You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
   The present invention relates to an improved hinge, in particular to a hinge for a lighting mast of the type which comprises an upper mast portion and a base, which upper mast portion is rotatable about a horizontal pivot axis in the base of the mast. 
   The present invention further rotates to an improved arrangement for the raising and lowering of masts, in particular lighting masts, which require to be raised and lowered for repair and routine maintenance. 
   In known mast systems, for example those described in GB Patent No 2 349 653, unauthorised lowering of the mast is prevented by the combination of a docking system preventing lowering of the mast when the docking system is engaged, a locking system manually operable selectively to allow or prevent disengagement of the docking system and a hinge operable to allow pivoting of the mast when the locking system is unlocked and the docking system is disengaged. 
   Currently available systems for raising and lowering masts of this type, for example the systems described in in GB Patent No 2 349 653, are manually operated and controlled, rather than automatic, and are suitable for use with the previously used smaller systems, but are not ideally suited for the raising and lowering of the larger systems, because of the need for manual intervention by an operator working in close proximity to the base of the mast. 
   It is an object of the present invention to provide a hinge mechanism in which the above disadvantages are reduced or substantially obviated. 
   SUMMARY OF THE INVENTION 
   The present invention provides a hinge mechanism which comprises first and second chock plates and an adjustable shaft, each of the chock plates including an aperture in the form of a figure of eight, the first and second circular portions of which are similar and are connected by a neck portion of a width less than the diameter of the circular portions and the shaft is longitudinally adjustable between a first retracted position in which the sections of the shaft aligned with each of the chock plates have a cross-section which corresponds to the circular portion of the aperture, and a second extended position in which the sections of the shaft aligned with each of the chock plates have a cross-section the diameter of which is less than the width of the neck portion. 
   In a preferred embodiment of a hinge according to the invention, the shaft comprises a set of two cam rams, one mounted at either end of a pivot shaft, each of the cam rams comprising a cylinder, a ram shaft which extends through the cylinder and is slideable therein and a circular cam plate mounted on a first end of the ram shaft, the ram shaft having a diameter less than the width of the neck portion and the cam plate having a cross-section which corresponds to a circular portion of the figure of eight aperture. 
   The present invention further provides an arrangement for the raising and lowering of a mast, which comprises an upper mast portion and a base, which upper mast portion is rotatable about a horizontal pivot axis, located in the base of the mast, which arrangement comprises
     (i) docking means for restraining the upper mast portion against rotation;   (ii) attachment means for attaching drive means for driving the docking means between a first engaged configuration wherein the upper mast portion is restrained against rotation and a second disengaged configuration wherein the upper mast portion is free to rotate and   (iii) locking means selectable via an intermediate unlocked configuration between a first locked figuration wherein the docking means is engaged and a second unlocked configuration wherein the docking means is disengaged characterised in that the locking means comprises a hinge mechanism which comprises first and second chock plates and an adjustable shaft, each of the chock plates including an aperture in the form of a figure of eight, the first and second circular portions of which are similar and are connected by a neck portion of a width less than the diameter of the circular portions and the shaft is longitudinally adjustable between a first retracted position in which the sections of the shaft aligned with each of the chock plates have a cross-section which corresponds to the circular portion of the aperture, and a second extended position in which the sections of the shaft aligned with each of the chock plates have a cross-section the diameter of which is less than the width of the neck portion.   

   In a preferred embodiment of the arrangement according to the invention the shaft comprises a set of two cam rams, one mounted at either end of a pivot shaft, each of the cam rams comprising a cylinder, a ram shaft which extends through the cylinder and is slideable therein and a circular cam plate mounted on a first end of the ram shaft, the ram shaft having a diameter less than the width of the neck portion and the cam plate having a cross-section which corresponds to a circular portion of the figure of eight aperture. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A hinge according to the invention and an arrangement for the raising and lowering of a lighting mast will now be described with reference to the accompanying drawings in which: 
       FIG. 1  is a perspective view of a mast (upper mast portion truncated) in the raised, docked and locked position, with hydraulic lift rams attached; 
       FIG. 2  is a perspective view of a pair of cam rams, with the rams retracted; 
       FIG. 3  is a view corresponding to  FIG. 2 , with the rams extended; 
       FIG. 4  is a perspective view of a hinge assembly, including a ram according to  FIG. 2 , forming part of a mast according to Figure, with the ram retracted; 
       FIG. 5  is a view corresponding to  FIG. 2 , with the ram extended; 
       FIG. 6   a  is a view of the attachment face of a mast of  FIG. 1  in the raised, docked and locked position, with hydraulic lift rams attached; 
       FIG. 6   b  is a section on the line B-B of  FIG. 6   a;    
       FIG. 6   c  is an enlarged view of the region C of  FIG. 6   b;    
       FIG. 6   d  is a side view of the mast of  FIG. 6   a;    
       FIG. 6   e  is a section on the line A-A of  FIG. 6   d;    
       FIGS. 7   a  to  7   e  are views corresponding to  FIGS. 6   a  to  6   e  in the raised, docked and unlocked position; 
       FIGS. 8   a  to  8   e  are views corresponding to  FIGS. 6   a  to  6   e  in the raised, undocked and unlocked position; 
       FIGS. 9   a  to  9   e  are views corresponding to  FIGS. 6   a  to  6   e  in the raised, undocked and locked position; and 
       FIGS. 10   a  to  10   e  are views corresponding to  FIGS. 2   a  to  2   e  in the lowered, undocked and unlocked position. 
