Abstract:
A dual-acting linear actuator including a hollow body and a stationary hollow rod. The stationary hollow rod includes a first rod segment separated from a second rod segment by a separator plate, the stationary hollow rod extending through the hollow body such that the separator plate forms a first chamber and a second chamber within the hollow body, wherein the first rod segment is configured to transmit a power medium to the first chamber to cause the hollow body to move along the stationary hollow rod toward the first rod segment and the second rod segment is configured to transmit the power medium to the second chamber to cause the hollow body to move along the stationary hollow rod toward the second rod segment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of Ser. No. 10/879,381 filed on Jun. 29, 2004, which is a continuation-in-part of Ser. No. 10/629,460 filed Jul. 29, 2003, each of which is incorporated by reference herein in its entirety. 
     
    
     THE FIELD OF THE INVENTION  
       [0002]     The present invention relates to concrete block molds, and more particularly to a concrete block mold adapted for use with a concrete block machine and having at least one moveable liner.  
       BACKGROUND OF THE INVENTION  
       [0003]     Concrete blocks, also referred to as concrete masonry units (CMU&#39;s), are typically manufactured by forming them into various shapes using a concrete block machine employing a mold frame assembled so as to form a mold box A mold cavity having a negative of a desired shape of the block to be formed is provided within the mold box. A support board, or pallet, is moved via a conveyor system onto a pallet table. The pallet table is moved upward until the pallet contacts and forms a bottom of the mold box. The cavity is then filled with concrete by a moveable feedbox drawer.  
         [0004]     As soon as the mold is filled with concrete, the feedbox drawer is moved back to a storage position and a plunger, or head shoe assembly, descends to form a top of the mold. The head shoe assembly is typically matched to the top outside surface of the mold cavity and is hydraulically or mechanically pressed down on the concrete. The head shoe assembly compresses the concrete to a desired pounds-per-square-inch (psi) rating and block dimension while simultaneously vibrating the mold along with the vibrating table, resulting in substantial compression and optimal distribution of the concrete throughout the mold cavity.  
         [0005]     Because of the compression, the concrete reaches a level of hardness that permits immediate stripping of the finished block from the mold. To remove the finished block from the mold, the mold remains stationary while the shoe and pallet table, along with the corresponding pallet, are moved downward and force the block from the mold onto the pallet. As soon as the bottom edge of the head shoe assembly clears the bottom edge of the mold, the conveyor system moves the pallet with the finished block forward, and another pallet takes its place under the mold. The pallet table then raises the next pallet to form a bottom of the mold box for the next block, and the process is repeated.  
         [0006]     For many types of CMU&#39;s (e.g., pavers, patio blocks, light weight blocks, cinder blocks, etc.), but for retaining wall blocks and architectural units in particular, it is desirable for at least one surface of the block to have a desired texture, such as a stone-like texture. One technique for creating a desired texture on the block surface is to provide a negative of a desired pattern or texture on the side walls of the mold. However, because of the way finished blocks are vertically ejected from the mold, any such pattern or texture would be stripped from the side walls unless they are moved away from the mold interior prior to the block being ejected.  
         [0007]     One technique employed for moving the sidewalls of a mold involves the use of a cam mechanism to move the sidewalls of the mold inward and an opposing spring to push the sidewalls outward from the center of the mold. However, this technique applies an “active” force to the sidewall only when the sidewall is being moved inward and relies on the energy stored in the spring to move the sidewall outward. The energy stored in the spring may potentially be insufficient to retract the sidewall if the sidewall sticks to the concrete. Additionally, the cam mechanism can potentially be difficult to utilize within the limited confines of a concrete block machine.  
         [0008]     A second technique involves using a piston to extend and retract the sidewall. However, a shaft of the piston shaft is coupled directly to the moveable sidewall and moves in-line with the direction of movement of the moveable sidewall. Thus, during compression of the concrete by the head shoe assembly, an enormous amount of pressure is exerted directly on the piston via the piston shaft. Consequently, a piston having a high psi rating is required to hold the sidewall in place during compression and vibration of the concrete. Additionally, the direct pressure on the piston shaft can potentially cause increased wear and shorten the expected life of the piston.  
       SUMMARY OF THE INVENTION  
       [0009]     One embodiment provides a dual-acting linear actuator including a hollow body and a stationary hollow rod. The stationary hollow rod includes a first rod segment separated from a second rod segment by a separator plate, the stationary hollow rod extending through the hollow body such that the separator plate forms a first chamber and a second chamber within the hollow body, wherein the first rod segment is configured to transmit a power medium to the first chamber to cause the hollow body to move along the stationary hollow rod toward the first rod segment and the second rod segment is configured to transmit the power medium to the second chamber to cause the hollow body to move along the stationary hollow rod toward the second rod segment. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a perspective view of one exemplary embodiment of a mold assembly having moveable liner plates according to the present invention.  
         [0011]      FIG. 2  is a perspective view of one exemplary embodiment of a gear drive assembly and moveable liner plate according to the present invention.  
         [0012]      FIG. 3A  is a top view of gear drive assembly and moveable liner plate as illustrated in  FIG. 2 .  
         [0013]      FIG. 3B  is a side view of gear drive assembly and moveable liner plate as illustrated in  FIG. 2 .  
         [0014]      FIG. 4A  is a top view of the mold assembly of  FIG. 1  having the liner plates retracted.  
         [0015]      FIG. 4B  is a top view of the mold assembly of  FIG. 1  having the liner plates extended.  
         [0016]      FIG. 5A  illustrates a top view of one exemplary embodiment of a gear plate according to the present invention.  
         [0017]      FIG. 5B  illustrates an end view of the gear plate illustrated by  FIG. 5A .  
         [0018]      FIG. 5C  illustrates a bottom view of one exemplary embodiment of a gear head according to the present invention.  
         [0019]      FIG. 5D  illustrates an end view of the gear head of  FIG. 5C .  
         [0020]      FIG. 6A  is a top view of one exemplary embodiment of a gear track according to the present invention.  
         [0021]      FIG. 6B  is a side view of the gear track of  FIG. 6A .  
         [0022]      FIG. 6C  is an end view of the gear track of  FIG. 6A .  
         [0023]      FIG. 7  is a diagram illustrating the relationship between a gear track and gear plate according to the present invention.  
         [0024]      FIG. 8A  is a top view illustrating the relationship between one exemplary embodiment of a gear head, gear plate, and gear track according to the present invention.  
         [0025]      FIG. 8B  is a side view of the illustration of  FIG. 8A .  
         [0026]      FIG. 8C  is an end view of the illustration of  FIG. 8A .  
         [0027]      FIG. 9A  is a top view illustrating one exemplary embodiment of a gear plate being in a retracted position within a gear track according to the present invention.  
         [0028]      FIG. 9B  is a top view illustrating one exemplary embodiment of a gear plate being in an extended position from a gear track according to the present invention.  
         [0029]      FIG. 10A  is a diagram illustrating one exemplary embodiment of drive unit according to the present invention.  
