Rolling goal frame mounted on ground contacting balls

Abstract

A goal frame (1, fig. 1) is supported on ground balls 11 that rotate omnidirectionally as the goal frame (1) rolls over the pitch; at least three swivelling casters 21 are spaced apart around a vertical central axis Xc to transfer the load from the goal frame (1) to each ground ball 11. Each caster 21 includes a swivel bearing 22 defining a swivel axis Xs and a caster wheel 23 rotating about a caster wheel axis Xw. When projected, the caster swivel axis Xs passes through the centre of ball 11. A load transfer mechanism 40 may be operable to raise and lower the goal frame to transfer the load selectively to and from the ground balls 11.

Description

This invention relates to portable goal frames that are rollable over the pitch between games. Wheeled goal frames are used for example in football (which is to say, soccer or association football) on both natural and artificial pitches. For better manoeuvrability it is known to mount the wheels on trailing axles to form swivelling casters, for example, as taught by WO2020128536 Al and GB2576113 A. However, if too much force is applied to a caster that happens to be misaligned with the direction in which the goal frame begins to move, then the wheel may scuff or gouge the playing surface, particularly on natural turf. GB 2599643 A (GB'643) teaches a goal frame mounted on omnidirectionally rotatable ground balls, each ground ball having a bearing assembly with at least one upper rolling body for transferring the mass of the goal frame to the ground ball. The upper rolling body may be configured as a caster but preferably is spherical and rotates omnidirectionally about its centre. A retaining structure with a set of fixed rollers reacts horizontal forces to maintain the ground ball vertically beneath the upper rolling body. The ground balls are retractable to lower the goal frame onto the pitch. GB2493370 A and US5455988 A teach support assemblies comprising a ball supported by smaller rolling wheels. In use, it is found that the rotating ground balls of the GB'643 arrangement provide improved performance compared with other prior art arrangements, particularly on a clean and flat pitch surface. It is a general object of the present invention to make it still easier to move a football goal frame, particularly when ground conditions vary. Accordingly, the invention provides a goal frame assembly as defined in the claims. The novel goal frame assembly includes a goal frame, and a plurality of rolling support assemblies for moving the goal frame over a ground surface. Each rolling support assembly includes a ball, and a bearing assembly. In a mobile state of the goal frame on a horizontal ground surface, the bearing assembly has a central vertical axis, the ball is arranged to rest on the ground surface, the bearing assembly is arranged to transfer a vertical load from the goal frame to the ball to support the goal frame on the ball, and the ball is rotatable omnidirectionally to roll over the ground surface. Each bearing assembly includes at least three casters spaced apart around the central vertical axis, each caster including a swivel bearing defining a swivel axis, and a caster wheel mounted for rotation about a caster wheel axis. The caster wheel axis is acollinear and nonparallel with the swivel axis, and the caster wheel and caster wheel axis are rotatable about the swivel axis. The caster wheel is arranged in rolling contact with the ball. In this specification, a ball means a spherical rolling body. Since the ball is arranged to rest on the ground surface, it is referred to herein as a ground ball or ground contacting ball. In use, it is found that the ground balls of the novel assembly move surprisingly easily over a natural grass pitch surface without marking or damaging the turf. The rolling resistance of the ground balls is low enough to allow a single person to move a full size football goal frame over the pitch, whereas two or three people would be required to move the same goal frame mounted on prior art ground contacting wheels or casters, which further disadvantageously tend to scuff the grass. Advantageously, by spacing the at least three casters apart around the central vertical axis of the bearing assembly, the casters contact the ground ball below its uppermost point so that a vector component of the vertical load applied by each caster urges the ground ball towards the central vertical axis. Preferably, while the goal frame is moving over the ground surface in normal use, the casters are arranged so that the ground ball is retained beneath the caster wheels only by this balanced self-centering action, and not by contact with any other part of the goal frame assembly. Further preferably, the swivel axis of each caster, when projected, passes through the centre of the ground ball when the centre of the ground ball lies on the