Abstract:
A clamping device suitable for lifting and handling of sheet like-objects has a fixed jaw parallel to a movable jaw. The movable jaw is supported for reciprocating, linear movement towards and away from the fixed jaw responsive to a tensile actuating force. The movable jaw comprises a clamping plate inclined at an angle of about 8 to 12 degrees with respect to first and second clamping surfaces. The clamping plate maintains said angle during movement to and from the fixed jaw. A tensile force transmitting member is guided and supported at an upper location of the clamping plate that substantially aligns with or overhangs the second clamping surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
   The present application is a national phase application under 35 U.S.C. § 371 of PCT/AU03/00032, filed Jan. 14, 2003, and claims priority under 35 U.S.C. § 119 from Viet Nam application number 2-2002-00004, filed Jan. 14, 2002. 
   FIELD OF THE INVENTION 
   The present invention relates to clamping devices that may find use in hoisting and handling heavy slab, panel or sheet materials such as stone, masonry, concrete, marble, metal and the like materials. Specifically, the invention relates to an improvement for lifting clamps of the type having a rigid frame having a fixed jaw plate that provides a first clamping surface, a movable jaw plate that provides a second clamping surface and which is supported at the frame for reciprocating movement towards and away from the fixed jaw plate, and an actuating mechanism to bias the movable jaw plate towards the fixed jaw plate whereby the respective clamping surfaces abut onto opposite faces of the sheet material and frictionally clamp same against displacement. 
   BACKGROUND OF THE INVENTION 
   Heavy slabs and sheets of material are usually stored and stacked upright standing or in slightly inclined orientation. Handling of this type of materials often entails use of lifting clamps that grip the sheet (hereinafter used generically to also encompass slabs and other planar objects) at its upper edge for hoisting. Consequently, it is convenient in the following description of known devices and the lifting device in accordance with the present invention to use reference terminology such as ‘vertical’, ‘horizontal’, ‘upper’, ‘lower’ and similar when describing operation and components of such clamping devices, bearing in mind that these devices may also be used in a ‘horizontal’ or other orientation, eg as a simple clamp or a haulage attachment. Thus, unless otherwise clear in the context, such reference terms are not to be interpreted as a limitation. 
   A lifting clamp of the type with which the present invention is concerned is known from U.S. Pat. No. 5,893,595 (Corbett). The lifting device includes a rigid frame comprising vertical, parallel spaced apart side plates, the upper ends of which are rigidly secured together by four tubular cross-members. The lower portion of one of the side plates is angled away in downward orientation from the other side plate, the latter providing a fixed clamping jaw of the device. A vertically extending plate is mounted for horizontal sliding movement on the cross-members between the side plates and provides a movable jaw of the device. An actuator carriage which is disposed for guided vertical up and down movement is located between the movable jaw and the lower, angled portion of the frame side plate, whereby a set of rollers of the carriage respectively engage the facing surfaces of the movable jaw plate and the angled portion of the frame side plate. The carriage is connected to a strip member that extends beyond the upper end of the frame and has a lifting lug to which is attachable a lifting cable or chain. In order to lift (or otherwise handle) sheet material, the device is placed over the upper edge of the sheet so that it is received between the fixed and movable jaw plates, the carriage is raised by lifting the strip member through pulling the lifting cable upwards, whereby the carriage travels on the angled frame side plate portion and displaces the movable jaw horizontally until it abuts on the facing surface of the sheet material. Upon increasing the upward pulling force, the sheet material is frictionally clamped sufficiently tightly for it to be lifted with the device. In essence, clamping of the sheet material between the plate jaws is achieved by wedging the carriage between the frame side plate and the movable jaw, and the clamping force is maintained for as long as there is upward force being on the lifting cable. 
