Abstract:
A gantry crane is provided which is equipped with standard wheels having rubber tires for free maneuverability on a road surface, such as asphalt, gravel, pavement, etc., wherein the crane is additionally equipped with a plurality of railwheels to provide auxiliary support one or more rails in the loading area. In a first mode, the crane is supported only on rubber tires for driving on a paved surface, and in other modes, one or both sides of the crane are supported via the railwheels on rails. In an embodiment, a railwheel may be securely mounted coaxially to the steel rim of the respective steel wheels, forming a dual or combination wheel. The railwheels are sized relative to the tires of the standard wheels so that the railwheel does not impair non-rail loading operations, i.e., the railwheel vertically clears the ground even when the tires deflect as the crane is fully loaded. Railwheels may be provided on one or both sides of the crane. Each of the railwheel has dual flanges to maintain rollable positioning of the railwheel on the rail, enabling the crane to be driven in a self-steering manner.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional of U.S. patent application Ser. No. 09/435,264, filed on Nov. 6, 1999, now U.S. Pat. No. 6,158,602 which is a Continuation-In-Part of U.S. Ser. No. 09/024,241, filed on Feb. 17, 1998, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     Gantry cranes are conventionally used in railyards or other types of shipping yards for loading and unloading large containers or other loads to and from railcars, trailers, etc. Such a crane typically has a steel frame including four vertical columns located at corners of the crane. The columns are joined by upper horizontal beams. These beams support a movably suspended lifting mechanism, known as a grappler or spreader, configured to engage, lift and lower a container or load. 
     The crane runway surface of a shipping yard may be asphalt, concrete, gravel, dirt, or another suitable surface, collectively referred to herein as a “road surface.” So that a conventional gantry crane can be driven around on the road surface of a shipping yard, the crane is equipped with four corner-mounted sets of one or more wheels, referred to herein as “roadwheels” (such cranes are generally described herein as “road traveling” cranes). More specifically, each of the roadwheels is mounted in a yoke or truck at the bottom of a respective one of the columns. Typically, at least two of the roadwheels are steerable, and at least two of the roadwheels are motor-driven. The operator can thereby maneuver the crane around the shipping yard as desired for lifting, lowering, and moving containers. 
     Conventionally, each of the roadwheels has a rigid steel center having a circumferential rim with a rubber off-highway tire mounted concentrically thereon. The rubber tires are compressible under a load. More specifically, the rubber tires deflect against the road surface under the weight of the crane. The amount of deflection increases due to weight of a container lifted by the lifting apparatus. Additionally, the lifting apparatus may be traversed along the horizontal upper beams of the gantry crane, shifting the weight distribution non-uniformly among the respective roadwheels depending on the particular position of the grappler or spreader. Additionally, tire deflection may result in increased tire wear against the road surface. 
     It is desirable to minimize tire deflection in order to reduce tire wear. It is also desirable to improve steering control of gantry cranes which lift heavy loads. 
     SUMMARY OF THE INVENTION 
     In shipping yards, standard railroad tracks are commonly present to accommodate the movement of freight trains, railcars and/or other rail-supported equipment. These tracks are often located immediately adjacent a location where containers must be loaded or unloaded from railcars, trucks, pallets, etc. The loading and unloading of containers in such environments is typically performed by a gantry crane. 
     Accordingly, a specific object of the invention is to provide a railwheel for a road-traveling gantry crane to utilize a rail of a standard railroad track for auxiliary load support. 
     Another specific object of the invention is to provide a railwheel for a road-traveling gantry crane to utilize a rail of a standard railroad track to provide crane-steering guidance. 
     The present invention achieves these objects by providing a gantry crane that may be selectively driven only on wheels having rubber tires and, the crane is additionally equipped with steel railwheels on one or both sides of the crane so the crane can be operated in a rail-supported mode. 
     The present invention also achieves these and other objects by providing a system including a plurality of railwheels which are respectively mounted relative to the roadwheels of a gantry crane. The system of the invention includes an apparatus and method for transferring crane loads through the railwheel to the rail. In a particular embodiment, the railwheel is mounted on a common axle to the rigid wheel center of the roadwheel. The rigid wheel center may have a flange to accommodate mounting of the railwheel thereto by bolts. Railwheels are provided on one or both sides of the crane. 
     The railwheel is sized relative to the tire so that a clearance exists between the rail wheel flange and the road surface when the tire is at maximum deflection under the maximum load. This allows the crane to travel freely about in the loading yard supported only by its rubber tires on the crane runways and to lift loads in a conventional manner. 
