Abstract:
An improved caster comprising a caster mount attachable to a frame of a mobile device, caster stem attached to the caster mount and operable to attach to a caster fork and wheel assembly, and a biasing means operable to restrict the rotation of the caster stem and thereby to prevent caster drift and flutter and to promote tracking of the mobile device. The biasing means can be in manner whereby the caster stem has at least one vertical notch defined therein and the biasing means comprises a spring plunger located in the caster mount. The spring plunger has an internal spring and a movable ball member attached to the spring and which faces the caster stem whereby the ball is biased against the caster stem and situated within the caster stem notch when the caster stem is in a desired rotational position. The biasing means can also employ magnets located in the caster stem and caster mount which align at opposite attracting poles when the caster is in a desired position.

Description:
PRIORITY 
       [0001]    This application hereby claims priority to provisional patent application Ser. No. 60/903,256, filed on Feb. 23, 2007. 
     
    
     STATEMENT OF FEDERAL FUNDING 
       [0002]    The United States government has rights to this invention, pursuant to Grant Number B3142C, through the Department of Veteran Affairs. 
     
    
     FIELD OF THE INVENTION 
       [0003]    This invention relates generally to a caster and caster system for a mobile device and more particularly relates to an improved caster and caster system which prevents caster drift and flutter and otherwise promotes tracking while a mobile device is traversing a side-sloped surface. 
       BACKGROUND OF THE INVENTION 
       [0004]    Wheelchair users constantly encounter difficulties while traversing various terrains during their daily activities. One specific problem a wheelchair user faces is caster drift while traversing side-sloped surfaces. As the wheelchair moves across a side-sloped surface, caster wheels tend to rotate down the slope (see e.g.,  FIG. 1 ). The user&#39;s downward limb must work very hard to keep the wheelchair climbing the incline or merely traveling straight across the side-sloped surface. This can result in pain or injury to the overworked limb. 
         [0005]    It is advantageous to eliminate the great force the user must exert on his or her downward limb for safety reasons. One particular design created for such purposes is U.S. Pat. No. 6,607,250, entitled “Caster block and wheel lock for wheelchair”. This reference features a locking mechanism that can be used to eliminate caster wheel rotation. Once engaged, this device locks the caster in the trailing position and fixes the path of the wheelchair. Although this system prevents caster drift, it also severely limits the viability of the caster for indoor use, on uneven surfaces, or in any condition under which regular turning is required. In contrast, the caster and caster system of the present invention allows users to turn the wheelchair or other device and make changes to the path of travel, while the caster is engaged and while still preventing caster drift (see, e.g.  FIG. 2 , where the path of a device using the caster and caster system of the present invention does not follow the slope). 
         [0006]    Outside of the wheelchair realm, delivery persons and others using carts and dollies frequently encounter trouble turning or navigating the related art devices, especially if the devices are heavily loaded. Further, because of caster drift, materials can fall off of the cart or dolly while the user is attempting to maintain a straight course on a sloped surface. 
         [0007]    Additionally, consumers frequently encounter wheel flutter in shopping carts because the current caster stems are freely rotating and somewhat loose fitting within the caster mount. This wheel flutter causes the entire cart to shake and/or become difficult to navigate. 
         [0008]    The present invention overcomes the disadvantages of the related art as discussed in detail below. 
       SUMMARY OF THE INVENTION 
       [0009]    An object of the caster and caster system of the present invention is to reduce the risk of upper limb pain and injury to wheelchair users by decreasing the forces required to traverse a side-sloped surface. The present invention further prevents caster drift and wheel flutter, by applying a forward or rearward biasing force on the caster wheel, thus lowering the forces required by the downhill limb to maintain a straight path. 
         [0010]    Another object of the present invention is to provide a caster biasing mechanism or means that can be retrofitted to an existing caster system of a mobile device such as a wheelchair, cart, shopping cart, or dolly. 
         [0011]    Additionally, the caster fork and wheel of the present invention rotate to allow a user to steer the wheelchair, cart, shopping cart, dolly or other mobile device, when necessary, and provide users with a greater degree of control while traversing a side-sloped surface. 
