Patent Publication Number: US-6910425-B2

Title: Trolley with tractor drive for use in curved enclosed tracks and system including the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a Continuation-In-Part of U.S. patent application Ser. No. 10/376,739, filed on Feb. 28, 2003, now Pat. No. 6,718,885 entitled Trolley With Tractor Drive For Use In Curved Enclosed Tracks And System Including The Same, which is assigned to the same assignee as this invention and whose disclosure is incorporated by reference herein. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable 
   INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISK 
   Not Applicable 

   SPECIFICATION 
   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   This invention relates to generally to overhead conveyor systems and more particularly to systems making use of an enclosed track in which a trolley is arranged to roll to support something from the trolley 
   2. Description of Related Art 
   Enclosed track conveyor systems are commonly used to support and carry items from a wheeled trolley located within the interior of an enclosed track. As is known enclosed tracks are hollow members having a top-wall, a pair of side walls projecting downward from the top wall and a pair of marginal flanges extending horizontally from respective ones of the side walls. The flanges are spaced from each other to form a slot therebetween. The trolley is located within the interior of the track, with its wheels or rollers disposed on the interior (upper) surface of the flanges. 
   Examples of enclosed track systems including internally located trolleys for rolling down the interior of the track are found in U.S. Pat. Nos.: 3,589,503 (Leach), 3,627,595 (Leach) and 6,450,326 (Hoffmann et al.). The trolleys of the foregoing patents are arranged so that they can negotiate curves in the track. 
   In some prior art system, the movement or rolling of a trolley down the interior of an enclose track is accomplished by use a tractor drive that is mounted on the trolley, but located outside of the track. Such tractor drives make use of a drive wheel which extends through the slot in the track to frictionally engage the inner surface of the top wall of the track. The drive wheel is rotated by a motor mounted on the externally located tractor. This arrangement requires that the drive wheel be of a relative large diameter. As a result such tractors are not suitable for use in systems wherein the enclosed track includes a relatively small radius curve, since the drive wheel would engage or bind in the slot. While some enclosed track systems make use of tractors having drive wheels that engage and ride on the bottom of the track, i.e., the inner surface of one or more of the flanges, such systems are not practical due to splices used on the track, which results in an uneven drive surface, and make it difficult to keep a constant pressure on the drive wheel. 
   Thus, the design of existing hoist trolley drives makes it impractical to drive a trolley through a curve in an enclosed track system and a need exists for an enclosed track system which achieves that end. 
   Other United States patents relating to the field of the subject invention are U.S. Pat. Nos. 3,774,548 (Borst), 3,518,947 (Borst), 3,855,941 (Fromme et al.), 6,178,891 (Ostholt et al.), 5,092,249 (Knuettel) and 2,710,319 (Bush) from Notice of References Cited in the Notice of Allowance of parent application Ser. No. 10/376,739. 
   All references cited herein are incorporated herein by reference in their entireties. 
   BRIEF SUMMARY OF THE INVENTION 
   This invention entails an enclosed track system including a trolley for use with an enclosed track. The enclosed track has at least one curved portion and is an elongated hollow member having a top wall portion, a pair of side wall portions and a pair of flange portions. The flanged portions are spaced from each other to define a slot therebetween extending longitudinally along the track. 
   The trolley comprises comprising a rolling section and a drive section. The rolling section comprises first and second roller portions. The first roller portion is located within the track and comprises at least one support roller arranged to roll on at least one of the flange portions of the track. The second roller portion comprises a driven wheel and a pair of support rollers. 
   The drive section is located outside of the track and includes a driving wheel. The driving wheel includes a peripheral portion extending through the slot in the track and arranged for engaging the driven wheel to cause the driven wheel to rotate about a horizontal axis. 
   The pair of support rollers of the second roller portion comprise an upstream support roller and a downstream support roller. The upstream support roller is located adjacent one side of the driving wheel and mounted on the drive section for pivoting action about an upstream vertical axis. The downstream support roller is located adjacent a diametrically opposed side of the driving wheel and mounted on the drive section for pivoting action about a downstream vertical axis, whereupon the upstream and downstream support rollers are enabled to roll along at least one of the flange portions of the track while the drive wheel rolls along the top wall portion of the track in the center thereof. 