central vertical axis of the bearing assembly. This ensures that the self-centering action of the bearing assembly retains the centre of the ground ball on the central vertical axis of the bearing assembly at all times while the goal frame is in motion, irrespective of its direction of motion, as further explained below. The self-centering action of the casters obviates the need for a prior art retaining structure with fixed axis rollers as taught by GB'643, which the applicant has found can sometimes retard the ground ball when rotating in certain directions oblique to the fixed roller axes. Optionally, the novel assembly may include a retaining structure to ensure that the ground ball remains captive so it cannot escape under exceptional use conditions, for example, if the goal frame has to move over a raised kerb or a hole in the ground. In this case, the self-centering action of the casters preferably retains the ground ball out of contact with the retaining structure in normal use (which is to say, while the goal frame is resting on the ground ball and moving at walking speed over horizontal ground). If the ground ball moves out of its use position under exceptional use conditions, it may contact the retaining structure, which then restrains it so that it is not separated from the assembly. Since the retaining structure does not need to contact the ground ball in normal use, it need not have rollers to engage the ground ball surface; a simple frame will suffice. Further advantageously, the novel assembly is relatively tolerant of contamination by grit and dirt from the ground surface. The ground contacting surface of the ground ball is in contact with the outer circumferential (rolling) surface or tyre of each caster wheel, but is spaced apart from the swivel bearing and from the caster wheel bearing that supports the caster wheel for rotation about the caster wheel axis. Thus, particles adhering to the surface of the ground ball can pass through the bearing assembly without entering either the swivel bearing or the caster wheel bearing, even if no special protection is provided for the bearings. The assembly thus continues to function with low rolling resistance even in dirty conditions, ensuring that the ground ball continues to rotate rather than damaging the grass by sliding over the pitch. Further advantageously, the at least three caster wheels forming the bearing assembly provide an enlarged combined contact area with the ground ball, compared with the single point contact between the ground ball and the spherical upper rolling body as taught by GB '643. This in turn makes it possible to provide the ground ball of the novel arrangement with a relatively softer surface than in the prior art arrangement, which requires a surface that is hard enough to support the load at the single point of contact without excessive deformation. A relatively softer ground ball surface, in turn, provides advantages when moving the goal frame over both harder and softer ground materials. A softer ground ball surface (or surface material) may be selected, compared with the relatively harder surface of the prior art ground ball, to have a relatively higher coefficient of static friction when in contact with the ground surface, particularly a natural grass (turf) pitch surface, so as to ensure that the ground ball will roll over the turf rather than sliding and scuffing the turf. A softer ground ball surface (or surface material) with relatively higher coefficient of friction is also less likely to be damaged when rolling under load over a hard ground surface such as a concrete made with exposed flint aggregates. The preferably relatively softer surface of the ground ball of the novel assembly gives a smoother and quieter ride over hard ground surfaces without incurring damage, while the enlarged contact area with the caster wheels is more tolerant of surface irregularities of the ground ball, whether caused by ground contact or by more relaxed manufacturing tolerances compared with the prior art assembly. However, a ground ball surface that is too soft will increase rolling resistance due to excessive local deformation where it contacts the caster wheels. Thus, the surface hardness of the ground ball is preferably selected to minimise total rolling resistance in normal use conditions on both hard and soft ground surfaces, and particularly natural grass football pitches. An optimal value is found to be in the range from 80 Shore A to 100 Shore A, for example, about 90 Shore A. This can be obtained by suitable choice of materials and method of manufacture, as further discussed below. Further features and advantages will be evident from the following illustrative embodiment which will now be described, purely by way of example and without limitation to the scope of the claims, and with reference to the accompanying drawings, in which: Fig. 1 shows a goal frame assembly