Abstract:
A slewing actuator system for rotating a boom structure comprises a cylindrical support member having lower and upper ends; a drive mounting member; gearing means comprising a gear assembly and at least one gear drive fixable to the drive mounting member and operatively connected to the gear assembly; and a boom support means to releasably secure the boom structure and operatively connectable to the gearing means. In an operating position, the boom support means is mounted proximate the upper end of the cylindrical support member and is rotatable about a substantially vertical axis; the gear assembly is substantially horizontal; and the gearing means imparts rotational movement to the boom support means. The system can include a pedestal surrounding at least part of the cylindrical support member, which can be a tube or kingpin. The compact modular system can be secured to various surfaces including ship decks and land surfaces.

Description:
[0001]    The present invention relates to a slewing actuator system for use with a boom, particularly for use in unloading cargo materials from a ship. 
       BACKGROUND OF THE INVENTION 
       [0002]    Booms for loading and unloading materials, whether on land or ship decks, are known, and typically are secured to a fixed supporting point and rotatable around at least a part of a circle. 
         [0003]    Such booms when intended for use secured to the deck of a ship, for use in unloading materials contained on the ship, are typically rotated (slewed) around a point on the deck. This rotating motion of the boom has been traditionally achieved by slewing actuators, with the boom structure typically being connected to the slewing actuator by trunnion pins. Typically, such a structure will slew from 90 degrees to 120 degrees in either direction to discharge material to another ship or on shore. Such range of motion generally relates to restrictions based on the space available on the ship deck, and not by any operational limits. Similar ground-surface based arrangements are also used for cargo movement. 
         [0004]    A conventional slewing actuator typically comprises a hydraulically actuated rack and pinion arrangement. In such an arrangement, hydraulically actuated steel racks move back and forth to effect rotation about the rotatable pinion, which cause the boom to move. The total length of the rack governs the amount of rotation and, typically, these arrangements can take up a large amount of space in order to operate. On some ships, and other loading and unloading locations, there may be a small clearance envelope available in which such an arrangement can be positioned, and this can limit the rotational range of the boom to, for example, 90 degrees in some situations, which may not be sufficient for unloading operations. 
         [0005]    There are other problems inherent in such slewing actuator arrangements. Firstly, slewing actuators are typically manufactured and shipped as integral units, and are not disassembled for shipping from the manufacturing facility to the shipyard or other intended use location, thus making transport by air freight or other standard means expensive and difficult, as the equipment used in these systems is very large, heavy and expensive. 
         [0006]    Secondly, such systems can be very difficult to install, particularly when intended for use on ships, as they are typically affixed to the ship deck by kingpin bushing arrangements mounted to the ship deck, in which a vertical pin is positioned through a key opening, to the hull. This requires cooperation between the key opening and the kingpin so that a proper fit is ensured. However, achieving such compatibility can be difficult since these are each usually fabricated by different manufacturers to very tight tolerances. 
         [0007]    Thirdly, conventional slewing actuators have a limited number of specific sizes available, often leading to extremely large, over-designed actuators, when the ideal size would have been in between two available sizes. 
         [0008]    It would therefore be advantageous to have a slewing actuator system suitable for use on ships, land surfaces or docks, and possessing a more compact design that allows for a greater amount of rotational range of the boom attached thereto, particularly on ships or at other use locations having a small clearance envelope. 
         [0009]    It would be further advantageous to have a slewing actuator system which is easy to install, lighter than conventional slewing actuator systems, and which can be easily disassembled into separate portions for shipping, if necessary. 
         [0010]    It would be still further advantageous to have a boom slewing actuator system which safely prevents boom slippage which securely locks the boom in place when hydraulic pressure is removed. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention discloses a rotary drive system comprising a plurality of fixed horizontally rotatable drives each of which drives a rotatable pinion gear for effecting the movement of a boom structure. 
         [0012]    The present invention safely prevents boom slippage through having an integral brake so that when hydraulic pressure is removed (or lost due to component failure), the boom remains securely locked in place. 
         [0013]    The present invention has a compact design, which may provide a greater amount of rotational range to a boom, is easy to install, and can be easily disassembled for shipping, if necessary. 
         [0014]    The present invention includes several degrees of redundancy in order to prevent a system shutdown in the event of a single component failure. 
         [0015]    The present invention is built up of several discrete components which are readily available for replacement in the event of component failure. 