         [0030]      FIG. 10B  is a partial top view of the drive unit of the illustration of  FIG. 10A .  
         [0031]      FIG. 11A  is a top view illustrating one exemplary embodiment of a mold assembly according to the present invention.  
         [0032]      FIG. 11B  is a diagram illustrating one exemplary embodiment of a gear drive assembly according to the present invention.  
         [0033]      FIG. 12  is a perspective view illustrating a portion of one exemplary embodiment of a mold assembly according to the present invention.  
         [0034]      FIG. 13  is a perspective view illustrating one exemplary embodiment of a gear drive assembly according to the present invention.  
         [0035]      FIG. 14  is a top view illustrating a portion of one exemplary embodiment of a mold assembly and gear drive assembly according to the present invention.  
         [0036]      FIG. 15A  is a top view illustrating a portion of one exemplary embodiment of a gear drive assembly employing a stabilizer assembly.  
         [0037]      FIG. 15B  is a cross-sectional view of the gear drive assembly of  FIG. 15A .  
         [0038]      FIG. 15C  is a cross-sectional view of the gear drive assembly of  FIG. 15A .  
         [0039]      FIG. 16  is a side view illustrating a portion of one exemplary embodiment of a gear drive assembly and noveable liner plate according to the present invention.  
         [0040]      FIG. 17  is a block diagram illustrating one exemplary embodiment of a mold assembly employing a control system according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]     In the following Detailed Description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the FIG.(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.  
         [0042]      FIG. 1A  is a perspective view of one exemplary embodiment of a mold assembly  30  having moveable liner plates  32   a ,  32   b ,  32   c  and  32   d  according to the present invention. Mold assembly  30  includes a drive system assembly  31  having side-members  34   a  and  34   b  and cross-members  36   a  and  36   b , respectively having an inner wall  38   a ,  38   b ,  40   a , and  40   b , and coupled to one another such that the inner surfaces form a mold box  42 . In the illustrated embodiment, cross members  36   a  and  36   b  are bolted to side members  34   a  and  34   b  with bolts  37 .  
         [0043]     Moveable liner plates  32   a ,  32   b ,  32   c , and  32   d , respectively have a front surface  44   a ,  44   b ,  44   c , and  44   d  configured so as to form a mold cavity  46 . In the illustrated embodiment, each liner plate has an associated gear drive assembly located internally to an adjacent mold frame member. A portion of a gear drive assembly  50  corresponding to liner plate  32   a  and located internally to cross-member  36   a  is shown extending through side-member  34   a . Each gear drive assembly is selectively coupled to its associated liner plate and configured to move the liner plate toward the interior of mold cavity  46  by applying a first force in a first direction parallel to the associated cross-member, and to move the liner plate away from the interior of mold cavity  46  by applying a second force in a direction opposite the first direction. Side members  34   a  and  34   b  and cross-members  36   a  and  36   b  each have a corresponding lubrication port that extends into the member and provides lubrication to the corresponds gear elements. For example, lubrication ports  48   a  and  48   b . The gear drive assembly and moveable liner plates according to the present invention are discussed in greater detail below.  
         [0044]     In operation, mold assembly  30  is selectively coupled to a concrete block machine. For ease of illustrative purposes, however, the concrete block machine is not shown in  FIG. 1 . In one embodiment, mold assembly  30  is mounted to the concrete block machine by bolting side members  34   a  and  34   b  of drive system assembly  31  to the concrete block machine. In one embodiment, mold assembly  30  further includes a head shoe assembly  52  having dimensions substantially equal to those of mold cavity  46 . Head shoe assembly  52  is also configured to selectively couple to the concrete block machine.  
         [0045]     Liner plates  32   a  through  32   d  are first extended a desired distance toward the interior of mold box  42  to form the desired mold cavity  46 . A vibrating table on which a pallet  56  is positioned is then raised (as indicated by directional arrow  58 ) such that pallet  56  contacts and forms a bottom to mold cavity  46 . In one embodiment, a core bar assembly (not shown) is positioned within mold cavity  46  to create voids within the finished block in accordance with design requirements of a particular block.  
         [0046]     Mold cavity  46  is then filled with concrete from a moveable feedbox drawer. Head shoe assembly  52  is then lowered (as indicated by directional arrow  545  onto mold  46  and hydraulically or mechanically presses the concrete. Head shoe assembly  52  along with the vibrating table then simultaneously vibrate mold assembly  30 , resulting in a high compression of the concrete within mold cavity  46 . The high level of compression fills any voids within mold cavity  46  and causes the concrete to quickly reach a level of hardness that permits immediate removal of the finished block from mold cavity  46 .  
         [0047]     The finished block is removed by first retracting liner plates  32   a  through  32   d . Head shoe assembly  52  and the vibrating table, along with pallet  56 , are then lowered (in a direction opposite to that indicated by arrow  58 ), while mold assembly  30  remains stationary so that head shoe assembly  56  pushes the finished block out of mold cavity  46  onto pallet  52 . When a lower edge of head shoe assembly  52  drops below a lower edge of mold assembly  30 , the conveyer system moves pallet  56  carrying the finished block away and a new pallet takes its place. The above process is repeated to create additional blocks.  
         [0048]     By retracting liner plates  32   a  through  32   b  prior to removing the finished block from mold cavity  46  liner plates  32   a  through  32   d  experience less wear and, thus, have an increased operating life expectancy. Furthermore, moveable liner plates  32   a  through  32   d  also enables a concrete block to be molded in a vertical position relative to pallet  56 , in lieu of the standard horizontal position, such that head shoe assembly  52  contacts what will be a “face” of the finished concrete block. A “face” is a surface of the block that will be potentially be exposed for viewing after installation in a wall or other structure.  
         [0049]      FIG. 2  is a perspective view  70  illustrating a moveable liner plate and corresponding gear drive assembly according to the present invention, such as moveable liner plate  32   a  and corresponding gear drive assembly  50 . For illustrative purposes, side member  34   a  and cross-member  36  are not shown. Gear drive assembly  50  includes a first gear element  72  selectively coupled to liner plate  32   a , a second gear element  74 , a single rod-end double-acting pneumatic cylinder (cylinder)  76  coupled to second gear element  74  via a piston rod  78 , and a gear track  80 . Cylinder  76  includes an aperture  82  for accepting a pneumatic fitting. In one embodiment, cylinder  76  comprises a hydraulic cylinder. In one embodiment, cylinder  76  comprises a double rod-end dual-acting cylinder. In one embodiment, piston rod  78  is threadably coupled to second gear element  74 .  
         [0050]     In the embodiment of  FIG. 2 , first gear element  72  and second gear. element  74  are illustrated and hereinafter referred to as a gear plate  72  and second gear element  74 , respectively. However, while illustrated as a gear plate and a cylindrical gear head, first gear element  72  and second gear element  74  can be of any suitable shape and dimension.  
