Abstract:
An elevatable conveyor for moving articles between different material handling equipment comprises a frame and a carriage mounted on the frame to move in a substantially vertical direction with respect to the frame. The carriage carries first and second conveyors capable of moving the material thereon and having different vertical elevations on the carriage. A cylinder is connected to the frame, and the cylinder has a cylinder rod connected to the carriage. The cylinder is operable to move the carriage and the first and second conveyors in a substantially vertical direction between first and second vertical positions.

Description:
FIELD OF THE INVENTION 
     This invention relates to material handling and more particularly, to a multilevel conveyor section movable to different elevations. 
     BACKGROUND OF THE INVENTION 
     Ongoing efforts to increase the efficiency and output of production operations have resulted in a continuing focus on how materials are moved in a manufacturing environment. In many environments, it is desirable to automatically move materials without human intervention. For example, parts are often buffered or temporarily stored in a material handler, for example, an automated parts buffer (“APB”), an automated storage and retrieval system (“ASRS”), etc.; and the parts are moved between the ASRS and part processing stations or equipment by material handlers, for example, an automated guided vehicle (“AGV”). In many applications, the AGV carries the parts in a stacked configuration, that is, at two different vertical levels or heights, thereby increasing the capacity and flexibility of the AGV. For example, being able to buffer or carry parts on conveyors on the AGV at two different heights, doubles the load carrying capacity of the AGV and permits an AGV to simultaneously transfer multiple parts with other part handling equipment, for example, an ASRS or a part processing station. Consequently, an AGV can transfer parts to or from, or simultaneously to and from, other part handling equipment. Thus, the AGV has significant flexibility in interfacing with other part handling equipment. 
     If an AGV must handle parts on conveyors at two different elevations, then the vertical distance separating the conveyors on the AGV must be the same as the vertical distance separating conveyors on the part handling equipment interfacing with the AGV. Commercial specifications of different equipment manufacturers facilitate obtaining a common separation between the conveyors on the AGV and its associated part handling equipment. However, the height or vertical position of the conveyors on the AGV with respect to a floor is often different from the vertical position of the conveyors on the part handling equipment associated with the AGV. Therefore, in transferring parts from stacked conveyors on the AGV, the parts must often be moved vertically up or down prior to the parts being at a height that matches the height of the conveyors on the associated part handling equipment. 
     If the vertical positions of the conveyors on the AVG and the vertical positions of the associated part handling equipment are different, then a direct transfer of parts between the AGV and the part handling equipment cannot occur. Under these conditions, in some applications, the parts are manually moved between the AGV to its associated equipment which is normally undesirable in an otherwise fully automated environment. In other applications, the parts are moved with inclined conveyors between the AGV and the associated part handling equipment. Inclined conveyors can take the form of linear or spiral conveyors, and both devices consume a significant area or floor space, thereby limiting their potential benefits and economies. Another option is to use a scissors lift. However, the requirement of providing a scissors lift with two vertically separated pass-through conveyors results in a scissors lift that is relatively complicated in design and expensive. 
     Consequently, there is a need for a material handling system that can transfer multiple parts from an AGV to associated part handling equipment having different elevations and that does not have the limitations and disadvantages of known devices. 
     SUMMARY OF THE INVENTION 
     The present invention provides a simple, compact, inexpensive and reliable elevatable conveyor section that may be used to interconnect existing conveyors having different heights. The elevatable conveyor section is especially useful for those applications in which floor space is at a premium, and it is desired to minimize the area consumed by the conveying elements. 
     According to the principles of the present invention and in accordance with the preferred embodiments, the invention provides an elevatable conveyor for moving articles between different material handling equipment. The elevator conveyor has a frame and a carriage supported by the frame which moves in a substantially vertical direction with respect to the frame. The carriage carries first and second conveyors capable of moving the material thereon and having different vertical locations on the carriage. A cylinder is connected to the frame, and the cylinder has a cylinder rod connected to the carriage. The cylinder is operable to move the carriage and the first and second conveyors in a substantially vertical direction between first and second vertical positions. 
     In one aspect of the invention, the conveyor elevator includes a first sensor for detecting a presence of an article on the first conveyor; and a second sensor for detecting a presence of an article on the second conveyor. 