         [0016]    In a broad embodiment, the invention therefore seeks to provide a slewing actuator system for rotating a boom structure and constructed and arranged to be secured to a support structure, the system comprising: 
         [0017]    (i) a cylindrical support member having a lower end constructed and arranged to be secured to the support structure, and an upper end; 
         [0018]    (ii) a drive mounting member; 
         [0019]    (iii) gearing means comprising:
       (a) a gear assembly; and   (b) at least one gear drive constructed and arranged to be fixed to the drive mounting member and operatively connected to the gear assembly; and       
 
         [0022]    (iv) a boom support means mounted to a securing means and having
       (a) a lower surface constructed and arranged to be operatively connected to the gearing means;   (b) an upper surface constructed and arranged to receive and releasably secure the boom structure; and   (c) a central region and two lateral regions having respective outer edges;       
 
         [0026]    wherein, when the system is in an operating position, 
         [0027]    (A) the boom support means is rotatably mounted proximate to the upper end of the cylindrical support member and is rotatable about a substantially vertical axis of rotation; 
         [0028]    (B) the gear assembly is positioned substantially horizontally; and 
         [0029]    (C) the gearing means imparts rotational movement to the boom support means. 
         [0030]    In a first more specific embodiment, the invention seeks to provide a system wherein the cylindrical support member is a tube, having an upper portion and a lower portion, and the boom support means is rotatable about the upper portion of the tube. 
         [0031]    Preferably, the system further comprises a pedestal constructed and arranged to surround the cylindrical support member substantially concentrically for at least a portion of a height of the cylindrical support member, and preferably the pedestal comprises a substantially cylindrical inner wall and an outer wall having a cross-sectional configuration of a regular polygon. 
         [0032]    In this embodiment, the gear assembly can be connected to the boom support means, and be rotatable about the tube, thus rotating the boom support means. Alternatively, the gear assembly can be fixedly connected to the tube, and the at least one gear drive rotates around the gear assembly and thereby rotates the boom support means. 
         [0033]    In a second more specific embodiment, the invention seeks to provide a system wherein the cylindrical support member is a solid kingpin, and the gear assembly is connected to the boom support means, and is rotatable about the kingpin, thus rotating the boom support means. 
         [0034]    Preferably, the support structure is selected from a base plate, a pedestal structure comprising a base plate, a pedestal structure comprising at least one support plate, a wharf, a stationary dock, a floating dock, and a ship deck. 
         [0035]    Where the cylindrical support member is a tube, it is preferably constructed and arranged to be secured at its lower portion to the support structure, and the securing means comprises the pedestal which is constructed and arranged to rotatably surround at least the upper portion of the tube and to be secured to the lower surface of the boom support means. Alternatively, where the support structure is a pedestal comprising a support plate, the tube is constructed and arranged to be secured to the support plate. 
         [0036]    Where the system includes a pedestal secured to the support structure, preferably the drive mounting member is constructed and arranged to be mounted on and secured by the upper surface of the pedestal. Alternatively, the drive mounting member can be mounted proximate a lower portion of the pedestal. 
         [0037]    Preferably each gear drive comprises an integral brake. More preferably, the system comprises a plurality of gear drives each fixed substantially equidistantly from the vertical axis of rotation of the boom support means, in which case preferably each of the plurality of gear drives shares a common power source, selected from hydraulic and electrical power. 
         [0038]    Preferably, the gear assembly has a proximal end constructed and arranged to be secured to the cylindrical support member; alternatively, the gear assembly has a proximal end constructed and arranged to be secured to the cylindrical connector. 
         [0039]    Preferably, the gear drive comprises a pinion gear, and the gear assembly comprises 
         [0040]    (i) a rotating gear constructed and arranged to be operatively driven by each pinion gear; and 
         [0041]    (ii) a gear plate in interlocking engagement with the rotating gear. More preferably, the gear plate has a plurality of spaced-apart openings from its upper surface through to its lower surface. 
         [0042]    Preferably, the boom support means comprises a central opening from its upper surface through to its lower surface and is constructed and arranged to rotatably surround the upper end of the cylindrical support member, and more preferably a cap plate is secured to the upper surface of the boom support means over the central opening. Further, the boom support means preferably also comprises at least one low friction bushing between the central opening and the upper end of the cylindrical support member. 
         [0043]    Preferably, the boom support means comprises a trunnion weldment and each lateral region includes a trunnion pin. More preferably, each trunnion pin is operatively connected to one of a pair of boom hubs each constructed and arranged to be operatively connected to the boom structure. 
         [0044]    Preferably, the cylindrical support member has an outer surface which includes an annular retaining location constructed and arranged to receive and support the lower surface of the boom support means, and more preferably the annular retaining location is selected from a protruding ledge and a detent provided at the outer surface of the cylindrical support member. Further, at least one low friction thrust washer is preferably provided between the annular retaining location and the boom support means. 
