Abstract:
A method of assembling a wheel rotatable about a main axis and having a plurality of peripheral rollers mounted on peripheral axles aligned tangentially about the wheel and radially spaced from the main axis, each peripheral axle joined to adjacent other peripheral axles to form a continuous ring comprising the peripheral axles, the method including the steps of: molding each peripheral axle in a die having a cylindrical cycle for forming the axle shaft of the peripheral axle without longitudinal separation tines, the peripheral axles each having a receiving head portion for receiving a free end of the axle shaft of an adjacent peripheral axle; mounting a roller on each axle shaft; joining the peripheral axles together to form a continuous ring of peripheral axles; and molding a wheel body including a support structure around the continuous ring.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 13/138,032, filed Oct. 7, 2011, pending, which is a national phase entry under 35 U.S.C. §371 of International Patent Application PCT/AU2010/001419, filed Oct. 25, 2010, published in English as International Patent Publication WO 2011/047443 A1 on Apr. 28, 2011, which claims the benefit under Article 8 of the Patent Cooperation Treaty to Australian Patent Application Serial No. 2009905168, filed Oct. 23, 2009, the disclosure of each of which is hereby incorporated herein in its entirety by this reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to a wheel. More particularly, this invention relates to a wheel frame. Still more particularly, this invention relates to a wheel frame for a wheel having a plurality of peripheral rollers capable of rotating about peripheral axes aligned normal to the main axis of rotation of the wheel. 
       BACKGROUND 
       [0003]    The following references to, and descriptions of, prior proposals or products are not intended to be, and are not to be construed as, statements or admissions of common general knowledge in the art. In particular, the following prior art discussion does not relate to what is commonly or well known by the person skilled in the art, but assists in the understanding of the inventive step of the present invention of which the identification of pertinent prior art proposals is but one part. 
         [0004]    A type of wheel capable of multiple directional travel (while the main axis of rotation remains oriented in the same direction) has been described variously as an omniwheel and multi-directional wheel. Such wheels include a central hub rotatable about a main axis and a plurality of independently mounted rotatable rollers located about the rim of the hub. The rollers are each capable of rotation about an axis normal to and radially spaced from the main axis, whereby the wheel is capable of moving in a first direction in which the wheel rotates about the main axis or in a transverse direction in which one or more rollers contacting the ground rotate about their corresponding axes. 
         [0005]    One such wheel has been described in International Patent Application No. PCT/AU01/01175 (publication No. WO02/24471). The multiple directional wheel described therein comprised a circular wheel frame having a plurality of circumferentially spaced peripheral axles on which were mounted a pair of parallel rings of rollers offset relative to one another. The frame included an integrally formed polyaxled ring that required the rollers to be molded over the axles and limited the range of molding techniques that could be utilized. The rollers rotated about multiple axes normal to a main axis of the wheel. The roller axes were each aligned substantially normal to lines extending radially from the main axis through the midpoint of each roller axis. The rollers were positioned to provide a large overlap (20%-35% in side view) of effective ground contacting surface between diagonally adjacent rollers. 
         [0006]    Further improvement to such types of wheels has been described in International Patent Application No. PCT/AU2003/001002 (publication No. WO2004/014667), the entire disclosure of which is incorporated herein by reference. That disclosure described a method of construction of a frame on which was mounted a single row of rollers. The single row frames were able to be joined to like frames, in offset orientation, to form multiple rowed wheels having good ground contacting overlap between diagonally adjacent rollers. The method of forming the wheel involved each roller being mounted onto a wrap-around bush, which, in turn, was mounted to one of the multiple axles. 
       DISCLOSURE 
       [0007]    Accordingly, in one aspect of the invention, there is provided a wheel rotatable about a main axis and having a plurality of peripheral rollers mounted on peripheral axles aligned tangentially about the wheel and radially spaced from the main axis, each peripheral axle joined to adjacent other peripheral axles to form a continuous ring. 
         [0008]    Each peripheral axle may be molded in a single molding process. The peripheral axle may include an axle shaft. In a particularly preferred embodiment, the peripheral axle die may be configured to produce no mold parting or separation lines extending longitudinally along the axle shaft. The peripheral axle may be molded in a die that produces no mold parting or separation lines on or around the axle shaft. Preferably, all or part of the length of the axle shaft of each peripheral axle is molded in a die that defines a cylindrical cavity. Although not preferred, if the relevant portion of the die must include two or more components that combine to define the required cavity, the mold separation line should extend laterally, but not longitudinally, on the axle shaft, for example, along an annular step portion at a location along the length of the axle shaft. 