   
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
     The invention will be described in conjunction with the following drawings in which like reference numerals designate like elements and wherein: 
       FIG. 1  is an isometric view, partially in section, of a portion of an enclosed track system making use of one embodiment of a trolley constructed in accordance with this invention shown in the process of negotiating a curve in the enclosed track; 
       FIG. 2  is a side elevational view of the portion of the track and trolley shown in  FIG. 1 ; 
       FIG. 3  is a sectional view of the track and trolley taken along line  3 — 3  of  FIG. 2 ; 
       FIG. 4  is an enlarged sectional view of the track and a portion of the trolley taken along line  4 — 4  of  FIG. 2 ; 
       FIG. 5  is an enlarged sectional view of a portion of the track and trolley shown in  FIG. 3 ; 
       FIG. 6  is a side elevational view, similar to  FIG. 2 , but showing an alternative embodiment of a trolley constructed in accordance with this invention; and 
       FIG. 7  is a sectional view of the track and trolley of the embodiment of  FIG. 6  taken along line  7 — 7  of FIG.  6 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 1  there is shown at  20  one exemplary embodiment of an enclosed track system constructed in accordance with this invention. The system  20  includes a conventional enclosed track  22  and a tractor-drive trolley  24 . The details of the trolley  24  will be described later. Suffice it for now to state that the trolley  24  includes two internal roller sections  26  and  28  arranged to be located within the track  22 , and an articulated externally located tow-arm assembly  30 . The tow-arm assembly  30  is located outside of, i.e., below, the track  22  and is coupled to the both internal roller sections for moving the trolley along the track. 
   Turning now to  FIGS. 1 and 5  the track  22 , the details of the track  22  will now be discussed. As can be seen the track is of the conventional “enclosed-type” construction. One particularly suitable enclosed track is that sold by SPANCO, a division of Transol Corporation, the assignee of this invention. The track  22  is an elongated member that can be linear or curved or both linear and curved, i.e., having at least one linear portion and at least one curved portion. In the exemplary system  20 , the portion of the track  22  that is shown in  FIG. 1  is curved. Other portions of the track  22  of the system  20  can be linear or curved, depending upon the application for the system. The track is formed of a strong material, e.g., steel, and has a horizontally disposed top wall  32 , a pair of vertical sidewalls  34  and  36  projecting downward from the top wall  32 , a pair of angularly located sidewalls  38  and  40  located below the vertical sidewalls  34  and  36 , respectively, and a pair of horizontally disposed flanges  42  and  44  projecting inward from the ends of the angularly located sidewalls  38  and  40 , respectively, to form a slot  46  also used for axles therebetween. 
   The track  22  is arranged to support at least one trolley  24  to enable the trolley to be driven, i.e., moved, along the track to any desired longitudinal position. The trolley  24  may be used to support or hold some other device or member from it. For example, the trolley  24  can be used to support a lifting device (not shown), such as a winch or hoist or one end of a bridge member to form a bridge crane. 
   Turning now to  FIGS. 1 and 2  the details of the trolley  24  will now be discussed. As can be seen the trolley  24 , basically comprises the heretofore identified two internal roller sections  26  and  28  and the tow-arm assembly  30 . The roller section  26  serves as the “front” roller section of the trolley  24  and includes two pairs of wheels or rollers located within the interior of the track  22 . One pair of rollers is designated by the reference numbers  26 A and  26 B, while the other pair is designated by the reference numbers  26 C and  26 D. In accordance with one preferred embodiment of this invention the rollers are formed of a tough, wear resistant material, such as polyamide, but can be formed of any other material used in conventional enclosed track trolleys. As best seen in  FIG. 1  the rollers  26 A and  26 B are mounted on an axle  47  and the rollers  26 C and  26 D are mounted on an axle  48 . The rollers are held in place on their associated axles by use of conventional snap-rings. The axles  47  and  48  are fixedly mounted on a roller support body, in the form of a weldment  50 , so that the longitudinal axis of each axle extends perpendicularly to the plane of the roller support body  50  and to the longitudinal axis of the track  22 . The axles  47  and  48  are spaced from each other longitudinally. The rollers  26 A- 26 D are arranged so that the periphery of each roller engages and rolls along the interior surface of a respective flange  42  or  44  of the track  22 . In particular, the rollers  26 A and  26 C are arranged to engage and roll on the inner surface of the flange  42  of the track  22 , while the rollers  26 B and  26 D are arranged to engage and roll on the inner surface of the flange  44  of the track  22 . The roller support body  50  also includes a downwardly depending plate-like portion  52  which extends through the slot  46  in the track. The plate-like portion  52  is pivotably coupled to a portion of the tow-arm assembly  30  (to be described later). 