         [0012]    Specifically, what is provided is an improved caster and caster system comprising a caster mount attachable to a frame of a mobile device, a caster stem attached to the caster mount and a biasing means for biasing the caster in a desired rotational position. In one embodiment, the biasing means involves having one or more notches defined in the caster stem, and at least one spring plunger, having a ball defined on one end. The ball is biased against the caster stem and situates within the one or more notches to prevent caster drift and flutter and to promote tracking. In this embodiment, an optional disengaging pin can be provided, which slideably engages the one or more notches to disengage the ball from a notch or notches and allow for the free rotation of the caster. 
         [0013]    Optionally, magnets located in the caster mount and stem, by using attracting and repelling poles, can also be used to bias the caster stem in a desired location. Other biasing means may also be used. 
         [0014]    The caster stem and caster mounts, as modified or retrofitted with the biasing means of the present invention are operable to attach to a standard caster fork and wheel assembly. As such, the improved caster of the present invention is adapted to replace any existing caster on a mobile device. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0015]      FIG. 1  shows a prior art caster system wherein, caster drift is shown while the wheelchair traverses a side-sloped surface. 
           [0016]      FIG. 2  shows one embodiment of the caster and caster system of the present invention traversing a side-sloped surface without caster drift. 
           [0017]      FIG. 3  is a side view of a typical caster system. 
           [0018]      FIG. 4A  is a top view of one embodiment of the caster stem and caster mount in the caster system of the present invention. 
           [0019]      FIG. 4B  is a side cut-away view of the caster stem and caster mount along line A-A shown in  FIG. 4A . 
           [0020]      FIG. 5  is a perspective cut-away view of the caster stem and caster of the present invention shown in  FIGS. 4A and 4B . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    The invention will now be described in detail in relation to the preferred embodiments and implementation thereof which is exemplary in nature and descriptively specific as disclosed. As is customary, it will be understood that no limitation of the scope of the invention is thereby intended. The invention encompasses such alterations and further modifications in the illustrated apparatus, system or method, and such further applications of the principles of the invention illustrated herein, as would normally occur to persons skilled in the art to which the invention relates. 
         [0022]    As an example of one embodiment and not by limitation, this specification discusses use of the caster and caster system on or for a wheelchair, however, the caster and caster system of the present invention is not limited to such. For example, the caster or caster system can be used on a cart, shopping cart, dolly, or any other similar mobile device having wheels capable of similar attachment. The caster and caster system of the present invention also reduces caster or wheel flutter and otherwise promotes tracking in mobile devices. 
         [0023]    For the purposes of this description, and as shown in  FIG. 3 , a caster is divided into four main components or assemblies, the caster mount  1 , the caster stem  2 , the caster fork  3 , and the wheel  4 . The present invention focuses on a biasing means or hardware that can be retrofitted to an existing caster system or incorporated as improvements to the caster mount  1  and caster stem  2 . Once assembled, the caster assembly of the present invention can be installed on any wheelchair, shopping cart, cart, dolly or other mobile device that has or is adaptable for detachable caster mounts. 
         [0024]    When a wheelchair, shopping cart, cart, dolly or like mobile device traverses a side-sloped surface, the center of mass of the user, cargo, load and/or mobile device, acting about the rear wheels, generates a “moment” on the casters. A “moment” is generally defined as a measure of the torque produced by a force which causes an object to rotate about an axis, which is equal to the force multiplied by the perpendicular distance of the axis from the line of action of the force. This moment causes casters to rotate downhill and causes the mobile device to drift in the same direction (see  FIG. 1 ). As shown in  FIG. 2 , the caster assembly of the present invention is designed to prevent unwanted caster rotation and/or drift by preferably biasing each caster in the trailing position, that is, when wheel  4  rotates so as to trail behind a mobile device when in motion, and optionally biasing each caster in a leading position (when wheels are facing forward). 