         [0051]     Gear plate  72  includes a plurality of angled channels on a first major surface  84  and is configured to slide in gear track  80 . Gear track  80  slidably inserts into a gear slot (not shown) extending into cross member  36   a  from inner wall  40   a . Cylindrical gear head  74  includes a plurality of angled channels on a surface  86  adjacent to first major surface  84  of female gear plate  72 , wherein the angled channels are tangential to a radius of cylindrical gear head  74  and configured to slidably mate and interlock with the angled channels of gear plate  72 . Liner plate  32   a  includes guide posts  88   a ,  88   b ,  88   c , and  88   d  extending from a rear surface  90 . Each of the guide posts is configured to slidably insert into a corresponding guide hole (not shown) extending into cross member  36   a  from inner wall  40   a . The gear slot and guide holes are discussed in greater detail below.  
         [0052]     When cylinder  76  extends piston rod  78 , cylindrical gear head  74  moves in a direction indicated by arrow  92  and, due to the interlocking angled channels, causes gear plate  72  and, thus, liner plate  32   a  to move toward the interior of mold  46  as indicated by arrow  94 . It should be noted that, as illustrated,  FIG. 2  depicts piston rod  78  and cylindrical gear head  74  in an extended position. When cylinder  76  retracts piston rod  78 , cylindrical gear head  74  moves in a direction indicated by arrow  96  causing gear plate  72  and liner plate  32  to move away from the interior of the mold as indicated by arrow  98 . As liner plate  32   a moves, either toward or away from the center of the mold, gear plate  72  slides in guide track  80  and guide posts  88   a  through  88   d  slide within their corresponding guide holes.  
         [0053]     In one embodiment, a removable liner face  100  is selectively coupled to front surface  44   a  via fasteners  102   a ,  102   b ,  102   c , and  102   d  extending through liner plate  32   a . Removable liner face  100  is configured to provide a desired shape and/or provide a desired imprinted pattern, including text, on a block made in mold  46 . In this regard, removable liner face  100  comprises a negative of the desired shape or pattern. In one embodiment, removable liner face  100  comprises a polyurethane material. In one embodiment, removable liner face  100  comprises a rubber material. In one embodiment, removable liner plate comprises a metal or metal alloy, such as steel or aluminum. In one embodiment, liner plate  32  further includes a heater mounted in a recess  104  on rear surface  90 , wherein the heater aids in curing concrete within mold  46  to reduce the occurrence of concrete sticking to front surface  44   a  and removable liner face  100 .  
         [0054]      FIG. 3A  is a top view  120  of gear drive assembly  50  and liner plate  32   a , as indicated by directional arrow  106  in  FIG. 2 . In the illustration, side members  34   a  and  34   b , and cross member  36   a  are indicated dashed lines. Guide posts  88   c  and  88   d  are slidably inserted into guide holes  122   c  and  122   d , respectively, which extend into cross member  36   a  from interior surface  40   a . Guide holes  122   a  and  122   b , corresponding respectively to guide posts  88   a  and  88   b , are not shown but are located below and in-line with guide holes  122   c  and  122   d . In one embodiment, guide hole bushings  124   c  and  124   d  are inserted into guide holes  122   c  and  122   d , respectively, and slidably receive guide posts  88   c and  88   d . Guide hole bushings  124   a  and  124   b  are not shown, but are located below and in-line with guide hole bushings  124   c  and  124   d . Gear track  80  is shown as being slidably inserted in a gear slot  126  extending through cross member  36   a  with gear plate  72  sliding in gear track  80 . Gear plate  72  is indicated as being coupled to liner plate  32   a  by a plurality of fasteners  128  extending through liner plate  32   a  from front surface  44   a.    
         [0055]     A cylindrical gear shaft is indicated by dashed lines  134  as extending through side member  34   a  and into cross member  36   a  and intersecting, at least partially with gear slot  126 . Cylindrical gear head  74 , cylinder  76 , and piston rod  78  are slidably inserted into gear shaft  134  with cylindrical gear head  74  being positioned over gear plate  72 . The angled channels of cylindrical gear head  74  are shown as dashed lines  130  and are interlocking with the angled channels of gear plate  72  as indicated at  132 .  
         [0056]      FIG. 3B  is a side view  140  of gear drive assembly  50  and liner plate  32   a , as indicated by directional arrow  108  in  FIG. 2 . Liner plate  32   a  is indicated as being extended, at least partially, from cross member  36   a . Correspondingly, guide posts  88   a  and  88   d  are indicated as partially extending from guide hole bushings  124   a  and  124   d , respectively. In one embodiment, a pair of limit rings  142   a  and  142   d  are selectively coupled to guide posts  88   a  and  88 , respectively, to limit an extension distance that liner plate  32   a  can be extended from cross member  36   a  toward the interior of mold cavity  46 . Limit rings  142   b  and  142   c  corresponding respectively to guide posts  88   b  and  88   c  are not shown, but are located behind and in-line with limit rings  142   a  and  142   d . In the illustrated embodiment, the limit rings are indicated as being substantially at an end of the guide posts, thus allowing a substantially maximum extension distance from cross member  36   a . However, the limit rings can be placed at other locations along the guide posts to thereby adjust the allowable extension distance.  
         [0057]      FIG. 4A  and  FIG. 4B  are top views  150  and  160 , respectively, of mold assembly  30 .  FIG. 4A  illustrates liner plates  32   a ,  32   b ,  32   c , and  32   d  in a retracted positions. Liner faces  152 ,  154 , and  154  correspond respectively to liner plates  32   b ,  32   c , and  32   d .  FIG. 4B  illustrates liner plates  32   a ,  32   b ,  32   c , and  32   d , along with their corresponding liner faces  100 ,  152 ,  154 , and  156  in an extended position.  
         [0058]      FIG. 5A  is a top view  170  of gear plate  72 . Gear plate  72  includes a plurality of angled channels  172  running across a top surface  174  of gear plate  72 . Angled channels  172  form a corresponding plurality of linear “teeth”  176  having as a surface the top surface  174 . Each angled channel  172  and each tooth  176  has a respective width  178  and  180 . The angled channels run at an angle (Θ) 182 from 0°, indicated at 186, across gear plate  72 .  
         [0059]      FIG. 5B  is an end view (“A”)  185  of gear plate  72 , as indicated by directional arrow  184  in  FIG. 5A , further illustrating the plurality of angled channels  172  and linear teeth  176 . Each angled channel  172  has a depth  192 .  
         [0060]      FIG. 5C  illustrates a view  200  of a flat surface  202  of cylindrical gear head  76 . Cylindrical gear head  76  includes a plurality of angled channels  204  running across surface  202 . Angled channels  204  form a corresponding plurality of linear teeth  206 . The angled channels  204  and linear teeth  206  have widths  180  and  178 , respectively, such that the width of linear teeth  206  substantially matches the width of angled channels  172  and the width of angled channels  204  substantially match the width of linear teeth  176 . Angled channels  204  and teeth  206  run at angle (Θ)  182  from 0°, indicated at 186, across surface  202 .  