     In another embodiment of the invention, a method is provided of transferring articles between first and second material handlers and first and second conveyors on a conveyor elevator. First, the conveyor elevator is located between the first and the second material handlers. Next, the first and second conveyors are moved in a vertical direction to a first vertical position aligning the first and second conveyors with the first material handler. Material is then transferred from the first material handler to the first conveyor of the elevator conveyor. The first and second conveyors are then moved in the vertical direction to a second vertical position aligning the first and second conveyors with the second material handler, and material is then transferred from the first conveyor of the elevator conveyor to the second material handler. 
     These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a bidirectional elevator conveyor in its lowered position in accordance with the principles of the present invention. 
     FIG. 2 is an end view of the elevator conveyor of FIG.  1 . 
     FIG. 3 is a schematic block diagram of the components used to control the elevator conveyor of FIG. 1 as well as control components of other devices operating with the elevator conveyor. 
     FIG. 4 is a flow chart illustrating a process by which material is transferred between an AGV and the elevator conveyor of FIG.  1 . 
     FIG. 5 is a flow chart illustrating a process by which material is transferred between an ASRS and the elevator conveyor of FIG.  1 . 
     FIGS. 6A and 6B are side views of the elevator conveyor of FIG. 1 in its respective lowered and raised positions. The structure of the outer frame has been removed for clarity. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, an elevator conveyor  20  is comprised of an outer or standing frame  22  mounted to a floor  24  and an inner frame or elevator carriage  26  mounted for vertical motion with respect to the outer frame  22  by means of linear guide rods  28 . Upper and lower conveyors  30 ,  32 , respectively, are mounted to the elevator carriage  26 . The elevator carriage  26  and conveyors  30 ,  32  are moved in the vertical direction by pneumatic fluid cylinders  34 . The elevator conveyor  20  functions to transfer materials between an AGV  125  (FIG. 6A) located at one end  36  of the conveyor elevator  20  and an ASRS  126  located at an opposite end  38  of the elevator conveyor  20 . The need to transfer materials between the AGV  125  and the ASRS  126  exists in many manufacturing and storage facilities. Further, it is common for both the AGV  125  and the ASRS  126  to have respective upper conveyors  127 ,  131  and respective lower conveyors  129 ,  133  that are a fixed distance apart, for example, 17 inches. However, it is equally common that the lower conveyor  129  on the AGV  125  is one height, for example, 10 inches, above the floor  24 , and the lower conveyor  12  of the ASRS  126  is a different height, for example, 19 inches, above the floor  24 . Thus, the conveyor elevator  20  is able to transfer material to and/or from an AGV  125  at the one end  36  with the elevator carriage  26  and conveyors  30 ,  32  in a lower position as illustrated in FIG.  6 A. Thereafter, the cylinders  34  are operated to raise the elevator carriage  26  and conveyors  30 ,  32  to a higher position (FIG.  6 B), thereby permitting the material on the elevator conveyor  20  to be transferred to and/or from conveyors  131 ,  133  of an ASRS  126  located at the opposite end  38  of the elevator conveyor  20 . 
     The outer frame  22  is a generally rectangular frame structure comprised of four vertical legs or posts  40  located at the corners of the outer frame  22 . Upper and lower pairs of outer siderails  42 ,  44 , respectively, are connected to respective upper and lower ends of the legs  40  to form a side frame member. Further, a pair of lower outer crossrails  46  is connected between the vertical posts  40  close to their lower end. First and second upper, outer crossrails  47 ,  48  are connected to the vertical posts  40  at different elevations to conform to the different elevations of the AGV and ASRS with which the elevator conveyor  20  operates. Further, the outer crossrails  46 ,  47 ,  48  provide a desired width to the outer frame  22 . The upper, outer crossrails  47 ,  48  have gussets  50  to further strengthen the outer frame. The upper, outer crossrails  47 ,  48  and associated gussets  50  are vertically adjustable on the legs  40 , so that any potential interference with between the crossrails  47 ,  48  material moving between the conveyor elevator  20  and interfacing material handling equipment such as an AGV or an ASRS can be eliminated. The legs  40  have respective fixed feet  52  and respective adjustable feet  54 . Each of the adjustable feet  54  has a threaded shaft that is screwed into the lower end of a respective vertical leg  40 . Thus, the outer frame  22  and the conveyor elevator  20  may be leveled on the floor  24  by utilizing the adjustable feet  54 . 