         [0045]    Preferably, the drive mounting member is constructed and arranged to be supported at its lower surface at least proximate its lateral edges by a plurality of wing support members connected to the pedestal. 
         [0046]    Preferably, the system further comprises an internal support ring which at least partially encloses the cylindrical support member proximate its upper end. More preferably, the cylindrical support member also has an internal reinforcing disc which substantially encloses the lower end of the cylindrical support member. 
         [0047]    Preferably, the system further comprises a retainer plate secured to the pedestal at a lower portion. 
         [0048]    Optionally, the system can be configured so that a horizontal distance between the vertical axis of rotation and each lateral edge of the boom support means exceeds a horizontal distance between a centre and an outer limit of the gear assembly. 
         [0049]    The structural system of the invention and its modular construction allow for each component to be easily shipped in conventional and relatively inexpensive fashion and then easily assembled during installation at the intended end use location, or subsequently disassembled for removal to another location. Similarly, maintenance and repairs are substantially simplified. As discussed further below, the features of the present invention result in a slewing actuator system suitable for use on ships, land surfaces or docks, with a more compact design, allowing for a greater amount of rotational range of the boom, and at the same time safely preventing boom slippage, and which securely locks the boom in place when hydraulic pressure is removed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0050]    The embodiments of the present invention will now be described by reference to the following figures, in which identical reference numerals in different figures indicate identical elements and in which: 
           [0051]      FIG. 1  is a vertical cross-sectional view of a first embodiment of the invention; 
           [0052]      FIG. 2  is an enlarged cross-sectional view showing the connection of the cylindrical support to the boom support means in the embodiment of  FIG. 1 ; 
           [0053]      FIG. 3  is a sectional view along the lines in  FIG. 1 ; 
           [0054]      FIG. 4  is an enlarged cross-sectional view of the gear assembly of the invention; 
           [0055]      FIG. 5  is a vertical cross-sectional view of a second embodiment of the invention; 
           [0056]      FIG. 6  is a sectional view along the lines VI-VI in  FIG. 5 ; 
           [0057]      FIG. 7  is a vertical cross-sectional view of a third embodiment of the invention; 
           [0058]      FIG. 8  is an enlarged cross-sectional view showing the connection of the kingpin to the boom support means in the embodiment of  FIG. 7 ; 
           [0059]      FIG. 9  is a top view of the third embodiment, taken along the lines IX-IX in  FIG. 7 ; and 
           [0060]      FIG. 10  is a sectional view along the lines X-X in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0061]    The invention will be described for the purposes of illustration only in connection with certain embodiments; however, it is to be understood that other objects and advantages of the present invention will be made apparent by the following description of the drawings according to the present invention. While a preferred embodiment is disclosed, this is not intended to be limiting. Rather, the general principles set forth herein are considered to be merely illustrative of the scope of the present invention and it is to be further understood that numerous changes may be made without straying from the scope of the present invention. 
         [0062]    Referring to  FIG. 1 , a first exemplary embodiment of the boom slewing actuator system  1  of the invention is shown in an operational position. The slewing actuator system  1  comprises a pedestal  10 , having an inner circular wall  12 , and an outer wall  14 , preferably having a cross-sectional configuration of a regular polygon, the pedestal  10  being mountable on a support, shown here as a base plate  13  affixed to the support  2 , such as a ship deck or a dock. Affixed to the upper surface  90  of the pedestal  10  is a drive mounting plate  20 , having a central opening providing a tube-receiving location  21 . At an upper surface of the drive mounting plate  20  adjacent the central opening, a support plate  80  contributing to the tube-receiving location  21  is affixed, for example secured by bolts  23 . Similarly mounted on an upper surface of the support plate  80  is a stiffening ring  81 . 
         [0063]    A cylindrical support, in this embodiment a tube  40 , is affixed with its lower end  42  secured by the perimeter of the tube-receiving location  21  provided by the central openings in the drive mounting plate  20  and the support plate  80 . 
         [0064]    Referring also to  FIG. 2 , the upper portion  41  of the tube  40  is provided with a mounting ring  82 , secured to an upper surface of the stiffening ring  81 . A boom support means, shown here as a structure known in the art as a trunnion weldment  50 , having a central cylindrical opening  66 , is mounted in a clearance fit over the upper portion  41 of the tube  40 , being dimensioned to be selectively rotatable about the tube  40 , and secured in the rotatable position by the cooperation of shoulders  53  adjacent the lower edge of the trunnion weldment  50 , the tube  40  and the stiffening ring  81 . Bushings  54  and thrust washers  52  are provided as shown between the trunnion weldment  50  and the tube  40 , to allow for smooth rotation of the trunnion weldment  50  and reduce wear. The upper portion  41  of the tube  40  is preferably covered by a very thin outer tube  46 , preferably of stainless steel, in order to provide the appropriate surface hardness required for the bushings  54 . Lubrication of the regions of contact can be effected through lubrication fittings  55 . 