         [0061]      FIG. 5D  is an end view  210  of cylindrical gear head  76 , as indicated by directional arrow  208  in  FIG. 5C , further illustrating the plurality of angled channels  204  and linear teeth  206 . Surface  202  is a flat surface tangential to a radius of cylindrical gear head  76 . Each angled channel has a depth  192  from flat surface  202 .  
         [0062]     When cylindrical gear head  76  is “turned over” and placed across surface  174  of gear plate  72 , linear teeth  206  of gear head  76  mate and interlock with angled channels  172  of gear plate  72 , and linear teeth  176  of gear plate  72  mate and interlock with angled channels  204  of gear head  76  (See also  FIG. 2 ). When gear head  76  is forced in direction  92 , linear teeth  206  of gear head  76  push against linear teeth  176  of gear plate  72  and force gear plate  72  to move in direction  94 . Conversely, when gear head  76  is forced in direction  96 , linear teeth  206  of gear head  76  push against linear teeth  176  of gear plate  72  and force gear plate  72  to move in direction  98 .  
         [0063]     In order for cylindrical gear head  76  to force gear plate  72  in directions  94  and  98 , angle (Θ)  182  must be greater than 0° and less than 90°. However, it is preferable that Θ  182  be at least greater than 45°. When Θ  182  is 45° or less, it takes more force for cylindrical gear head  74  moving in direction  92  to push gear plate  72  in direction  94  than it does for gear plate  72  being forced in direction  98  to push cylindrical gear head  74  in direction  96 , such as when concrete in mold  46  is being compressed. The more Θ  182  is increased above  450 , the greater the force that is required in direction  98  on gear plate  72  to move cylindrical gear head  74  in direction  96 . In fact, at  900  gear plate  72  would be unable to move cylindrical gear head  74  in either direction  92  or  96 , regardless of how much force was applied to gear plate  72  in direction  98 . In effect, angle (Θ) acts as a multiplier to a force provided to cylindrical gear head  74  by cylinder  76  via piston rod  78 . When Θ  182  is greater than 45°, an amount of force required to be applied to gear plate  72  in direction  98  in order to move cylindrical gear head  74  in direction  96  is greater than an amount of force required to be applied to cylindrical gear head  74  in direction  92  via piston rod  78  in order to “hold” gear plate  72  in position (i.e., when concrete is being compressed in mold  46 ).  
         [0064]     However, the more Θ  182  is increased above 45°, the less distance gear plate  72 , and thus corresponding liner plate  32   a , will move in direction  94  when cylindrical gear head  74  is forced in direction  92 . A preferred operational angle for Θ  182  is approximately 70°. This angle represents roughly a balance, or compromise, between the length of travel of gear plate  72  and an increase in the level of force required to be applied in direction  98  on gear plate  72  to force gear head  74  in direction  96 . Gear plate  72  and cylindrical gear head  74  and their corresponding angled channels  176  and  206  reduce the required psi rating of cylinder  76  necessary to maintain the position of liner plate  32   a  when concrete is being compressed in mold cavity  46  and also reduces the wear experienced by cylinder  76 . Additionally, from the above discussion, it is evident that one method for controlling the travel distance of liner plate  32   a  is to control the angle (Θ)  182  of the angled channels  176  and  206  respectively of gear plate  72  and cylindrical gear head  74 .  
         [0065]      FIG. 6A  is a top view  220  of gear track  80 . Gear track  80  has a top surface  220 , a first end surface  224 , and a second end surface  226 . A rectangular gear channel, indicated by dashed lines  228 , having a first opening  230  and a second opening  232  extends through gear track  80 . An arcuate channel  234 , having a radius required to accommodate cylindrical gear head  76  extends across top surface  220  and forms a gear window  236  extending through top surface  222  into gear channel  228 . Gear track  80  has a width  238  incrementally less than a width of gear opening  126  in side member  36   a  (see also  FIG. 3A ).  
         [0066]      FIG. 6B  is an end view  250  of gear track  80 , as indicated by direction arrow  240  in  FIG. 6A , further illustrating gear channel  228  and arcuate channel  234 . Gear track  80  has a depth  252  incrementally less than height of gear opening  126  in side member  36   a  (see  FIG. 3A ).  FIG. 6B  is a side view  260  of gear track  80  as indicated by directional arrow  242  in  FIG. 6A .  
         [0067]      FIG. 7  is a top view  270  illustrating the relationship between gear track  80  and gear plate  72 . Gear plate  72  has a width  272  incrementally less than a width  274  of gear track  80 , such that gear plate  72  can be slidably inserted into gear channel  228  via first opening  230 . When gear plate  72  is inserted within gear track  80 , angled channels  172  and linear teeth  176  are exposed via gear window  236 .  
         [0068]      FIG. 8A  is a top view  280  illustrating the relationship between gear plate  72 , cylindrical gear head  74 , and gear track  80 . Gear plate  72  is indicated as being slidably inserted within guide track  80 . Cylindrical gear head  74  is indicated as being positioned within arcuate channel  234 , with the angled channels and linear teeth of cylindrical gear head  74  being slidably mated and interlocked with the angled channels  172  and linear teeth  176  of gear plate  72 . When cylindrical gear head  74  is moved in direction  92  by extending piston rod  78 , gear plate  72  extends outward from gear track  80  in direction  94  (See also  FIG. 9B  below). When cylindrical gear head  74  is moved in direction  96  by retracting piston rod  78 , gear plate  72  retracts into gear track  80  in direction  98  (See also  FIG. 9A  below).  
         [0069]      FIG. 8B  is a side view  290  of gear plate  72 , cylindrical gear head  74 , and guide track  80  as indicated by directional arrow  282  in  FIG. 8A .  
         [0070]     Cylindrical gear head  74  is positioned such that surface  202  is located within arcuate channel  234 . Angled channels  204  and teeth  206  of cylindrical gear head  74  extend through gear window  236  and interlock with angled channels  172  and linear teeth  176  of gear plate  72  located within gear channel  228 .  FIG. 8C  is an end view  300  as indicated by directional arrow  284  in  FIG. 8A , and further illustrates the relationship between gear plate  72 , cylindrical gear head  74 , and guide track  80 .  
         [0071]      FIG. 9A  is top view  310  illustrating gear plate  72  being in a fully retracted position within gear track  80 , with liner plate  32   a  being retracted against cross member  36   a . For purposes of clarity, cylindrical gear head  74  is not shown. Angled channels  172  and linear teeth  176  are visible through gear window  236 . Liner plate  32   a  is indicated as being coupled to gear plate  72  with a plurality of fasteners  128  extending through liner plate  32   a  into gear plate  72 . In one embodiment, fasteners  128  threadably couple liner plate  32   a  to gear plate  72 .  
         [0072]      FIG. 9B  is a top view  320  illustrating gear plate  72  being extended, at least partially from gear track  80 , with liner plate  32   a  being separated from cross member  36   a . Again, cylindrical gear head  74  is not shown and angled channels  172  and linear teeth  176  are visible through gear window  236 .  
