Abstract:
A conveyor is disclosed comprising a track having a pair of side rails and a plurality of interconnected carts which are reciprocally moveable along the track by a pneumatic cylinder. Each of the carts includes a pair of air bladders which are selectively inflatable to move a respective load between a lowered position wherein the load rests on upper surfaces of the rails and a raised position wherein the load is lifted clear of the rails. A pneumatic system controls the cylinder and each pair bladders so as to advance a load from a loading position toward an unloading position by cyclically moving the load into the raised position, moving the carts forward, moving the load into the lowered position, and moving the carts back beneath the load. Individual pairs of bladders may be kept from inflating so as to accumulate loads near the unloading position.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to conveyors for moving load units, such as loaded pallets or slip sheets, from a first location to a second location, and in particular to such a conveyor which acts to accumulate the load units proximate the second location for subsequent removal from the conveyor. 
     2. Description of the Related Art 
     Conveyors of various kinds are generally used in industry to move load units from one location to another. An accumulating conveyor differs from an ordinary transfer conveyor in that it has the ability to retain load units and store them until they can be removed as needed. One well known type of accumulating conveyor generally comprises a track upon which load units are placed and a lifting mechanism for lifting the load units relative to the track. The load units are indexed forward by sequentially lifting a load unit off of the track, moving it forward, and then setting it back down on the track. The lifting mechanism is then moved rearwardly, and the process is repeated. 
     For example, Milazzo, U.S. Pat. No. 3,322,259, discloses (as an alternative embodiment) an Accumulating Pallet Conveyor having a horizontal track with parallel side rails and a carriage with multiple lifting plates or platforms which are reciprocally moveable, with the carriage, along the track by a pneumatic cylinder. The carriage rolls on top of a plurality of rollers which are secured to the side rails. An air bladder is positioned beneath the lifting platform and can be inflated and deflated to raise and lower the lifting platform relative to the carriage. When the platform is lowered, any load unit positioned above the carriage will rest on the track side rails. When the platform is raised the load unit will be lifted clear of the side rails and supported by the carriage. The carriage is reciprocally moveable along the track by a pneumatic cylinder. 
     Inflation and deflation of the bladders in the Milazzo conveyor is controlled by a pneumatic system which includes a normally closed limit switch mounted on the first platform which actuates a solenoid valve controlling airflow through a main line. Each of the bladders is connected to the main line through a respective normally open sensor valve which is mounted in association with the next lifting plate ahead of the lifting plate under which the bladder is mounted. Note that the sensor valves are not mounted directly to the lifting platforms, therefore they only detect the presence of a load unit that is resting on the side rails, not ones that are lifted by the bladders. A normally open sensor valve mounted to the track in the unload position controls airflow to the bladder under the last lifting platform. 
     Thus, when a load is placed on the first lifting platform, each bladder will inflate unless there is a load unit sitting on the side rails above the next lifting platform. As the bladders inflate, the respective load units are lifted off of the side rails. Any of the remaining bladders which are controlled by sensor valves of the lifting platforms with raised loads will also inflate once the load units clear the side rails. In effect, this means that each of the bladders will eventually inflate unless all of the remaining positions nearer to the unloading position are already filled. While this system provides a means of accumulating pallets near the unloading position, it creates an undesirable time delay during the inflation process. 
     There remains a need for an accumulating pallet conveyor and storage system which is relatively inexpensive to manufacture while still providing for the automatic accumulation of palletized loads. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a conveyor for advancing load units from a loading position toward an unloading position. The conveyor includes a track having a pair of side rails, each of the which have an upper surface and an inwardly extending flange spaced downwardly from the upper surface. The track defines a plurality of sequential load unit positions, a first of the load unit positions being a loading position proximate a first end of the track, an d a last of the load unit positions being an unloading position proximate a second end of the track. 
     The conveyor further includes a plurality of interconnected carts which include wheels that ride on the inwardly extending flanges of the track. The number of carts is at least one fewer than the number of load positions. A first one of the carts is located proximate the first end of the track and a last of the, carts is located proximate the second end of the track. Each of the carts is adapted to receive a respective load unit and includes a pair of lifting platforms for raising the respective load unit relative to the track. An air bladder positioned beneath each lifting platform is selectively inflatable and debatable for moving the respective lifting platform between a lowered position wherein the upper surface of the lifting platform is below the upper surface of the track side rails and a raised position wherein the upper surface of the lifting platform is above the upper surface of the track side rails. A load unit positioned on a cart will, therefore, rest on the side rail upper surfaces when the lifting platforms are lowered and be lifted clear of the side rail upper surfaces when the lifting platforms are raised. 