     For placing the railwheels to ride along the rail, according to one embodiment of the invention, a ramp is positioned adjacent the rail. The crane is driven so that the rubber tires adjacent the railwheels travel down the sloping ramp until the railwheels contact the rail. In this embodiment, it is possible that the roadwheels may become suspended over the ground, with all of the wheel loading being transmitted from the associated railwheel to the rail. More specifically, as the crane continues moving, the railwheel rides along the rail, while the ramp may continue to slope downwardly, away from the tire of the associated roadwheel. Thereby, one side of the crane is supported by the railwheels on the rail, and the other side of the crane is supported by the rubber tires on the road. Also, in an embodiment wherein railwheels are equipped on both sides of a crane, a second rail and ramp arrangement may be provided at the other side of the crane to position both sides of the crane supported on the rails. 
     According to an embodiment of the invention, the railwheel is normally raised over the ground a distance greater than the height of a rail during unloaded or light loading conditions. In such a state, the railwheel does not contact the rail, and, therefore, all of the axle load is borne by the roadwheel and is transmitted to the ground via the tire. The difference in circumference between the railwheel and the tire provides vertical clearance of the railwheel over the ground sufficient that the crane may be maneuvered to position the railwheel vertically over the rail. However, the difference in circumference is also such that when the tire deflects against the road a predetermined amount due to increased loading, the railwheel moves downwardly to contact against the rail. At this point, additional loading is transmitted to the rail through the railwheel, and further deflection of the tire of the associated roadwheel is prevented. Such an embodiment advantageously limits the amount of tire deflection beyond a predetermined amount. 
     An advantage of the present invention is that it provides a crane that can operate fully on rubber tires on both sides of the crane, or supported on a rail on one side, or supported on a rail on both sides. 
     The crane utilize one of the rails of a railroad track, independently of the gauge of the track. This advantageously permits the crane to be used in an environment with a railroad track without a need to adapt the crane for a particular track gauge. 
     A further advantage of the present invention is that it provides a dual-flange railwheel which guides movement of the gantry crane along the rail, eliminating steering effort by the operator. 
     Additional features and advantages of the present invention are described in, and will be apparent from, the disclosure herein, including the drawings, description, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is left side elevational view of a gantry crane according to the invention. 
     FIG. 2 is right side elevational view of the gantry crane of FIG.  1 . 
     FIG. 3 is a rear elevational view of the gantry crane of FIGS. 1 and 2 on the road surface during a lifting operation to move a container between a truck chassis and a rail car. 
     FIG. 4 is a rear sectional, elevational view of a truck assembly having a railwheel according the invention. 
     FIG. 5 is an exploded perspective view of a mountable railwheel and roadwheel according to the invention. 
     FIG. 6 is a rear elevational view of a truck assembly subjected to a loading condition resulting in a sufficiently low amount of tire deflection such that the railwheel is vertically elevated above the rail. 
     FIG. 7 is a rear elevational view of the railwheel and roadwheel assembly of FIG. 6 subjected to a relatively higher loading condition resulting in a greater amount of tire-deflection such that the railwheel is lowered to contact the rail. 
     FIG. 8 is a schematic side view of the railwheel and roadwheel assembly riding on the rail, as positioned near a sloping ramp on which the tire can be driven for setting the railwheel on the rail. 
     FIG. 9 is a rear elevational view of the railwheel and roadwheel assembly wherein the truck load is carried by the railwheel on the rail, the tire carrying none of the load or a negligible load. 
     FIG. 10 is a rear elevational view of another embodiment of a gantry crane constructed in accordance with teachings of the invention, wherein both sides of the crane are equipped with railwheels, and wherein the railwheels are located interiorly of respective roadwheels. 
     FIG. 11 is a schematic view of a drive system for a gantry crane according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now turning to the Figures, wherein like numerals designate like components, in FIGS. 1-3 there is shown a gantry crane  10  constructed in accordance with teachings of the present invention. As shown, the crane  10  rests on an operating surface  12 , such as the ground surface of a shipping yard. In a first mode, the crane  10  is operable to freely maneuver around on the ground surface, supported only on rubber tires. Additionally, as described below in greater detail, the crane according to the invention can be operated in modes by which one or both sides of the crane are supported on rails. 