     The elevator carriage  26  is also a generally rectangular frame structure comprised of four vertical inner posts  56  located generally at the corners of the elevator carriage  26 . Upper and lower pairs of inner siderails  58 ,  60 , are connected at the upper and lower ends, respectively, of the vertical posts  56 . Wear strips  59  are applied to the opposed inner surfaces of the siderails  58 ,  60 , and guides  61  are attached at the ends of the siderails  58 ,  60 . The carriage  26  has a width that provides a clearance of approximately 0.5 inches between the siderails  58 ,  60  and the largest pallet or tote to be conveyed across the elevator conveyor  20 . The guides  61  have an angled surface so that the pallets or totes are steered toward the middle of the conveyors  30 ,  32 . The wear strips  59  and guides  61  are made of a durable, low friction material, for example, an ultrahigh molecular weight material. Gussets  57  strengthen the connection between the vertical posts  56  and the siderails  58 ,  60 . Upper conveyor siderails  62  are mounted to, and below, the upper, inner siderails  58 ; and lower conveyor siderails  64  are mounted to, and below, the lower, inner siderails  60 . A motorized roller  66  is drivingly connected by means of drive belts  68  to a plurality of, for example, five, idler rollers  70 , thereby forming the upper conveyor  30 . The ends of the rollers  66 ,  70  are rotatably mounted within the upper conveyor siderails  62 . The lower conveyor  32  is formed by a motorized roller  72  connected by drive belts  74  to a plurality of, for example, five, idler rollers  76 . The motorized roller  72  and idler rollers  76  are rotatably connected at their ends to the lower conveyor siderails  64 . Upper and lower pairs of inner crossrails  77 ,  79 , respectively, are mounted to the respective upper and lower ends of the vertical posts  56  below the respective conveyors  30 ,  32 . 
     Each of the vertical posts  40  has upper and lower bearing blocks  78 ,  80  mounted thereto. The bearing blocks  78 ,  80  slide on respective linear guides  28 . The bearing blocks  78 ,  80  and linear guides  28  combine to form a linear bearing and can be implemented with many known devices, for example, center bronze bushing or a recirculating ball linear bearing commercially available from Tompson of Port Washington, N.Y. The upper ends of the guide rods  28  are inserted in holes drilled in lower surfaces of the upper, outer siderails  42 . The lower ends of the guide rods  28  are mounted to blocks  82  that, in turn, are adjustably mounted to the lower, outer siderails  44 . The lower ends of the guide rods  28  are adjustable so that the guide rods  28  can be made as parallel as possible. Thus, the elevator carriage  26  is mounted for motion in the vertical direction on the outer frame  22 . The bases of the cylinders  34  are rigidly connected to a mounting plate  84  that, in turn, is mounted by fasteners or other means to one of the lower, outer siderails  44 . The cylinders  34  further have rods  86  extending from an upper end thereof and which are connected at their distal ends to respective floating joint blocks  88 . The floating joint blocks  88  are implemented with a pivoting joint, for example, a ball and knuckle joint, and are specified and purchased with the cylinders  34 . The floating joint blocks  88  are mounted by fasteners or other means to one of a pair of tie bars  90  connected to the vertical posts  56  on opposite sides of the elevator conveyor  26 . Thus, by selectively activating the cylinders  34 , their respective rods  86  can be extended or retracted to, in turn, respectively raise and lower the elevator carriage  26  with the upper and lower conveyors  30 ,  32 . 
     The vertical posts  40 , 56 , siderails  42 ,  58 ,  60  and crossrails  46 ,  47 ,  48 ,  77 ,  79  are normally of the same cross-sectional area, whereas, the siderails  44  and tie bars  90  are approximately twice the width for additional strength. The vertical posts  40 , 56 , siderails  42 ,  44 ,  58 ,  60 , tie bars  90  and crossrails  46 ,  47 ,  48 ,  77 ,  79  can be made from any material suitable for such structural members, for example, extruded aluminum beams and associated fasteners commercially available from MB Kit Systems Ltd. of Akron, Ohio. The outer and inner frames are assembled by tapping a center through-hole in the ends of the siderails and crossrails, drilling clearance holes through the sides of the vertical posts at the appropriate locations, and using a threaded fastener to connect the rails together. The conveyor siderails can be made from any stock material that provides sufficient strength, for example, an extruded aluminum right angle bar stock or angle iron. The roller assemblies comprising the conveyors  30 ,  32  are commercially available from Interroll Corporation of Wilmington, N.C. and the cylinders  34  are commercially available from SMC Pneumatics of Indianapolis, Ind. 