     A pneumatic cylinder is connected between the track and the carts for reciprocally moving the carts along the track between a home position wherein the first cart is in the loading position and a forward position wherein the last cart is in the unloading position. A pneumatic system controls the cylinder and each pair of bladders so as to advance a load unit from the loading position toward the unloading position by cyclically moving the load unit into the raised position, moving the carts into the forward position, moving the load unit into the lowered position, and moving the carts back into the home position beneath the load unit. 
     The pneumatic system includes a plurality of sensor cams, one of which is mounted to a lifting platform of each cart. Each sensor cam is operable in response to a load unit being positioned on the respective cart to actuate a respective sensor valve. Similarly, an unloading position sensor is mounted on the track at the unloading position. The unloading position sensor is operable in response to a load unit being positioned in the unloading position to actuate an unloading position sensor valve. 
     The pneumatic system includes a bladder inflation circuit for selectively inflating the air bladders. Airflow into the bladder inflation circuit is controlled by a bladder inflation valve which is actuated upon the carts arriving in the home position. The bladder inflation circuit includes a first branch which is connected to all of the air bladders. Airflow into the first branch is controlled by the unloading position sensor valve, such that all of the air bladders may be inflated when no load unit is positioned on the track at the unloading position. 
     A second branch of the bladder inflation circuit is connected in parallel to the first branch. Airflow into the second branch is controlled by the sensor valve on the first cart such that no air will flow into the second branch unless a load unit is positioned on the first cart. The second branch includes a plurality of parallel legs. Airflow through each of the legs is controlled by a respective one of the cart sensor valves. Each of the legs is connected to the pair of air bladders on the cart which is one cart nearer to the first cart than is the cart to which the sensor cam actuating the respective sensor valve is mounted. 
     The first branch and the second branch are interconnected such that a respective air bladder is not inflatable if load units are positioned at the unloading position and on any of the carts which are nearer to the last cart than the cart on which the respective air bladder is mounted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a horizontal pallet conveyor embodying the present invention. 
     FIG. 2 is a cross-sectional view of the pallet conveyor taken generally along line  2 — 2  in FIG.  1  and showing the lifting platforms thereof in a lowered position. 
     FIG. 3 is a cross-sectional view of the pallet conveyor taken generally along line  2 — 2  in FIG.  1  and showing the lifting platforms thereof in a raised position. 
     FIG. 4 is an enlarged fragmentary view of a portion of a lifting platform of the pallet conveyor showing a sensor cam and sensor valve attached thereto. 
     FIG. 5 is a schematic diagram showing the pneumatic circuit controlling the pallet conveyor. 
     FIGS. 6-16 are motion studies showing movement of pallets along the conveyor under various loading conditions. 
     FIG. 17 is a perspective view of a slip sheet conveyor which represents an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof 
     Certain terminology will be used in the following description for convenience in reference only and will not be limiting. For example, the words “upwardly,” “downwardly,” “rightwardly,” and “leftwardly” will refer to directions in the drawings to which reference is made. The words “inwardly” and “outwardly” wills refer to directions toward and away from, respectively, the geometric center of the embodiment being described and designated parts thereof. Said terminology will include the words specifically mentioned, derivatives thereof and words of a similar import. 
     Referring to the drawings in more detail, and in particular to FIG. 1, the reference number  1  generally designates a horizontal pneumatic conveyor  1  which embodies the fly present invention. The conveyor  1  is adapted for moving pallets or other load units  3  from a first end  5  of the conveyor  1  toward a second end  7  of the conveyor  1 . The pallets  3  are indexed along the conveyor  1  through a plurality of positions  9 , for example six positions  9   a - 9   f  as shown in FIG.  1 . The first position  9   a  is a loading position into which the pallets  3  are placed, such as by a forklift (not shown). The last position  9   f  is an unloading position from which the pallets  3  are removed from the conveyor  1 , typically by a forklift. 