     Generally, the crane  10  has a four vertical columns  14  which are joined by upper beams  16 . Although crane various structures are possible, the illustrated embodiment includes vertically-movable beams  18  which are driven along a track  20  (FIG. 3) fixed to the respective columns  14 . A lifting mechanism  22  is movably mounted to the beams  18 . More particularly, as shown in FIG. 3, each of the beams  18  is equipped with a traversible trolley  24  which is movable along the respective beam  18 . Although other vertical lifting devices are possible, the illustrated embodiment includes a hoist cylinder  26  mounted on each horizontal member  16 . Each hoist cylinder is attached to its respective vertically movable beam  18  by a set of chain or cables  119 , and rollers or sheaves  120 . Extending or retracting the hoist cylinder  26  lowers or raises vertically movable beam  18 . By moving the trolleys  24 , the lifting mechanism  22  may be traversed from side to side as desired. The lifting mechanism  22  is configured to engage an object in an appropriate manner as is known in the art, such as, for example, with twistlocks  32  and/or grabbing arms  34 . Each of the columns  14  is attached to a column support member  114  formed by a portion of a baseframe  116 . A cab  118  is mounted to occupy an operator, and the cab  118  contains controls for operating the crane  10 . 
     Still referring to FIG. 3, the crane  10  is illustrated in an exemplary loading position over a rail car  36  and a road trailer  38 . A container  40  is illustrated which can fit on either the rail car  36  or the trailer  38 . The rail car  36  rests on rails  42 , and another rail  44  is disposed nearby. Such a container  40  is commonly moved between road and rail transportation modes, as shown. 
     In the embodiment illustrated in FIG. 3, the operating surface  12  includes truck portion  46  supporting the road trailer  38  and an adjacently-positioned rail portion  48  on which the rails  42 ,  44  are fixed or embedded. While the operating surface  12  is shown as having different elevations among the portions  46 ,  48 , the invention may also be used in environments where this is not the case. 
     So that the crane  10  can be driven and maneuvered on the operating surface  12  and around the shipping yard, the crane  10  includes four rotatably mounted roadwheels, a front right roadwheel  50 , a rear right roadwheel  52 , a front left roadwheel  54  and a rear left roadwheel  56 . More specifically, each of the roadwheels  50 ,  52 ,  54 ,  56  is rotatably mounted in a respective truck  58 , which is steerably mounted at the bottom of the respective column support member  114 . Typically, at least the rear two wheels  52 ,  56  are steerable, and at least the rear two wheels  52 ,  56  are drivable in a manner described below. 
     As illustrated in FIG. 4, the roadwheel  56  (as well as each of the other respective roadwheels  50 ,  52 ,  54 ) generally has a rigid wheel center  60 , having a circumferential rim  62  and a tire  64  which is concentrically mounted on the rim  62 . Each of the roadwheels  50 ,  52 ,  54 ,  56  includes a resilient tire  64  made of rubber or some other suitable material. Preferably, the tire  64  is a large diameter tire having a construction of the off-highway class. Off-highway tires can experience much greater deflection than highway tires because off-highway tires are typically capable of supporting wheel loading in excess of thirteen times the wheel loading capacity of highway tires. 
     As also shown in FIG. 4, the truck  58  includes a first side  66  and a second side  68 , and an axle  70  fixed to extend between the first and second sides  66 ,  68 . The axle  70  is disposed along an axis  72 . The wheel center  60  is axially mounted to the axle  70  to centrally rotate around the axis  72 . The truck  58  is mounted to the side beam or column  14  by a swivel joint  74 . A hydraulic drive motor  76  is mounted to the truck  58  and drives a sprocket  78  which transfers drive rotation to a sprocket  80  fixed to the wheel center  60  via a chain (not shown) or gearing. 
     When the column  14  exerts a downward load against the truck  58 , the truck  58  transfers force through the axle  70 , which in turn transfers the load to the rigid wheel center  60  of the roadwheel  56 , which in turn transfers the load to the tire  64  which deflects against the operating surface  12 . Accordingly, the rigid wheel center  60  moves downwardly a corresponding amount. The amount of downward deflection at a particular tire  64  corresponds to the amount of loading on the respective truck  58 , which may vary depending on the weight of the container  40  (FIG.  3 ), or the position of the lifting mechanism  22  affecting weight distribution. Typically, the tire  64  is hollow and filled with pressurized air, however, resilient solid tires or non-pressurized tires may be possible in some embodiments. This description also applies to the roadwheels  50 ,  52  and  54 . 
