Abstract:
A materials handling vehicle may include a vehicle frame, a drive wheel, a first caster assembly located on the right side of the drive wheel, and a second caster assembly located on the left side of the drive wheel. A weight distribution assembly connects the first or second caster assembly to the vehicle frame and may include an adjustable preload mechanism to provide a preload force to the first or second caster assembly.

Description:
[0001]     This application claims priority from U.S. Provisional Application 60/671,548, filed Apr. 14, 2005, and herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The invention relates to a transportation device used primarily in a materials handling vehicle such as an industrial pallet truck.  
         [0003]     Industrial pallet trucks typically include a drive or steer wheel located proximately under a vehicle frame of the pallet truck. The drive wheel may include a single tire or dual-tire construct. Two casters/wheels are typically employed which are located adjacent and on opposite sides of the drive wheel. The casters provide additional support of the vehicle frame weight as well as provide additional stability as the pallet truck is being turned or operated on an incline.  
         [0004]     The pallet trucks may be powered by an electric motor or may be manually pulled or pushed by an operator. Electrically powered pallet trucks may further include a platform upon which an operator may ride during transport of a load. For an electrically powered pallet truck, the steer wheel may additionally be used as the drive wheel, such that the steer wheel also provides a traction force that drives the pallet truck.  
         [0005]     Pallet trucks may operate in a variety of operating conditions and locations including, for example, a warehouse, truck yard, grocery store, sidewalk or even an automobile road. Operating surfaces associated with these different locations also vary significantly. For example, the pallet trucks may be required to traverse over relatively smooth paved surfaces or relatively rough unimproved and uneven surfaces such as dirt or gravel roads. Other operating surfaces may include cobbled roads or grooved or siped pavement.  
         [0006]     As the pallet truck is moved by either an electric motor or by manual effort of an operator, the drive wheel and casters rotate in the direction of vehicle travel. As the pallet truck is operated over uneven or unimproved surfaces, the steer wheel and casters tend to move up and down in irregular patterns. As a result of the vertical movement, casters may temporarily lose contact with the ground or lose traction, making it harder to operate the vehicle. For example, if the casters lose contact with the operating surface when steering or turning around a corner, a load may move or the pallet truck may tip.  
         [0007]     Casters that are rigidly attached to the pallet truck create an additional problem. The drive wheel may partially lose contact with the operating surface, or slip, when either of the casters travels over uneven terrain and moves vertically up and down. If the lose of pressure with the operating surface is significant enough, a loss of traction or braking ability may be experienced.  
         [0008]     The present invention addresses these and other problems associated with the prior art.  
       SUMMARY OF THE INVENTION  
       [0009]     A materials handling vehicle may include a vehicle frame, a drive wheel, a first caster assembly located on the right side of the drive wheel, and a second caster assembly located on the left side of the drive wheel. A weight distribution assembly connects the first or second caster assembly to the vehicle frame and may include an adjustable preload mechanism to provide a preload force to the first or second caster assembly.  
         [0010]     The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention which proceeds with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a perspective view of a pallet type forklift truck that may include a novel weight distribution assembly;  
         [0012]      FIG. 2  is a perspective view of the weight distribution assembly that may be used with the forklift truck of  FIG. 1 ;  
         [0013]      FIG. 3  is a perspective rear view of the forklift truck including the weight distribution assembly of  FIG. 2  attached to a vehicle frame;  
         [0014]      FIG. 4  is a rotated bottom view of the weight distribution assembly shown in  FIG. 2 ;  
         [0015]      FIG. 5  is an exploded view of the weight distribution assembly;  
         [0016]      FIG. 6  illustrates an operation of a three wheel system as is known in the art when traveling over a level surface;  
         [0017]      FIG. 7  illustrates an operation of the three wheel system shown in  FIG. 6  as is known in the art when traveling over an uneven surface;  
         [0018]      FIG. 8  illustrates an embodiment of the novel weight distribution assembly when traveling over a level surface;  
         [0019]      FIG. 9  illustrates an operation of the novel weight distribution system shown in  FIG. 8  when traveling over an uneven surface;  
         [0020]      FIG. 10  illustrates a further embodiment of the novel weight distribution assembly when traveling over a level surface; and  
         [0021]      FIG. 11  illustrates an operation of the novel weight distribution system shown in  FIG. 10  when traveling over an uneven surface.  