         [0009]    The peripheral axle may include a head. The head may be sized and configured to receive a free end of an axle shaft of an adjacent peripheral axle. A portion of the die for molding the head may comprise components that form a lateral or longitudinal separation line during molding of the peripheral axle. 
         [0010]    The head may define a bore configured to receive a free end of an axle shaft of an adjacent peripheral axle. The bore may be defined by a mouth extending from the head. The mouth and the bore may be cylindrical and/or conical in shape. Preferably, the mouth and the bore are cylindrical. 
         [0011]    In a particularly preferred embodiment, the mouth is molded in a component of the die that produces no mold parting lines along the length of the cylinder or cone forming the mouth. Accordingly, the peripheral axle die preferably further includes a component that defines a cylindrical or conical cavity to form the mouth without producing mold parting lines and, particularly, mold parting lines extending longitudinally along the mouth. 
         [0012]    The mouth may be wholly or partially cylindrical and/or wholly or partially conical in internal and external shape. Similarly, the axle may be uniformly cylindrical or may be frusto-conical in shape. The axle shaft may be partly conical in shape, particularly proximal to its free end. Accordingly, the axle shaft and mouth may each be molded in a die component defining a cylindrical or conical structure, or a combination of both. This enables the axle shaft and mouth to be formed in a single die component defining a cylindrical or conical cavity that completely surrounds the component to be formed. This die arrangement may have the effect that no mold parting lines extending longitudinally are formed on the axle shaft and mouth components so these components, having a closely circular cross-section at any point along their length, may be formed to have a round cross-section to a high degree of accuracy. This enables the axle shaft to mouth components to be uniformly and consistently molded accurately round without blemishes and shape inconsistencies, enabling manufacturing to low tolerances with resultant minimal play between moving parts. 
         [0013]    The peripheral axle head portion may be separately formed from the peripheral axle shaft portion, although this is not preferred as it would add to the number of components required for a particular wheel. 
         [0014]    Each head effectively forms a corner of the continuous ring. The head may include an annular shoulder at one end to retain the roller on the axle shaft. The other end of the head includes the mouth. The head includes a corner portion wherein the mouth bore has a longitudinal axis set at an angle θ (theta) relative to the longitudinal axis of the axle shaft. The general angle θ (theta) may be determined by the algorithm 180-360/n, where n equals the number of peripheral rollers lying in a single plane on the continuous ring. Correspondingly, n equals the number of peripheral axles constituting the continuous ring. 
         [0015]    Accordingly, the wheel may comprise three or more rollers up to a practical maximum of ten rollers. Based on the above formula, the angle θ (theta) for a head of a triple roller wheel is about 60°. The angle θ (theta) for a head of a four roller wheel is about 90°. 
         [0016]    The axle shaft may terminate in a free end that is the same radius as the remainder of the axle shaft. That is, the axle shaft has a consistently round cross-section throughout its length. Alternatively, the axle shaft may include a stepped terminating portion that has a smaller or larger radius than the main body of the axle shaft. The terminating portion may be sized and configured to be received in the mouth. Accordingly, the radius of the mouth bore may be smaller than the radius of the main body of the axle shaft and the external cylindrical or conical wall of the mouth may have a greater diameter than the diameter of the axle shaft main body at the juncture. In this way, the mouth may perform as an annular shoulder effective to retain a roller on the axle shaft between the head of the peripheral axle on which the roller is mounted on the head of an adjacent peripheral axle. 
         [0017]    The mouth bore may be a through-bore extending through from the mouth to an open end at the back of the head. The through-bore may be effective to permit the terminal end of the axle shaft to be in contact with an over-molded support of the wheel body. The through-bore may thus provide an opening into which the over-molded wheel body may extend during the molding process to reinforce the connection between the axle shaft and the head to provide the combined structure of the axles and the over-molded wheel body that is of greater strength and rigidity than if the over-molding did not so extend into the interstitial spaces of the axle head. The terminal end butt of the axle shaft may be shaped at an angle transverse to the longitudinal axis of the axle shaft to permit a greater length of axle shaft to be inserted into the mouth bore while conforming to the corner portion shape. The joint between the axle shaft and the head of an adjacent peripheral axle may be further strengthened by adhesive, heat fusion, ultrasonic or other known joining or welding techniques. 
         [0018]    The wheel preferably further comprises a wheel body or frame. The wheel body may include a hub or central aperture defining a main axle or central bore. Extending from the hub or an inner rim of the wheel body may be a plurality of outwardly extending supports. The outwardly extending supports may extend radially from the center of the wheel body. The outer ends of the supports may secure the heads. Preferably, the wheel body is formed at least partially around the heads. The heads may be mounted on or in the outwardly extending supports. The outwardly extending supports may partially or wholly surround the heads, provided that the supports do not interfere with the movement of the rollers. 