     Referring to FIG. 3, the elevator conveyor  20  includes an elevator controller  100  which provides output signals on lines  102 ,  104  to operate the upper and lower roller motors  66 ,  72 , respectively. In addition, at the appropriate times, the elevator controller  100  provides command signals on an output  106  to a solenoid valve  107  that appropriately ports pressurized air to the cylinders  34 , thereby causing the cylinders  34  to either extend or retract their respective cylinder rods  86 . The presence and absence of a pallet or tote on the conveyors  30 ,  32 , is detected by respective proximity sensors  108 ,  110 . Referring to FIG. 1, in this example, the proximity sensor  108  is mounted on the inner, upper side rail  58  at the opposite end  38  of the conveyor  30 ; and the proximity sensor  110  is mounted on the inner, upper side rail  58  at the one end  36  of the conveyor  32 . The proximity sensors  108 ,  110  are often photoreflective sensors, but may be any other type of known proximity sensor suitable for the purpose and environment. The elevator controller  100  is in electrical communication via a communications link  112  with a master controller  114 . The elevator controller  100  is normally a commercially available programmable logic controller, and the and the master controller  114  is normally a commercially available personal computer which is more rugged for use in a manufacturing environment. The master controller  114  provides communications with other controllers in the manufacturing environment, for example, a shop floor controller  116  and an AGV controller  118 . The communications link  120  between the master controller  114  and the shop floor controller  116  is normally a hard wired link but may also be an RF wireless communications link. Since the AGV  125  and its associated controller  118  are moving along paths within the manufacturing or warehousing facility, the communications link  122  is normally a wireless communications link, for example, an RF communications link. The RF link is implemented using a transmitter/receiver or transceiver  123  located on the conveyor elevator  20  and a transmitter/receiver or transceiver  124  located on the AGV. The ASRS  126  has material handling devices controlled by an ASRS controller  128 . The ASRS  126  is a stationary device and therefore, is in electrical communications with the shop floor controller  116  via a communications link  130  that is normally a wired communications link. However, as will be appreciated, the communications link  130  may also be a wireless link. 
     In use, the shop floor controller  116  coordinates the flow of material through the facility. Material, for example, optical discs, are stacked on spindles which, in turn, are loaded into totes or pallets. During the manufacturing process, a tote of discs is moved by the AGV  125  between a processing station and temporary storage such as the ASRS  128 ; and when at the ASRS  128 , one or more totes of discs is transferred between the AGV  125  and the ASRS  128 . In order to move the AGV  125  carrying a tote from one location to another, the shop floor controller  116  commands the AGV  125  along a path between the first location and coordinates the transfer of a tote onto the AGV. Thereafter, the shop floor controller  116  commands the AGV  125  to move to the second location, for example, the ASRS  128 . The shop floor controller coordinates a transfer of the tote from the AGV  125  to the elevator conveyor  20  and thereafter, from the elevator conveyor  20  to the ASRS  128 . 
     Upon the AGV  125  receiving a tote from, for example, a processing station, a transceiver on the processing station transmits the identification code of the transferred tote to the AGV controller  118  via the transceiver  124 . The identification codes of totes being carried by the AGV  125  are stored in the AGV controller  118 . Assume it is desired to transfer a tote from the AGV  125  to the elevator conveyor  20 . The shop floor controller  116  first provides a message over the communications link  120  to the master controller  114  of the identification code of the AGV traveling to the elevator conveyor  20  and the identification code of the tote to be transferred to the elevator conveyor  20 . The shop floor controller  116  then provides commands to the AGV controller  118  over a communications link  132  causing the AGV  125  to travel to the elevator conveyor  20 . Control of the AGV  125  is well known and is not a part of the present invention. As the AGV approaches one end  36  (FIG. 1) of the elevator conveyor  20 , it moves into a docking station (not shown) in a known manner and the receiver/transmitters  123 ,  124  initiate communication between the AGV controller  118  and the master controller  114  apprising the master controller  114  that the AGV  125  having a particular identification code is positioned in the docking station and ready to initiate a transfer. 