     The conveyor  1  comprises a frame  11  having a pair of parallel spaced tracks or rails  13  which may be formed of steel channel stock. The rails  13  are supported on a framework (not shown) so as to extend generally horizontally. Each rail includes an upper surface  15  and an inwardly oriented channel  17  formed between opposing webs  19 . A plurality of carts  21 , which are one fewer in number than the number of positions  9  (five carts  21   a - 21   e  shown) are mounted for movement along the rails  13 . The carts  21  are linked together so as to move in unison. 
     Each cart  21  includes a pair of cross members  23 , which form the front and rear of the respective cart  21 . Referring to FIG. 2, a pair of longitudinal members  25  are connected between the cross members  23 . The cross members  23  may be lengths of angle stock, whereas the longitudinal members  25  are inverted channels. The cross members  23  have, at each end, a wheel mounting flange  27  to which a respective wheel  29  is rotatably mounted. The wheels  29  of the carts  21  ride within the channels  17  of the rails  13 . The wheels  29  may be mounted at an angle relative to vertical in order to conform to and ride upon the lower webs  19  of the rails  13 , which are typically canted relative to horizontal. 
     Each cart  21  further includes a pair of lifting platforms  31  which are in the form of inverted channels sized to receive the longitudinal members  25  of the cart  21 . Each lifting platform  31  has an upper face  33 . As best seen in FIG. 4, the platforms  31  are each connected to a respective one of the longitudinal members  25  by a pair of bolts  35  which pass through respective pairs of elongated holes  37  in the lifting platforms  31 . The elongate holes  37  allow the platforms  31  to be moved vertically between raised and lowered positions. In the lowered position, the upper faces  33  of the platforms  31  are spaced downwardly from the upper surfaces  15  of the rails  13 , as shown in FIG.  2 . In the raised position, the upper faces  33  of the platforms  31  are spaced upwardly from the upper surfaces  15  of the rails  13 , as shown in FIG.  3 . 
     Each cart  21  also includes a pair of air bladders  39 , one of which is interposed between each lifting platform  31  and the respective longitudinal member  25 . The bladders  39  may be constructed of lengths of fire hose having the ends crimped together to form an airtight seal. Each bladder  39  includes a threaded inlet/outlet port. (not shown) for connecting the respective bladder  39  into a pneumatic system  43  of the conveyor  1  to be described later in detail. 
     As best seen in FIG. 4, each cart  21  further includes a sensor cam  45  which is pivotally connected to a respective one of the lifting platforms  31  by a mounting plate  46  and is biased upwardly by a respective spring  47 . Each sensor cam  45  includes a first or upper cam face  49  which will be engaged by a pallet positioned on the respective cart  21  so as to pivot the sensor cam  45  downwardly. A second or end cam surface  51  on each sensor cam  45  engages a roller cam or trigger  52  of a respective sensor valve  53  so as to depress the trigger  52  when the sensor cam  45  is pivoted downwardly by a pallet  3  and thereby actuate the valve  53 . 
     Referring again to FIG. 1, an unloading position sensor  55  is mounted on the frame  11  at the unloading position  9   f  so as to be pivoted downwardly by any pallet  3  which is located in the unloading position  9   f . The sensor  55  engages a trigger  56  of an unloading position sensor valve  57  (shown schematically in FIG. 5) so as to depress the trigger  56  when the sensor  55  is pivoted downwardly by a pallet  3 . 
     The carts  21  are moved reciprocally back and forth along the rails  13  by a double-acting pneumatic indexing cylinder  59  which is connected between the frame  11  and one of the carts  31 . The cylinder  59  is shown as having a cylinder body  61  secured to cart  21   c by brackets  63 . A rod  65  of the cylinder  59  is secured to a cross member  67  which forms a part of the frame  11 . When the rod  65  is fully extended, the carts  21  are moved rearwardly, toward the first end  5  of the conveyor  1 , such that cart  21   a  is located in the loading position  9   a  and cart  21   e  is in position  9   e  (hereinafter the “home position”). When the rod  65  is fully retracted, the carts  21  are moved forwardly, toward the second end  7  of the conveyor  1 , such that cart  21   a  is in position  9   b  and cart  21   e  is located in the unloading position  9   f  (hereinafter the “forward position”). It is foreseen that the orientation of the indexing cylinder could be reversed such that when the rod  65  is retracted, the carts are positioned such that cart  21   a  is located in the loading position  9   a , and extension of the rod  65  advances the carts toward the second end  7  of the conveyor  1  and advances cart  21   e  to the unloading position  9   f.    