   A lifting clamp of similar design to that of Corbett is disclosed in German patent publication DE 199 23 788 A1 (Scheibenbogen GmbH &amp; Co KG), where, however, the mechanism employed to move the movable jaw into gripping engagement with the sheet material comprises a cam pulley instead of a wedging carriage. The pulley is mounted on the movable jaw plate with its axis of rotation perpendicular to the plane of the movable jaw plate, four rollers serving to support the pulley in parallel relationship at the facing jaw surface. Two cams are arranged on the opposite face of the pulley, concentrically with and symmetrical about the rotation axis. The cams extend equiradially over a sector of the of the pulley circumference. The height of the cams increase from near the pulley face to a maximum height that is dictated by the maximum spacing between the fixed and movable jaws of the clamp in their fully spaced apart position. The cams thus provide sloping guide and bearing surfaces for respective actuator rollers that are secured in fixed relationship on the frame plate of the device that faces the pulley. An actuator cable that is secured to the pulley perimeter is used to rotate the pulley, the actuator cable advantageously terminating in a hoop on which a hoisting cable or chain may be attached. In operation of the clamp, when a pulling force is exerted on the actuator cable, rotation of the pulley causes the actuator rollers to travel along the inclined (or curved) bearing surfaces of the cams, thereby displacing the movable jaw away from the facing frame plate towards the fixed jaw and clamping sheet material received between the jaws. 
   One disadvantage that has been observed with lifting clamps made in accordance with Corbett as well as Scheibenbogen is the tendency of the clamp frame, on lifting of in particular thin sheet material from the ground, of rotating into a different spatial attitude from the initially given “no load” attitude, the later being characterised by the substantially vertical orientation of the clamping surfaces of the jaws when these are clamped onto a sheet of material resting upright on the ground. This rotation induces swaying and swinging of the sheet material at the lifting cable that makes precise hoisting difficult, and increases breakage risk upon hitting against objects in the vicinity of the sheet. The moment that causes this rotation is induced by the presence of an out-of-alignment force pair on initial lifting off of the sheet attached to the clamp frame (upward directed lifting force vs downward directed weight of sheet material and clamp). This movement inducing moment decreases and ceases as the clamp frame and the sheet material rotate into and eventually assume a final, slightly inclined orientation with respect to the vertical, as the respective centres of gravity of the clamp and the sheet material seek to and ultimately align themselves along the same (vertical) line along which the lifting force is being exerted. 
   This problem stems from the lay-out of the clamp as such, ie due to the presence of a fixed clamp against which a single movable clamping jaw plate is forced. With such lay out, the line along which gravity force acts on the upright sheet material locates within the clamp frame at different traverse locations, depending on the thickness of the sheet, and does not align with the point at which the upward directed hoisting force acts on the clamp frame, notwithstanding the lifting force ultimately acts on the carriage or cam pulley of the clamp actuating mechanism in close vicinity of the secured sheet material. 
   Equally, whilst the cam pulley clamping mechanism of Scheibenbogen has the advantage over Corbett&#39;s of providing a more even distribution of clamping pressure onto the sheet material, the Scheibenbogen lifting clamp has a more pronounced tendency of swaying under load, which results in difficulties in controlling movement of sheet material during transport. 
   One object of the present invention is to provide a lifting device of the aforementioned type in which the tendency of the clamp of swaying into an inclined position during lifting of sheet material is minimised. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the invention there is provided a clamping device suitable for lifting and handling of sheet like-objects, having a rigid frame with a fixed jaw that provides a first clamping surface, a movable jaw that provides a second clamping surface that is substantially parallel to the first clamping surface, the movable jaw being supported at the frame for reciprocating, linear movement towards and away from the fixed jaw, and an operating mechanism arranged to bias the movable jaw towards the fixed jaw in response to an actuating force being exerted on a tensile force transmitting member of the operating mechanism, thereby to abut the respective clamping surfaces onto opposite faces of an object received between the jaws and frictionally clamping same against displacement, characterised in that the movable jaw comprises a sliding plate on which the second clamping surface is provided, the sliding plate being inclined at an angle of about 8 to 12 degrees with respect to the first and second clamping surfaces and being supported at the frame to maintain said angle during movement to and from the fixed jaw, and in that the tensile force transmitting member is guided and supported at an upper location of the sliding plate that substantially aligns with or overhangs the second clamping surface. 
   It has been found that the inclined arrangement of the sliding plate and location of the force transmitting member reaction point at the sliding plate have positive effects in minimising the presence of swing-inducing moments when a slab or sheet material of relatively small thickness is hoisted with the clamp, as the reaction point of the lifting force at the sliding plate is brought in closer alignment with the line along which gravity exerts force on the sheet material. An inclination angle of the sliding plate of about 11 degrees renders particularly good results in minimising the gravity and lifting force induced moment at the clamp frame, providing thus better lift/hoisting control. 
   Advantageously, the operating mechanism of the lifting clamp comprises a cam pulley system whose operating principle (but not necessarily its constructional layout) is similar to the one described in above mentioned German document, the contents of which is incorporated herein by way of cross-reference. Such actuating system provides for a more uniform and even clamping load distribution as compared to the solution of Corbett. 