     The process of a transfer of a tote between the AGV  125  and the elevator conveyor  20  is illustrated in FIG.  4 . As shown at process step  402 , the master controller  114  first determines whether the AGV  125  is properly docked. If so, the master controller  114  then determines, at  404 , whether the elevator  26  is in its lower position at which the conveyors  30 ,  32  are at the same elevation as the upper and lower conveyors  127 ,  129 , respectively, on the AGV  125 . If not, the master controller  114 , at  406 , commands the elevator controller  100  to provide command signals over the output  106  to the solenoid valve  107  to operate the cylinders  34  such that the elevator carriage  26  is moved to its lower position as shown in FIG.  6 A. If the elevator conveyor  20  is ready to receive a tote from the AGV  125 , that state of readiness is transferred from the master controller  114  to the shop floor controller  116 . The shop floor controller  116  then provides a tote transfer command to the master controller  114  and the AGV controller  118 . 
     Upon receiving that command, as detected at  408 , the master controller  114  provides, at  410 , commands to the elevator controller  100  to start the appropriate motorized conveyor roller  66 ,  72 . Assuming the upper conveyor  30  is receiving a tote from the AGV  125 , the elevator controller  100  provides a command on the output  102  to operate the upper motorized roller  66  in a counterclockwise direction as viewed in FIG.  1 . As the motorized roller  66  rotates counterclockwise, the drive belt  68  causes the idler rollers  70  to also rotate counterclockwise. Simultaneously, the AGV controller  118  is commanding a conveyor on the AGV  125  to operate in a similar manner to move a tote generally from right to left as viewed in FIG. 1, thereby transferring the tote to the upper conveyor  30 . The elevator controller  100  then detects, at  412 , that the sensor  108  changes state indicating the tote is loaded on the upper conveyor  30 . Immediately thereafter, at  414 , the elevator controller  100  changes the state of the signal on the output  102  to cause the upper motorized roller  66  to stop. The elevator controller  100  then transfers data to the master controller  114  indicating that the tote has been successfully loaded onto the upper conveyor  30 , and the master controller  114  then passes that data onto the shop floor controller  116 . 
     The AGV  125  has two conveyors  30 ,  32  and therefore, has the capability of transferring a tote between the elevator conveyor  20  and the AGV  125  via either one or both of the conveyors  30 ,  32 . For example, while a tote is being transferred from the AGV  125  to the upper conveyor  30 , it may be desirable to transfer a second tote from the lower conveyor  32  to the AGV  125 . The method illustrated in FIG. 4 is also applicable to that process. Upon the master controller  114  receiving a tote transfer command, at  408 , the master controller  114  commands the elevator controller  100  to provide signals on the output  104  to operate the lower motorized roller  72  in the clockwise direction. The AGV controller  118  is simultaneously commanding a lower conveyor on the AGV  125  to operate in the clockwise direction. Thus, with the motorized roller  74  and idler rollers  76  rotating clockwise, a tote on the lower conveyor  32  is translated generally left to right as viewed in FIG. 1 off of the lower conveyor  32  and onto a lower conveyor of the AGV  125 . As described above, the elevator controller  100  then detects, at  412 , a change of state of the sensor  110  indicating that the tote has been transferred off of the lower conveyor  32 . Immediately thereafter, at  414 , the elevator controller  100  changes the state of the signal on the output  104  to stop the lower motorized roller  66 . Next, at  416 , the elevator controller  100  provides a signal on output  106  commanding the solenoid valve  107  to port pressurized air to the cylinders  34  to raise the elevator carriage  26  with the conveyors  30 ,  32  to a position shown in FIG.  6 B. The elevator controller  100  then transfers data to the master controller  114  indicating that the tote has been successfully transferred off of the lower conveyor  30 , and the master controller  114  then passes that data onto the shop floor controller  116 . 
     In the process just described, a tote was loaded from the AGV  125  to the upper conveyor  30 , and now it is desirable to move that tote to the ASRS. A process for moving material between the ASRS and the elevator conveyor  20  is illustrated in FIG.  5 . In the process of loading the tote onto the elevator conveyor, the master controller  114  has transmitted information to the shop floor controller  116  that a tote having a particular identification is loaded on the upper conveyor  30  of the elevator conveyor  20 . The shop floor controller  116  then, at  502 , confirms the tote identification to the master controller  114 . At  504 , the master controller  114  detects whether the elevator  26  is in its upper position at which the conveyors  30 ,  32  are at the same elevation as the upper and lower conveyors  131 ,  133 , respectively, on the ASRS  126 . If not, the master controller  114 , at  506 , provides command signals over the communications link  112  to the elevator controller  100 , requesting the elevator  26  be raised to the position shown in FIG.  6 B. The elevator controller  100  then provides signals on the output  106  to the solenoid  107  that ports fluid to the cylinders  34  such that the cylinder rod  86  is extended to its uppermost position, thereby raising the elevator  26  to its upper position as shown in FIG.  6 B. 