     
    
     DETAILED DESCRIPTION  
       [0022]      FIG. 1  shows a pallet type forklift truck  5  that includes forks  10 , a vehicle frame  8  and a steer arm  4  by which the forklift truck is guided. The steer arm  4  may include electronic or mechanical controls that raise and lower the forks  10 , for example, or that activate a traction motor  7  ( FIG. 3 ) residing in the vehicle frame  8 . A weight distribution assembly  100  is located in the back of the forklift truck  5  to improve support and stabilization of the forklift truck  5  during operation.  
         [0023]     It should be understood that the forklift truck  5  shown is merely one example of a type of forklift truck that could be used with the weight distribution assembly  100 . For example, a motorized rider pallet truck may include an extended frame upon which an operator may stand while the motorized rider pallet truck is being operated. Other industrial lift trucks can similarly use the weight distribution assembly  100 , and their applications and embodiments used with the stability system are herein claimed.  
         [0024]     Forklift trucks, such as forklift truck  5 , may be pulled and guided by an operator by means of the steer arm  4 , or they may be powered by the traction motor  7  ( FIG. 3 ) and guided by the steer arm  4 . In either case, the forklift truck  5  efficiently transports or moves a load which may be placed on one or more forks such as forks  10 .  
         [0025]     Pallet trucks may frequently be required to operate over rough pavement or other traveling surfaces. For example, it is common to have traveling surfaces that include stones or are cobbled. The load wheels on previous pallet trucks would tend to bounce up and down while traversing over uneven terrain. This subjected the pallet truck, as well as any load on the forks, to increased vibration and irregular shifts in direction. The weight distribution assembly  100  reduces these previous problems.  
         [0026]     Referring to  FIG. 2 , the weight distribution assembly  100  includes a first caster assembly  120  and a second caster assembly  125 . The caster assembly  120  and  125  include casters  20  and  25 , respectively, as well as dampening devices  30  and  35 . The first and second caster assembly  120  and  125 , respectively are coupled together with a coupling bar  15 .  
         [0027]      FIG. 3  shows a rear view of the forklift truck  5  with the steer arm  4  and a back cover removed to show internal components such as the electric motor  7 . A drive wheel  9  is located directly below the electric motor  7 , along an approximate centerline of the truck in a fore and aft direction. The weight distribution assembly  100  is shown mounted to the vehicle frame  8  such that caster assembly  120  and caster assembly  125  are located on opposite sides of the drive wheel  9 .  
         [0028]     In a preferred operation of the forklift truck  5 , the drive wheel  9  and the casters  20  and  25  all maintain contact with the ground as much as possible. This three point contact helps distribute the supported weight of the forklift truck  5  and load that may be carried on forks  10 . Additionally, the casters  20  and  25  provide additional lateral stability when the forklift truck  5  is being turned or when a change in a vehicle center of gravity may otherwise cause the forklift truck  5  to lean or fall over if the casters  20  and  25  were not present.  
         [0029]      FIG. 4  shows a rotated bottom isolated view of the weight distribution assembly  100 . The weight distribution assembly  100  allows the drive wheel  9  ( FIG. 3 ) to pivot at a constant, fixed height, while the casters  20  and  25  are supported by the dampening devices  30  and  35  and the coupling bar  15 .  
         [0030]     The casters  20  and  25  are preferably allowed to swivel about an axis perpendicular to the traveling surface. This allows the casters  20  and  25  to follow the direction of the drive wheel  9  as the drive wheel  9  is turned during operation of the forklift truck  5 . Swiveling casters are well known in the art and is therefore not described in any further detail. The coupling bar  15  further supports the synchronization of the caster orientation with that of the drive wheel  9 .  
         [0031]      FIG. 5  shows an exploded view of the weight distribution assembly  100 . The caster assembly  125  has three subcomponents that include the dampening device  35 , the caster  25  and a mounting plate  75 . The mounting plate  75  attach the dampening device  35  and the caster  25  with coupling bar  15 . Similarly, the caster assembly  120  has three subcomponents that include the dampening device  30 , the caster  20  and a mounting plate. The mounting plate  70  attaches the dampening device  30  and the caster  20  with coupling bar  15 .  
         [0032]     The dampening device  30  includes a hydraulic shock absorber  40  that works in compression and an internal helical spring  50 . The dampening device  30  may further include an external helical spring  71  that slides over the shock absorber  40 . The external spring  71  works in compression and may be retained in position with a threaded nut  60 , such as a gland nut. The dampening device  35  includes similar components and operates in a similar manner.  