including a football goal frame with four rolling support assemblies, showing the goal frame in the rest state with each ground ball retracted into its support frame. Fig. 2 is a side view of one of the rolling support assemblies in the rest state. Fig. 3 is an end view of the same rolling support assembly in the rest state, looking along the bar of the goal frame which is cut through in the drawing. Fig. 4 is the same view as Fig. 3, showing the goal frame supported on the ground ball in the mobile state. Figs. 5 and 6 show the rolling support assembly in the rest state with the housing removed. Fig. 7 shows the rolling support assembly in the mobile state with the housing removed. Fig. 8 shows the rolling support assembly partially assembled. Fig. 9 shows a further step in assembly. Fig. 10 is a top view of the rolling support assembly in the rest state. Figs. 10A and 10B are sections taken respectively at A - A and B - B of Fig. 10. Fig. 11 is a schematic view showing the orientation of the swivel axes relative to the ground ball. Fig. 12 illustrates the geometric relationship between the caster wheel axis and the swivel axis. Figs. 13 and 14 are sections through the ground ball illustrating two alternative structural arrangements. Reference numerals appearing in more than one of the figures indicate the same or corresponding features in each of them. Referring to Fig. 1, the goal frame 1 may be for example a football goal frame, and may have dimensions as defined in the regulations of the International Football Association Board or the relevant national Football Association. The goal frame may include a horizontal crossbar 2 supported between a pair of vertical posts 3. The vertical posts may extend from the forward ends of a pair of parallel, horizontal bottom bars 4 which are joined by a horizontal rear bar 5 extending in parallel with the crossbar 2 to support a lower edge of the net (not shown). A diagonal brace 8 may be arranged between each vertical post 3 and the adjacent bottom bar 4. A pair of net support posts 6 may extend upwardly from the corners where the bottom bars 4 join the rear bar 5. Each bar of the goal frame 1 may have a channel 7 for use in fixing the net to the frame as known in the art. The goal frame assembly includes a plurality of rolling support assemblies 10 for moving the goal frame 1 over the generally horizontal ground surface G, which may be a natural or artificial turf. For example, as illustrated, there may be four rolling support assemblies 10, two of which are arranged respectively on the two bottom bars 4, and the other two on the rear bar 5. Referring also to Figs. 2 - 10B, each rolling support assembly 10 includes a ground ball 11. In a mobile state of the goal frame, as shown in Fig. 4, the mass of the goal frame 1 is supported on the ground ball, and the ground ball 11 rests on the ground surface G. The ground ball 11 is rotatable omnidirectionally about its centre C to roll over the ground surface G, so that the goal frame 1 can be pushed in any direction without reorienting the ground ball. The ground ball 11 may have a diameter D of at least 150mm, for example, about 200mm or more. Each rolling support assembly 10 also includes a bearing assembly 20 which is arranged to transfer a vertical load L from the goal frame 1 to the ground ball 11 to support the goal frame 1 on the ground ball 11. The goal frame may be supported entirely on the ground balls 11 in the mobile state, so that in aggregate, the vertical loads L transferred to the ground balls 11 include the entire weight of the goal frame. When considered in the mobile state of the goal frame when resting on a horizontal ground surface G, the bearing assembly 20 has a central vertical axis Xc. For reference, Fig. 10B shows a horizontal plane through the centre C of the ground ball 11 and perpendicular to the central vertical axis Xc. Each bearing assembly 20 includes at least three casters 21 spaced apart around the central vertical axis Xc. Preferably as shown the casters form an axisymmetric array about the central vertical axis Xc, being spaced apart at equal angular intervals around the central vertical axis Xc. The at least three casters may consist of exactly three (and no more) casters, or alternatively there may be four, five or more casters. Preferably all of the total vertical load L imposed by the goal frame 1 on the ground ball 11 is transferred to the ground ball 11 by the at least three casters 21. Referring now particularly to Figs. 5 - 7 and 10A - 10B, each caster 21 includes a swivel bearing 22 defining a swivel axis Xs, and a caster wheel 23 mounted for rotation about a caster wheel axis Xw. The caster wheel axis Xw is acollinear (i.e., not collinear) and nonparallel with the swivel axis Xs. The caster wheel 23 and caster wheel axis Xw are rotatable about the swivel axis Xs. The caster wheel 23 is arranged in rolling contact with the