving, which avoids the chain  14  from dangling or hanging freely with respect to the boom arm  31 . As a result, there is then little or no risk of the chain  14  coming into unforeseen contact with an operative associated with the excavator  30 , while the act of stowing the chain  14  avoids the need to remove and re-install the chain between use of the excavator  30 . 
     Although not shown for the purposes of clarity, the retaining assembly  13  may be covered by a removable lid, cover or cap to prevent debris and contaminants (e.g. dirt, stones, dust, mud etc.) from entering into the mechanism of the assembly. 
     The aperture in the biased plate  13   d  of the retaining assembly  13  may have a tapered edge (not shown) to receive a reciprocally shaped neck of the hook  15 . This is to ensure that the hook  15  fully engages with the plate  13   d,  so that the ejection mechanism provides the maximum ejection force when the hydraulic ram  11  is extended. 
     The use of a hook  15  with a substantially conical shaped neck  50  is most preferable, as not only will such a hook engage fully with the biased plate  13   d,  but this will also ensure that the hook lies essentially flat when it enters the retaining assembly  13  (cf.  FIG. 3 ). 
     As shown in  FIGS. 6 a  and 6 b   , the conical shaped neck  50  may be truncated such that it has a flat surface  52  at an end region adjacent the linkage  14 . Conveniently, the hook  15  has flat faces  53  which help to ensure that the hook lies essentially flat within the retaining assembly. In the embodiment being described, the conical shaped neck  50  has a cross section which is sometimes referred to as “obround”; a cross-section consisting of two semicircles connected by parallel lines tangent to their endpoints. 
     As shown in  FIG. 5 , the retaining assembly  13  also includes a wear plate  13   f  that serves as an inclined surface (e.g. ramp) for guiding the hook  15  into the biased plate  13   d,  while also facilitating ease of displacement of the chain  14  through the assembly  13 . In addition, the wear plate  13   f  may also reduce any noise associated with the chain ‘clattering’ or ‘clunking’ through the biased plate  13   d  and the assembly itself. 
     The conical shaped neck  50  may be narrower adjacent the linkage  14  than adjacent the hook  15 . 
     The linkage  14  may be pivotably connected to the hook  15  by means of a bar, bolt, pin or the like  54  through the conical shaped neck  50  of the hook  15 . 
     As mentioned previously, the lifting system  10  of the present invention may be retro-fitted to an existing excavator  30  or installed as part of the fabrication of a new excavator. In some cases, it may be necessary to modify the position and arrangement of the crowning ram  32 . Hence, it is envisaged that for at least some types of excavator, the brackets  33  of the crowning ram may need to be increased by around 10-15 mm or more to create space for the hydraulic ram  11 . Moreover, it may also be the case that a recess or channel of around 10-15 mm be cut or drilled and welded in the upper surface of the boom arm  31 , again to accommodate the axial movement of the hydraulic ram  11 . 
     Where any structural changes are made to the boom arm  31 , one or more metal strengthening plates (not shown) of around 5-6 mm in thickness may be fixed to the arm to provide additional support. However, in most case, this would not be necessary. 
     Further modifications may also include the need to relocate/re-position one or more hydraulic feed lines and/or the pipes associated with the quick-hitch  35 . However, all of the modifications would be well within the capabilities of a skilled person in this field. 
     To prevent wear to the boom arm  31 , and also reduce noise, plastic wear plates or shims (not shown) may be fitted to the surface of the boom arm  31  to avoid the chain  14  from rubbing or clattering against the arm. Therefore, a shim may be fitted between the piston block housing  12  and the retaining assembly  13 . The thickness of the shims would be around 5 mm. 
     In addition, a metal wear plate (not shown) may be fixed to the end of the boom arm  31 , which has upturned edges to serve as a guide for the chain  14  and to prevent the chain from entering into the gap where the quick-hitch  35  is fixed to the boom arm  31 . Again, the wear plate would be around 5 mm in thickness, with the upturned edges being about 20 mm in height. 
     As will be appreciated from the foregoing embodiments, the present invention provides a reliable and safe lifting system for an excavator that is scalable and easy to install. However, it should be recognised that one or more of the principles of the invention may also extend to other lifting systems, both mobile and static and irrespective of size, where there is a risk that a linkage or similar component may cause harm or injury to an operative associated with the system. 
     The above embodiments are described by way of example only. Many variations are possible without departing from the invention.