   The roller section  28  serves as the “rear” roller section of the trolley  24  and also includes two pairs of wheels or rollers located within the interior of the track  22 . One pair of rollers is designated by the reference numbers  28 A and  28 B, while the other pair is designated by the reference numbers  28 C and  28 D. The rollers  28 A- 28 D are of similar construction to the rollers  26 A- 26 D, but are smaller in diameter, for reasons to become apparent later. The rollers  28 A and  28 B are mounted on an axle  54  and the roller pair  26 C and  26 D are mounted on an axle  56 . The axle  54  is mounted on a roller support body  58  (FIG.  2 ). The roller support body  58  is in the form of a plate-like member projecting upward from a portion of the externally located tractor  30  and extending through the slot  46  in the track  22 . The axle  58  is mounted perpendicularly to the roller support body  58 . The rollers  28 A and  28 B are dimensioned so that their peripheries engage and roll along the interior surface of flanges  42  or  44 , respectively, of the track  22 . The axle  56  is mounted on a roller support body  60  (FIG.  1 ). The roller support body  60  is in the form of a plate-like member. That member is mounted on a spring-biased rod (to be described later) forming another portion of the externally located tractor drive  30 . A portion of the roller support body  60  extends through the slot  46  in the track  22 . The rollers  28 C and  28 D are dimensioned so that their peripheries engage and roll along the interior surface of flanges  42  or  44 , respectively, of the track  22 . 
   The roller section  28  serves as the driving assembly of the trolley  24 . In particular, section  28  includes a roller or wheel  62  which, as shown in  FIGS. 1 and 2 , is disposed between the pairs of rollers  28 A,  28 B and  28 C,  28 D. The wheel  62  is formed of polyamide, but can be formed of other suitable materials, if desired, and is in turn seated or disposed on a drive or driving wheel  64  ( FIG. 2 ) forming another portion of the tractor drive trolley  24 . The wheel  62  is a passive device that is engaged and driven by the drive wheel  64 . The drive wheel  64  is formed of steel or any other suitable material and is also located between the pairs of rollers  28 A,  28 B and  28 C,  28 D. The wheel  62  serves as a driven wheel of the trolley  24  and is held in position by the rollers  28 A- 28 D and  62 , so that it effectively “floats” on the drive wheel  64 , i.e., its periphery frictionally engages the periphery of the drive wheel. In order to expedite the frictional engagement between the wheels  62  and  64 , the outer periphery of the drive wheel  64  is knurled. When driven by the drive wheel  64  (as will be described later) the top portion of the periphery of the floating wheel  62  frictionally engages the inner surface of the top wall  32  of the track to cause the trolley to move longitudinally along the interior of the track. The drive wheel  64  forms a portion of the tractor drive trolley  24  and is a thin disk-like wheel having its top peripheral portion extending minimally through the slot  46  in the track. The drive wheel includes an axle fixedly secured thereto and located at the center of the wheel and having end portions projecting perpendicularly outward from the wheel and defining a rotation axis that extends perpendicularly to the longitudinal axis of the track  22 . The axle of the drive wheel  64  is mounted within a pair of bearings on a frame portion of the tractor drive trolley  24  located outside, i.e., below, the track  22 . The axle of the drive wheel  64  is coupled via a transmission to a motor forming another portion of the tractor drive trolley  24 , so that upon operation of the motor the drive wheel  64  is rotated about an axis extending perpendicularly to the longitudinal axis of the track  22 . This action causes the concomitant, albeit opposite, rotation of the floating wheel  62  about its axis, which is also perpendicular to the longitudinal axis of the track. The floating wheel  62  and the drive wheel  64  are dimensioned so that the top portion of the periphery of the floating wheel  62  frictionally engages the inner surface of the top wall  32  of the track, as shown in FIG.  2 . The spring-biased rod mentioned earlier, and to be discussed later, helps ensure that the wheel  62  makes good frictional engagement with the interior surface of the top wall  32  of the track. Accordingly, when the drive wheel  64  is rotated by the motor, the floating or driven wheel  62  is rotated in the opposite rotational direction to frictionally engage the interior surface of the top wall  32  of the track  22  and hence push or pull (as the case may be-depending upon the direction of rotation of the wheel  62 ) the trolley along the track. In  FIG. 2  the curved arrows represent the direction of rotation of the wheels  62  and  64  to cause the trolley to move in a forward direction along the track  22 , i.e., the tractor drive pushes the trolley to the left in that figure. Rotation of the wheels  62  and  64  in the opposite directions causes the tractor drive to pull the trolley in the opposite longitudinal direction, i.e., rearwardly. 