ground ball 11. Preferably, in normal use, in the mobile state of the goal frame 1, while the goal frame 1 is moving over the ground surface G, the at least three caster wheels 23 that transfer the vertical load L from the goal frame 1 to the ground ball 11 are arranged also to urge the ground ball 11 towards the central vertical axis Xc of the bearing assembly, so that in use, the ground ball 11 is retained in its use position beneath the caster wheels 23 by the caster wheels 23 and not by contact with any other part of the goal frame assembly. This can be accomplished by spacing the casters 21 apart by a sufficient distance, as shown, so that the caster wheels 23 rest on the ground ball 11 at a position far enough below its uppermost point to obtain a sufficiently large balanced self-centering force, as discussed above. That is to say, the casters are positioned to ensure that the horizontal vector component of the force applied by each caster 21 to the ground ball 11, acting in the radially inward direction towards the central vertical axis Xc, is large enough to prevent significant displacement of the ground ball 11 away from the central vertical axis Xc as the goal frame 1 moves over the ground surface G in normal use (i.e. at walking pace). This can be determined by trial and error on a horizontal natural grass pitch surface. For the purpose of the test, the pitch should meet the FIFA Basic field test requirements for certification extension as defined in Annex C of FIFA Test Manual II (Test requirements), when tested in accordance with FIFA Test Manual I (Test Methods), both Test Manuals as published under the FIFA Quality Programme for Football Turf - October 2015 edition, v3.4 01.02.2022. For natural grass pitches, the relevant tests are the ball / surface and player / surface tests. Both Test Manuals are available online at https: / / www.fifa.com / technical / football-technology / standards / football-turf. In practice, the novel assembly may retain the ground ball securely in its use position even when moving over pitch surfaces that do not meet the test requirements. Referring now to Fig. 11, preferably, in the mobile state of the goal frame 1, a centre of the ground ball 11 lies on the central vertical axis Xc of the bearing assembly 20, and the swivel axis Xs of each caster 21, when projected, passes through the centre C of the ground ball 11. This relationship ensures that the ground ball 11 remains in the same position without oscillating around the central vertical axis Xc when the casters swivel around their swivel axes Xs as the moving goal frame 1 changes direction over the ground surface. Since the ground ball 11 remains on the central vertical axis Xc, the remaining parts of the rolling support assembly 10 surrounding the ground ball can be advantageously compact to provide only a small clearance from the ground ball surface, without interfering with the ground ball 11 when in motion. For ease of understanding, Fig. 11 illustrates this geometrical relationship showing two casters 21 with their swivel axes Xs in the plane of the drawing, which contains the central vertical axis Xc. (Note, the two casters are shown in diametrically opposed relation for ease of understanding, instead of the 120° angular spacing as shown in the other figures.) Each caster 21 is shown in two angular positions rotated 180° apart about its swivel axis Xs. It will be noted that the tyre of the caster wheel 23 lies on the surface 13 of the ground ball 11 in both positions, and it can be appreciated that, as the caster 21 rotates about its swivel axis Xs, a central point in the region of contact between the tyre of the caster wheel 23 and the spherical surface 13 of the ground ball 11 will define a circular track on the surface 13 of the ground ball 11, bounding a segment of the sphere. Thus, it can be appreciated that the caster wheel 23 remains in contact with the ground ball 11 as the caster 21 swivels, while the centre C of the ground ball 11 remains on the central vertical axis Xc of the bearing assembly 20 at the point of intersection of the projected swivel axes Xs. Preferably, for each caster 21, the swivel axis Xs, when projected, does not intersect the caster wheel axis Xw, whereby the caster wheel 23 and caster wheel axis Xw are rotatable to trail behind the swivel axis Xs. That is to say, the caster wheel axis Xw is offset from the swivel axis Xs so that the caster wheel 23 can follow the swivel axis Xs in the direction of relative motion of the caster wheel 23 over the surface 13 of the ground ball 11. Referring to Fig. 12, for each caster 21, preferably the caster wheel axis Xw defines a chord of a circle XwC lying in a plane P (Fig. 11), wherein the swivel axis Xs is perpendicular to the plane P and, when projected, passes through a centre of the circle XwC. The caster wheel axis Xw trails behind the swivel axis Xs by the distance of the