   

   DETAILED DESCRIPTION 
   As can be seen from  FIG. 1 , a mast shown generally at  10  comprises an upper mast portion  2  and a base portion  4 . The upper mast portion  2  is in the form of a hollow tapering cylinder and is shown truncated in the drawings. 
   The base portion  4  comprises an upper base portion  6  and a lower base portion  8 . The upper base portion  6  is in the form of a substantially hollow square section box. The upper mast portion  2  extends into the upper base portion  6  and is secured thereto. The lower base portion  8  is secured to the ground (not shown). A docking latch  12  is formed at the lower end of the upper base portion  6 , for engagement with a co-operating notch  14  in the upper end of the lower base portion  8 . 
   As can be seen more clearly in  FIGS. 1 to 5 , a horizontal pivot axis  16  extends through the upper base portion  6  and terminates at each end in a cam ram  18 , each of which cam rams  18  comprises a cylinder  20 , a shaft  22  which extends through the cylinder  20  and is slidable therein and a cam plate  24  mounted on a first end of the shaft  22 . The diameter of the shaft  22  varies along its length. The second end of the shaft  22  terminates in a spigot  29  of smaller diameter than the shaft  22  and projects through an aperture in the end of the cylinder  20  remote from the cam plate  24 . One external end face only of one cam plate  24  is visible in  FIG. 1 . 
   A mounting groove  27 , the function of which will be described below, is provided in the outer wall of the cylinder  20 . The cylinder  20  is further provided in a manner known per se with an inlet  20 ′ and an outlet  20 ″ for the supply and removal of hydraulic fluid. 
   A chock plate  26 , having an aperture  28  in the form of a figure of eight, is provided on a side wall of the upper base portion  6 . A similar chock plate  26  is located on the opposite sidewall of the upper base portion  6  in alignment with the first chock plate  26 . 
   Each circular portion of the figure of eight aperture  28  has an internal diameter corresponding to the diameter of the cam plate  24 . 
   In the position shown in  FIGS. 1 ,  3  and  5 , the cam plate  24  of the cam ran  18  engages in the upper circular portion of the figure of eight aperture  28 . 
   The neck portion of the aperture  28  is sized so as to allow the shaft  22  of the cam ram  18  to pass freely between the circular portions of the aperture  28 . 
   In the configuration shown in  FIG. 2 , the cam ram  18  is in the retracted configuration, with the cam plate  24  in contact with the cylinder  20 . 
   As can be seen from  FIG. 4 , the shaft  22  extends through the cylinder  20 . The cam plate  24  is mounted on the free end of the shaft  22 . 
   In the configuration shown in  FIG. 3 , the cam ram  18  is in the extended configuration, with the cam plate  24  spaced from the cylinder  20  and supported by the shaft  22 . 
     FIGS. 4 and 5  show a cam ram according to  FIGS. 2 and 3 , in situ in a lighting mast as shown in  FIG. 1 . A mounting bracket  31  for sensors, the function of which will be described later, is located within the upper base portion  6 . 
   In the configurations shown in  FIGS. 1 ,  4  and  5 , the cam ram  18  is shown aligned vertically with the upper circular portion of the aperture  28 , which corresponds to the raised, docked and locked position of the mast  10 . 
   In the configuration shown in  FIG. 5 , the cam ram  18  is in the extended configuration, with the cam plate  24  spaced from the cylinder  20  and supported by the ram shaft  22 . 
   The upper base portion  6  of the mast  10  includes a collar  33  for mounting engagement in the mounting groove  27 . 
   In the position shown in  FIG. 1 , the cam plate  24  of the cam ram  18  engages in the upper circular portion of the figure of eight aperture  28 . 