   In order to ensure that the portions of the tractor drive that extend through the slot  46  in the track  22  into its interior, e.g., the drive wheel  64  of the rear roller section  28  and the plate-like portion  52  of support body  50  of the front roller section  26 , do not engage or bind on the edges of the slot  46  when the trolley moves along the track, each roller section  26  and  28  includes a pair of cam rollers to center the roller sections with respect to the track. In particular, as best seen in  FIG. 5 , a cam roller  66  is mounted on a vertically extending bolt  68  secured to the roller support body  60  of the rear roller section  28 . The axis of rotation of the cam roller  66  is vertical and centered between the peripheral edges of the flanges  42  and  44  forming the track&#39;s slot  46 . The diameter of the cam roller  66  is slightly smaller than the width of the track so that it can be centered therein. An identical cam roller  66  is mounted on a vertically extending bolt  68  secured to the roller support body  58  of the rear roller section  28 . The axis of rotation of the cam roller  66  is vertical and centered between the peripheral edges of the flanges  42  and  44  forming the track&#39;s slot  46 . As best seen in  FIGS. 1 and 4 , the front roller section  26  also includes cam rollers  66  and bolts  68 , that are identical in construction to the cam rollers  66  and bolts  68 , respectively, of the rear roller section  28 . The cam rollers  66  are mounted via bolts  68  to the roller body  50  of the front roller section  26 . 
   The tractor drive trolley  24  basically comprises a frame  74  (FIG.  2 ), the heretofore mentioned motor  76 , a speed reducer  78 , the heretofore mentioned transmission assembly  80 , a drive wheel assembly  82  ( FIG. 3 ) including the heretofore identified drive roller  64 , and a spring biasing assembly  84  including the heretofore mentioned spring-biased rod. The frame  74  is in the form of a weldment having a upper portion  86  supporting the drive wheel assembly  82 , and a lower flange  88  ( FIG. 3 ) mounting the speed reducer  78  and a portion of the transmission assembly  80 . The speed reducer is secured to the flange  88  via plural bolts and nuts. 
   A pair of flanged bearings  90 , forming a portion of the drive wheel assembly  82 , are mounted on the upper portion  86  of the frame  74  and serve to journal respective portions of the axle of the drive wheel  64 . As best seen in  FIG. 3 , a sprocket  92  is mounted on one end portion  94  of the axle of the drive wheel  64 . The sprocket  92  forms a portion of the transmission assembly  80 . That assembly also includes a drive chain  96  and another sprocket  98 . The sprocket  98  is mounted on one end of an rotary output shaft  100  of the speed reducer  78 . The drive chain  96  is a continuous chain in the form of a loop which extends about the sprockets  92  and  98 . The speed reducer is a conventional device which is connected to the rotary output shaft of the motor  76  and includes gearing to reduce its rotary output shaft&#39;s rotational speed, e.g., 1,725 rpm, to a lower rotational speed, e.g., a 40 to 1 speed reduction, and to provide that at its output shaft  100 . The rotation of the output shaft  100  of the speed reducer causes the concomitant rotation of the sprocket  98 , which is coupled via the drive chain  86  to the sprocket  92  and to the axle  94  of the drive wheel  64  to cause it to rotate at the desired speed. It should be appreciated by those skilled in the art that the number of teeth on the two sprockets can be selected to provide a different rotational speed reduction, if desired. Moreover, the motor&#39;s speed and the amount of reduction of it by the speed reducer (or by the sprockets) is a matter of choice by the designer of the system. 