         [0019]    The wheel components may be made from a number of different materials and may comprise a composite of components made from different materials. However, the skilled person will appreciate that a wide variety of suitable materials may be employed, depending on the application and strength and wear demands on the wheels. 
         [0020]    For example, the peripheral axles may be made from high-strength molded plastic, cast steel, or a composite of two or more different materials. For example, axles may include a molded plastic casing reinforced with inner metal rods or other reinforcing structures. 
         [0021]    Alternatively, the peripheral axle may be constructed such that the axle shaft consists of a metal rod with no plastic over-mold, except about the shaft end attached to the axle head, the head consisting of a plastic molded component. 
         [0022]    The bushings may be made from high-strength extruded plastic over which the rollers may be molded. Accordingly, the bushings may be inserts molded inside, for example, polyurethane elastomeric rollers. 
         [0023]    The bushings may be shaped to follow the contours of the roller tyre. For example, the external surface of the bushing may have a plurality of annular ribs aligned laterally relative to the bushing&#39;s longitudinal axis to better grip the tyre (preferably over-molded) and be effective against longitudinal slippage or movement of the tyre relative to the bushing. Preferably, however, the external surface has a plurality of longitudinal ribs. Advantageously, the longitudinal ribs are arc- or bow-shaped to follow the general cigar shape of the roller&#39;s external shape. The arched ribs are preferably solid and integral to the general cylindrical core of the bushing throughout their lengths, although they may be supported only at their ends and bowed in the middle. Longitudinally aligned ribs will reduce relative movement between the ribs and the tyre as the roller rolls about its longitudinal roller axis, strengthening the joint and extending the life of the roller. 
         [0024]    In another aspect, the invention provides a method of assembling a wheel rotatable about a main axis and having a plurality of peripheral rollers mounted on peripheral axles aligned tangentially about the wheel and radially spaced from the main axis. Each peripheral axle is joined to adjacent other peripheral axles to form a continuous ring, the method including the steps of:
       molding each peripheral axle in a die having a cylindrical cavity for forming the axle shaft of the peripheral axle without longitudinal separation lines, the peripheral axles each having a receiving head portion for receiving a free end of the axle shaft of an adjacent peripheral axle;   mounting a roller on the axle shaft;   joining the peripheral axles together to form a continuous ring of peripheral axles; and   molding a wheel body including a support structure around the continuous ring.       
 
         [0029]    The wheel body may include a central hub to be rotatably mounted on the main axle. 
         [0030]    The support structure may include a plurality of outwardly extending arms that support the receiving heads in spaced relationship to the main axis. 
         [0031]    The step of mounting a roller on each axle shaft may be preceded by the step of first forming a bush as a sleeve to be mounted on the axle shaft. The corresponding roller may be molded around the bush prior to mounting on the axle shaft. The step of mounting the roller on each axle shaft may include inserting the bush into a roller and then inserting the axle shaft into the corresponding bush and roller. 
         [0032]    The head may further include spurs, ridges or other surface features that permit greater adherence to a structure molded about the head or a segment thereof. The spur or other surface features may serve to increase the surface area between the adhering wheel body and the receiving head, thereby strengthening and reinforcing the joint and increasing the strength and rigidity of the wheel structure. 
         [0033]    The receiving head may include a mouth defining a short bore, the mouth forming an annular shoulder on the axle shaft to trap the corresponding roller on the shaft between the receiving head and the mouth. 
         [0034]    The receiving head including the corner portion may be shaped so that the longitudinal axis of the axle shaft is set at an angle θ (theta) to the longitudinal axis of the short bore. The angle θ (theta) may be determined by the algorithm 180-360/n, wherein n equals the number of peripheral rollers lying in a single plane on the continuous ring. 