         [0073]      FIG. 10A  is a diagram  330  illustrating one exemplary embodiment of a gear drive assembly  332  according to the present invention. Gear drive assembly  332  includes cylindrical gear head  74 , cylinder  76 , piston rod  78 , and a cylindrical sleeve  334 . Cylindrical gear head  74  and piston rod  78  are configured to slidably insert into cylindrical sleeve  334 . Cylinder  76  is threadably coupled to cylindrical sleeve  334  with an  0 -ring  336  making a seal. A window  338  along an axis of cylindrical sleeve  334  partially exposes angled channels  204  and linear teeth  206 . A fitting  342 , such as a pneumatic or hydraulic fitting, is indicated as being threadably coupled to aperture  82 . Cylinder  76  further includes an aperture  344 , which is accessible through cross member  36   a.    
         [0074]     Gear drive assembly  332  is configured to slidably insert into cylindrical gear shaft  134  (indicated by dashed lines) so that window  338  intersects with gear slot  126  so that angled channels  204  and linear teeth  206  are exposed within gear slot  126 . Gear track  80  and gear plate  72  (not shown) are first slidably inserted into gear slot  126 , such that when gear drive assembly  332  is slidably inserted into cylindrical gear shaft  134  the angled channels  204  and linear teeth  206  of cylindrical gear head  74  slidably mate and interlock with the angled channels  172  and linear teeth  176  of gear plate  72 .  
         [0075]     In one embodiment, a key  340  is coupled to cylindrical gear head  74  and rides in a key slot  342  in cylindrical sleeve  334 . Key  340  prevents cylindrical gear head  74  from rotating within cylindrical sleeve  334 . Key  340  and key slot  342  together also control the maximum extension and retraction of cylindrical gear head  74  within cylindrical sleeve  334 . Thus, in one embodiment, key  340  can be adjusted to control the extension distance of liner plate  32   a  toward the interior of mold cavity  46 .  FIG. 10A  is a top view  350  of cylindrical shaft  334  as illustrated in  FIG. 10B , and further illustrates key  340  and key slot  342 .  
         [0076]      FIG. 11A  is a top view illustrating one exemplary embodiment of a mold assembly  360  according to the present invention for forming two concrete blocks. Mold assembly  360  includes a mold frame  361  having side members  34   a  and  34   b  and cross members  36   a  through  36   c  coupled to one another so as to form a pair of mold boxes  42   a  and  42   b . Mold box  42   a  includes moveable liner plates  32   a  through  32   d  and corresponding removable liner faces  33   a  through  33   d configured to form a mold cavity  46   a . Mold box  42   b  includes moveable liner plates  32   e  through  32   h  and corresponding removable liner faces  33   e  through  33   h configured to form a mold cavity  46   b.    
         [0077]     Each moveable liner plate has an associated gear drive assembly located internally to an adjacent mold frame member as indicated by  50   a  through  50   h . Each moveable liner plate is illustrated in an extended position with a corresponding gear plate indicated by  72   a  through  72   h . As described below, moveable liner plates  32   c  and  32   e  share gear drive assembly  50   c/e , with gear plate  72   e  having its corresponding plurality of angled channels facing upward and gear plate  72   c  having its corresponding plurality of angled channels facing downward.  
         [0078]      FIG. 11B  is diagram illustrating a gear drive assembly according to the present invention, such as gear drive assembly  50   c/e .  FIG. 11B  illustrates a view of gear drive assembly  50   c/e  as viewed from section A-A through cross-member  36   c  of  FIG. 11A . Gear drive assembly  50   c/e  includes a single cylindrical gear head  76   c/e  having angled channels  204   c  and  204   e  on opposing surfaces. Cylindrical gear head  76   c/e  fits into arcuate channels  234   c  and  234   e  of gear tracks  80   c  and  80   d , such that angled channels  204   c  and  204   e  slidably interlock with angled channels  172   c  and  172   e  of gear plates  72   c  and  72   e respectively.  
         [0079]     Angled channels  172   c  and  204   c , and  172   e  and  204   e  oppose one another and are configured such that when cylindrical gear head  76   c/e  is extended (e.g. out from  FIG. 11B ) gear plate  72   c  moves in a direction  372  toward the interior of mold cavity  46   a  and gear plate  72   e  moves in a direction  374  toward the interior of mold cavity  46   b . Similarly, when cylindrical gear head  76   c/e  is retracted (e.g. into  FIG. 11B ) gear plate  72   c  moves in a direction  376  away from the interior of mold cavity  46   a  and gear plate  72   e  moves in a direction  378  away from the interior of mold cavity  378 . Again, cylindrical gear head  76   c/e  and gear plates  72   c  and  72   c  could be of any suitable shape.  
         [0080]      FIG. 12  is a perspective view illustrating a portion of one exemplary embodiment of a mold assembly  430  according to the present invention. Mold assembly includes moveable liner plates  432   a  through  4321  for simultaneously molding multiple concrete blocks. Mold assembly  430  includes a drive system assembly  431  having a side members  434   a  and  434   b , and cross members  436   a and  436   b . For illustrative purposes, side member  434   a  is indicated by dashed lines. Mold assembly  430  further includes division plates  437   a  through  437   g.    
         [0081]     Together, moveable liner plates  432   a  through  4321  and division plates  437   a  through  437   g  form mold cavities  446   a  through  446   f , with each mold cavity configured to form a concrete block. Thus, in the illustrated embodiment, mold assembly  430  is configured to simultaneously form six blocks. However, it should be apparent from the illustration that mold assembly  430  can be easily modified for simultaneously forming quantities of concrete blocks other than six.  
         [0082]     In the illustrated embodiment, side members  434   a  and  434   b  each have a corresponding gear drive assembly for moving moveable liner plates  432   a through  432   f  and  432   g  through  4321 , respectively. For illustrative purposes, only gear drive assembly  450  associated with side member  434   a  and corresponding moveable liner plates  432   a  through  432   g  is shown. Gear drive assembly  450  includes first gear elements  472   a  through  472   f  selectively coupled to corresponding moveable liner plates  432   a  through  432   f , respectively, and a second gear element  474 . In the illustrated embodiment, first gear elements  472   a  through  472   f  and second gear element  474  are shown as being cylindrical in shape. However, any suitable shape can be employed.  
         [0083]     Second gear element  474  is selectively coupled to a cylinder-piston (not shown) via a piston rod  478 . In one embodiment, which is described in greater detail below (see  FIG. 12 ), second gear element  474  is integral with the cylinder-piston so as to form a single component.  
         [0084]     In the illustrated embodiment, each first gear element  472   a  through  472   b farther includes a plurality of substantially parallel angled channels  484  that slidably mesh and interlock with a plurality of substantially parallel angled channels  486  on second gear element  474 . When second gear element  474  is moved in a direction indicated by arrow  492 , each of the moveable liner plates  432   a  through  432   f  moves in a direction indicated by arrow  494 . Similarly, when second gear element  474  is move in a direction indicated by arrow  496 , each of the moveable liner plates  432   a  through  432   f  moves in a direction indicated by arrow  498 .  