         [0033]     Advantageously, the threaded nut  60  may be screwed upward to tighten the compression of spring  71  or screwed downward to loosen the compression of spring  71 . Increasing the compression of compression spring  71  by screwing nut  60  upwards increases the amount of preload downward force applied to the caster  20 . Decreasing the compression of compression spring  71  by screwing nut  60  downwards decreases the amount of downward preload force applied to the caster  20 . Thus, the threaded nut  60  can be selectively adjusted prior to operation to vary the preload force according to individual forklift truck capacity and application requirements.  
         [0000]     Other Features  
         [0034]     In one embodiment, the weight distribution assembly  100  is assembled as a complete modular unit that may be attached to the vehicle frame  8  as a pre-assembled unit. For example, the weight distribution assembly  100  may be attached to the vehicle frame  8  by means of attaching blocks  90  and flanged bushings  80  shown in  FIG. 5 . The dampening devices  30  and  35  further attach the mounting plates  70  and  75 , respectively, to the vehicle frame  8 .  
         [0035]     The flanged bushings  80  allow the ends of the coupling bar  15  to be inserted into the attaching blocks  90 , such that the coupling bar  15  is free to rotate about its longitudinal axis when either of the dampening devices  30  and  35  compresses and decompresses.  
         [0036]     In another embodiment, the coupling bar  15  is made from a unitary non-hollow piece of solid metal. The weight distribution system  100  allows a relatively simple coupling bar  15  to be used for connecting the two caster assemblies  125  and  175  together.  
         [0000]     Operation  
         [0037]     An operation of the novel weight distribution assembly is described after first explaining an operation of a three wheel system known in the art, and as illustrated in  FIGS. 6 and 7 , in order to further clarify some of the improvements. Alternative embodiments of the novel weight distribution assembly are then described and illustrated making reference to  FIGS. 8-11 .  
         [0038]      FIG. 6  illustrates an operation of the three wheel system as is known in the art when traveling over a level surface  150 . Casters  220  and  225  are shown attached to a vehicle frame  208  and provide support of a vehicle weight of a pallet truck. Similarly, a drive wheel  209  also attached to the vehicle frame  208 , supports the remainder of the vehicle weight of the pallet truck, not supported by casters  220  and  225 . The casters  220  and  225  as well as the drive wheel  209  are all shown to be in contact with the level surface  150 . Significantly, the casters  220  and  225  as well as the drive wheel  209  may be considered as rigidly attached to the vehicle frame  208 , other than an allowed rotation.  
         [0039]      FIG. 7  illustrates an operation of the three wheel system shown in  FIG. 6  as is known in the art when traveling over an obstacle  160  located on the level surface  150 . In this figure, caster  225  is shown at an elevated position above the obstacle  160 , while the caster  220  remains on the level surface  150 . Significantly, because the casters  220  and  225  and the drive wheel  209  are considered as rigidly attached to a vehicle frame  208  having a tilt angle  222 , the drive wheel  209  is caused to lift off the ground by a distance A, thereby losing an ability to provide traction and braking. In addition to creating an inability to accelerate or brake the pallet truck, an instability of the pallet truck also occurs.  
         [0040]      FIG. 8  illustrates an embodiment of the novel weight distribution assembly when traveling over the level surface  150 . In this embodiment, casters  320  and  325  are mounted to dampening devices  330  and  335 , respectively, of caster assemblies  420  and  425 . Dampening devices  330  and  335  are shown as including external springs  371  and nuts  360 , in order to provide a variable preload force. The caster assemblies  420  and  425  are mounted on the vehicle frame  308  and provide support of a vehicle weight of a pallet truck. Similarly, a drive wheel  309  supports the remainder of the vehicle weight of the pallet truck, not supported by casters  320  and  325 . The casters  320  and  325  as well as the drive wheel  309  are all shown to be in contact with the level surface  150 .  
         [0041]     The dampening devices  330  and  335  of  FIG. 8  are shown to be in a compressed state, with dampening device  330  compressed to a distance M and dampening device  335  compressed to a distance L. In a static condition, the distance M and distance L are the same, provided the dampening devices  330  and  335  have been adjusted similarly.  