     According to the invention, a railwheel is mounted relative to the roadwheel, and the railwheel can be positioned to ride in contact on a rail, carrying some or all of the load from a respective wheel truck on a gantry crane. The railwheel has a smaller radius than the tires of the roadwheels so that the so that the crane can be supported and driven only on the tires while maintaining a suitable clearance between the railwheel and the road surface. 
     In an embodiment, the railwheel can be positioned onto the rail by tire deflection in excess of a predetermined amount, transferring excess loading to a standard traintrack rail adjacent the loading site. To facilitate a transfer of the excess load to the rail  44 , instead of to the operating surface  12  via the tire  64 , the crane  10  includes at least one railwheel  82  which is sized and mounted relative to the roadwheel  56  such that loading on the axle  70  exceeding a predetermined limit is delivered from the railwheel  82  to the adjacent railroad rail  44  under certain loading conditions. 
     On the illustrated crane  10 , as shown in FIGS. 1 and 3, each of the left roadwheels  54  and  56  is preferably equipped with a respective railwheel  82  in the manner described herein. Depending on the particular application of the crane  10 , it may be desirable to equip some or all of the roadwheels with respective railwheels  82 . The railwheels  82  respectively mounted on the roadwheels  54 ,  56  are located at an outer side of the roadwheels  54 ,  56  and are both positionable against the track  44  in the manner described below in greater detail. 
     As illustrated in FIGS. 4-7, the railwheel  82  is a dual-flange type wheel. The railwheel  82  is generally formed of a rigid steel disc-shaped body  84  (FIG.  5 ), having a circular support surface  86  for contacting the rail  44 . The railwheel  82  additionally includes first and second circumferential flanges,  88  and  90 , respectively located at opposite sides of the support surface  86 . The railwheel  82  is configured to rollably receive the standard-width rail  44  against the support surface between the first and second flanges  88 ,  90 . The flanges  88 ,  90  maintain the position of the railwheel  82  on the rail  44 . Preferably, some amount of play exists between the flanges  88 ,  90  to assist in positioning the railwheel  82  on the rail  44 . 
     As shown in FIG. 5, the railwheel body  84  has boltholes  92  disposed therein to permit mounting of the railwheel  82  relative to the roadwheel  56  with bolts  94 . In the illustrated, embodiment, the railwheel  82  is mounted on the common axis  72  with an associated one of the roadwheels  56  by securing the railwheel  82  to the rigid wheel center  60  of the roadwheel  56 . So that the railwheel  82  may be mounted to the wheel center  60 , the wheel center  60  includes a mounting flange  96 . The mounting flange  96  has threaded boltholes  98  to receive the bolts  94 . The bolts  94  are extended through the boltholes  92  in the railwheel  82  and threaded into the mounting flange  96 , so that the railwheel  82  is secured to the roadwheel  56 . In another embodiment, the bolts  94  are long enough to extend though unthreaded holes  98  in the mounting flange  96 , and the protruding portion of the bolts  94  can be secured with nuts. The flange  96  holds the railwheel  82  at a sufficient axially-spaced position relative to the roadwheel  56  such that the railwheel  82  will not be contacted by the laterally deflectable tire  64 . The mounted railwheel  82  and associated roadwheel  56  are mounted within the truck  58  to commonly rotate on the axis  72 . 
     The structure described herein for mounting the railwheel to the roadwheel is exemplary, and it is noted that other mounting means may be used. The scope and advantages of this invention are not limited to a particular structure for attaching the railwheel to the rigid rim assembly of the rubber tire. 
     In FIG. 6, the roadwheel  82  is shown positioned on the operating surface  12  near the rail  44 . The railwheel  82  is sized relative to the outer diameter of the tire  64  of the associated roadwheel  56  so that when normal or light load transferred to the roadwheel  56  from the axle  70 , the tire  64  deflects only a small amount and bears all of the load from the truck  58  against the operating surface  12 . In this condition, the railwheel  82  is spaced a distance above the operating surface  12 , permitting the crane to be freely driven about on the tires only, even in a fully loaded condition. More specifically, the flanges  88 ,  90  of the railwheel  82  have an outer diameter less than an outer diameter of the tire  64  of the roadwheel  56 , so that the flanges  88 ,  90  of the railwheel  82  are normally held a clearance distance above the height of the rail  44 . Thus, the railwheel  82  does not interfere with normal driving of the crane  10  on the operating surface  12  during a low-loading condition, and the crane  10  can be driven into a position wherein the railwheel  82  is positioned vertically above a standard rail  44 . The crane  10  is then in position to perform a loading operation. In this low-loading condition, the support surface  86  of the railwheel  82  is vertically spaced a distanced above the rail  44 . 