   Accordingly, in accordance with a preferred implementation of the invention, the operating mechanism includes a drive pulley that is mounted on the sliding plate of the movable jaw with its axis of rotation perpendicular to the plate plane, a plurality (preferably four or more) of bearing rollers arranged to support the pulley in parallel relationship at the facing sliding plate surface, at least one, preferably two actuating cams protruding from the pulley surface that faces away from the sliding plate, the actuating cams being symmetrically disposed about the rotation axis of the pulley and extending along an arc sector radially inwards of the pulley circumference, the cams having a height that increases from near the pulley face to a maximum height that is related to the maximum spacing between the fixed and movable jaws of the clamp in their fully spaced apart position, the cams each defining a sloped guide and bearing surface for respective actuator rollers that are secured in fixed relationship on a stationary part of the clamp frame that faces the pulley, the arrangement being such that upon rotation of the pulley, the actuator rollers travel along the guide and bearing surfaces of the cams thereby displacing the pulley and therewith associated sliding plate along the axis of rotation of the pulley relative to the stationary clamp frame part. 
   The tensile force transmitting member used to rotate the pulley is preferably an actuator cable that is suitably secured to the pulley perimeter and partially wound on and running in a peripheral groove of the pulley. The actuator cable may advantageously terminate at its other, free end in a hoop on which a hoisting cable or chain may be attached. 
   In operation of the clamp, when a pulling force is exerted on the actuator cable, eg when hoisting a slab, rotation of the pulley causes the actuator rollers to travel along the bearing surfaces of the cams. The height increase of the cams can be constant, progressive or decreasing, ie the contour of the bearing surfaces can be rectilinear, curved or otherwise shaped, thereby to achieve a desired displacement movement of the sliding plate away from the stationary frame part, eg a frame plate, towards the fixed jaw. The sheet material received between the jaws is thus clamped with a force that is a function of the pulley diameter, the distance of the arc-shaped cams from the axis of rotation of the pulley, and the weight of the material being hoisted with the clamp. 
   When employing such cam pulley operating mechanism, the rigid frame may preferably be an assembly consisting of a side plate that provides the fixed jaw, a substantially rectangular parallelepiped shaped housing part with an open side disposed to face the side plate, whereby a bottom wall or side walls of the housing may provide the stationary frame part that supports and locates the actuating rollers that engage the cam pulley, and a plurality of (preferably four) cross struts rigidly joining the side plate and the housing part in spaced apart relation ship at upper ends thereof, the housing part having a cavity in which the cam pulley of the operating mechanism locates when the clamp is in a fully open state, eg non-clamping state, in which the movable sliding plate comes to rest adjacent the housing part and covers the open side face. The partially encased operating mechanism is thus protected from damage to its moving parts. 
   In a further preferred embodiment, the clamping device may incorporate a locking mechanism that is arranged to lock the cam pulley against rotation. The locking mechanism may be designed to be engageable only when the movable jaw is in its non-clamping position, or in any position. 
   One embodiment of the locking mechanism includes a rotatable locking plate attached to or otherwise operably connected to a handle, knob or other manual actuating member, the locking plate being supported at the housing part in a location where it can be rotated or displaced in and out of engagement with a locking receptacle provided either on the cam pulley or a support shaft thereof. A spring-loaded actuator rod that acts on the latch mechanism, located at the housing part front or side wall, is preferred. 
   A preferred embodiment of a clamping device as used for lifting slab materials in accordance with the present invention, and further features and advantages of the invention, will be described with the accompanying drawing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a  is a side view of a lifting clamp in accordance with a first embodiment of the invention, its gripping position with a sheet material suspended there from; 
       FIG. 1   b  is a front elevation of the clamp of  FIG. 1   a;    
       FIG. 2  is a longitudinal section of the clamp along line A-A of  FIG. 1   b;    
       FIG. 3  is a plan section of the clamp taken along line B-B of  FIG. 2 ; 
       FIGS. 4   a  to  4   c  are, respectively, a front elevation, a side view and a section along line D-D of a housing part of the clamp that provides a stationary clamp part and protective cover for the operating and clamping mechanism of the clamp device of  FIG. 1   a;    
       FIGS. 5   a  to  5   c  are, respectively, a front elevation, a side section along line E-E and a side view of a sliding plate that forms part of a movable jaw member of the clamp device of  FIG. 1   a;    
       FIG. 6  shows in cross-section a partial view of the cam pulley of the operating and clamping mechanism for actuating the movable jaw of the clamp device; 
       FIGS. 7   a  and  7   b  are an isometric top and bottom view of the cam pulley of the operating and clamping mechanism of the clamp device; and 
       FIG. 8  is a plan section similar to  FIG. 3  of the clamp device incorporating a second embodiment of a cam pulley locking device in accordance with an aspect of the present invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENT 
   In the following description, reference terms such as ‘upper’, ‘lower’, ‘vertical’, ‘horizontal’, ‘left’ and ‘right’ are chosen with regard to the drawing plane of the relevant figures and to aid in referencing clamp components with respect to one another. 