   In order to protect the drive chain and sprockets of the transmission assembly  80 , a hollow housing or cover  102  is provided on the frame  74  and extends over the sprockets and the belt. The cover  102  is held in place on the frame  74  via plural screws and lock washers  103 . 
   As mentioned earlier it is the frictional engagement and rotation of the driven wheel  62  on the inner surface of the top wall  32  of the track which effects the movement of the trolley  24  along the track. In order ensure that the driven wheel  62  makes good frictional engagement with the interior surface of the top wall  34  of the track  22  to effectively and efficiently move the trolley along the track without slippage, the heretofore spring biasing assembly  84  is provided. That assembly is mounted on the upper portion of the frame  74  and basically comprises the heretofore mentioned rod, now designated by the reference number  104  (FIG.  2 ), a helical compression spring  106  and an associated pair of nuts  110  and a flat washer  109 . The rod  104  is an elongated member having an upper end to which the roller supporting body  60  is fixedly secured. The upper portion of the rod  104  extends through and is journalled in a bore in a projection  108  at the upper portion of the frame  74 . The rod can thus rotate about its longitudinal axis in the bore. The lower portion of the rod  104  extends out the bottom of the projection  108  and down through the longitudinal center of the spring  106  and out its lower end. The lower end of the rod is threaded. A flat washer  109  is mounted on the lower end of the rod  104  so that the spring  106  is interposed between it and the projection  108  of the upper portion of the frame  74 . A pair of threaded nuts  110  are mounted on the lower threaded end of the rod to hold the washer  109  in place and to adjust the amount of compression applied to the spring  106  by the tightening of the nuts  110 . 
   As should be appreciated by those skilled in the art, by tightening the nuts  110  on the rod  104 , the spring  106  is compressed. The natural bias of the spring  106  tends to oppose this compression to thereby pull downward on the rod  104 . This downward pulling of the rod  104  pulls the roller mounting body  60  and the rollers  28 C and  28 D mounted thereon downward. Since the rollers  28 C and  28 D are in engagement with the inner surfaces of the track&#39;s flanges  42  and  44 , this downward pull is resisted by the flanges and is translated into an upwardly directed force on the frame  74  and the drive wheel assembly  82  carried thereby. Accordingly, an upward force is applied through the drive wheel  64  to the floating wheel  62  to force it into good frictional engagement with the inner surface of the top wall  32  of the track  22 . Thus, when the driven roller  62  is driven by rotation of the drive roller  64 , the driven roller  62  will roll on the inner surface of the top wall  32  of the track without slippage. This results in the movement of the trolley  24  down the track at a desired speed, e.g., 50 feet per minute using the exemplary rotational speeds of the shafts as discussed above. 
   In order to ensure that the two roller sections  26  and  28  can readily negotiate curves in the track  22 , those roller sections are coupled together by a dual-hinged, articulated tow-arm assembly  30 . To that end, as best seen in  FIGS. 1 and 2 , the tow-arm assembly  30  basically comprises a tow-arm member  114  and a pair of brackets  116  and  118 . The bracket  116  is fixedly secured to a front edge portion of the frame  74  and includes a flanged bushing (to be described later). The bracket  118  also includes a flanged bushing (to be described later) and is in the form of a clevis that is secured to a portion of the plate-like member  52  of the roller support body  50  of the front roller section  26  (as will be described later with reference to FIG.  4 ). Respective pivot or hinge bolts  120  extend through respective ones of the flanged bushings making up brackets  116  and  118 . 
   The tow-arm member  114  is a weldment in the form of an elongated plate-like member  115  having a pair of linear reinforcing webs  122  secured along the top and bottom edges of it. Each web  122  terminates beyond the associated end of the plate-like member  115  to form a gap therebetween in which a respective one of the flanged bushings of the brackets  116  and  118  is located. In particular, the pivot bolt  120  of the tow arm  114  closest to the frame  74  extends through aligned holes in the ends of the reinforcing webs  122  closest to the frame  74 . That bolt also extends through the flange bushing making up the bracket  116  and includes a head on its upper end and a nut on its lower end to secure it to the tow-arm. Thus, the rear end of the tow-arm member  114  is hingedly secured to the frame  74  by the bolt  120  and its associated flanged bushing  116 . In a similar manner the pivot bolt  120  of the tow-arm member closest to the plate-like member  52  of the support body  50  of the front roller section  26  extends through aligned holes in the ends of the reinforcing webs  122  closest to the plate-like member  52 . That bolt also extends through the flange bushing making up the bracket  118  and also includes a head on its upper end and a nut on its lower end to secure it to the other end of the tow-arm member  114 . 