     When the crane  10  lifts a container  40  (FIG. 3) or other load, the load is distributed from the trucks  58  among the respective roadwheels  50 ,  52 ,  54 ,  56 , causing the tires  64  to correspondingly deflect. Referring to FIG. 7, deflection of the tire  64  causes a corresponding downward movement of the axis  72  and the railwheel  82 . According to the invention, the loading from the truck exceeds a predetermined limit, the tire  64  has deflected a corresponding amount resulting in downward movement of the axis  72  by the distance d. This moves the railwheel  82  downwardly to contact the rail  44 , thereby transferring excess loading from the truck  58  to the rail  44 . At this point, loading on the tire  64  is limited, and further deflection of the tire  64  is prevented. 
     FIGS. 6 and 7 illustrate the rail  44  as having a portion embedded into the operating surface  12  in a sunken manner, although the rail  44  partially protrudes above the operating surface  12 . The rail  44  is supported by a railroad tie  100  which lies under the operating surface  12 . It is noted that the invention may be used with other mounting configurations of the rail  44  relative to the operating surface  12 , so long as the dimensional differences between the railwheel  82  and the tire  64  result in the effect described herein. 
     In connection with the embodiment of FIGS. 6 and 7, depending on the spring rates of the tires  64 , structural dimensions of the crane  10  (FIG.  3 ), and other design considerations, the amount of load on the particular truck  58  which exceeds the load amount required to displace the railwheel  82  against the rail  44  can be, for example, the result of the lifting of the container  40  from either the road trailer  38  at the right side of the crane  10  or from the rail car  36  at the left side of the crane  10 . The crane  10  could be designed so that the limit is exceeded as a result of moving the suspended container from right to left (relative to FIG. 3) via the traversible trolleys  24 , altering the weight distribution among the trucks  58 . 
     When a container  40  has been lifted by the crane  10 , it is sometimes desirable to drive the crane  10  forwardly or rearwardly along the direction of the rail  44  in order to move the lifted container  40  to a desired location. When loading is sufficient such that the front and rear railwheels  82  are contacting the rail  44 , the crane  10  may be driven in a self-steering manner. In particular, the railwheels  82  roll along the rail  44 , tracking the movement of the crane  10  in a self-guiding manner along the direction of the rail  44 . This eliminates a need for the crane operator to provide steering input. 
     At some shipping yards, the rail  44  has an end which tapers downwardly to become flush with the ground. Thus, when the crane  10  is driven far enough along the rail  44 , the railwheels  82  eventually lose contact with the tapered rail, transferring all of the load from the respective truck  58  to the operating surface  12  through the tire  64 . In the embodiment shown in FIGS. 6 and 7, the tires  64  of the associated roadwheels  54 ,  56  fixed with the railwheels  82  can advantageously provide driving traction against the road surface when the railwheels  82  track along the rail  44 . 
     Referring now to FIG. 8, according another embodiment of the invention, the transfer of truck loading to the rail  44 , and relieving the load on the tires  64 , can also be accomplished by providing a ramp  112  adjacent to rail  44  to accommodate the rubber tires  64  of the left-side roadwheels  54 ,  56 . The crane  10  is driven so that the rubber tires  64  adjacent the railwheels  82  travel down the sloping ramp  112  until the railwheels  82  respectively contact the rail  44 . In particular, as the crane continues moving, the railwheel  82  rides along the rail  44 , while the ramp  112  may continue to slope downwardly away from the tire of the associated roadwheel. Thereby, one side of the crane  10  is fully supported by the railwheels  82  on the rail  44 . The other side of the crane may be supported by the rubber tires  64  on the operating surface  12 , or in an appropriate situation, in an embodiment wherein both sides of the crane are equipped with railwheels, the other side of the crane may also be supported on a parallel rail. 
     When the railwheel is supporting the crane on a rail, as illustrated in FIG. 9, depending on the environment, it is possible that the tires may contact the ground or railroad ties adjacent the rail, however the railwheel is dimensioned relative to the roadwheel such contact is insignificant. Such contact, if it occurs, is so slight that the tires will not exert any substantial amount of force to the ground, and accordingly, the tires will cause no appreciable damage the ground or railroad ties. It is also possible that the tires  64  may become suspended over the ground by a vertical clearance. 