     FIG. 1   a  illustrates a clamping device  10  in accordance with the present invention as used for lifting and hoisting sheet-like materials  15  of substantial dimensions and weight. The expression “sheet materials” is here generically also used herein to describe slabs of stone, marble, sheet or sheets of metal tightly bundled into a stack, or the like. 
   The lifting clamp  10  illustrated in greater detail in  FIGS. 1   b  to  3  comprises substantially three subassemblies or groups, a fixed, stationary jaw section  20 , a movable jaw section  22  and a stationary housing section  24 , wherein an actuating mechanism  90  employed to move the movable jaw section  22  with respect to the fixed jaw section and housing section  24  is located between the housing and movable jaw sections. 
   Jaw section  20  and housing section  24  define a rigid clamp frame and are joined together at an upper end in spaced apart relationship by means of 4 cross bars  26  and  28 , the terminal ends of which are fixedly secured to the respective sections  20  and  24  by appropriate screws  30 ,  32 , see below. The upper pair of cross bars  28  also serve to suspend in guided manner the movable jaw section  22  so that it may be caused to slide there along upon actuation of the operating and clamping mechanism described below toward and away from the stationary jaw section  20  of the device. 
   Stationary jaw section  20  is comprised of a substantially rectangular steel plate member  34 , having four bores disposed on an upper section to receive and secure the screws for fixing the cross bars  26 ,  28  to it. A rubber glove  36  is fitted to the lower portion of steel plate member  34 . As shown in  FIG. 2 , the rubber glove  36  has a trapezoidal cross-section with a lower portion that is angled slightly inwardly to allow for easy manoeuvring of the jaw  20  onto the work piece or slab and into gripping position for lifting. Glove  36  provides a high friction material between the stationary jaw and the slab being lifted to prevent the slab from slipping or being damaged during lifting operation. 
   As best seen in  FIGS. 4   a  to  4   c,  housing section  24  includes a substantially rectangular parallelepiped shaped housing box  38 , that has a solid material upper portion  40  that is angled in side view about 9 to 12, preferably 11 degrees with respect to a lower housing portion  42 . Upper portion has formed therein four bores of stepped diameter that serve to form-fittingly receive the terminal ends of cross rods  26  and  28  and secure same against displacement therein using screws  30 ,  32  ( FIG. 2 ). Lower portion  42  includes an open cavity  44  bordered by opposing side walls  50 , a bottom wall  51 , a base wall  48  and said upper solid material portion  40 . Within cavity  40  are arranged two diametrically opposing bearing pedestals  52   a  and  52   b  that are made integral with the respective side walls  50  and the base wall  48  and protrude therefrom into the cavity  40 . Pedestals  52   a  and  52   b  serve to respectively support an actuator roller  86  (see  FIGS. 2 and 3 ) of the operating mechanism for the movable jaw  22  (described below). Not illustrated in greater detail, actuating rollers  86  are supported on respective shafts that extend substantially parallel to base wall  42 , the shafts being secured in known manner to the pedestals. Cavity  44  in lower housing portion  42  is dimensioned such as to cover and receive therein a cam pulley  94  of the operating mechanism  90  described below. A protective rubber shoe  54  covers the lower terminal end of housing box  38 . 
   As can be best gleaned from  FIGS. 1   a  and  2 , housing section  24  also carries the components of a locking or arresting mechanism  65  employed to lock the movable jaw section  22  of the clamp against axial displacement along cross rods  28 , whilst an additional arresting mechanism  60 , described in greater detail below, serves to lock the actuating mechanism  90 . 