   As best seen in  FIG. 4 , and as mentioned earlier, the front end portion of the bracket  118  is in the form of a clevis having a pair of spaced apart arms  118 A and  118 B. The plate-like member  52  of the front roller support body  50  is located between the arms  118 A and  118 B of the clevis and is secured in place via a pin  126  extending through it and through the arms of the clevis. The ends of the pin  126  are held in place by any conventional means, e.g., respective cotter pins (not shown) and associated flat washers  128 . Thus, the front end of the tow-arm member  114  is hingedly secured to the plate-like member  52  by the bolt  120  and its associated flanged bushing  118 . With this arrangement, the tow-arm assembly  30  can pivot independently with respect to the frame  74  carrying the rear roller section  28  and with respect to the roller support body  50  carrying the front roller section  26 . 
   As best seen in  FIGS. 2 and 4  a pair of hanger plates  130  and  132  forming a hoist hook bracket are suspended from the plate-like member  52 . In particular, the hanger plate  130  is an elongated plate like member having a opening adjacent its upper end through which one end of the pin  126  extends. The hanger plate  130  is located between the plate-like member  52  and the washers  128  on one side of that member. A plurality of flat washers  134  are located between the hanger plate  130  and the plate-like member  52 . The hanger plate  132  is identical in construction to the hanger plate  130  and also has a opening adjacent its upper end through which the other end of the pin  126  extends. The hanger plate  130  is located between the plate-like member  52  and the washers  128  on the opposite side of that member. A plurality of flat washers  136  are located between the hanger plate  132  and the plate-like member  52 . The hanger plates  130  and  132  in turn serve to support a pin  138  which can support a hook, a hoist or any other member to be supported by the trolley  24 . To that end each plate  130  and  132  includes a hole through which a respect portion of the pin  128  extends. Each end of the pin is secured in place via a cotter pin (not shown) and associated washers  140 . 
   Referring now to  FIGS. 6 and 7  there is shown an alternative embodiment of a trolley  220  constructed in accordance with this invention. The trolley  220  is constructed to ensure that its driven wheel (to be described later) stays centered in the track to render it resistant to abrasion damage. The track is identical to that described with reference to  FIGS. 1-5 , while the trolley  220  is basically constructed in the same manner as trolley  24  described heretofore, except for some features, which will be described later. Thus, in the interests of brevity the common components of the trolley units  24  and  120  will be given the same reference numbers and a description of their construction, arrangement and operation will not be reiterated. 
   As can be seen in  FIGS. 6 and 7  the rollers  28 A and  28 B are mounted on the axle  54  and the rollers  26 C and  26 D are mounted on the axle  56 . The axle  54  is mounted on a roller support body  222  which is similar in construction to the roller support body  60  described earlier. The roller support body  222  is mounted on the upper end of a vertically oriented spring biased rod  224 . The rod  224  is of rectangular cross-section and projects upward vertically from a portion of the frame  74 . The upper end of the rod  224  extends through the slot  46  in the track  22 . The rod  224  is in turn mounted for sliding longitudinal movement within a swing plate  226 . The swing plate includes a laterally extending upper portion  226 A and a recessed lower portion  226 B. A linear square cross-section passageway (not shown) extends vertically through the upper portion  226 A of the swing plate for receipt of the rod  224 . Thus, the rod  224  is able to slide along its longitudinal axis within the passageway in the swing plate  226 . The swing plate  226  is in turn pivotably mounted on the frame  74  via a pivot rod  228  extending between a pair of spaced projections of the frame. The axis of the pivot rod  228  thus forms the pivot axis for the swing plate  226 . A linear, circular cross-section passageway (not shown) extends vertically through the recessed lower portion  226 B of the swing plate for receipt of the rod  228 . With such an arrangement the rollers  28 A and  28 B mounted on the top of the rod  224  can pivot through an arc about the vertical pivot axis formed by the pivot rod  228 . 