         [0025]      FIGS. 4A and 4B  show one embodiment of the caster mount  11  and caster stem  12  of the present invention. The front of the caster mount housing  15  contain a biasing means in that form of at least one ball-nose spring plunger  16 . Optionally, the present invention can use two or more spring loaded ball-nose plungers  16  with the number of spring plungers  16  preferably corresponding to an equal number of notches  18  within caster stem  12 . However, in other embodiments, multiple plungers  16  may lie within a single notch  18 . For purposes of this description, an embodiment with a single plunger  16  is described, but the invention is not limited to this embodiment. The ball  17  of the spring plunger  16  protrudes into the bored center of the housing and interfaces with the notch  18  in the caster stem  12 . A spring  26  within the plunger  16  holds ball  17  into the notch  18  and thereby creates a lateral restriction against caster rotation, wobbling and/or drift. Ball  17  can have varying shapes, including a rounded, angled or pointed surface, provided that the shape of ball  17  is operable to interact with notch  18  to provide the desired lateral restriction against caster rotation, wobbling and/or drift. 
         [0026]    The interior of the notch  18  (toward the core of the caster stem  12 ) is cut and/or fabricated to accept ball  17  in such a manner that rotation of stem  12  causes the sides of notch  18  to exert a force on ball  17  and force it backwards into plunger  16 . By way of example, but not limitation, notch  18  can be rounded or V-shaped. Other shapes may also be used. The shape and size of the groove can be varied to provide for different levels of resistance to caster rotation and/or flutter. 
         [0027]      FIG. 5  shows a detailed view of the notch  18  in the caster stem  12 . As shown, notch  18  runs lengthwise from about the midpoint  21  of the caster stem  12  and upward to the top of caster stem  12  (i.e., upper thread  13 ). However, notch  18  can be of a shorter length, so long as it is placed to receive ball  17 . 
         [0028]    Both of the ends of the caster stem  12  are preferably threaded in one embodiment. The upper thread  13  allows the caster stem  2  to be secured against the bushing  20  in the top of the caster mount  11 . The lower thread  14  allows for the attachment of a standard caster fork  3 . Just above the lower thread  14 , is a bossed, preferably cylindrical section  22 . This cylindrical section  22  abuts the bearing  23  in the bottom of the caster mount  11  and serves as an anchor point for attaching the caster fork  3  and for securing the caster stem  12  to the caster mount  11 . 
         [0029]    In operation, when ball  17  of plunger  16  is fully engaged and caster stem  12  of the present invention begins to rotate, notch  18  in caster stem  12  turns away from ball  17  of the spring plunger  16 . As notch  18  initially turns (before full disengagement of ball  17  from notch  18 ), the surface of the notch  18 , opposite the direction of rotation, pushes the ball back towards the spring  26  within the plunger  16 . This compresses spring  26  and generates an increased spring force (according to Hooke&#39;s Law), which is proportional to the angle of caster stem  12  rotation. The magnitude of the spring force can be adjusted by changing either the strength or stiffness of spring  26  or the position (depth) of the spring plunger  16  within the caster mount housing  11 . The latter adjustment can occur, for example, if plunger  16  threadably attaches to cylinder  28  in caster mount  11  and end  27  of plunger  16  is adapted to have a screw head whereby the depth of plunger  16  into notch  18  can be adjusting by screwing plunger  16  inward or outward. Other means of spring adjustment, known to those skilled in the art, are also within scope of the caster and caster system of the present invention. Further, the spring plunger  16  optionally can be replaced with other biasing means known to those skilled in the art. 
         [0030]    When ball  17  of spring plunger  16  pushes against the side of the notch  18  upon initial rotation of caster stem  12 , the compression force of spring  26  generates a moment about the caster stem  12 . This moment increases while ball  17  remains in notch  18  and is forced backward against the spring  26  or other biasing mechanism. This moment or force is opposite in sign to, i.e., resists, the moment generated by the effects of gravity on the center of mass of the user and the wheelchair on a sloped surface. Once the magnitude of the plunger moment matches the magnitude of the moment generated by the center of mass, the caster ceases to rotate. The amount of allowable rotation can be adjusted to meet the request of a specific user or a user&#39;s activity. For example, with an increased spring strength and/or increased depth of plunger  16  into notch  18 , all caster rotation can be prevented such that the caster and caster system of the present invention serves as a caster lock. 