   Turning next to  FIG. 2 and 5   a  to  5   c , the movable jaw section  22  comprises a sliding plate (assembly)  70  that includes a generally rectangular base plate  72  to which are welded at the upper width-wise edges thereof two side plates  73 ,  74  that extend perpendicular to base plate  72 . Parallel to the latter and received between side plates  73 ,  74  is welded a smaller face plate  75 . Guide tubes  76  that extend parallel to one another are received in openings in the face and base plates  72 ,  75  and welded to the plates. Ring seals  78  and a guide bush  79  are secured within the guide tubes as illustrated in  FIG. 5   b.    
   Guide tubes  76  serve the purpose of guiding and suspending the sliding plate assembly  70  in a defined orientation at the upper pair of cross rods  28  that extend between fixed jaw section  20  and housing section  24 , that is with the plate assembly  70  maintaining an inclined attitude with respect to a vertical line as shown in  FIGS. 1   a  and  2  whilst it displaces along the horizontally orientated cross rods (bars)  28 . This angle is preferably set to about 11 degrees. Heretofore, the tubes  78  are welded to the base plate  72  with their longitudinal axis inclined about 11 degrees with respect to a line that extends orthogonally with respect to the plate&#39;s plane. As can be further seen from  FIGS. 5   a  and  5   b,  an additional pair of openings  76   a  and  76   b  is respectively provided in face and base plates  72  and  75  below the guide tubes  76  thereby to allow the lower pair of cross rods  26  to pass extend through the sliding plate assembly  70  with play, as seen in  FIG. 2 . 
   As best is also seen in  FIG. 2 , a clamping plate  77 , which in longitudinal section is about triangular, is mounted in fixed manner to the lower portion of base plate  72  of sliding plate assembly  70 . The clamp plate  77  is rubber coated and defines the clamping surface of the movable jaw section  22 , wherein the inclination angle at the pointed upper end of clamping plate is about 11 degrees, such that the inclination angle maintained between guide tubes  76  and base plate  72  is off-set and the clamping surface  77   a  of clamp plate  77  is held about parallel to the clamping surface  36   a  provided at the fixed jaw plate  34 ; this parallel relation ship being maintained as the sliding plate assembly  70  moves along cross rods  28  of the clamp device  10 . 
   As best seen in  FIGS. 1   a  and  2 , a set of tension springs  80 ,  82  is located between the movable jaw section  22  and the housing section  24  such as to bias the sliding plate assembly  70  towards the housing box portion  42  and cover cavity  44 . This position represents the fully open clamp jaws position, and depending on the spring constants of the tension springs employed, see below, an additional locking member may be required to secure the open position. For symmetry reasons, four tension springs are used, located and secured in appropriate and known manner at the respective retention elements  84  at the corners of sliding base plate  72  (see  FIG. 5   a ) and receptacle bores  83  in the housing part/section  38  (see  FIG. 4 ). It is noted that the number of springs, the type of springs and the spring constants of the springs used to pull the movable jaw into the open position varies depending on several design parameters set for the lifting devices such as its size and weight, etc. 
   The sliding plate assembly  70  also carries at its lower portion a mounting axle  92  for a cam pulley  94  of the operating mechanism  90  of the clamp device, as will now be described with reference to  FIGS. 2 ,  3 ,  6  and  7  in particular. 
   Mounting axle (or shaft)  92  is welded onto the side of base plate  72  that faces away from the fixed jaw section  20 , and supports in axially fixed manner a cam pulley  94  for rotation. Cam pulley  94  has a disc portion  97  with a peripheral groove  91 , a lower circular face that is held with small clearance parallel to the facing surface of base plate  72  and a protruding hub portion  95  on the opposite face housing a pair of thrust bearings  96  that serve to support pulley  94  at shaft  92  and react forces in axial direction of the shaft. As the cam pulley  94  may be subjected to quite considerable loads acting in shaft direction (ie perpendicular to the extension of the pulley disc portion  97 ) during clamping operation, a set of four, additional bearing wheels or rollers  98  are provided to maintain the clearance gap, reduce frictional load on the shaft bearings  96  and more evenly distribute loads past the pulley  94  onto the adjoining sliding plate assembly  70 . Each roller  98  is received in a respective one of four housing openings  99  that extend through the disc portion  97  and which are located equidistantly spaced apart and with equal radius about the axis of rotation represented by the pulley shaft  92 . Rollers  98  are supported at the disc portion  97  via respective axles  101  that are press-fitted or treaded into respective radially extending sack bores  102  whose axis extend radially away from and perpendicular to the pulley&#39;s axis of rotation, such that the outer surfaces of the rollers are in rolling contact with the facing surface of base plate  72  of the sliding plate assembly  70 . 