   The roller support body  60  on which the rollers  28 C and  28 D are mounted is in turn mounted on the upper end of a vertically oriented spring biased rod  230 . The  230  is rectangular cross-section and projects upward vertically from another portion of the frame  74 . The upper end of the rod  230  extends through the slot  46  in the track  22 . The rod  230  is mounted for longitudinal sliding movement within a swing plate  232 . The swing plate is similar to swing plate  226  and includes a laterally extending upper portion  232 A and a recessed lower portion  232 B. A linear square cross-section passageway (not shown) extends vertically through the upper portion  232 A of the swing plate for receipt of the rod  230 . Thus, the rod  230  is able to slide along its longitudinal axis within the passageway in the swing plate  232 . The swing plate  232  is in turn pivotably mounted on the frame  74  via a pivot rod  234  extending between a pair of spaced projections of the frame. The axis of the pivot rod  234  thus forms the pivot axis of the swing plate  232 . A linear circular cross-section passageway (not shown) extends vertically through the recessed lower portion  232 B of the swing plate for receipt of the rod  234 . With such an arrangement the rollers  28 C and  28 D mounted on the top of the rod  230  can pivot through an arc about the vertical pivot axis formed by the pivot rod  234 . 
   The roller section  28  includes a driven wheel  240  that is disposed between the pairs of rollers  28 A,  28 B and  28 C,  28 D. The driven wheel  240  is mounted in a yoke or fork assembly  242  comprising a pair of planar generally V-shaped members projecting upward from the upper portion of the frame  74  and spaced apart from each other to form a gap in which the drive wheel  240  is disposed. Each of the V-shaped members includes a vertically oriented slot  244 . The driven wheel  240  is mounted on an horizontally disposed axle  246 , whose ends extend into the slots  244  on opposite sides of the yoke/fork assembly  242 . The slots  244  are provided to enable the driven wheel  240  to move up and down with respect to the frame  74  to ensure that the periphery of the driven wheel engages the inner surface of the top wall of the track. 
   The driven wheel  240 , like the driven wheel  62  described earlier, is a passive device that is engaged and driven by the driving wheel  64 . When mounted by the yoke assembly  242 , the driven wheel  240  effectively “floats” on the drive wheel  64 , i.e., its periphery frictionally engages the periphery of the drive wheel. In order to expedite the frictional engagement between the wheels  240  and  64 , the outer periphery of the driving wheel  64  is knurled. When driven by the driving wheel  64  the top portion of the periphery of the driven wheel  240  frictionally engages the inner surface of the top wall  32  of the track to cause the trolley to move longitudinally along the interior of the track. 
   The driving wheel  64  forms a portion of the tractor drive of the trolley  220  and is a thin disk-like wheel having its top peripheral portion extending minimally through the slot  46  in the track. The driving wheel  64  includes an axle fixedly secured thereto and located at the center of the wheel and having end portions projecting perpendicularly outward from the wheel and defining a rotation axis that extends perpendicularly to the longitudinal axis of the track  22 . The axle of the drive wheel  64  is mounted within a pair of bearings on the frame  74 , below, the track  22 . The axle of the driving wheel  64  is coupled via a transmission to the motor  76 , so that upon operation of the motor the driving wheel  64  is rotated about an axis extending perpendicularly to the longitudinal axis of the track  22 . This action causes the concomitant, albeit opposite, rotation of the driven wheel  240  about its axis, which is also perpendicular to the longitudinal axis of the track. The driven wheel  240  and the drive wheel  64  are dimensioned so that the top portion of the periphery of the driven wheel frictionally engages the inner surface of the top wall  32  of the track  22 . 