         [0042]      FIG. 9  illustrates an operation of the novel weight distribution assembly shown in  FIG. 8  when traveling over an obstacle  160  located on the level surface  150 . In this figure, caster  325  is shown at an elevated position above the obstacle  160 , while the caster  320  remains on the level surface  150 . Dampening device  330  is shown compressed to a distance O and dampening device  335  is shown compressed to a distance N. Because of an increased reaction force of the caster  325  and the obstacle  160 , the dampening device  335  is compressed more than when the caster  325  is on the level surface  150 , as shown in  FIG. 8 . As a result, distance N in  FIG. 9  is substantially less than distance L of  FIG. 8 . The dampening device  330 , on the other hand, may undergo a moderate decrease in reaction force, such that the distance O in  FIG. 9  is approximately the same, or slightly greater than distance M of  FIG. 8 .  
         [0043]     Distances N and O may vary with time according to dynamic reaction forces being applied to casters  320  and  325  when traversing over uneven terrain. For example, the distance O may be observed when caster  325  initially comes into contact with obstacle  160 . The distance O may thereafter decrease after the reaction forces become static and redistribute the weight of the vehicle frame  308  between casters  320  and  325 , in part due to the tilt angle  333 .  
         [0044]     As a result of the compression of the dampening device  335 , the tilt angle  333  of the vehicle frame  308  in  FIG. 9  is less than the tilt angle  222  of the vehicle frame  208  in  FIG. 7 , and the drive wheel  309  is able to maintain contact with the level surface  150 . The drive wheel  309  may be partially lifted from the level surface  150 , depending on the amount of compression of the dampening devices  330  and  335 . The tilt angle  333  may be adjusted by varying the compression of the dampening devices  330  and  335 .  
         [0045]     One or both of the nuts  360  may be tightened in order to further compress the springs  371  and make more rigid dampening devices  330  and  335 . Similarly, one or both of the nuts  360  may be loosened in order to allow the springs  371  to decompress and make less rigid dampening devices  330  and  335 . Adjusting the amount of preload of springs  371  may therefore affect a resultant force acting through the casters  320  and  325 , varying the degree of the tilt angle  333 , and ultimately varying a resultant force acting through the drive wheel  309 . An increased resultant force acting through the drive wheel  309  may provide for an increase in vehicle traction and braking ability.  
         [0046]      FIG. 10  illustrates a further embodiment of the novel weight distribution assembly  100  when traveling over a level surface  150 . Casters  20  and  25  are shown mounted to mounting plates  70  and  75 , which are in turn attached to dampening devices  30  and  35 , respectively of caster assemblies  120  and  125 . Dampening devices  30  and  35  are shown as including the external springs  71  and the nuts  60 , in order to provide variable preload forces. The caster assemblies  120  and  125 , are mounted on the vehicle frame  8  and provide support of a vehicle weight of the forklift truck  5 . Similarly, a drive wheel  9  supports the remainder of the vehicle weight of the forklift truck  5 , not supported by casters  20  and  25 . The casters  20  and  25  as well as the drive wheel  9  are all shown to be in contact with the level surface  150 .  
         [0047]     The dampening devices  30  and  35  of  FIG. 10  are shown to be in a compressed state, with dampening device  30  compressed to a distance B and dampening device  35  compressed to a distance C. In a static condition, the distance B and distance C are the same, provided the dampening devices  30  and  35  have been adjusted similarly. In addition caster assemblies  120  and  125  are connected together by the coupling bar  15 , having a longitudinal axis  140  about which it may rotate.  
         [0048]      FIG. 11  illustrates an operation of the novel weight distribution system  100  shown in  FIG. 10  when traveling over an obstacle  160 . Caster  25  is shown at an elevated position above the obstacle  160 , while the caster  20  remains on the level surface  150 .  
         [0049]      FIG. 11  shows in more detail how the preloading by the dampening devices  30  and  35  can be selectively varied to control a torsional force or torsional moment about the coupling bar  15 . As the forklift truck  5  moves, one or both of the dampening device  30  or  35  may compress. In this example, dampening device  35  experiences compression force  175 , for example, when the forklift truck  5  is being turned or maneuvered or when traveling over an obstacle  160 . Compression force  175  results in a torsional force  132  being transferred through the coupling bar  15  to the caster assembly  120 .  