   Cam pulley  90  includes, on the face spotting the hub portion  95 , two identical cam members  105  symmetrically located about the axis of rotation of the pulley. In plan view (as per  FIG. 3 ) cam members  105  have a curved or arcuate shape and are equiradially and concentrically spaced from the rotation axis of the pulley. Each of the two curved cam members  105  extends along an arc of at least 90 degrees and defines a sloped guiding surface  106  (see  FIG. 7   a ) on its free terminal ridge. In other words, each guiding cam surface  106  increases in height from its lowest point at  107   a,  where it is at the level with the top surface of pulley disc portion  97 , to a highest point at  107   b,  where it is farthest from the surface of the pulley disc, a planar end portion  107   c  of constant height adjoining the highest point. 
   The slope direction of the guide surface  106 , ie in clockwise or anti-clockwise direction, at the cams will be dictated by the rotational direction in which the pulley disc is rotated by an operating cable  110  received in the pulley&#39;s groove  91  in order to displace the movable jaw section  22 , ie whether the actuating cable unwinds clock or anticlockwise. In the embodiment shown in  FIG. 3 , cable  110  is wound to cause anti-clockwise rotation of pulley  94  upon cable  110  being pulled upwards. 
   As shown in  FIG. 3 , the upper portion of the base plate  72  of sliding plate assembly  70  carries a pair of guide rollers  87  in between which actuating cable  110  is guided at the upper end of the assembly  70  so that the cable extends therefrom in a line that coincides with the axis of rotation of cam pulley  94  and avoids an unsymmetrical loading of the clamp assembly  10  when hoisted (suspended). A deflection roller  88  is located below guide rollers  87 , the centre of rotation of deflection roller  88  is vertically aligned with the right upper roller  87  because, in this embodiment, the cam rotates counterclockwise when force is exerted upwardly on cable  110  in order displace the movable jaw into its clamping position. The upper portion of cable  110  is formed into a loop such that the clamp device  10  can be hooked and lifted by a lifting crane or the like (not shown) during lifting operation. The other (lower) terminal end of actuating cable  110  is removably secured to the perimeter of cam pulley  74  at a recess using known fastening techniques. 
   As noted above, the operating mechanism  90  includes the actuating rollers  86  that are mounted in fixed locations in cavity  44  of stationary housing part  38 . The location of these rollers  86  is such that these bear on and can travel along the bearing and guiding surface  106  of the respectively associated one of the cams  105 , wherein the arrangement is such that rotation of the pulley  74  causes axial displacement of the sliding plate assembly  70  (with the attached cam pulley  74 ) along the upper guide cross bars  28  of the clamp frame and coaxial with the axis of rotation of the pulley. It can be seen from the arrangement of the pulley-cam mechanism that the length and the sloping angle of the cam surface  106  maybe chosen as desired, depending on the space or gap between the gripping surfaces  36   a  and  77   a  of the fixed jaw and the movable jaw, and thus the distance the movable jaw travels from its open position toward the fixed jaw in order to clamp the thinnest slab the device is designed to grip. It should be noted also that in its closed position for gripping the thinnest slab the device is designed to lift, the actuating rollers  86  must not contact the cam surfaces at their highest point  107   b.    
   Another important point to note is that because the entire clamp device  10  is lifted using the same tensile force transmitting member that is used to cause the movable jaw assembly  22  to be displaced into a sheet material clamping engagement towards the fixed jaw assembly  20 , a locking mechanism  60  is provided which when activated prevents the cam pulley  74  from being rotated and consequently keeps the jaw members  20 ,  22  in a fixed relation to one another. The locking mechanism  60  illustrated in  FIGS. 2  and partly in  6 , is designed to maintain the movable jaw section  22  adjacent the housing section  24  irrespective of whether the tension springs  80 ,  82  intended to bias the movable clamp section  22  into the open position are weak or strong, and irrespective of an actuating force being exerted on the actuating cable  110  that would otherwise cause the cam pulley  74  to rotate. 