         [0035]    The die may include an additional cylindrical cavity for forming the mouth devoid of longitudinal separation lines thereon. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    The invention may be better understood from the following non-limiting description of preferred embodiments, in which: 
           [0037]      FIGS. 1 a -1 i    are various views of a peripheral axle according to one embodiment; 
           [0038]      FIGS. 1 j -1 p    are various views of a peripheral axle according to another embodiment; 
           [0039]      FIGS. 1 q -1 v    are various views of a peripheral axle according to yet another embodiment; 
           [0040]      FIGS. 2 a -2 c    are various views of a bush according to one embodiment; 
           [0041]      FIGS. 2 d -2 f    are various views of a bush according to another embodiment; 
           [0042]      FIGS. 2 g -2 i    are various views of a bush according to another embodiment; 
           [0043]      FIGS. 3 a -3 d    are various views of roller; 
           [0044]      FIGS. 4 a -4 f    are various views of a wheel body; 
           [0045]      FIGS. 5 a -5 d    are various views of a wheel; 
           [0046]      FIGS. 6 a -6 d    are various partially transparent views of the wheel shown in  FIGS. 5 a   - 5   d;    
           [0047]      FIGS. 7 a  and 7 b    are sectional views of the wheel shown in  FIGS. 5 a   - 5   d;    
           [0048]      FIG. 8  is an exploded view of the wheel shown in  FIGS. 5 a -5 d   , noting intersections between the peripheral axles, with the wheel body supports not being shown; 
           [0049]      FIGS. 9 a  and 9 b    are perspective and ghosted views, respectively, of a continuous ring assembled without rollers; 
           [0050]      FIGS. 10 a -10 c    are perspective and ghosted views of a continuous ring and rollers preassembly; 
           [0051]      FIGS. 11 a  and 11 b    are cut-away and ghosted views, respectively, of a peripheral axle and roller preassembly; 
           [0052]      FIG. 12  is a cut-away sectional view of the peripheral axle and roller preassembly shown in  FIGS. 10 a   - 10   c;    
           [0053]      FIG. 13  is a perspective view of a peripheral axle and a head of an adjacent peripheral axle; 
           [0054]      FIG. 14  is a perspective view or a pair of peripheral axles mated together without a roller; 
           [0055]      FIG. 15  is a ghosted perspective view of a pair of peripheral axles with mounted rollers and an unmated pair of peripheral axles with mounted rollers prior to complete assembly; 
           [0056]      FIG. 16  is a perspective view of the peripheral axles and rollers shown in  FIG. 15  assembled prior to insertion into a wheel body injection mold; 
           [0057]      FIG. 17  is a perspective ghosted view of the assembled wheel completed in an injection mold; 
           [0058]      FIG. 18  is a perspective view of the completed wheel after removal from an injection mold; 
           [0059]      FIG. 19  is a perspective cut-away view of the joint between a peripheral axle head, adjacent peripheral axle shaft and wheel body support; and 
           [0060]      FIG. 20  is a perspective view of a completed twin wheel made according to an aspect of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0061]    Preferred features of the present invention will now be described with particular reference to the accompanying drawings. However, it is to be understood that the features illustrated in and described with reference to the drawings are not to be construed as limiting on the scope of the invention. 
         [0062]    Referring to  FIGS. 1 a  to 1 i   , there is shown a peripheral axle  10  comprising a head  20  and an axle shaft  50 . The peripheral axle  10  shown is one of four peripheral axles  10  shaped to form a continuous ring  80  (see below) with three other like peripheral axles  10 . However, the skilled person will appreciate that the continuous ring  80  may be configured to comprise a lesser or greater number of peripheral axles, for example, between three and ten axles making up the continuous ring of a wheel (see below). In each case, the peripheral axle  10  components may be identical, having a male end (the axle shaft  50 ) and a female end (the head  20 ), so that each peripheral axle  10  may mate with a like, adjacent peripheral axle  10  to form a continuous ring  80  comprising between three and eight peripheral axles  10 . 
         [0063]    The head  20  includes a mouth  30  and corner portion  40 . 
         [0064]    The mouth  30  is a short cylinder defining a bore  32 . The bore  32  may be cup-shaped and may terminate in the corner body  42  of the corner portion  40 . However, preferably, the bore  32  may be a through-bore extending fully through to the rear of the corner body  42 . The bore  32  internal walls may be keyed with longitudinal surface features that cooperate with corresponding features on the axle shaft  50  to mitigate against rotation of the axle shaft  50  in the bore  32 , but preferably the bore  32  and the corresponding axle shaft are round in cross-section. 
         [0065]    The corner portion  40  sets the angle θ (theta) at which the cylindrical axis of the bore  32  is set relative to the longitudinal axis of the axle shaft  50 . Based on the formula 180-360/n, where n equals the number of peripheral axles  10  lying in a single plane on the continuous ring  80 , the corner angle θ (theta) for a head of a four-axled embodiment is about 90°. 
         [0066]    The corner portion  40  further comprises a generally cylindrical corner body  42  extending between the mouth  30  and an annular shoulder  44  aligned coaxially with the axle shaft  50 . The corner body  42  includes surface features in the form of a spur  46  having two lateral triangular projections extending either side of the corner body  42 , and bridged to each other in the elbow of the corner body  42 . 