         [0065]    For additional strength and stability, and to maintain the integrity of the load-bearing tube  40 , it can be provided with one or more reinforcing rings, shown in  FIG. 2  as upper and lower reinforcing rings  43  and  45  respectively, the upper reinforcing ring preferably comprising a disc  48  to seal the upper end of the tube  40 . 
         [0066]    At each lateral portion of the trunnion weldment  50 , trunnion pins  57  are provided to enable the securing of a boom hub  60 . A proximal end  58  of each trunnion pin  57  is secured to the trunnion weldment  50  by locking pins  59 , and the boom hub  60  is mounted on a distal end  61  of each trunnion pin  57 , each of the boom hubs  60  having respective ends of the selected boom structure (not shown) secured and mounted thereon. 
         [0067]    Within the intermediate portion  51  of the trunnion weldment  50 , reinforcement means can be provided, preferably steel stiffeners  65 . 
         [0068]    A cap plate  62  is secured, for example by bolts  56 , to the upper surface of the trunnion weldment  50  adjacent the central opening  66 , so as to cover the top of the central opening  66 , and inhibit or prevent water, dirt or other particulates from entering the central opening  66  and coming into contact with or potentially inhibiting the movement of trunnion weldment  50  around the tube  40 , or becoming lodged in spaces between the tube  40  and thrust washers  52  and bushings  54 . 
         [0069]    The pedestal  10  has vertical stiffeners, such as stiffening wings  11 , secured to the outer pedestal wall  14 , and extending radially outward from the central vertical axis  18  of the system, to provide support to the drive mounting plate  20  at its lower surface along a desired distance, which may be up to the outer perimeter of the drive mounting plate  20 , while not interfering with any portion of the plurality of gear drives  30 , discussed further below, as may extend below the drive mounting plate  20 . 
         [0070]    Referring now also to  FIG. 3  together with  FIG. 1 , the drive mounting plate  20  comprises a substantially planar surface having a plurality of openings  22  in spaced-apart relation a constant distance radially outward from the central vertical axis  18 . The thickness (in the vertical direction when in operation) of the drive mounting plate  20  is selected so as to minimize the weight of the plate, while providing sufficient strength for its support functions. Increased thickness can advantageously be provided in the region between the central opening and the location of the outer wall  14  of the pedestal  10 , for example as shown at  25 . 
         [0071]    Referring again to  FIG. 1 , each of the plurality of gear drives  30  is a rotational drive system and is adapted to be mounted in one of the openings  22  provided in the drive mounting plate  20 , so as to extend downwardly and substantially perpendicular to the drive mounting plate  20 . Each of the gear drives  30  mounted on the drive mounting plate  20  drives a corresponding pinion gear  36  which meshes with the gear assembly  70  (discussed in more detail below in relation to  FIG. 4 ), whereby the pinion gears  36 , when driven by the gear drives  30 , cause the gear assembly  70  to rotate, and thus to provide the desired selected rotation of the trunnion weldment  50 . 
         [0072]    Although a substantial portion of each of the gear drives  30  extends downwardly from the drive mounting plate  20 , an upper portion  31  of the gear drives  30  extends upwardly above the drive mounting plate  20 . 
         [0073]    Referring now to  FIG. 4 , the gear assembly  70  comprises a connecting ring  76 , a gear pedestal plate  71  and a large circular gear  72 . 
         [0074]    The connecting ring  76  is rigidly secured on an underside of the trunnion weldment  50 , preferably by welding, such that it descends downwardly from the trunnion weldment  50  and is positioned, when the trunnion weldment  50  is mounted upon the tube  40 , around an outside circumference of both the central opening  66  in the trunnion weldment  50  and the tube  40 . 
         [0075]    The connecting ring  76  is secured to the gear pedestal plate  71  by any suitable means, such as by pins or bolts (not shown), and the gear pedestal plate  71  supports the large circular gear  72 . 
         [0076]    The gear pedestal plate  71  comprises a large substantially planar surface having a central opening  73  (see  FIG. 1 ), which is configured to be coaxial, in an operational position, with the central vertical axis  18 . 