   The locking mechanism  60  includes a locking bar  64  that is excentrically fixed to the terminal end of the shaft of a knob  62  that extends through and is secured in axially fixed but rotation permitting manner at the bottom wall  48  of housing part  38 . Locking bar  64  is disposed to co-operate with a notched slot or groove  63  provided at the terminal free end of cam pulley support shaft  92 . The shaft  92  must hereby be long enough to allow locking bar  64  to engage slot or groove  63  of the support shaft when the movable jaw section  22  is in the fully open position. When locking bar  64  is engaged in slot  64 , the movable jaw section  22  is held stationary relative to the housing section  24 . 
   An alternative locking mechanism  160  is illustrated in the clamp device embodiment of  FIG. 8 , the clamp device otherwise remaining unchanged. The manual actuating member  161  includes a spring-loaded actuator rod  162  attached outside to the housing part  38 . A latching plate  164  is fixed within the housing part cavity.  44  for rotation about an axis parallel to the rotation axis of the cam pulley  74 . An actuator arm  163  is secured rigidly to latching plate  164  and extends through an opening in the housing part  38  into engagement with the terminal end of actuator rod  162 . The cam pulley  74  has a cut-out  166  formed in its periphery into which the latching plate may be rotated into form-locking engagement to prevent rotation of the pulley  74 . 
   Operation of the clamp device will now be described. As noted above, the entire device  10  may be hoisted through actuating cable  110  being secured to a hoisting cable. Normally, a lifting force exerted on cable  110  will cause cam pulley  74  to rotate and force the movable sliding plate assembly  70  which carries clamping plate  77  at its lower end to move toward the fixed jaw section  20  and thus close the clamp&#39;s jaws. Depending on the type of bias springs  80 ,  82  used, the movable jaw assembly  22  plate will or will not move when “normal” lifting force is initially exerted on cable  110 : If springs  80 ,  82  are relatively weak, then the cam pulley  74  of operating mechanism  90  will rotate and drive the movable jaw toward the fixed jaw. If relatively stiff (high spring constant) springs are employed, the gravity force exerted by device  10  on the cable  110  by itself will not be sufficient to overcome the bias force of the springs when the device  10  is lifted. As such, the pulley-cam mechanism  90  will not move and the movable jaw  22  will remain open and stationary until an actuating force is exerted that is greater than the weight of the device as a whole. This additional force element can be exerted by an operator pulling the clamping frame downwards over the slab to be lifted 
   When operating the clamp device with activated locking mechanism  60 , regardless of spring type employed, the locking bar or plate must be disengaged from engagement with the cam pulley in order to allow rotation thereof and close the movable clamping jaw  22 . 
   When the lifting clamp device  10  is located directly above the slab material to be lifted, a human operator will pull the device down toward the slab material whilst the hoop of actuating cable  110  is kept stationary. A downward directed pulling force exerted by an operator will causes the cam pulley to rotate, resulting in the movable jaw being displaced and moved into gripping position. The entire clamp device  10  is hereby lowered as the actuating cable  110  is unwound from the pulley. When stop cross bars provided below the cross bars  28  come into contact with the upper edge of the sheet material (the width of the slab material), the movable jaw  22  contacts the sheet and, together with the fixed jaw  20 , grips the slab for lifting and transporting. 
   Once the slab material has been moved to its intended position, the lifting force is released form the cable  110  allowing the cam pulley  74 , which advantageously is provided with a helical winding spring to induce rotation of the pulley in a direction counter the cable unwinding direction, to turn and partially wind cable  110  around pulley  74 , thereby allowing the movable jaw  22  to return to the open/rest position under the influence of the biasing springs  80 ,  82 . The lifting operation is repeated in the same manner. 
   As can be seen from the above discussion, the lifting force exerted upwardly on the upper portion of the cable  110  and the centre of gravity of the slab and the lifting device are substantially aligned vertically by inclining the movable jaw member at an angle with respect to the vertical. It has been found that an angle between 9 degrees to 11 degrees with respect to the vertical gives the best results in term of enhancing the stability of the device during lifting operation. The even distribution of the gripping force provided by the cam mechanism and the substantial alignment of the lifting force and the gravity force of the device and slab material being lifted of the improved lifting device result in the device being able to lift large and heavy slab material with less incident of breakage and/or deforming of the slab material, depending on whether the material is stone, marble or metal. Of course, modifications such as the profile of the cam surface of the cam members, the number and locations of the tension springs, the location of the pulleys, the size and cross-sectional shape of the cross bars, the material of various components of the lifting device to form a light weight yet strong enough to withstand the heavy duty performance, etc. are all within the scope contemplated by the disclosed invention.