         [0050]     The coupling bar  15  couples the mounting plate  75  to the mounting plate  70 . A bending moment of the torsion bar between the first and second caster assembly may be varied according to an amount of compression of the dampening devices  30  and  35 . At the same time the coupling bar  15  may rotate  138  about a longitudinal axis  140  of coupling bar  15  according to the amount of compression of the dampening devices  30  and  35 .  
         [0051]     To adjust a torsional relationship between the two caster assemblies  120  and  125 , the preload forces against the two caster wheels  20  and  25  may be adjusted by screwing the nuts  60  up or down. When caster assembly  125  is then further compressed during vehicle travel, the given amount of compression  175  will vary the torsional force  132  transferred through coupling bar  15  to the caster assembly  120 .  
         [0052]     Dampening device  30  is shown compressed to a distance E and dampening device  35  is shown compressed to a distance D. Because of an increased reaction force of the caster  25  and the obstacle  160 , the dampening device  35  is compressed by a greater distance than when the caster  25  is on the level surface  150 , as shown in  FIG. 10 . As a result, distance D in  FIG. 11  is substantially less than distance B of  FIG. 10 .  
         [0053]     The torsional force  132  transferred through the coupling bar  15  acts against the mounting plate  70  and results in a further compression of the dampening device  30 . As a result of the increased torsional force  132  acting through the coupling bar  15  to the dampening device  132 , the compression distance E may be less than distance C of  FIG. 10 . The distance E in  FIG. 10  of dampening device  30  shown in  FIG. 11  may be less than the distance O of the dampening device  330  shown in  FIG. 9 .  
         [0054]     Distances D and E may vary with time according to dynamic reaction forces being applied to casters  20  and  25  when traversing over uneven terrain. For example, the distance E may be observed when caster  25  initially comes into contact with obstacle  160 . The distance E may thereafter decrease after the reaction forces become static and redistribute the weight of the vehicle frame  8  between casters  20  and  25 , in part due to the tilt angle  111 .  
         [0055]     As a result of the compression of the dampening device  30 , the tilt angle  111  of the vehicle frame  8  in  FIG. 11  may be equal to or less than the tilt angle  333  of the vehicle frame  308  in  FIG. 9 , and the drive wheel  9  is able to maintain contact with the level surface  150 . The drive wheel  9  may be partially lifted from the level surface  150 , depending on the amount of compression of the dampening devices  30  and  35 . The tilt angle  111  may also be adjusted by varying the compression of the dampening devices  30  and  35 .  
         [0056]     The contact between the drive wheel  9  and the level surface  150  may be controlled by varying the amount of preload force in springs  71 . Significantly, because of the additional compression of the dampening device  30  as a result of the transferred torsional force through the coupling bar  15 , and overall decrease in height of the vehicle frame  8  may be achieved when traversing over the obstacle  160  as compared to vehicle frames  208  and  308  of  FIGS. 7 and 9 . As a result, a contact pressure between the drive wheel  9  of  FIG. 11  and the level surface  150  may be greater than that of drive wheels  209  and  309  of  FIGS. 7 and 9 , if the tilt angle  111  is equal to the tilt angles  222  and  333 , or even if the tilt angle  111  is greater than the tilt angles  222  and  333 .  
         [0057]     As described above, the preload forces placed on the mounting plates  70  and  75  by compression springs  71  of dampening devices  30  and  35 , maintain corresponding contact pressures between the casters  20  and  25  and the ground. This allows the weight distribution assembly  100  to maintain contact of the drive wheel  9  with the ground in different driving and surface conditions while also improving vertical stability of the forklift truck  5 .  
         [0058]     For forklift trucks having a low lifting capability and that are traveling in a straight line, the described weight distribution assembly  100  provides a relatively static system. The supporting force on each of the caster assemblies  125  and  175  is approximately the same, with the pressure force applied to the drive wheel  9  being load dependent. Furthermore, a load in a central location on the forks  10  contributes to a vertical stabilization of the forklift truck  5 .  
         [0059]     Forklift trucks having a high lifting capability, however, with increasing lifting height may need a more rigid undercarriage structure. The torsional force  132  exerted on the coupling bar  15  is controlled by the weight distribution assembly  100  so that significant pressure of the drive wheel  9  against the ground surface is maintained even when there are ground irregularities or wear on the drive wheel  9 .  
         [0060]     Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention may be modified in arrangement and detail without departing from such principles. I claim all modifications and variation coming within the spirit and scope of the following claims.