         [0077]    Referring again to  FIG. 3 , the gear pedestal plate  71  has a plurality of inner bolt circles  74  defined thereon, positioned proximate to an outside circumference of the central opening  73  in the gear pedestal plate  71 . These inner bolt circles  74  correspond to openings  78  in the lower surface of the connecting ring  76 , as seen in  FIG. 4 , whereby the gear pedestal plate  71  can be secured to the connecting ring  76  by any suitable means, such as pins or bolts (not shown). 
         [0078]    The gear pedestal plate  71  also has a plurality of outer bolt circles  75  positioned thereon proximate its outer circumference, corresponding to securing openings  79  provided in the circular gear  72 , whereby the plate  71  can be secured to the circular gear  72  by any suitable means, such as by pins or bolts (not shown). 
         [0079]    Preferably, a plurality of holes  77  extend throughout the surface of the gear pedestal plate  71 , so as to reduce the weight of the plate  71 , and to provide for drainage. These holes  77  are arranged around a circumference of the gear pedestal plate  71 , and between the inner bolt circles  74  and the outer bolt circles  75 . 
         [0080]    Referring to  FIGS. 1 and 4 , the circular gear  72  meshes with the pinion gears  36 , and extends around an outside circumference of the plate  71 . The circular gear  72  has a thickened lower portion having securing openings  79  therein, upon which a portion of the plate  71  near to and inside its outer circumference can be secured. In operation, the pinion gears  36 , when driven by the gear drives  30 , cause the circular gear  72  to rotate, which correspondingly causes the trunnion weldment  50 , and the boom structure rigidly secured thereto, to rotate around the tube  40  to a desired position, without interference from the gear drives  30  positioned underneath the trunnion weldment  50 . 
         [0081]    Preferably, all of the pinion gears  36  are simultaneously supplied from a suitable common power source, including hydraulic power and electric power. If the common power source is hydraulic, so that the pressure between the pinion gears  36  will automatically equalize, thus keeping the pinion gears  36  in synchronization. 
         [0082]    Additionally, each of the gear drives  30  preferably has a normally locked in line, spring applied integral brake  32  (see  FIG. 1 ). When there is no hydraulic pressure, the springs within the brake will lock the drive shaft, preventing it from rotation. When hydraulic pressure is applied, the spring pressure will be counteracted, and the lock will release. In the absence of hydraulic power, there is no further rotation of the pinion gears  36  or unloading boom structure indirectly attached thereto, so that the boom structure at all times remains positively locked in place, thus preventing slippage of the boom, without the application of hydraulic power. 
         [0083]    Referring now to  FIG. 5 , a second exemplary embodiment  501  of the boom slewing actuator system of the present invention is shown. The configuration of this embodiment is particularly advantageous in situations where there is limited clearance space available on the support surface, such as a ship deck or a land surface. In this embodiment, the gear drives  30  and the pedestal  10  are inverted from their respective positions in the first embodiment discussed above in relation to  FIGS. 1 to 4 , to provide a low clearance solution and a more compact design. 
         [0084]    Furthermore, unlike its position in the first embodiment, the trunnion weldment  550  is rigidly secured directly to the pedestal  510 , the trunnion weldment  550  and the pedestal  510  combining to form a single unit that is rotatably mounted upon, and substantially covers, the tube  540 . 
         [0085]    The cylindrical support, shown here as tube  540 , is preferably constructed of steel and rigidly mounted on the deck or other support surface (not shown), either directly or, as shown in  FIG. 5 , to a supporting platform  503 , which is in turn mounted to an intermediate support structure  502 , which is itself rigidly mounted on the support surface. The tube  540  is preferably connected to the support structure  502  by welding, though it will also be readily apparent to one skilled in the art that other means could be used. Support platform  503 , which is optionally positioned between the support structure  502  and the tube  540 , serves to provide a stable flat surface on which to mount the tube  540 . 
         [0086]    In this embodiment, the tube  540  consists of lower, middle and upper portions, respectively  541 ,  542  and  543 , the outermost diameter of each of the lower portion  541  and the upper portion  543  being slightly greater than the outermost diameter of the middle portion  542 . Preferably, such difference is achieved by thickening the lower portion  541  and upper portion  543  relative to the middle portion  542 . These thickened portions bear a greater share of the load on the tube  540  imposed by pedestal  510  being rotatably mounted thereon, as discussed below, while the reduced thickness of the middle portion  542  reduces points of contact with the pedestal  510  for ease of installation, and incidentally results in cost savings associated with the types of bushings  516  which can be used. 