         [0067]    The surface features may also be in the form of ridges, knobs, other protrusions or grooves that increase the surface area of the corner body  42 . This increases the strength of the joint between the peripheral axle  10  and an over-molded wheel body  100  ( FIG. 4 a   ) as will be described below. The spur&#39;s  46  lateral projections  46   a ,  46   b  extending in opposed lateral directions, and the elbow bridge  46   c  extending inwardly toward a main axis of the wheel (see below), provide structures in three different directions for optimum rigidity and strength in the over-molded joint between the peripheral axle  10  and the wheel body  100 . 
         [0068]    Coaxial with the annular shoulder  44  is the axle shaft  50 . The axle shaft  50  comprises an elongate, uniformly cylindrical axle  51  stepped down in diameter relative to the shoulder  44 , so that the shoulder  44  provides an annular retaining wall  45  for trapping one end of a roller  140  (see below). 
         [0069]    It will be appreciated by the skilled person that in molding an axle shaft  50  or mouth  30  structure so that the structures are accurately round at any cross-sectional slice along their lengths, structures of either a conical or cylindrical shape may be utilized within the scope of this invention. The axle shaft  50  may be marginally truncated conical in shape, as may the mouth  30  and its bore  32 . The conical shape may be useful to achieve length limiters that restrict the axial movement of the axle shaft  50  relative to a roller  120 , or the axle shaft  50  relative to the bore  32 . However, in the preferred arrangement shown, the structures  50 ,  30  are shown as cylindrical. 
         [0070]    The axle shaft  50  may be of consistent cross-section throughout its length and may be in the form of a cylinder. However, preferably the terminal end  52  of the axle shaft  50  terminates in an angled wall  54  having a plane lying at an angle α (alpha) to the longitudinal axis  56  of the axle shaft  50 . The greater the number of rollers  120  (see below) in a single wheel plane, and the greater the corresponding number of corner portions  40 , the larger the angle θ (theta). The angle α (alpha) is inversely proportional to the angle θ (theta). For a four roller wheel, the angle α (alpha) is 90°. 
         [0071]    Furthermore, the terminal end  52  may have a step-down terminal portion  58  connected to the main cylindrical axle  51  by an annular step  59 . The terminal portion  58  is shaped and configured for insertion in the mouth  30  of bore  32 , so that the angled wall  54  extends to the outer wall  43  of the corner body  42 . The angled wall  54  preferably does not extend beyond the outer wall  43 , but may be flush therewith. 
         [0072]    The axle shaft  50  is of a sufficient thickness to provide a structurally rigid and strong axle along its length. For example, in the embodiment shown in  FIGS. 1 a -1 i   , the axle shaft  50  diameter may be 3.8 mm for a 48 mm sized wheel  160  ( FIG. 5 a   ), although the axle shaft  50  diameter may vary depending on the application and wheel  160  size. The mouth  30  wall may be sufficiently thick for joint strength with the terminal portion  58 , as well as to provide a step (in the form of mouth face  31 ) from the axle shaft  50  to the mouth  30  outer wall. The spur  46  adds mass to the corner or bridge portion  46   c  and to increase the surface area for over-mold adhesion with the wheel body  100 . 
         [0073]    The axle shaft  50  may be solid or hollow in structure. The axle shaft  50  for heavy load applications is a composite structure having a metal rod extending centrally and longitudinally along a substantial proportion of its length to provide a central and longitudinal reinforcing beam. Alternatively, the axle shaft  50  may be wholly made of metal or wholly made of plastic. 
         [0074]    Referring to  FIGS. 1 j -1 p   , there is shown an alternative embodiment of a peripheral axle  310 . The peripheral axle  310  comprises a steel axle shaft  350  and a plastic over-molded head  320 , but is otherwise of the same shape and configuration as the peripheral axle  10 . Referring to  FIGS. 1 q -1 v   , there is shown an alternative peripheral axle  410  comprising a composite or steel and plastic over-molded materials, wherein the axle shaft  450  includes an inner steel core rod or pin  451  and a plastic over-molded sleeve  452 . The steel core rod  451  is continuous with a terminal portion  458  and the outer extent of the plastic over-molded sleeve  452  forms an annular step  459 . As best seen in  FIG. 1 v   , the plastic over-molded sleeve  452  is contiguous with the head  420 . The head  420  is formed with plastic material integrally formed with the plastic over-molded sleeve  452 . 
         [0075]    Referring to  FIGS. 2 a  to 2 c   , there is shown a bush or roller  140  in the form of a hollow cylinder having a longitudinal axis  142  and defining an internal cylindrical bore  144  shaped and configured to receive the axle shaft  50 . The internal cylindrical bore  144  is preferably shaped and configured to receive the main cylindrical axle  51  (not shown). The bush  140  may be mounted on the main cylindrical axle  51  between the annular step  59  and the annular retaining wall  45  (see  FIG. 1 e   ). 