apothem a in the plane P. A full size football goal frame is large and heavy, and so it is desirable to lower the goal frame 1 so that its bottom bars 4, 5 rest directly on the ground surface G when it is not being moved. This helps to stabilise the goal frame 1 in use, and to prevent the ground balls 11 from gradually becoming embedded in a soft pitch surface. Referring again to Figs. 4 and 5, this can be achieved by arranging each rolling support assembly 10 to include a load transfer mechanism 40, which is operable in use to transfer a mass of the goal frame 1 selectively to and from the ground ball 11 so as to define the mobile state (Fig. 4, Fig. 7) and a rest state (Figs. 1, 2, 3, 5, 6,10,10A, 10B) of the goal frame 1. In the rest state, as best seen in Figs. 2 and 3, the mass of the goal frame 1 is not supported on the ground ball 11, so that the ground ball 11 is relieved of the load. For example, the bottom bars 4 and 5 could be arranged to rest directly on the ground surface G in the rest state, as shown. The load transfer mechanism 40 may define the mobile state and the rest state by lowering and raising the bearing assembly 20 relative to the goal frame 1 (which is to say, raising and lowering the goal frame 1 relative to the bearing assembly 20). As shown, each rolling support assembly 10 may include a support structure 50 fixed to the goal frame 1, wherein the at least three casters 21 of the bearing assembly 20 are supported on the support structure 50 and movable, relative to the support structure 50, between the mobile state and the rest state. As illustrated, the bearing assembly 20 may be movable, in use, in translation relative to the support structure 50 and in a direction perpendicular to the ground surface G, between the mobile state and the rest state. As illustrated, the support structure 50 may form a frame that connects together two collinear bars which together form one horizontal bar 4 or 5 of the goal frame 1, wherein the two bars extend in use in parallel with the ground surface G to rest on the ground surface in the rest state. The support structure maintains the respective bars of the goal frame in rigid relation. This can be accomplished as shown by fixing two connectors 51 to the support structure so that the connectors 51 extend in opposite directions in collinear relation and are fixed fittingly inside the bars 4, 5 of the goal frame which are fixed, e.g. screwed or clamped to the connectors 51. Figs. 8 and 9 show how the support structure 50 may be formed as a frame with two halves which are connected together to form an aperture through which the ground ball 11 extends to contact the ground surface. As shown, the bearing assembly 20 may include a bearing frame 24, with the swivel bearings 22 of the at least three casters 21 being mounted on the bearing frame 24. The bearing frame 24 is mounted to the support structure 50 and movable relative to the support structure 50 between the mobile state and the rest state. The bearing frame 24 may be supported by a movable linkage or linkages 25 mounted on the support structure 50; the or each linkage may have an over-centre action as shown, which is to say, they may be movable through a metastable position between two stable positions defining respectively the rest and mobile states of the goal frame. The goal frame may be lowered, relative to the bearing assembly 20, from the metastable position of the linkage to each of its stable positions, so that linkage is maintained in the selected stable position by the weight of the goal frame 1, and the goal frame 1 must be lifted to transition the linkage from one stable position to the other. As illustrated, the load transfer mechanism 40 may include a handle 41, and may be operable, in use, to move the bearing frame 24, relative to the support structure 50, from the rest state to the mobile state by raising the handle 41 away from the ground surface G, and to move the bearing frame 24, relative to the support structure 50, from the mobile state to the rest state by lowering the handle 41 towards the ground surface G. Rotating and lifting the handle 41 from the position of Fig. 3 to the position of Fig. 4 transitions the linkage 25 from its initial stable position to the metastable position as the goal frame 1 is lifted off the ground, and then lowers the goal frame slightly as the linkage 25 continues to move through the metastable position to the other stable position corresponding to the mobile state (Fig. 4). Rotating and lowering the handle 41 back from the position of Fig. 4 to the position of Fig. 3 first raises the goal frame 1 slightly as the linkage 25 transitions from its initial stable position to the metastable position, and then lowers the goal frame 1 to the ground as the linkage 25 continues to move through the metastable position to the other stable position corresponding to the rest state (Fig. 3). Each rolling