     The pneumatic system  43  is schematically depicted in FIG.  5 . For each cart  21   a - 21   e , the respective pairs of air bladders  39  are designated as  39   a - 39   e  and the respective sensor valves  53  are designated as  53   a - 53   e . Compressed air is supplied to the pneumatic system  43  through a main line  68  from an air source  69  such as an air compressor. A lockout valve  71  controls airflow through the main line  68  and serves to turn the system  43  off and on. A pressure regulator  73 , also installed in the main line  68 , is adjustable to vary the air pressure within the system  43  to match the weight of the pallets  3  being moved by the conveyor  1 . The lighter the pallets  3 , the less air pressure is required. 
     The main line  68  connects to a junction  75  where the flow splits into three branches; an inflation branch  77 , an indexing branch  79 , and a deflation branch  81 . The inflation branch  77  provides air to a bladder inflation circuit  82  and a cylinder extension control circuit  83 . The indexing branch  79  provides airflow to a conveyor indexing circuit  84 , and the deflation branch  81  provides airflow to a bladder deflation circuit  85 . 
     The inflation branch  77  connects to a four-way inflation control valve  87  which is actuated by the cylinder  59  becoming fully extended such that the carts  21  are in the home position. When actuated, the inflation control valve  87  directs air to the bladder inflation circuit  82  which acts to inflate the air bladders  39 . When deactuated, the valve  87  directs air to the cylinder extension control circuit  83 , which triggers extension of the cylinder  59  as will be described hereinafter. 
     Downstream from the inflation control valve  87 , the bladder inflation circuit  82  divides into two parallel branches  89  and  91  through which the air bladders  39  may be inflated. A first branch  89  of the bladder inflation circuit  82  is controlled by the unloading position sensor valve  57 , which is normally open, and will be closed only upon actuation by a pallet  3  moving into the unloading position  9   f . Thus, if a pallet  3  is positioned in the unloading position  9   f , no air will flow through the first branch  89 . From the unloading position sensor valve  57 , air flows to all five pairs of the air bladders  39   a - 39   e , which are connected in parallel to one another. Therefore, if no pallet  3  is located in the unloading position  9   f , all of the air bladders  39  will inflate upon actuation of the inflation control valve  87  by the cylinder  59  becoming fully extended. Check valves  92  positioned on each side of each pair of bladders  39  allow airflow from the unloading position sensor valve  57  toward the bladders  39  but prevent airflow in the opposite direction. 
     A second branch  91  of the bladder inflation circuit  82  is controlled by sensor valve  53   a , which is normally closed, and will be opened only upon actuation by a pallet  3  being placed on cart  21   a . Thus, if no pallet  3  is positioned on cart  21   a , no air will flow through the second branch  91 . From the sensor valve  53   a , air flows to the remainder of the sensor valves  53   b - 53   e  which are normally open and connected in parallel to one another. 
     Each of the sensor valves  53   b - 53   e  controls airflow through a respective leg  93   b - 93   e  which is connected to the air bladders  39  of the cart  21  behind the cart on which the respective sensor valve  53   b - 53   e  is mounted. For example, sensor valve  53   b  controls airflow through leg  93   b  which is connected to bladders  39   a , sensor valve  53   c  controls airflow through leg  93   c  which is connected to bladders  39   b , etc. Each leg  93   b - 93   e  includes a respective check valve  94  which allows airflow from the respective sensor valve  53  toward the respective air bladders  39  but prevents flow in the opposite direction. 
     The second branch  91  of the bladder inflation circuit  82  allows each pair of bladders  39   a - 39   d  to be individually locked out and thereby prevented from inflating if all of the positions  9   a - 9   f  nearer to the second end  7  of the conveyor  1  are already filled by respective pallets  3 . For example, if positions  9   c - 9   f  are already filled, sensor valves  53   c - 53   e  will be actuated (closed) and bladders  39   b  will not inflate upon actuation of the inflation control valve  87  (upon retraction of the cylinder  59 ). Note that bladders  39   e  are not operable by the second branch  91  and can, therefore, only be inflated through the first branch  89 . 