         [0076]    A first bush end  146  may abut, be flush with, or lie close to or in the same plane as, the annular retaining wall  45  and the other bush end  148  may be flush with, or lie close to, or in the same plane as, the annular step  59 . 
         [0077]    Instead of the bushing  140 , there is shown in  FIGS. 2 d  and 2 e    an alternative bushing  340  comprising a plurality of longitudinal ribs  342  (in this case twelve). The longitudinal ribs  342  are arced along their length, whereby their respective mid-sections  345  are radially spaced further away from the longitudinal axis  356  than the ribs&#39;  342  respective ends  346 . The longitudinal ribs  342 , therefore, form solid arcs extending longitudinally along the external surface of the bushing  340 , whereby the bushing  340  generally has a longitudinally ribbed cigar shape that may correspond to the general cigar shape of roller tyres to be over-molded thereon. The longitudinal ribs  342  are preferably integrally formed with the main body or core  341  of the bushing  340 . 
         [0078]    Turning to  FIGS. 2 g -2 i   , yet another alternative bushing embodiment is shown in the form of barrel-shaped bushing  440 . The internal bore  344 ,  444  is cylindrical in order to conform to the general cylindrical shape of the axle shaft  50 . However, the external surface  442  of the barrel-shaped bushing  440  is, indeed, cigar- or barrel-shaped in order to correspond to the generally cigar- or barrel-shaped rollers  120  that may be molded thereon. The provision of a cigar-shaped bushing  440 , made of necessarily hardened plastic, relative to the softer elastomeric plastics of which the tyre is made, may serve to strengthen the overall roller  120  construction, save on elastomeric material and give greater strength to the roller  120  throughout its length relative to a straight cylindrical bushing  140 . The arced curvature of the bushing  440  represents a stronger structure with regard to lateral forces to which a roller  120  may be subjected to transverse to the longitudinal roller axis  142 , compared to the straight cylindrical bush  140 . 
         [0079]    Referring to  FIGS. 3 a  to 3 d   , preferably the bush  140  is retained within a cylindrical cavity  122  in the roller  120 . The roller  120  includes a pair of opposed openings at each end  124  that are coaxial with the cylindrical cavity  122 . The bush  140  has a smaller internal diameter and a larger outer diameter than the openings  124 . The roller tyre  126  may be over-molded on the bush  140  as will be described below. 
         [0080]    The roller  120  is a bi-truncated fusiform, barrel or cigar shape. The thickness of the roller tyre  126  at its respective ends as indicated by reference no.  128  is critical for a number of reasons. The end portion  128  of the roller  120  must be sufficiently thick to preclude failure through normal wear and tear as the stresses and potential for failure are greatest at the end portion  128 . However, the thicker the end portion  128  is, the more difficult it is to achieve significant overlap between diagonally adjacent rollers in a wheel such as the twin wheel  180  shown in  FIG. 20 . This is because the more elongate bi-truncated fusiform shape better accommodates greater overlap between diagonally adjacent rollers  120   a ,  120   b , but a finely tapered end portion  128  will make the roller  120  less robust and likely to smoothly rotate, and more prone to failure and less resilient to normal wear and tear. 
         [0081]    The dimensions of wheel  160  and the rollers  120  may vary for different applications. The radius of curvature of the rollers  120 , when viewed in side elevation, generally will follow the radius of curvature of the wheel periphery. Larger wheels will have rollers having a larger radius of curvature. The number of rollers per frame may be determined according to criteria such as the relative strength of component materials and structures and the capacity of individual rollers to bear the load transferred through the main axle  15 . High load wheels will require barrel-shaped, proportionally shorter rollers having thicker roller tyres to withstand the large forces, particularly those borne at the remote ends of the rollers when in ground contact. Lighter load applications may utilize cigar-shaped, more-elongate and small diameter rollers that may be employed to reduce production and raw material costs. 
         [0082]    In  FIGS. 4 a -4 f   , there is shown the wheel body  100  as it would be shaped without the intersecting peripheral axles  10 . The wheel body  100  comprises a central hub  102  journaled for rotation about a main bearing  104  that is keyed by longitudinal slot  106  for mounting on a main axle  15  (see  FIG. 20 ). The wheel body  100  further includes a plurality of outwardly extending arms  108 . The arms preferably extend radially. The arms  108  terminate in support heads  110  that are, in side elevation as shown in  FIG. 4 a   , substantially triangular in shape. Adjacent support heads  110  present opposed substantially parallel planar faces  112  between which extend the axle shaft  50  when the wheel  160  is assembled. In the embodiment shown, the wheel body  100  is shaped to accommodate four rollers  120 , thereby comprising four radial arms  108 . 