         [0031]    Under normal circumstances, though, the caster and caster system of the present invention allows for at least a minimal amount of rotation to aid in navigation. Where some amount of rotation is intended, the user must overcome the force of the spring  26  or other biasing means that acts to resist caster rotation. In such an embodiment, the ball-nose spring plunger  16  (or other biasing means) is configured to provide just enough force to resist caster drift. By altering the position of the spring plunger  16 , the biasing force can be raised or lowered to meet the needs of the user. 
         [0032]    Once the caster stem  12  is rotated far enough in either direction, ball  17  exits notch  18 , whereby the force of the spring  26  or other biasing means acts directly through the center of the caster stem  12  and no moment is applied to the caster stem  12 . This allows the user to turn a mobile device, such as a wheelchair, with only a slight resistance (due to the friction of the ball on the caster stem  12 ). When a turn is complete and each caster is realigned in the trailing position, the ball  17  reengages the notch  18  in the caster stem  12 . 
         [0033]    When wheeling indoors or in places where frequent turning is necessary, the forward or rearward bias can be removed from the casters. In the embodiment discussed above, such disengagement can occur by screwing plunger  16  outward so that ball  17  does not rest in notch  18 . 
         [0034]    In an alternative embodiment of the present invention, disengaging pin  19  (shown in  FIGS. 4B and 5 ) can act to force ball  17  into plunger  16 . Disengaging pin  19  fits within notch  18  when it runs the length of stem  12 . When push top  24  is pushed downward, the bottom end  25  of disengaging pin  19  disengages the ball  17  of the spring plunger  16  from insertion into notch  18  (i.e., ball  17  is forced into and plunger  16 ), thereby allowing the caster stem  12  to rotate without lateral restriction and with only limited friction of ball  17  pressing against the outer surface of the caster stem  12 . The bottom end  25  of disengaging pin  19  is preferably tapered to gradually push ball  17  of plunger  16  out of and away from notch  18 . The push top  24  of the disengaging pin  19  is preferably large, making it easier to handle for users with limited hand function. The top of the disengaging pin  9  can optionally be any other shape or size. 
         [0035]    In embodiments with multiple spring plungers  16  and notches  18 , a plurality of disengaging pins  19  can be optionally used. The number of disengaging pins  19  is preferably the same as the number of notches  18 . Disengaging pin  19  is an optional feature of the present invention, and the caster and caster system of the present invention can function properly with or without the disengaging pin  19 . 
         [0036]    Other biasing means can also be used, as noted above. For example, in another embodiment, a central rod can run up and down the axle or lie outside the caster stem to create a similar caster bias. This rod interacts with the stem, either directly or indirectly, to prohibit stem rotation when the rod is engaged. Such engagement can occur, for example, when the rod is pushed downward to engage a locking mechanism associated with the rotation of the caster stem. 
         [0037]    In yet another embodiment of the present invention, the biasing force can be provided magnetically, e.g., by two sets of magnets. In particular, magnets can be positioned in the caster stem  2  and the caster housing  1  such that attracting poles face each other (e.g. the south pole of the housing magnet faces inward and the north pole of the caster magnet faces outward) and are closest when the caster assembly and wheel rotates toward a desired position (i.e., a trailing or leading position). Two additional magnets can be further positioned in the caster housing  1  on both sides of the initial housing magnet such that like repelling poles face the caster stem  2  (e.g. the north poles of the outer housing magnets face inward). The attraction of the north pole within the caster stem magnet to the south pole in internal magnet of the caster housing and the repulsion of the north pole within the caster magnet to the south poles of the outer magnets in the caster housing  1  act to provide a further bias toward a desired position of the caster assembly and wheel. The strength of the bias is determined by the type, area, and configuration of the magnets in the caster stem  2  and housing  1 .