     Thereafter, at  508 , the master controller  114  awaits a tote transfer command from the shop floor controller  116 . In this situation, the shop floor controller  116  is coordinating the operation of the ASRS  126  via the ASRS controller  128  and communications link  130 . When the shop floor controller  116  determines that the elevator conveyor  20  and the ASRS  126  are in respective states suitable to execute a transfer of the tote from the elevator conveyor  20  to the ASRS  126 , a tote transfer command is issued to the master controller  114  and ASRS controller  128 . The master controller  114  detects the tote transfer command, at  508 , and at  510 , initiates a command to the elevator controller  100  to provide a signal on the output  102  causing the upper motorized roller  66  to rotate counterclockwise, thereby moving the tote in a generally right to left direction as viewed in FIG.  1 . Simultaneously, the ASRS controller  128  provides a signal to a conveyor on the ASRS  126  to rotate the conveyor rollers counterclockwise. When the elevator controller  100 , at  512 , receives a signal from the sensor  108  indicating that the tote has moved past the sensor  108 , the elevator controller  100  changes the state of the signal on output  102  causing the upper motorized roller  66  to stop. 
     As in the prior example, while a tote is being transferred from the elevator conveyor  20  to the ASRS  126  via the upper conveyor  30 , a tote may be transferred from the ASRS  126  to the lower conveyor  32 . In that situation, a process similar to the previously described process of FIG. 5 is executed except that the lower motorized roller  72  is commanded to operate in a clockwise direction, thereby moving the tote in a right to left direction as viewed in FIG.  1 . Upon the elevator controller  100  detecting either, at  512 , a change in state in the sensor  110  indicating that the tote is no longer on the lower conveyor  32 , a command signal, at  514 , is given to stop the operation of the lower motorized motor  72 . Thereafter, the master controller  114  provides a command, at  516 , to the elevator controller  100  to move the elevator  26  to its lower position as shown in FIG. 6A in anticipation of an exchange of totes with an AGV  125 . 
     While the invention has been illustrated by the description of one embodiment and while the embodiment has been described in considerable detail, there is no intention to restrict nor in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those who are skilled in the art. For example, in the described example, the upper conveyor  30  is used to transfer material in one direction across the elevator conveyor and the lower conveyor  32  is used to transfer material in an opposite direction. As will be appreciated, the motorized rollers  66 ,  72  can be operated by the elevator controller  100  in either direction; and therefore, the conveyors  30 ,  32  can be used singularly or together to transfer material in the same or different directions. In addition, the motorized rollers  66 ,  72  may be placed at any location on the respective conveyors  30 ,  32  with respect to the respective idler rollers  70 ,  76  that is consistent with the manufacturer&#39;s instructions for use. Further, as will be appreciated, the location of the sensors  108 ,  110 , or the use of additional sensors, is well within the skill of the art to detect the presence and absence of material on the conveyors  30 ,  32  as required by the desired use of the conveyors  30 ,  32 . 
     The conveyor elevator  20  is described as being made from extruded aluminum, however, other materials may also be used that sufficiently support the totes which, when fully loaded, weigh up to 100 pounds apiece and the conveyors  30 ,  32  which together weigh up to 100 pounds. Further, in the described embodiment, a pair of cylinders  34  is used to lift and lower the elevator carriage  26  because of their size and cost. As will be appreciated, a single cylinder may be used in place of the pair of cylinders  34 . However, the single cylinder will probably be physically larger and may be more expensive than the pair of cylinders  34 . In addition, it will be appreciated that the cylinders  34  may be mounted on either side of the conveyor elevator  20  or on both sides. Mounting a cylinder on each side of the conveyor elevator  20  may be preferred if the conveyor elevator  20  is made wider and mounting the cylinders  34  on one side results in unacceptable moment arms and torque. Further, in the described embodiment of the conveyor elevator  20 , the linear bearings  28 ,  78 ,  80  are spread out and separated from the cylinders  34  in order to reduce moments in the operation of the cylinders  34 , however, as will be appreciated, the linear bearings  28 ,  78 ,  80  may be integrated within, and purchased as part of, the cylinders  34 . 
     Therefore, the invention in its broadest aspects is not limited to the specific details shown and described. Consequently, departures may be made from the details described herein without departing from the spirit and scope of the claims which follow.