   Like the tractor  24  described earlier the tractor  220  includes spring biasing means to ensure that the driven wheel  240  makes good frictional engagement with the interior surface of the top wall  32  of the track. The details of that spring biasing means will now be discussed. To that end, as can be seen, a helical compression spring  106  is mounted on the lower end of the rod  224  below the laterally extending upper portion  126 A of the swing plate  126 . The lower end of the rod  224  is threaded. A flat washer  109  is mounted on the lower end of the rod  204  so that the spring  106  is interposed between it and the laterally projecting portion  226 A of the swing plate  226 . A pair of threaded nuts  110  are mounted on the lower threaded end of the rod  124  to hold the washer  109  in place and to adjust the amount of compression applied to the spring  106  by the tightening of the nuts  110 . In a similar manner the rod  230  mounted in the swing plate  232  includes a helical compression spring  106  on its lower end interposed between the laterally extending upper portion  132 A of the swing plate  132  and a flat washer  109 . A pair of threaded nuts  110  are mounted on the lower threaded end of the rod  230  to hold the washer  109  in place and to adjust the amount of compression applied to the spring  106  by the tightening of the nuts  110 . 
   As should be appreciated by those skilled in the art, by tightening the nuts  110  on the rods  224  and  230 , the associated springs  106  are compressed. The natural bias of the springs  106  tends to oppose this compression to thereby pull downward on the rods  224  and  230 . This downward pulling of the rods pulls the roller mounting body  60  and the rollers  28 C and  28 D mounted thereon downward while at the same time pulling downward the roller mounting body  122  and the rollers  28 A and  28 B mounted thereon. Since the rollers  28 A- 28 B and  28 C- 28 D are in engagement with the inner surfaces of the track&#39;s flanges  42  and  44 , this downward pull is resisted by the flanges and is translated into an upwardly directed force on the frame  74  and the drive roller assembly  82  carried thereby. Accordingly, an upward force is applied through the driving wheel  64  to the driven wheel  240  to force it into good frictional engagement with the inner surface of the top wall  32  of the track  22 , with the slots  244  in the yoke assembly enabling the driven wheel to move upward. Thus, when the driven wheel  240  is driven by rotation of the drive roller  64 , the driven wheel  240  will roll on the inner surface of the top wall  32  without slippage. This results in the movement of the trolley  220  down the track at a desired speed. 
   In the embodiment of  FIGS. 6 and 7  the tow-arm member  114  is secured to the drive assembly frame  74  via a pair of tow arm brackets  260  and associated bolts  262 . The brackets are mounted on opposite sides of the frame  74  and are spaced apart so that the swing plate  226  can freely pivot therebetween. 
   It should be pointed out at this juncture that the rollers  28 A- 28 B may be considered as being “upstream” rollers or “downstream rollers,” depending upon the direction of movement of the tractor  120  along the track. Thus, either one of the pairs of rollers, the rod on which they are mounted, the associated swing plate supporting that rod, and the associated pivot rod for pivoting that swing plate can be deemed the “upstream” components, while the other of the pair or rollers, the rod on which they are mounted, the associated swing plate supporting that rod, and the associated pivot rod for pivoting that swing plate can be deemed the “downstream” components. 
   As should be appreciated by those skilled in the art from the foregoing, the fact that the upstream rollers are arranged to be swung in an arc about the offset upstream pivot axis, while the downstream rollers are arranged to be swung in an arc about the offset downstream pivot axis, ensures that the trolley  120  can negotiate tight curves in the track while maintaining the driven wheel  240  centered laterally in the track. This action results in increased life for the driven wheel  240  due to lack of abrasion and pinch points on that wheel as it rolls along the track. In order to help in keeping the driven roller within the center of the track a pair of cam rollers  66  are provided coupled to the driven wheel  240 . In particular, a pair of cam rollers are mounted on respective vertically extending bolts  68  secured to the yoke assembly  242  and on opposite sides of the driving wheel  64  so that they are located within the slot in the track as best seen in FIG.  6 . 
   Thus, the systems of the subject invention, and in particular their trolleys, are particularly well suited for use in any enclosed track system, even those having a relatively tight or small radius of curvature curves. The trolleys  24  and  220  can be constructed in various ways and need not include all of the rollers shown and described heretofore. Moreover, other arrangements than that specifically described above can be used to effect the driving or movement of the trolley along the track by means of some motor located outside the track. Further still, this system is not limited to use with powered trolleys. Thus, the trolleys of this invention can be passives one that are pulled along the track by hand or by some other mechanism located below the track. 
   While the invention has been described in detail and with reference to specific examples thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.