     Because the first branch  89  and second branch  91  of the bladder inflation circuit  82  are commonly connected to each of the pairs of bladders  39   a - 39   d , the two branches share a common outlet line  95  which will receive air if either sensor valve  53   a  or the unloading position sensor valve  57  is open. Connected to the inflation circuit outlet line  95  is a retraction timer  97  which starts running upon inflation of one or more pairs of the air bladders  39 . The timer  97  is adjustable to create a time delay of from 0-30 seconds after which the timer sends a pilot signal though a pilot line  98  to a cylinder control valve  99 , to be described later, which controls flow through the conveyor indexing circuit  84 . In response to the pilot signal, the valve  99  causes the indexing cylinder  59  to retract. 
     When the inflation control valve  87  is deactuated by the cylinder  59  moving out of its fully extended position, air flows into the cylinder extension control circuit  83 . Airflow through the circuit  83  is controlled by a pilot operated extension control valve  101 . The extension control valve  101  is normally open and is closed by a pilot signal transmitted from the inflation circuit outlet line  95  through a pilot line  102 . The valve  101  is thus closed when there is pressure in the outlet line  95  and returns to an open condition upon release of pressure from the bladder inflation circuit  82 . (Pressure is released from the bladder inflation circuit  82  by the bladder deflation circuit  85 , to be described hereinafter.) 
     Upon opening of the extension control valve  101 , air flows to an extension timer  103 . The timer  103  is adjustable to create a time delay of from 0-120 seconds after which the timer sends a pilot signal to the cylinder control valve  99  through a pilot line  104 . In response to the pilot signal, the valve  99  causes the indexing cylinder  59  to extend. 
     The indexing branch  79  supplies air to the conveyor indexing circuit  84  through the cylinder control valve  99 , which is a four-way two position pilot-operated valve. The cylinder control valve  99  acts to control extension and retraction of the cylinder  59 . The indexing circuit  84  includes a base end line  105  and a rod end line  107  which connect the cylinder control valve  99  to the cylinder  59 . Each line  105  and  107  includes a respective speed control valve  109  which is adjustable to vary the speed at which the cylinder  59  extends or retracts. 
     The cylinder control valve  99  is actuated to retract the cylinder  59  by a retraction signal from the retraction timer  97  mounted in the bladder inflation circuit  82 . In response to the retraction signal, the cylinder control valve  99  directs air through the rod end line  107  to the rod end of the cylinder  59 , causing it to retract and move the carts  21  to the forward position. Similarly, the cylinder control valve  99  is actuated to extend the cylinder  59  by an extension signal from the extension timer  103  in the cylinder extension control circuit  83 . In response to the extension signal, the cylinder control valve  99  directs air through the base end line  105  to the base end of the cylinder  59 , causing it to extend and move the carts  21  to the home position. 
     Air is released from the bladders  39  through the bladder deflation circuit  85 . The bladder deflation circuit  85  is connected to the bladder inflation circuit  82  through a deflation valve  111  which is actuated by the cylinder  59  becoming fully retracted (carts  21  in the forward position). The deflation valve  111  is normally closed and opens upon actuation to deflate the air bladders  39 . Also actuated by the cylinder  59  becoming fully retracted is a vacuum generator valve  113  which supplies air from the deflation branch  81  to a vacuum generator  115 . The vacuum generator  115  is connected to the outlet from the deflation valve  111  and acts to accelerate deflation of the bladders  39  by pulling air out of the inflation circuit  82 . In order to further accelerate deflation, one or more supplementary vacuum generators  117  (one shown in FIG. 4) may be added to the deflation circuit  85  in parallel with the vacuum generator  115 . These supplementary vacuum generators  117  are connected to the bladder inflation circuit  82  through respective pilot operated deflation valves  119  which open upon receiving a pilot signal from the bladder deflation circuit  85 . 
     In use, a first pallet  3   a  is placed on cart  21   a  in the loading position  9   a  (as shown in FIG.  6 ), and the conveyor  1  is turned on by opening the lockout valve  71  (see FIG.  5 ). Because the cylinder  59  is fully extended, the inflation control valve  87  will be actuated, allowing air to flow into the bladder inflation circuit  82 . Air will flow through the first branch  89  of the bladder inflation circuit  82  inflating all of the air bladders  39   a - 39   e  because no pallet  3  is located in the unloading position  9   f  and the unloading position sensor valve  57  will, therefore, remain open. Inflation of the bladders  39   a  will cause the respective lifting platforms  31  to engage the pallet  3   a  and lift it clear of the upper surfaces  15  of the frame rails  13 . Inflation of the bladders  39  will activate the retraction timer  97 . After the preset delay period, the timer  97  will send a retraction signal to the cylinder control valve  99 , which will supply air to the rod end of the cylinder  59 , causing the cylinder  59  to begin to retract and the carts  21  to move toward the forward position. As the cylinder  59  moves out of its fully extended position, the inflation control valve  87  will be deactuated, shutting off flow to the bladder inflation circuit  82  and supplying air to the cylinder extension control valve  101 . 