         [0087]    The upper portion  543  of the tube  540  has an annular internal support ring  544  formed at the top thereof, partially enclosing (but for a central opening  545 , which acts as a sighting and alignment hole) the interior of the upper portion  543  of the tube  540 . Similarly, the lower portion  541  of the tube  540  preferably also has an internal reinforcing disc  546  which can enclose the lower portion  541  proximate the top of that portion, but for a central opening  547 . 
         [0088]    Preferably, the support ring  544  and the reinforcing disc  546  are both made of steel. The dimensions and configuration of the support ring  544  and reinforcing disc  546  can be varied, as appropriate, corresponding to the amount of load expected to be borne by the tube  540 . 
         [0089]    In the operating position, the pedestal  510  slides over and surrounds the tube  540 , in a loose fit, the pedestal  510  having a vertical cylindrical cavity  512  defined therethrough by an interior pedestal wall  519  which is coaxial with the central vertical axis  18 . 
         [0090]    At the upper surface of the pedestal  510 , support plates  515  are affixed, on an upper surface of which a mounting portion  525  is secured, to receive and secure the trunnion weldment  550 . At the upper and lower ends of the pedestal  510 , bushings  516  are fixed to the interior of the interior pedestal wall  519 , to cooperate in the operating position with the upper portion  543  and bottom of the lower portion  541  of the tube  540 , respectively. 
         [0091]    In this manner, the pedestal  510  may be rotated about the vertically extending tube  540 , without interference from the support structure  502  upon which the tube  540  rests. 
         [0092]    Preferably the pedestal  510  has vertical stiffeners, such as stiffening wings  514 , secured to the interior pedestal wall  519 , and extending radially outward from the central vertical axis  18  of the system, to provide support to the drive mounting plate  520  at its upper surface along a desired distance, which may be up to the outer perimeter of the drive mounting plate  520 , while not interfering with any portion of the plurality of gear drives  30 , discussed further below, as may extend above the drive mounting plate  520 . 
         [0093]    The pedestal  510  preferably also has a steel stiffening band  513  formed internally within an upper portion of each of the vertical stiffeners  514 , which extends laterally across a partial internal width of the vertical stiffeners  514 . The stiffening band  513  provides additional support to the pedestal  510  in handling loads placed upon the pedestal  510  by trunnion weldment  550 . Similarly, the interior pedestal wall  519  of the pedestal  510  can also be thickened or reinforced if desired. 
         [0094]    The interior pedestal wall  519  of the pedestal  510  extends at its lower edge  561  below the drive mounting plate  520 , so as to provide sufficient contact area for the lower bushing  516 . At the edge  561 , a securement ring  562  can be attached to assist in maintaining the desired positioning of the associated bushing  516 . 
         [0095]    The inverted positioning of the gear drives  30  in this embodiment, as compared with the embodiment shown in  FIG. 1 , thus permits a lower clearance structure, and allows the boom to be mounted closer to the support surface such as a ship&#39;s deck, with a lower centre of gravity and, in the case of mounting on a ship or other movable support, a correspondingly increased stability despite any movement of the support surface. 
         [0096]    The drive mounting plate  520  comprises a large substantially planar surface, and is secured to a lower surface of the pedestal  510 , preferably by welding. The drive mounting plate  520  supports and positions the gear drives  30 , as discussed below, and has a central opening  523  coaxial, in an operational position, with the central vertical axis  18 . The central opening  523  is dimensioned so that the drive mounting plate  520  abuts the interior pedestal wall  519 . 
         [0097]    In the same manner as for drive mounting plate  20  in  FIG. 1 , drive mounting plate  520  comprises a plurality of openings  22  in spaced apart relation a constant distance radially outward from the central opening  523 , each sized to accept and support one of the plurality of gear drives  30 . In this embodiment, a substantial portion of each of the gear drives  30  will extend vertically above the drive mounting plate  520 , while a lower portion  35  of the gear drives  30  and the gear assembly  570  will extend below the drive mounting plate  520 . In the illustrated embodiment, as shown in  FIG. 6 , four separate gear drives  30  are shown, though it will be readily apparent to one skilled in the art that variations as to the actual number of gear drives present can be made. 
         [0098]    Although the gear drives  30  are identical to those shown in 
         [0099]      FIG. 1 , they are each provided with a mounting portion  533  which has a greater diameter than that of the openings  22 , to provide a mounting surface for the gear drives  30 . 
         [0100]    As can be seen from  FIG. 5 , the gear assembly  570  is substantially identical to gear assembly  70  shown in  FIGS. 1 and 2 , and comprises a gear ring support  572 , a gear pedestal plate  571  and a large circular gear  72 . 