support assembly 10 may include a rounded or domed housing 52 to enclose the ground ball 11 and support structure 50 and bearing assembly 20. The ground ball 11 may be retractable into the housing 52 in the rest state, as shown. Optionally, as best seen in Fig. 8, each rolling support assembly 10 may include a retaining structure 30 which defines at least one retaining surface 31. The at least one retaining surface 31 is arranged to contact the ground ball 11 to prevent disassembly of the ground ball 11 from the respective rolling support assembly 10. Preferably, the caster wheels 23 are arranged (by virtue of their self-centering action as discussed above) to retain the ground ball 11 out of contact with the at least one retaining surface 31 while the goal frame 1 is moving over the ground surface G in normal use in the mobile state of the goal frame 1. The retaining structure may be arranged so that the ground ball 11 is captive between the bearing assembly and the retaining structure. As illustrated, the retaining structure 30 may form part of the bearing frame 24 so that it moves as part of the bearing assembly 20 between the mobile and rest states of the goal frame. In the illustrated example, the retaining structure 30 includes an annular frame 32 defining a circular aperture 33 slightly larger than the diameter of the ground ball 11, and three retaining tabs 34 that extend radially inwardly below the equator of the ground ball 11. The radially inwardly facing surface of the aperture 33 and the radially inwardly facing surfaces of the tabs 34 define respective retaining surfaces 31. The retaining surfaces 31 are spaced apart from the ground ball 11 while it rests on the ground in the mobile state of the goal frame, but are positioned to contact the surface 13 of the ground ball 11 if it moves substantially off the central vertical axis Xc. Since the tabs 34 project inside a circle corresponding to the diameter of the ground ball 11, they will also retain the ground ball 11 when it is unloaded to ensure it remains retracted into the housing 52 and does not escape from the assembly 10, for example, if the goal frame 1 moves over a hole in the ground. The ground ball 11 may be solid or hollow and may be made from metal or plastics or any other suitable material. It may include a hollow or solid, spherical core enclosed in a moulded spherical shell. The shell may be made from plastics material, while the core could be, for example, a hollow metal sphere or a solid foamed plastics sphere. Preferably, the ground ball 11 has a surface hardness from 80 Shore A to 100 Shore A, for example, about 90 Shore A. This can be achieved in various ways while retaining adequate sphericity under load. For example, as illustrated in Fig. 13, the ground ball 11 may be formed as a unitary moulding, e.g. a reaction injection moulding, e.g. in a polyurethane material, having a foamed core 12 and an outer surface 13, the outer surface 13 having a higher density than the core 12. In another arrangement, as illustrated in Fig. 14, the ground ball 11 includes an inner body 14 covered by an outer shell 15, wherein the outer shell 15 is softer than the inner body 14 and defines an outer surface 13 of the ground ball 11. The inner body could be solid or, as shown, hollow. For example, the inner body could be a hollow metal or plastics sphere formed by rotomoulding. The outer shell 15 may be made from a polyurethane material or a rubber or other elastomer. In these and other embodiments, the ground ball 11 may include a polyurethane, plastics or rubber material which forms the outer surface 13 of the ground ball 11. Preferably, this material forming the outer surface 13 of the ground ball 11 is selected to define a coefficient of static friction between the ground ball 11 and a natural grass pitch surface that is higher than a coefficient of dynamic friction via the bearing assembly 20 between the ground ball 11 and the goal frame 1. That is to say, the total dynamic friction or rolling resistance between the moving ground ball 11 and the goal frame 1, defined by the functional characteristics of the bearing assembly 20, is preferably lower than the static friction between the ground ball 11 and the natural turf of the pitch surface G. This ensures that the ground ball 11 will roll over the turf rather than sliding and scuffing the turf. (It will be understood that since the ground ball 11 rolls over the turf, there is no relative sliding displacement between the surface 13 of the ground ball 11 and the turf, and for this reason the coefficient of friction between ground ball 11 and turf is referred to as the coefficient of static friction.) The coefficients of friction may be tested by rolling the goal frame over a natural grass football pitch meeting the FIFA Basic field test requirements for certification extension as mentioned above; the test is satisfied if the ground ball 11 