         [0083]    The wheel body  100  may be bi-symmetrical through a plane B as shown in  FIG. 4 b   . More preferably, the wheel body  100  includes an outer rim  114  and an inner rim  116 . The inner rim  116  includes locking elements  118  comprising a pair of raised arc tracks  111 ,  113  that include complementary grooves and ridges adapted to lockably engage each other when like wheel bodies  100  are abutted with their respective inner rims  116  facing each other. The respective wheel bodies  100  are positioned so that the raised arc locking elements  111 ,  113  are positioned in arc gaps  115  and rotated 90° so that the respective locking elements  111 ,  113  engage. 
         [0084]    In another embodiment, the locking elements  111 ,  113  are identical in shape and configuration, but rotated 180° relative to each other. The locking element  111   a ,  113   a  shown in  FIG. 4 d    include a receding ramp  117  including a trapped groove into which corresponding lead projections  119  enter to positively engage the respective wheel bodies  100 . The respective wheel bodies  100  may be further chemically or mechanically bonded by subjecting the combined twin wheel body  180  to, for example, ultrasonic welding to provide a strong bond between the respective surfaces of the facing inner rims  116  so that they are locked in non-reversible engagement. 
         [0085]    As shown in  FIG. 4 f   , the support heads  110  are slightly biased toward the inner side of the main axle bore  101  of the wheel body  100  when viewed in transverse section along line A-A of  FIG. 4   e.    
         [0086]    In  FIGS. 5 a -5 d   , a completed wheel  160  is shown with the wheel body  100  over-molded onto the peripheral axles  10 . 
         [0087]    In  FIGS. 6 a -6 d   , the internal detail of the over-molded connection between the peripheral axles  10  and the supports  108  are shown in greater detail. It can be seen that the spurs  46  provide rigid internal structures around which the over-molded support  108  strongly contact over a large surface area to ensure a strong and rigid connection between the peripheral axles  10  and the support  108 . It can also be seen that the wheel body  100  is shaped and configured to provide a cradle or recess  105  within which the roller  120  is suspended by its mounting to the peripheral axle  10 . 
         [0088]    In  FIGS. 7 a  and 7 b   , sectioned views of the wheel  160  show the relationship between the over-molding of the support  108  and the terminal end  58  of the peripheral axle  10 , together with the mouth bore  32 . The terminal end  58  terminates just short of lying flush with the outer opening of the mouth bore  32 , so that mold material of the support  108  penetrates into the mouth bore  32 , strongly connecting with the peripheral axle  10 , both at the head  20  and the peripheral end  58 . Also indicated is the relationship between the axle shaft  50 , the annular retaining wall  45  and the bush  140 , stepped wall  59  and the mouth  30 . It can be seen that the annular retaining wall  45  and mouth facing wall  31  are inserted into the openings  124  and abut the ends  146 ,  148  of the bush  140 , so that the roller  120  is trapped for rotation within the recess  105  and able to freely rotate about the axle shaft  50 . 
         [0089]    In  FIG. 8  there is shown the components of the wheel  160  in exploded view. The wheel  160  comprises the wheel body  100 , four peripheral axles  10  and four rollers  120 .  FIGS. 9 a  and 9 b    show how the peripheral axles  10  join together to form a continuous ring  80  by the joining of four identical peripheral axles  10  by the insertion of the axle shaft  50  of each peripheral axle  10  into a mouth  30  of an adjacent peripheral axle so that the terminal end  58  extends almost, but not fully, through the mouth bore  32 . It is noted that the rollers  120  may be over-molded over the axle shafts  50 , so that the continuous ring  80  may be formed before the rollers  120  are mounted to the continuous ring  80 . However, it is preferred that the rollers  120  are first formed about a corresponding bush  140  and mounted onto the corresponding peripheral axle, while the terminal end  58  is free and unattached to an adjacent peripheral axle  10 . In  FIG. 9 a   , it can be seen that the annular shoulder  44  is integrally formed with the axle shaft  50 . 
         [0090]    In  FIGS. 10 a -10 c   , the rollers  120  are shown assembled onto the continuous ring  80  preparatory to the wheel body  100  being over-molded onto the heads  20  of the preformed peripheral axles  10 . 