   The neck portion of the aperture  28  is sized so as to allow the shaft  22  of the cam ram  18  to pass freely between the circular portions of the cam ram  18 . Upper U-shaped mounting brackets  32  and lower mounting brackets  34  are located on the upper base portion  6  and lower base portion  8  respectively, for receiving lift ram cylinders  36 . The left ram cylinders  36  are supplied in a manner known per se by a manifold  38  mounted on a bracket  40  secured to the cylinders  36  by means of yokes  42 . 
   In  FIG. 1 , the hydraulic ram cylinders  36  are shown in a partially extended configuration with the mast in a raised, docked and locked position. 
     FIG. 6   a  is a view of the mast of  FIG. 1  in the same configuration, but showing the attachment face  44  of the upper base portion  6 . 
   A second manifold  46 , for supply to the cam ram  18  is provided. 
   As can be seen from  FIGS. 6   c  and  6   e , the mast is provided with a plurality of sensors S 1  to S 8 , which have the following functions: 
   Sensors S 1  and S 2  together sense the start of the extension of the lift rams  36  and full closure of these rams; 
   Sensors S 1  and S 3  together sense the vertical alignment of the mast  10 ; 
   Sensors S 1  and S 4  together sense the full extension of the lift rams  36  and the start of closure of these rams; 
   Sensors S 5 , S 6  and S 7  together sense the extension and closure of the cam rams  18 ; 
   Sensors S 1 , S 2  and S 4  together sense the engagement/disengagement of the docking latch  12  relative to the notch  14  and 
   Sensor S 8  senses the proximity to the ground of the lowered mast. 
   As can be seen from  FIGS. 6   e  and  7   e , a pair of cam rams  18  are located aligned on the horizontal pivot axis of the mast  10 . 
   Each of the cam rams  18  is moveable between a first, locking position as shown in  FIG. 6   d  and  6   e , in which the cam plate  24  of the cam ram  18  is engaged in the upper cylindrical portion of the aperture  28  in the chock plate  26 , and an intermediate position, which can be seem most clearly in  FIG. 7   e , in which the cam plate  24  of the cam ram  18  projects from the side wall  30  of upper base portion  6  and the shaft  22  of the cam ram  18  extends through the aperture  28 . 
   The operation of the system will now be described with reference to  FIGS. 6 to 10  of the accompanying drawings. 
   The normal operational position of the mast  10  is shown in  FIGS. 6   a  to  6   e . In this position, the mast  10  is raised, locked by means of the cam plate  24  in engagement with the upper circular portion of the aperture  28  in the chock plate  26 . The vertical alignment of the mast  10  is checked by sensor S 2  and the extension of the lift rams  36  is sensed by sensors S 1 , S 2  and S 4 . The control system then extends the cam rams  18  until full extension is confirmed by the sensors S 6 , S 7 . 
   The position of the mast  10  is now as shown in  FIGS. 7   a  to  7   e , i.e., raised, docked and unlocked. The control system then extends the lift rams  36  so as to lift the upper mast portion  2  and the upper base portion  6  relative to the lower base portion  8  and to disengage the docking latch  12  from the notch  14 . The sensors S 1  and S 4  confirm that the disengagement is complete. 
   The position of the mast  10  is now as shown in  FIGS. 8   a  to  8   e , i.e., raised, undocked and unlocked. The control system then retracts the cam rams  18  and hence the cam plates  24  into engagement with the lower circular portion of the aperture  28  and the sensors S 5  and S 6  confirm that the retraction is complete. 
   The position of the mast  10  is now as shown in  FIGS. 9   a  to  9   e , i.e., raised, undocked and locked. The control system then retracts the lift rams  36 , and the mast is lowered about the horizontal pivot axis  16  until the ground proximity sensor S 8  senses a predetermined proximity to the ground, and retraction of the lift rams  36  is terminated. 
   The mast  60  is now in the position shown in  FIGS. 10   a  to  10   e , i.e., lowered, undocked and locked. 
   After the required repair or maintenance work has been carried out, the lowering sequence is repeated in reverse, so as to return the mast  10  to the position shown in  FIGS. 6   a  to  6   e.

Summary:
A hinge mechanism comprises first and second chock plates and an adjustable shaft. Each of the chock plates includes an aperture in the form of a figure of eight, the first and second circular portions of which are similar and are connected by a neck portion of a width less than the diameter of the circular portions. The shaft is longitudinally adjustable between a first retracted position in which the sections of the shaft aligned with each of the chock plates have a cross-section which corresponds to the circular portion of the aperture and a second extended position in which the sections of the shaft aligned with each of the chock plates have a cross-section of a diameter which is less than the width of neck portion.