         [0101]    With reference also to  FIG. 6 , gear pedestal plate  571  has a plurality of inner bolt circles  74  defined therealong which surround an outside circumference of the central opening  73  in the gear pedestal plate  571 , whereby openings (not shown) in the gear ring support  572  are aligned with the inner bolt circles  74  to secure the gear pedestal plate  571  to the gear ring support  572  by any suitable means, such as pins or bolts (not shown). 
         [0102]    The gear pedestal plate  571  also has a plurality of outer bolt circles  75  positioned thereon proximate its outer circumference. These outer bolt circles  75  correspond to securing openings (not shown) in the circular gear  72 , whereby the gear pedestal plate  571  can be secured to the circular gear  72  by any suitable means, such as pins or bolts (not shown). 
         [0103]    Gear ring support  572  is connected, preferably by welding, to an outside surface of lower portion  541  of the tube  540 , such that it extends outwardly therefrom while, at the same time, being vertically positioned at a sufficient height whereby there is no interference with rotation of the pedestal  510 , and correspondingly the trunnion weldment  550 , about the tube  540 , from any of the support structure  502  or the support platform  503 . 
         [0104]    In operation, none of the components of the gear assembly  570  rotates, but instead the pinion gears  36 , when driven by the gear drives  30 , rotate and move about the stationary circular gear  72  in a desired direction, causing the pedestal  510 , trunnion weldment  550  and the boom structure (not shown) attached thereto, to correspondingly rotate about the tube  540  and be positioned where desired. The inverted gear drives  30 , by virtue of their connection to the drive mounting plate  520  secured to the pedestal  510 , will also likewise correspondingly rotate about the tube  540  when the pedestal  510  rotates about the tube  540 . 
         [0105]    The rotational range of the pinion gears  36  in moving around the stationary circular gear  72 , and the corresponding rotational movement of the boom structure (not shown) is only a portion of a complete revolution. Nevertheless, the rotating portion is not restricted from so doing by the space available on the support surface (not shown) as is the case with conventional slewing actuator systems. In this embodiment, rotation of such a structure may be restricted and in a range of 180-200° from a center position. Since the boom structure is constrained from complete rotation, the hydraulic lines (not shown) connected to the gear drives  30  will not get crossed during the slewing motions. 
         [0106]    Preferably, the gear drives  30  are powered and supplied with an integral brake in the same manner as for the first embodiment, as discussed above. 
         [0107]    This embodiment thus provides a smaller support structure, and a vertically more compact design than the first embodiment or known structures. 
         [0108]    Referring again to  FIG. 5 , the trunnion weldment  550  is mounted onto the pedestal  510 , and, as with the first embodiment described in relation to  FIGS. 1 to 4 , is used to support a boom structure (not shown). However, in this embodiment, the trunnion weldment  550  is preferably constructed as a substantially solid piece. In this manner, when secured to the pedestal  510 , the trunnion weldment  550  effectively covers the top of the tube  540 , thus rendering superfluous the cap plate  62  (shown in  FIG. 1 ). 
         [0109]    As noted above, mounting portion  525  of the trunnion weldment  550  is mounted and secured to support plate  515  by suitable means such as bolts  555 , and is thereby rigidly secured to and covers the pedestal  510 . 
         [0110]    Thrust washers  560  are provided between a lower surface of cylindrical mounting portion  525  of the trunnion weldment  550  and an upper lip  549  of the tube  540 , so as to provide a low friction sliding surface for the rotation of the pedestal  510  and the trunnion weldment  550  about the tube  540 . 
         [0111]    The lateral portions of the trunnion weldment  550  are preferably constructed in the manner described in relation to the embodiment shown in  FIG. 1 , and reinforcing stiffeners  65  are preferably also provided in the same manner as described above. 
         [0112]    Referring now to  FIGS. 7 to 10 , a third embodiment  701  of a boom slewing actuator system of the invention is shown. 
         [0113]    This embodiment has a substantially similar construction to that of the first embodiment, in relation to the features, configuration, orientation and securing of each of the pedestal  10 , drive mounting plate  20 , gear drives  30 , gear assemblies  70 , and the general construction of the trunnion weldment  50 , so that these will not be described further in relation to this embodiment. 
         [0114]    However, instead of the tube  40  of the first embodiment, this third embodiment comprises a substantially cylindrical kingpin  740 , which is secured directly or indirectly to the support surface  2 , as discussed below, and is secured to the drive mounting plate  20  at the upper surface of pedestal  10 . The kingpin  740  provides by means of a selected variation in its outer surface a support location on which the lower portion of the trunnion weldment  50  can rest securely, the kingpin being received in the central opening  51  of the trunnion weldment  50 , whereby the trunnion weldment  50  can rotate around the upper portion of the kingpin  740  about the central vertical axis  18 . 