         [0085]     In the illustrated embodiment, the angled channels  484  on each of the first gear elements  432   a  through  432   f  and the angled channels  486  are at a same angle. Thus, when second gear element  474  moves in direction  492  and  496 , each moveable liner plate  432   a  through  432   f  moves a same distance in direction  494  and  498 , respectively. In one embodiment, second gear element  474  includes a plurality of groups of substantially parallel angled channels with each group corresponding to a different one of the first gear elements  472   a  through  472   f  In one embodiment, the angled channels of each group and its corresponding first gear element have a different angle such that each moveable liner plate  432   a  through  432   f  move a different distance in directions  494  and  498  in response to second gear element  474  being moved in direction  492  and  496 , respectively.  
         [0086]      FIG. 13  is a perspective view illustrating a gear drive assembly  500  according to the present invention, and a corresponding moveable liner plate  502  and removable liner face  504 . For illustrative purposes, a frame assembly including side members and cross members is not shown. Gear drive assembly  500  includes double rod-end, dual-acting pneumatic cylinder-piston  506  having a cylinder body  507 , and a hollow piston rod  508  with a first rod-end  510  and a second rod-end  512 . Gear drive assembly  500  further includes a pair of first gear elements  514   a  and  514   b  selectively coupled to moveable liner plate  502 , with each first gear element  514   a  and  514   b  having a plurality of substantially parallel angled channels  516   a  and  516   b.    
         [0087]     In the illustrated embodiment, cylinder body  507  of cylinder-piston  506  includes a plurality of substantially parallel angled channels  518  configured to mesh and slidably interlock with angled channels  516   a  and  516   b . In one embodiment, cylinder body  507  is configured to slidably insert into and couple to a cylinder sleeve having angled channels  518 .  
         [0088]     In one embodiment, cylinder-piston  506  and piston rod  508  are located within a drive shaft of a frame member, such as drive shaft  134  of cross-member  36   a , with rod-end  510  coupled to and extending through a frame member, such as side member  34   b , and second rod-end  512  coupled to and extending through a frame member, such a side member  34   a . First rod-end  510  and second rod-end  512  are configured to receive and provide compressed air to drive dual-acting cylinder-piston  506 . With piston rod  508  being fixed to side members  34   a  and  34   b  via first and second rod-ends  512  and  510 , cylinder-piston  506  travels along the axis of piston rod  508  in the directions as indicated by arrows  520  and  522  in response to compressed air received via first and second rod-ends  510  and  512 .  
         [0089]     When compressed air is received via second rod-end  512  and expelled via first rod-end  510 , cylinder-piston  506  moves within a drive shaft, such as drive shaft  134 , in direction  522  and causes first gear elements  514   a  and  516   b and corresponding liner plate  502  and liner face  504  to move in a direction indicated by arrow  524 . Conversely, when compressed air is received via first rod-end  510  and expelled via second rod-end  512 , cylinder-piston  506  moves within a gear shaft, such as gear shaft  134 , in direction  520  and causes first gear elements  514   a  and  516   b  and corresponding liner plate  502  and liner face  504  to move in a direction indicated by arrow  526 .  
         [0090]     In the illustrated embodiment, cylinder-piston  506  and first gear elements  514   a  and  514   b  are-shown as being substantially cylindrical in shape. However, any suitable shape can be employed. Furthermore, in the illustrated embodiment, cylinder-piston  506  is a double rod-end dual-acting cylinder. In one embodiment, cylinder piston  506  is a single rod-end dual acting cylinder having only a single rod-end  510  coupled to a frame member, such as side member  34   b . In such an embodiment, compressed air is provided to cylinder-piston via single rod-end  510  and a flexible pneumatic connection made to cylinder-piston  506  through side member  34   a  via gear shaft  134 . Additionally, cylinder-piston  506  comprises a hydraulic cylinder.  
         [0091]      FIG. 14  is a top view of a portion of mold assembly  430  (as illustrated by  FIG. 12 ) having a drive assembly  550  according to one embodiment of the present invention. Drive assembly  550  includes first drive elements  572   a  to  572   f that are selectively coupled to corresponding liner plates  432   a  to  432   f  via openings, such as opening  433 , in side member  434   a  Each of the first drive elements  572   a  to  572  if further coupled to a master bar  573 . Drive assembly  550  further includes a double-rod-end hydraulic piston assembly  606  having a dual-acting cylinder  607  and a hollow piston rod  608  having a first rod-end  610  and a second rod-end  612 . First and second rod-ends  610 ,  612  are stationary and are coupled to and extend through a removable housing  560  that is coupled to side member  434   a  and encloses drive assembly  550 . First and second rod ends  610 ,  612  are each coupled to hydrautic fittings  620  that are configured to connect via lines  622   a  and  622   b  to an external hydraulic system  624  and to transfer hydraulic fluid to and from dual-acting cylinder  607  via hollow piston rod  608 .  
         [0092]     In one embodiment, as illustrated, first drive elements  572   b  and  572   e include a plurality of substantially parallel angled channels  616  that slideably interlock with a plurality of substantially parallel angled channels  618  that form a second drive element. In one embodiment, as illustrated above by  FIG. 12 , angled channels  618  are formed on dual-acting cylinder  607  of hydraulic piston assembly  606 , such that dual-acting cylinder  607  forms the second drive element. In other embodiments, as will be described by  FIGS. 15A-15C  below, the second drive element is separate from and operatively coupled to dual-acting cylinder  607 .  
         [0093]     When hydraulic fluid is transmitted into dual-acting cylinder  607  from second rod-end  612  via fitting  620  and hollow piston rod  608 , hydraulic fluid is expelled from first rod-end  610 , causing dual-acting cylinder  607  and angled channels  618  to move along piston rod  608  toward second rod-end  612 . As dual-acting cylinder  607  moves toward second Tod-end  612 , angled channels  618  interact with angled channels  616  and drive first drive elements  572   b  and  572   e , and thus corresponding liner plates  432   b  and  432   e , toward the interior of mold cavities  446   b  and  446   e , respectively. Furthermore, since each of the first drive elements  572   a  through  572   f  is coupled to master bar  573 , driving first gear elements  572   b  and  572   e  toward the interiors of mold cavities  446   b  and  446   e  also moves first drive elements  572   a ,  572   c ,  572   d , and  572   f  and corresponding liner plates  432   a ,  432   c ,  432   d , and  432   e  toward the interiors of mold cavities  446   a ,  446   c ,  446   d , and  446   f , respectively. Conversely, transmitting hydraulic fluid into dual-acting cylinder  607  from first rod-end  610  via fitting  620  and hollow-piston rod  608  causes dual-acting cylinder  607  to move toward first rod-end  610 , and causes liner plates  432  to move away from the interiors of corresponding mold cavities  446 .  