rolls rather than sliding over the turf. The caster wheels 23 may have a surface hardness and material similar to that of the ground ball 11. For example, each caster wheel 23 may include a metal body covered by a solid tyre of polyurethane or rubber or plastics material. The caster wheels 23 may have a diameter substantially smaller than that of the ground ball 11, as illustrated. Either or both of the caster wheel bearing and the swivel bearing of each caster may be provided with seals to exclude dirt as is common in the art. Since the novel bearing assembly is generally tolerant of dirty conditions, it is not necessary to provide special protection. If desired however, a wiper (not shown) may be included in each rolling support assembly 10. The wiper may be mounted on the bearing frame 24 or retaining structure 30, and may be a ring or sheet of plastics or rubber or other flexible or resilient material. An edge of the wiper is arranged in sliding contact with the ground ball 11 to remove dirt from the ground ball as it rotates. In summary, a goal frame 1 is supported on ground balls 11 that rotate omnidirectionally as the goal frame rolls over the pitch. At least three swivelling casters 21 are spaced apart around a central vertical axis Xc to transfer the load L from the goal frame 1 to each ground ball 11. A load transfer mechanism 40 may be operable to raise and lower the goal frame 1 to transfer the load selectively to and from the ground balls 11. In alternative embodiments, the goal frame could include one or more, additional rolling support assemblies, for example, at the bottom bars 4 or the rear bar 5, to reduce the load on each ground ball. Alternatively, the goal frame could include only two or three rolling support assemblies, optionally in combination with conventional rollers or wheels. In alternative embodiments, rather than mounting the casters 21 on a moving bearing frame 24 as shown, each of the casters may be mounted to the fixed support structure 50 (or other fixed part of the goal frame 1) via a respective individual movable connection, e.g. a linkage or a simple pivot. Each caster is movable on its respective connection to swing in an arc extending radially and axially with respect to the central vertical axis Xc, upwards to allow the ground ball 11 to rise in the rest position of the goal frame, and downwards to raise the goal frame and support it on the ground ball 11 in the mobile state. The motion of the three casters can be coordinated and controlled by a suitable linkage that connects the casters together. In these and other alternative embodiments, the load transfer mechanism could include a spring (e.g. a torsion spring or coil spring) to urge the bearing assembly to the mobile (load supporting) state, and a sprung pin or other locking arrangement to retain it in that state. Alternatively or additionally, the load transfer mechanism could be arranged to transition between the mobile and rest states by a mechanism having a toggling or bistable action as known in principle in the art, with the action being triggered merely by lifting the goal frame. In this way, the bearing assembly is locked in the mobile (ground ball extended) state by the load of the goal frame, and then unlocked and transitioned or toggled from that state to the rest state by lifting the goal frame at one corner to relieve the load from the respective rolling support assembly, releasing the bearing assembly so that it retracts with the ground ball as the goal frame is lowered to the ground in the rest state. Then, the goal frame may be lifted again at the corner to allow the ground ball and bearing assembly to drop down relative to the goal frame, causing the bistable toggle mechanism to lock and transition again to the mobile state so that, when the goal frame is lowered, the bearing assembly will support it on the ground ball 11. Thus, by lifting and then lowering the goal frame at each corner, the rolling support assemblies can be transitioned one by one from the rest state to the mobile state, and then, when the goal frame has been moved to the desired location on the pitch, by repeating the same action, they can be transitioned back again one by one to the rest state. In alternative embodiments, the load transfer mechanism could include a different, geared or hydraulic actuator assembly, e.g. a screwjack or a lever providing additional mechanical advantage. Alternatively, the bearing assembly 20 could be fixed without any load transfer mechanism, so that the goal frame is permanently supported on the ground balls 11. The bars of the goal frame could be configured other than as shown, either for football or for another sport. Many further adaptations are possible within the scope of the claims. In the claims, reference numerals and characters are provided in parentheses, purely for ease of reference, and should not be construed as limiting features.