         [0115]    As can be seen from  FIG. 7 , the pedestal  10  has a vertical cylindrical cavity  91  coaxial with the central vertical axis  18 , in which the kingpin  740  can be supported and retained in a vertical position. The lower edge of the pedestal  10  is preferably secured to a circular interior base wall  15  to form a cylindrical shoulder which engages a detent  41  on the kingpin  740 . For ease in installation, preferably, the circular interior base wall  15  has an opening  19 , which can be closed by a retainer plate  16  which is preferably welded in place to the interior base walls  15 . The kingpin  740  can be secured to the retainer plate by any suitable means, for example by bolts  17  through the retainer plate  16 , and can thereby be secured to the support surface  2 . 
         [0116]    When in such position, the kingpin  740  is rigidly mounted, so that unlike conventional systems, no keying system is required, and the close fit which can be achieved by this construction provides accurate mounting and secure support against tilting. 
         [0117]    The kingpin  740  is secured within the central opening  21  provided adjacent the drive mounting plate  20 , and preferably the thickness of the drive mounting plate  20  is increased at that location as shown at  25 , so as to provide additional support to the kingpin  740  in an upright position against stresses resulting from those imparted on the trunnion weldment  50  by the weight of the boom (not shown). 
         [0118]    Referring now to  FIG. 7  together with  FIG. 8 , at an upper portion  745 , the outside diameter of the kingpin  740  is narrowed, for example by a detent or upper shoulder  743  machined upon the upper portion  745 , configured to receive and retain in a sliding fit the lower edge  53  of the trunnion weldment  50 . As shown in  FIG. 8 , preferably thrust washers  52  are provided between the lower edge  53  of the trunnion weldment  50  and the upper shoulder  743  of the kingpin  740 . As in the embodiment shown in  FIGS. 1 and 2 , the thrust washers  52  are low friction rings that transmit any vertical load from the trunnion weldment  50  into the kingpin  740 , and provide a sliding surface for the rotation of the trunnion weldment  50  about the kingpin  740 . 
         [0119]    Similarly bushings  54  are preferably fixed to the interior of the trunnion weldment  50 . Additionally, grease or other lubricants such as would be known to those having ordinary skill in this art may be fed by way of lubrication fittings  55  into the bushings  54  and thrust washers  52  to further reduce friction between the kingpin  740  and the trunnion weldment  50  positioned thereon. 
         [0120]    As in the embodiment shown in  FIG. 1 , preferably a cap plate  62  is secured to the upper surface of the trunnion weldment  50 , by any suitable means such as bolts  56 . 
         [0121]    Referring now to  FIG. 9 , it can be seen that the configuration and the relationship of the drive mounting plate  20  and the pinion gears  36  are substantially as in the embodiment shown in  FIG. 1 , and the configuration of the boom hubs  60  and the cap plate  62  can also be seen, as well as the location of the kingpin  740 . 
         [0122]    Referring to  FIG. 10 , in addition to the features of the pinion gears  36 , drive mounting plate  20 , gear pedestal plate  71  and circular gear  72  described above in relation to the first embodiment, the location and configuration of the kingpin  740  and the upper shoulder  743  can be seen. 
         [0123]    Those having ordinary skill in this art will appreciate that the structural system of the invention is modular in construction, such that each component may be easily shipped in conventional and relatively inexpensive fashion and then easily assembled during installation at the intended end use location. Similarly, maintenance and repairs are substantially simplified. 
         [0124]    In addition, the present invention provides a slewing actuator system suitable for use on ships, land surfaces or docks, which possesses a more compact design that allows for a greater amount of rotational range of the boom attached thereto, particularly on ships or at other use locations having a small clearance envelope. 
         [0125]    The present invention also provides a slewing actuator system which is easy to install, lighter than conventional slewing actuator systems, and which can be easily disassembled into separate portions for shipping, if necessary, and subsequent reassembly. 
         [0126]    The present invention further provides a boom slewing actuator system which safely prevents boom slippage, and which securely locks the boom in place when hydraulic pressure is removed. 
         [0127]    Other embodiments consistent with the present invention will become apparent from consideration of the specification and the practice of the invention disclosed therein. 
         [0128]    Accordingly, the specification and the embodiments are to be considered exemplary only, with the true scope and spirit of the invention being disclosed by the following claims.