     When the cylinder  59  reaches its fully retracted position and the carts  21  reach the forward position, the deflation valve  111  and the vacuum generator valve  113  will be actuated. Air will then flow out of the bladders  39  assisted by vacuum from the vacuum generator  115 . A pilot signal will also be sent to the pilot operated deflation valves  119 , creating flow through the valves  119  which is assisted by the respective supplementary vacuum generators  117 . 
     As the bladders  39  deflate, the lifting platforms  31  will drop, and the pallet  3   a  will come to rest on the upper surfaces  15  of the frame rails  13  (as shown in FIG. 7) in the second position  9   b . As the pressure drops in the bladder inflation circuit  82 , the extension control valve  101  will open, activating the extension timer  103 . After the preset delay period, the timer  103  will send an extension signal to the cylinder control valve  99 , which will supply air to the base end of the cylinder  59 , causing the cylinder  59  to extend and the carts  21  to move back to the home position. Because the pallet  3   a  is supported by the frame rails  15 , it will remain in the second position  9   b  (as shown in FIG. 7) as the carts  21  move back to the home position beneath it. 
     Once the carts  21  return to the home position, the cycle begins over again; this time with the single pallet  3   a  starting in the second position  9   b  over the cart  21   b  and actuating sensor valve  53   b , which is thereby moved to a closed condition. Even though the valve  53   b  is closed, all of the bladders  39   a - 39   e  will continue to inflate because the unloading position sensor valve  57  remains open, and air can reach the bladders  39   a - 39   e  through the first branch  89  of the bladder inflation circuit  82 . The cycle, therefore operates exactly as described above and moves the pallet  3   a  to the third position  9   c . The cycle continues to be repeated, moving the pallet  3   a  one position forward per cycle until it reaches the unload position  9   f  as shown in FIG.  8 . 
     With the pallet  3   a  located in the unloading position  9   f , the unloading position sensor valve  57  is moved to a closed condition. After the carts  21  return to the home position, the conveyor  1  will stop until either the pallet  3   a  is removed from the conveyor or another pallet  3  is placed in the loading position  9   a  so as to actuate (and thereby open) sensor valve  53   a . This is because no air will flow through the bladder inflation circuit  82  with the unloading position sensor valve  57  and sensor valve  53   a  both closed (again, sensor valve  53   a  is normally closed and only opened when a pallet  3  is placed in the loading position  9   a ). With no airflow through the bladder inflation circuit  82 , the retraction timer  97  will not being activated to cause retraction of the cylinder  59 . 
     If a second pallet  3   b  is placed on the conveyor  1  at the loading position  9   a , as shown in FIG. 9, sensor valve  53   a  will be actuated, allowing air to flow through the second leg  91  of the bladder inflation circuit  82  and thereby causing bladders  39   a - 39   d  to inflate. Bladders  39   e  will not inflate because the unloading position sensor valve  57  is closed due to the presence of pallet  3   a  in loading position  9   f  acting on sensor  55 . The system is designed to prevent bladders  39   e  from inflating when a pallet  3 , such as pallet  3   a , is in the unloading position  9   f  in order to prevent the lifting platforms  31  on cart  21   e  from raising and thereby pushing pallet  3   a  off the end of the rails  13  when cart  21   e  is subsequently advanced beneath the unloading position  9   f.    
     In addition to inflating the bladders  39   a - 39   d , airflow into the bladder inflation circuit  82  also starts the retraction timer  97 . The conveyor  1  can then complete its cycle, and move pallet  3   b  to the second position  9   b . After the pallet  9   b  reaches the second position  9   b  (as shown in FIG. 10) and the carts  21  return to the home position, the conveyor  1  will again stop until either the pallet  3   a  is removed from the unloading position  9   f  or another pallet  3  is placed in the loading position  9   a . This is an energy saving feature of the conveyor  1  which takes advantage of the idea that there is no need to continue to advance pallets  3  along the conveyor  1  so long as there is a pallet  3  already located in the unloading position  9   f  and the loading position  9   a  is open to accept a new pallet  3 . 