         [0094]     In one embodiment, drive assembly  550  further includes support shafts  626 , such as support shafts  626   a  and  626   b , which are coupled between removable housing  560  and side member  434   a  and extend through master bar  573 . As dual-acting cylinder  607  is moved by transmitting/expelling hydraulic fluid from first and second rod-ends  610 ,  612 , master bar  573  moves back and forth along support shafts  626 . Because they are coupled to static elements of mold assembly  430 , support shafts  626   a  and  626   b  provide support and rigidity to liner plates  432 , drive elements  572 , and master bar  573  as they move toward and away from mold cavities  446 .  
         [0095]     In one embodiment, drive assembly  550  further includes a pneumatic fitting  628  configured to connect via line  630  to and external compressed air system  632  and provide compressed air to housing  560 . By receiving compressed air via pneumatic fitting  628  to removable housing  560 , the internal air pressure of housing  560  is positive relative to the outside air pressure, such that air is continuously “forced” out of housing  560  through any non-sealed openings, such as openings  433  through which first drive elements  572  extend through side member  434   a . By maintaining a positive air pressure and forcing air out through such non-sealed opening, the occurrence of dust and debris and other unwanted contaminants from entering housing  560  and fouling drive assembly  550  is reduced.  
         [0096]     First and second rod ends  610 ,  612  are each coupled to hydraulic fittings  620  that are configured to connect via lines  622   a  and  622   b  to an external hydraulic system  624  and to transfer hydraulic fluid to and from dual-acting cylinder  607  via hollow piston rod  608 .  
         [0097]      FIG. 15A  is a top view illustrating a portion of one embodiment of drive assembly  550  according to the present invention. Drive assembly  550  includes double-rod-end hydraulic piston assembly  606  comprising dual-acting cylinder  607  and a hollow piston rod  608  with first and second rod-ends  610  and  612  being and coupled to and extending through removable housing  560 .  
         [0098]     As illustrated, dual-acting cylinder  607  is slideably-fitted inside a machined opening  641  within a second gear element  640 , with hollow piston rod  608  extending through removable end caps  642 . In one embodiment, end caps  646  are threadably inserted into machined opening  641  such that end caps  646  butt against and secure dual-acting cylinder  607  so that dual-acting cylinder  607   30  is held stationary with respect to second drive element  640 . Second drive element  640  includes the plurality of substantially parallel angled channels  618 , in lieu of angled channels being an integral part of dual-acting cylinder  607 .  
         [0099]     With reference to  FIG. 14 , angled channels  618  of second gear element  640  are configured to slideably interlock with angled channels  616  of first gear elements  572   b  and  572   e.    
         [0100]     Second gear element  640  further includes a guide rail  644  that is slideably coupled to linear bearing blocks  646  that are mounted to housing  560 . As described above with respect to  FIG. 14 , transmitting and expelling hydraulic fluid to and from dual-acting cylinder  607  via first and second rod-ends  610 ,  612  causes dual-acting cylinder  607  to move along hollow piston-rod  608 . Since dual-acting cylinder  607  is “locked” in place within machined shaft  641  of second gear element  640  by end caps  642 , second gear element  640  moves along hollow piston-rod  608  together with dual-acting cylinder  607 . As second drive element  640  moves along hollow piston-rod  608 , linear bearing blocks  646  guide and secure guide rail  644 , thereby guiding and securing second drive element  640  and reducing undesirable motion in second drive element  640  that is perpendicular to hollow piston rod  608 .  
         [0101]      FIG. 15B  is a lateral cross-sectional view A-A of the portion of drive assembly  550  illustrated by  FIG. 15A . Guide rail  644  is slideably fitted into a linear bearing track  650  and rides on bearings  652  as second drive element  640  is moved along piston rod  608  by dual-acting cylinder  607 . In one embodiment, linear bearing block  646   b  is coupled to housing  560  via bolts  648 .  
         [0102]      FIG. 15C  is a longitudinal cross-sectional view B-B of the portion of drive assembly  550  of  FIG. 15A , and illustrates dual-acting cylinder  607  as being secured within shaft  641  of drive element  640  by end caps  642   a  and  642   b . In one embodiment, end caps  642   a  and  642   b  are threadably inserted into the ends of second drive element  640  so as to butt against each end of dual-acting cylinder  607 . Hollow piston rod  608  extends through end caps  642   a  and  642   b and has first and second rod ends  610  and  612  coupled to and extending through housing  560 . A divider  654  is coupled to piston rod  608  and divides dual-acting cylinder  607  into a first chamber  656  and a second chamber  658 . A first port  660  and a second port  662  allow hydraulic fluid to be pumped into and expelled from first chamber  656  and second chamber  658  via first and second rod ends  610  and  612  and associated hydraulic fittings  620 , respectively.  
         [0103]     When hydraulic fluid is pumped into first chamber  656  via first rod-end  610  and first port  660 , dual-acting cylinder  607  moves along hollow piston rod  608  toward first rod-end  610  and hydraulic fluid is expelled from second chamber  658  via second port  662  and second rod-end  612 . Since dual-acting cylinder  607  is secured within shaft  641  by end caps  642   a  and  642   b , second drive element  640  and, thus, angled channels  618  move toward first rod-end  610 . Similarly, when hydraulic fluid is pumped into second chamber  658  via second rod-end  612  and second port  662 , dual-acting cylinder  607  moves along hollow piston rod  608  toward second rod-end  612  and hydraulic fluid is expelled from first chamber  656  via first port  660  and first rod-end  610 .  
         [0104]      FIG. 16  is a side view of a portion of drive assembly  550  as shown by  FIG. 14  and illustrates a typical liner plate, such as liner plate  432   a , and corresponding removable liner face  400 . Liner plate  432   a  is coupled to second drive element  572   a  via a bolted connection  670  and, in-turn, drive element  572   a is coupled to master bar  573  via a bolted connection  672 . A lower portion of liner face  400  is coupled to liner plate  432   a  via a bolted connection  674 . In one embodiment, as illustrated, liner plate  432   a  includes a raised “rib”  676  that runs the length of and along an Lipper edge of liner plate  432   a . A channel  678  in liner face  400  overlaps and interlocks with raised rib  676  to form a “boltless” connection between liner plate  432   a  and an upper portion of liner face  400 . Such an interlocking connection securely couples the upper portion of liner face  400  to liner plate  432  in an area of liner face  400  that would otherwise be too narrow to allow use of a bolted connection between liner face  400  and liner plate  432   a  without the bolt being visible on the surface of liner face  400  that faces mold cavity  446   a.    
         [0105]     In one embodiment, liner plate  432  includes a heater  680  configured to maintain the temperature of corresponding liner face  400  at a desired temperature to prevent concrete in corresponding mold cavity  446  sticking to a surface of liner face  400  during a concrete curing process. In one embodiment, heater  680  comprises an electric heater.  
         [0106]      FIG. 17  is a block diagram illustrating one embodiment of a mold assembly according to the present invention, such as mold assembly  430  of  FIG. 14 , further including a controller  700  configured to coordinate the movement of moveable liner plates, such as liner plates  432 , with operations of concrete block machine  702  by controlling the operation of the drive assembly, such as drive assembly  550 . In one embodiment, as illustrated, controller  700  comprises a programmable logic controller (PLC).  