         [0091]    In  FIG. 11 a   , the formation of the roller  120  and peripheral axle  10  is shown. The peripheral axle  10  is separately molded, as is the bush  140 . The roller tyre  126  is then molded over the bushing  140  to form a roller module comprising the roller tyre  126  and bushing  140 . The roller module  120 ,  140  is then mounted onto the axle shaft  50  by insertion of the axle shaft  50  through either end of the roller module  120 ,  140 . The roller module  120 ,  140  is mounted onto the axle shaft  50  so that either end  146 ,  148  of the bush  140  abuts the annular retaining wall  45 . The opening  124  maybe of a marginally greater diameter than the annular shoulder  44  to minimize friction as the roller  120  rotates, although the gap between the end portions  128  of the roller  120  and the annular shoulder  44  are minimal to reduce the effect of compressive forces applied to the end roller portions  128 . 
         [0092]    It will be appreciated that the external diameter of the mouth  30  is preferably identical to that of the annular shoulder  44 , so that the insertion of the terminal end  58  in the mouth bore  32  until the mouth  30  open face  31  abuts the stepped wall  59 , the end roller portion  128  partially extending over the mouth  30  as best seen in  FIG. 7 b   . In  FIG. 11 b   , the bushing  140  is shown mounted onto the peripheral axle  10 . The peripheral axle  10  may be molded from high-strength plastic material, the bushing  140  from high-strength, low-friction plastic material and the roller tyre  126  from a high-strength and moderately high-friction polymer. 
         [0093]    As with  FIG. 7 b   ,  FIGS. 12 and 19  clearly demonstrate the configuration of the joint between the terminal end  58  and head  20 , as well as the mounting of the roller  120  over the extruded plastic bushing  140 . 
         [0094]      FIGS. 13 and 14  demonstrate the insertion of the terminal portion  58  into the mouth bore  32  in a “line-to-line fit” that holds the adjacent peripheral axles  10  together. As shown in  FIG. 14 , the joining of the axle shaft  50  with the mouth  30  creates a second annular step formed by mouth face  31  abutted against the stepped wall  59 , whereby the mouth face  31  and annular retaining wall  45  are effective to trap the roller bushing  140  on the axle shaft  50 . 
         [0095]    In  FIG. 15 , the step of joining pairs of sub assemblies of peripheral axle  10  and roller module  120 ,  140  combinations is shown, whereby pairs are formed that are then joined to form a continuous ring  80  on which n number of rollers are mounted. 
         [0096]      FIG. 16  shows the assembled pairs of subassemblies joined to form a continuous ring  80  prior to it being inserted into a wheel body  100  injection mold  200 , whereby the continuous ring  80  and rollers  120  are nestled into the mold  200 , which is then shut as shown in  FIG. 17  to lock the continuous ring  80  and rollers  120  in place. Shut offs in the mold  200  secure the continuous ring  80  for molding and then the wheel body  100  is injected over the continuous ring  80 . 
         [0097]    The dye or mold  200  includes a central insert  202  that defines the cavity corresponding to the main axle bore  101  shown in  FIG. 4 a    and  FIG. 4 f   . The insert  202  also includes a longitudinal ridge  204  that forms the keyed slot  106 . As shown in  FIG. 18 , the completed wheel  160  may then be removed from the mold  200 . 
         [0098]    As shown in  FIG. 19 , the terminal portion  58  extends through the mouth bore  32 , but leaves a small area adjacent to the outer opening of the mouth bore  32  to permit the over-molded wheel body  100  in the area of the support heads  110  to penetrate and mold into and around the mouth bore  32  to increase the adhesion of the wheel body  100  to the peripheral axle  10  and more particularly to the head  20 . 
         [0099]    A pair of completed wheels  160  are shown as a pair of wheels joined to form a twin wheel  180  mounted to a main axle  15  having a hexagonal cross-sectional bore. The twin wheel  180  is useful as an omni-directional or multi-directional wheel and may be used as a substitute, for example, in place of castor wheels. 
         [0100]    Throughout the specification and claims, the word “comprise” and its derivatives are intended to have an inclusive rather than exclusive meaning unless the contrary is expressly stated or the context requires otherwise. That is, the word “comprise” and its derivatives will be taken to indicate the inclusion of not only the listed components, steps or features that it directly references, but also other components, steps or features not specifically listed, unless the contrary is expressly stated or the context requires otherwise. 
         [0101]    Orientational terms used in the specification and claims such as “vertical,” “horizontal,” “top,” “bottom,” “upper” and “lower” are to be interpreted as relational and are based on the premise that the component, item, article, apparatus, device or instrument will usually be considered in a particular orientation, with the context indicating which component is uppermost. 
         [0102]    It will be appreciated by those skilled in the art that many modifications and variations may be made to the methods of the invention described herein without departing from the spirit and scope of the invention.