     Placement of a third pallet  3   c  in the loading position  9   a  as shown in FIG. 11, causes sensor valve  53   a  to again be actuated, allowing air to flow through the second leg  91  of the bladder inflation circuit  82  and inflating bladders  39   a - 39   d . The conveyor  1  can again cycle, moving pallet  3   b  to the third position  9   c  and pallet  3   c  to the second position  9   b  (as shown in FIG.  12 ). After the carts  21  return to the home position, the conveyor  1  will again stop. 
     If pallet  3   a  is removed from the unloading position  9   f  (as shown in FIG.  13 ), the unloading position sensor valve  57  will be deactuated, allowing air to flow through the first branch  89  of the bladder inflation circuit  82  and thereby inflating bladders  39   a - 39   e  and starting the retraction timer  97 . Because the unloading position sensor valve  57  is open, the conveyor  1  will not stop after moving the pallets  3   b  and  3   c  ahead a single position  9 , but instead will continue to cycle and move pallets  3   b  and  3   c  in unison until pallet  3   b  is in the unloading position  3   f  and pallet  3   c  is in position  3   e  (as shown in FIG.  14 ). After the pallets  3   b  and  3   c  have reached their respective positions  9   f  and  9   e , and the carts  21  have returned to the home position, the conveyor  1  will stop moving. 
     Adding an additional pallet  3   d  to the conveyor  1  (as shown in FIG.  15 ), will again cause sensor valve  53   a  to be actuated, allowing air to flow through the second leg  91  of the bladder inflation circuit  82 . The conveyor  1  can then move pallet  3   d  to the second position  9   b  (as shown in FIG.  16 ). Note that the conveyor  1  does not attempt to move either pallet  3   b  or pallet  3   c  as pallet  3   d  is indexed forward. Pallet  3   c  is not lifted by bladders  39   e  when the bladder inflation circuit  82  is pressurized because pallet  3   b  actuates and closes the unloading sensor valve  57 , cutting off airflow to bladders  39   e  through the first branch  89 . Furthermore, pallet  3   c  actuates sensor valve  53   e  and cuts off flow to bladders  39   d  through the second branch  91 . Because the respective bladders  39   e  and  39   d  are not inflated, carts  21   e  and  21   d  slide beneath pallets  3   b  and  3   c  as carts  21  reciprocate without moving them. 
     As additional pallets  3  are added to the conveyor  1 , more pairs of bladders  39  are locked out by the second branch  91  of the bladder inflation circuit  82  such that the additional pallets  3  can accumulate behind pallet  3   b  without attempting to push pallet  3   b  off the second end  7  of the conveyor  1 . 
     FIG. 17 depicts a slip sheet conveyor  200  which represents an alternative embodiment of the present invention. The conveyor  200  is adapted to convey goods supported on a slip sheet, such as a sheet of corrugated paper, instead of on a pallet. The conveyor  200  is generally identical to the conveyor  1  previously described except that it includes frame side rails  213  which are wider than the rails  13  of the conveyor  1 , and further includes a plurality of plates  220  which each cover a respective cart  21 . The plates  220  and wider side rails  213  both serve lend additional support to the slip sheets. The side rails  213  each include an inwardly extending flange  222  upon which the wheels  29  of the carts  21  roll. 
     The plates  220  are generally supported on the upper surfaces  33  of the lifting platforms  31  of each cart  21  and thereby acts as an extension of the platforms  31 . In order to further support the plates  220 , additional cross members  224  may be added to the carts  21  between the respective lifting platforms  31 . The upper surface of each plate  220  is spaced below the upper surfaces  215  of the rails  213  when the bladders  39  on the respective cart  21  are deflated and extends above the upper surfaces  215  when the when the respective bladders are inflated. Each plate  220  includes an elongate opening  226  through which the sensor cams  45  extend. 
     It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown. For example, the conveyor  1  has been described and depicted herein as being a first-in-first-out type conveyor where pallets or other loads are put on the conveyor at a first end and moved toward a second end where they are unloaded. It is to be understood that the same novel design and pneumatic circuitry could also be adapted to a first-in-last-out type conveyor where the loads are both added and removed at the first end and accumulated toward the second end.