Claims

1. A goal frame assembly including:a goal frame (1), anda plurality of rolling support assemblies (10) for moving the goal frame (1) over a ground surface (G);each rolling support assembly (10) including:a ball (11), anda bearing assembly (20);wherein, in a mobile state of the goal frame (1) on a horizontal ground surface(G):the bearing assembly (20) has a central vertical axis (Xc),the ball (11) is arranged to rest on the ground surface (G),the bearing assembly (20) is arranged to transfer a vertical load from thegoal frame (1) to the ball (11) to support the goal frame (1) on the ball (11), andthe ball (11) is rotatable omnidirectionally to roll over the ground surface (G);wherein:each bearing assembly (20) includes at least three casters (21) spaced apart around the central vertical axis (Xc);each caster (21) including a swivel bearing (22) defining a swivel axis (Xs), and a caster wheel (23) mounted for rotation about a caster wheel axis (Xw);the caster wheel axis (Xw) being acollinear and nonparallel with the swivel axis (Xs);the caster wheel (23) and caster wheel axis (Xw) being rotatable about the swivel axis (Xs);the caster wheel (23) being arranged in rolling contact with the ball (11).

2. A goal frame assembly according to claim 1, wherein, in normal use, in the mobile state of the goal frame (1), while the goal frame (1) is moving over the groundsurface (G), said caster wheels (23) that transfer said vertical load from the goal frame (1) to the ball (11) are arranged also to urge the ball (11) towards the central vertical axis (Xc), whereby the ball (11) is retained beneath said caster wheels (23) by said caster wheels (23) and not by contact with any other part of the goal frame assembly.

3. A goal frame assembly according to claim 2, wherein each rolling support assembly (10) further includes a retaining structure (30), the retaining structure (30) defining at least one retaining surface (31), the at least one retaining surface (31) being arranged to contact the ball (11) to prevent disassembly of the ball (11) from the respective rolling support assembly (10);wherein, in normal use, in the mobile state of the goal frame (1), while the goal frame (1) is moving over the ground surface (G), the caster wheels (23) are arranged to retain the ball (11) out of contact with the at least one retaining surface (31).

4. A goal frame assembly according to any preceding claim, wherein the ball (11) has a surface hardness from 80 Shore A to 100 Shore A.

5. A goal frame assembly according to any preceding claim, wherein, in the mobile state of the goal frame (1), a centre of the ball (11) lies on the central vertical axis (Xc) of the bearing assembly, and the swivel axis (Xs) of each caster (21), when projected, passes through the centre of the ball (11).

6. A goal frame assembly according to any preceding claim, wherein for each caster (21), the swivel axis (Xs), when projected, does not intersect the caster wheel axis (Xw), whereby the caster wheel (23) and caster wheel axis (Xw) are rotatable to trail behind the swivel axis (Xs).

7. A goal frame assembly according to claim 6, wherein for each caster (21), thecaster wheel axis (Xw) defines a chord of a circle (C) lying in a plane (P), wherein theswivel axis (Xs) is perpendicular to the plane (P) and, when projected, passes through a centre of the circle (C).

8. A goal frame assembly according to any of claims 1 - 7, wherein the ball (11) is a unitary moulding having a foamed core (12) and an outer surface (13), the outer surface (13) having a higher density than the core (12).

9. A goal frame assembly according to any of claims 1 - 7, wherein the ball (11) includes an inner body (14) covered by an outer shell (15), wherein the outer shell (15) is softer than the inner body (14) and defines an outer surface (13) of the ball (11).

10. A goal frame assembly according to any of claims 1 - 9, wherein the ball (11) includes a polyurethane, plastics or rubber material, said material forming an outer surface (13) of the ball (11).

11. A goal frame assembly according to claim 10, wherein said material forming the outer surface (13) of the ball (11) is selected to define a coefficient of static friction between the ball (11) and a natural grass pitch surface that is higher than a coefficient of dynamic friction via the bearing assembly (20) between the ball (11) and the goal frame (1).

12. A goal frame assembly according to any preceding claim, wherein each rolling support assembly (10) includes a load transfer mechanism (40);the load transfer mechanism (40) being operable in use to transfer a mass of the goal frame (1) selectively to and from the ball (11) so as to define the mobile state and a rest state of the goal frame (1);wherein in the rest state, the mass of the goal frame (1) is not supported on the ball (11).

13. A goal frame assembly according to claim 12, wherein each rolling support assembly (10) includes a support structure (50) fixed to the goal frame (1), and the at least three casters (21) are supported on the support structure and movable, relative to the support structure (50), between the mobile state and the rest state.

14. A goal frame assembly according to claim 13, wherein the bearing assembly (20) includes a bearing frame (24), the swivel bearings (22) of the at least three casters (21) being mounted on the bearing frame (24);the bearing frame (24) being mounted to the support structure (50) and movable relative to the support structure (50) between the mobile state and the rest state.

15. A goal frame assembly according to claim 14, wherein the load transfer mechanism (40) includes a handle (41), and is operable, in use:to move the bearing frame (24), relative to the support structure (50), from the rest state to the mobile state by raising the handle (41) away from the ground surface (G), andto move the bearing frame (24), relative to the support structure (50), from themobile state to the rest state by lowering the handle (41) towards the ground surface (G).

Images