     The ramp is formed in the operating surface  12  on the rail side of the crane in this embodiment so that the load is substantially transmitted from the railwheel  82  to the rail  44 , substantially relieving the tire  64  from supporting the load. The tire  64  may incur negligible contact with the tie  100  when the crane is fully supported on the rail  44 , as shown in FIG.  9 . The embodiment of FIGS. 8 and 9 also provides self-guiding advantages when the railwheels  82  are in contact with the rail  44 . Moreover, the embodiment of FIGS. 8 and 9 facilitates a transfer of load to the rail independently of the amount of the load carried by the crane  10 . 
     A crane  210  is illustrated in FIG. 10 according to another embodiment of the invention. The crane  210  is similar to the crane  10  illustrated in FIG. 3, except the crane  210  of FIG. 10 has left side wheel trucks  258  and right side wheel trucks  260  each being equipped with a railwheel  82  and a roadwheel  56  having a rubber tire  64 . Additionally, as shown, each of the railwheels  82  is mounted interiorly relative to the roadwheels  56 . The crane  210  of FIG. 10 is also illustrated in a loaded condition, lifting a container  40 , yet the railwheels remain positioned above the ground by a vertical clearance. Because of the clearance, the crane  210  can freely drive around on the ground, even in a loaded condition; supported only on the road wheels. 
     The crane  210  can be also operated in rail-supported modes whereby one or both sides of crane  210  are supported by its railwheels  82  on respective rails. As in the embodiment described in connection with FIGS. 1-9, the railwheels  82  are preferably dual-flange railwheels to provide self-steering guidance along the rails. 
     A drive system  300  is illustrated in FIG. 11 for controllably driving a pair of wheel assemblies  301  each assembly including a roadwheel  56 , on opposite sides of the crane  10  (FIG.  3 ),  210  (FIG.  10 ). Additionally, at least one of the wheel assemblies  301  includes a railwheel  82 , and in the illustrated embodiment, both of the wheel assemblies  301  include a railwheel  82 . The drive system  300  includes a hydraulic pump  302  driven by an engine  304 , such as an internal combustion engine. Also, each of the driven wheels assemblies  301  is drivably coupled to a respective independent hydraulic motor  306 . The hydraulic pump  302  delivers a pressurized flow of hydraulic fluid though a supply line  308  that commonly supplies both of the hydraulic motors  306 . A common return line  310  delivers a return flow of the hydraulic fluid to the pump  302 . The system  300  is advantageously simple and cost effective to manufacture and to maintain. The system  300  provides automatic compensation for minor variations in drive motor rotational velocity due to variations in rolling resistance and/or radius. More specifically, because each of the drive wheel assemblies  301 , is driven by a respective hydraulic motor  306 , slight differences in actual linear velocity due to differences in rolling radius of the drive wheel assemblies  301  (e.g., one side of the crane supported by the tire on the road surface, and the opposite side of the crane  10 ,  210  being supported by the railwheel  82  on the rail  44 ) are compensated by the self-steering guidance provided by the flanges  88 ,  90  (FIGS. 5,  6 ) of rail wheels  82  as the crane  10 ,  210  travels along the rail  44 . This system  300  is advantageous over a conventional drive system because the system used in the described embodiment of the invention eliminates a need for multiple independent drive systems for independently driving the drive wheels as in a conventional drive arrangement. 
     Although the invention is described herein with reference to certain preferred embodiments, it is recognized that various changes and modifications will be apparent to those skilled in the art. For example, instead of mounting the railwheel  82  directly to the roadwheel  56 , the railwheel  82  could be mounted to another component fixed relative to the crane structure. Also, while the illustrations herein depict the crane  10  moving a container  40  between road and rail modes, the invention could be used for transferring objects and loads in other situations as well. For example, the container  40  may be moved from rail-to-rail modes, road-to-road modes, to or from the ground or a pallet, etc. Also, the railwheel  82  may have a single flange in a situation where such a configuration would provide sufficient guidance, e.g., where the top of the rail is flush with the road surface such that the flange rides in a single groove between the rail  44  and the operating surface  12 . Such changes and modifications may be made without departing from the spirit and scope of the invention. Accordingly, the appended claims are intended to cover all such changes and modifications within the spirit and scope of the invention.