         [0107]     As described above with respect to  FIG. 1 , mold assembly  430  is selectively coupled, generally via a plurality of bolted connections, to concrete block machine  702 . In operation, concrete block machine  702  first places pallet  56  below mold box assembly  430 . A concrete feedbox  704 ,then fills mold cavities, such as mold cavities  446 , of assembly  430  with concrete. Head shoe assembly  52  is then lowered onto mold assembly  430  and hydraulically or mechanically compresses the concrete in mold cavities.  446  and, together with a vibrating table on which pallet  56  is positioned, simultaneously vibrates mold assembly  430 . After the compression and vibration is complete, head shoe assembly  52  and pallet  56  are lowered relative to mold cavities  446  so that the formed concrete blocks are expelled from mold cavities  446  onto pallet  56 . Head shoe assembly  52  is then raised and a new pallet  56  is moved into position below mold cavities  446 . The above process is continuously repeated, with each such repetition commonly referred to as a cycle. With specific reference to mold assembly  430 , each such cycle produces six concrete blocks.  
         [0108]     PLC  700  is configured to coordinate the extension and retraction of liner plates  432  into and out of mold cavities  446  with the operations of concrete block machine  702  as described above. At the start of a cycle, liner plates  432  are fully retracted from mold cavities  446 . In one embodiment, with reference to  FIG. 14 , drive assembly  550  includes a pair of sensors, such as proximity switches  706   a  and  706   b  to monitor the position of master bar  573  and, thus, the positions of corresponding moveable liner plates  432  coupled to master bar  573 . As illustrated in  FIG. 14 , proximity switches  706   a  and  706   b  are respectively configured to detect when liner plates  432  are in an extended position and a retracted position with respect to mold cavities  446 .  
         [0109]     In one embodiment, after pallet  56  has been positioned beneath mold assembly  430 , PLC  700  receives a signal  708  from concrete block machine  702  indicating that concrete feedbox  704  is ready to deliver concrete to mold cavities  446 . PLC  700  checks the position of moveable liners  432  based on signals  710   a  and  710   b  received respectively from proximity switches  706   a  and  706   b . With liner plates  432  in a retracted position, PLC  700  provides a liner extension signal  712  to hydraulic system  624 .  
         [0110]     In response to liner extension signal  712 , hydraulic system  624  begins pumping hydraulic fluid via path  622   b  to second rod-end  612  of piston assembly  606  and begins receiving hydraulic fluid from first rod-end  610  via path  622   a , thereby causing dual-acting cylinder  607  to begin moving liner plates  432  toward the interiors of mold cavities  446 . When proximity switch  706   a  detects master bar  573 , proximity switch  706   a  provides signal  710   a  to PLC  700  indicating that liner plates  432  have reached the desired extended position. In response to signal  710   a , PLC  700  instructs hydraulic system  624  via signal  712  to stop pumping hydraulic fluid to piston assembly  606  and provides a signal  714  to concrete block machine  702  indicating that liner plates  432  are extended.  
         [0111]     In response to signal  714 , concrete feedbox  704  fills mold cavities  446  with concrete and head shoe assembly  52  is lowered onto mold assembly  430 . After the compression and vibrating of the concrete is complete, concrete block machine  702  provides a signal  716  indicating that the formed concrete blocks are ready to be expelled from mold cavities  446 . In response to signal  716 , PLC  700  provides a liner retraction signal  718  to hydraulic system  624 .  
         [0112]     In response to liner retraction signal  718 , hydraulic system  624  begins pumping hydraulic fluid via path  622   a  to first rod-end  610  via path  622  and begins receiving hydraulic fluid via path  622   b  from second rod-end  612 , thereby causing dual-acting cylinder  607  to begin moving liner plates  432  away from the interiors of mold cavities  446 . When proximity switch  706   b  detects master bar  573 , proximity switch  706   b  provides signal  710   b  to PLC  700  indicating that liner plates  432  have reached a desired retracted position. In response to signal  710   b , PLC  700  instructs hydraulic system  624  via signal  718  to stop pumping hydraulic fluid to piston assembly  606  and provides a signal  720  to concrete block machine  702  indicating that liner plates  432  are retracted.  
         [0113]     In response to signal  720 , head shoe assembly  52  and pallet  56  eject the formed concrete blocks from mold cavities  446 . Concrete block machine  702  then retracts head shoe assembly  52  and positions a new pallet  56  below mold assembly  430 . The above process is then repeated for the next cycle.  
         [0114]     In one embodiment, PLC  700  is farther configured to control the supply of compressed air to mold assembly  430 . In one embodiment, PLC  700  provides a status signal  722  to compressed air system  630  indicative of when concrete block machine  702  and mold assembly  430  are in operation and forming concrete blocks. When in operation, compressed air system  632  provides compressed air via line  630  and pneumatic fitting  628  to housing  560  of mold assembly  420  to reduce the potential for dirt/dust and other debris from entering drive assembly  550 . When not in operation, compressed air system  632  does not provide compressed air to mold assembly  430 .  
         [0115]     Although the above description of controller  700  is in regard to controlling a drive assembly employing only a single piston assembly, such as piston assembly  606  of drive assembly  500 , controller  700  can be adapted to control drive assemblies employing multiple piston assemblies and employing multiple pairs of proximity switches, such as proximity switches  706   a  and  706   b . In such instances, hydraulic system  624  would be coupled to each piston assembly via a pair of hydraulic lines, such as lines  622   a  and  622   b . Additionally, PLC  700  would receive multiple position signals and would respectively allow mold cavities to be filled with concrete and formed blocks to be ejected only when each applicable proximity switch indicates that all moveable liner plates are at their extended position and each applicable proximity switch indicates that all moveable liner plates are at their retracted position.  
         [0116]      FIGS. 18A through 18C  illustrate portions of an alternate embodiment of drive assembly  550  as illustrated by  FIGS. 15A through 15C .  FIG. 18A  is top view of second gear element  640 , wherein second gear element  640  is driven by a screw drive system  806  in lieu of a piston assembly, such as piston assembly  606 . Screw drive system  806  includes a threaded screw  808 , such as an Acme or Ball style screw, and an electric motor  810 . Threaded screw  808  is threaded through a corresponding threaded shaft  812  extending lengthwise through second gear element  640 . Threaded screw  808  is coupled at a first end to a first bearing assembly  814   a  and is coupled at a second end to motor  810  via a second bearing assembly  814   b.    
         [0117]     In a fashion similar to that described by  FIG. 15A , second gear element  640  includes the plurality of angled channels  616  which slideably interlock and mesh with angled channels  616  of first gear elements  572   b  and  572   e , as illustrated by  FIG. 14 . Since second gear element  640  is coupled to linear bearing blocks  646 , when motor  810  is driven to rotate threaded screw  808  in a counter-clockwise direction  816 , second gear element  640  is driven in a direction  818  along linear bearing track  650 . As second gear element  640  moves in direction  818 , angled channels.  
         [0118]     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.