Abstract:
A lockable shopping cart wheel ( 10 ) comprises a plastics hollow casing ( 12 ), an axle ( 22 ) on which the casing is rotatably mounted and a resetable locking mechanism that is mounted on the axle in the casing. The casing has an end wall ( 14 ), a cylindrical wall ( 16 ) having a tire ( 20 ) and an end wall ( 18 ). The locking mechanism includes a locking plate ( 34 ) which is axially displaceable but rotationally fixed to the axle ( 22 ). A coil spring ( 38 ) acts between wall ( 18 ) and plate ( 34 ) urging it towards wall ( 14 ). The plate ( 34 ) defines formations which can engage the wall ( 16 ) thereby locking the wheel. Between the plate ( 34 ) and wall ( 14 ) are three gear wheels ( 44.1 ), ( 44.2 ) and ( 44.3 ) which meter the number of revolutions of the casing and release the plate ( 34 ) when a predetermined number of revolutions has taken place. The plate ( 34 ) is ferromagnetic, allowing the plate to be displaced against the spring action to reset the locking mechanism by using a magnet from outside the casing.

Description:
FIELD OF INVENTION 
   THIS INVENTION relates to a lockable wheel. 
   More particularly, the invention relates to a lockable wheel of the type that is used for shopping carts or trolleys, and is provided with a locking mechanism for locking the wheel after the cart has travelled a predetermined distance, thereby immobilising the cart. The provision of a shopping cart with such a wheel will prevent or discourage shoppers from removing the cart beyond the predetermined distance from the shopping centre. 
   SUMMARY OF INVENTION 
   According to the invention there is provided a lockable wheel which comprises 
   an axle; 
   a ground-engaging hollow casing that is mounted for rotation on the axle; and 
   a resetable locking mechanism that is mounted on the axle and housed in the casing, the locking mechanism comprising: a locking element which is axially displaceable in the casing between a first position in which it permits rotation of the casing with respect to the axle and a second position in which it engages with the axle and the casing to lock the casing against rotation with respect to the axle; biasing means for biasing the locking element towards the second position; and metering means for metering the number of revolutions of the casing after the locking mechanism has been reset and releasing the locking element for displacement to the second position when a predetermined number of revolutions of the casing has taken place; the locking element being of a ferro-magnetic material so that the locking element can be displaced against the bias of the biasing means to reset the locking mechanism by the application of a magnetic field from outside the casing. 
   The metering means may comprise a train of at least two gear wheels housed in the hollow casing, with a first of the gear wheels being advanced by each revolution of the casing, and a second of the gear wheels being advanced by each revolution of the first gear wheel. 
   The metering means may comprise a train of three gear wheels, the train including a third gear wheel which is advanced by each revolution of the second gear wheel. 
   Each gear wheel may be provided with an eccentrically arranged element of ferromagnetic material, whereby the gear wheel can be reset from outside of the casing to a predetermined rotational position with respect to the axle, upon resetting of the locking mechanism, by the application of a magnetic field. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in more detail, by way of example, with reference to the accompanying drawings. In the drawings: 
       FIG. 1  is an axial section of a shopping cartwheel in accordance with the invention, showing the locking mechanism thereof in a normal, unlocked condition; 
       FIG. 2  is a section similar to  FIG. 1 , showing the locking mechanism in a locked condition; 
       FIG. 3  is a section similar to  FIG. 1 , showing the locking mechanism in a resetting condition; 
       FIG. 4  is an exploded three-dimensional view of the wheel; 
       FIG. 5  is a view similar to  FIG. 4 , but showing the various component parts of the wheel in section; 
       FIG. 6  is a sectioned three-dimensional view of the wheel, shown in an assembled condition, with the one end wall thereof having been removed, and with the locking mechanism thereof in the locked condition; and 
       FIGS. 7  to  13  are exploded 3-dimensional views of various parts of the mechanism. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawings in more detail, reference numeral  10  generally indicates a shopping cart wheel which comprises a hollow casing  12  which has an end wall  14  on one side of the wheel and a cylindrical wall  16  which extends from said one side of the wheel to the other. On the other side of the wheel there is a removable end wall  18 . On the outside of the cylindrical wall  16  there is a tyre  20 . The casing  12 , the end wall  18 , and the tyre  20  are of a non-magnetic material such as, for example, a suitable plastics material. 
   The wheel further comprises a fixed axle  22 , the axle being a steel component which has a threaded socket  24  at each opposite end thereof, the threaded sockets being able to receive the threaded fasteners screws whereby the wheel is mounted on a wheel bracket (not shown). The assembly comprising the casing  12 , the end wall  18 , and the tyre  20  are rotatably mounted on the axle  22  via a pair of bearing bushes  26 . 1  and  26 . 2 , which may conveniently be of Teflon or the like material. The bushing  26 . 1  is adjacent the end wall  14  and the bushing  26 . 2  adjacent the end wall  18 . 
   The axle  22  comprises a cylindrical section  28 , a splined section  30 , and a collar  32 . 
   Inside the casing  12 , in the space between the end walls  14  and  18 , there is a locking plate  34 . The locking plate has an internally splined opening  36  therein, whereby it cooperates with the splined section  30  of the axle. The locking plate  34  is therefore axially displaceable but rotationally fixed with respect to the axle  22 . Between the collar  32  and the locking plate  34  there is a coil spring  38 , the coil spring acting to urge the locking plate away from the end wall  18  and towards the end wall  14 . 
   The locking plate  34  has a number of circumferentially spaced teeth  40  on the periphery thereof. These teeth  40  are able to enter into recesses  42  which are provided in a shoulder on the inside of the cylindrical wall  16 . When the teeth  40  have entered into the recesses  42 , the assembly of casing  12 , end wall  18 , and tyre  20  is prevented from rotating, thus locking the wheel. During normal operation, however, as illustrated in  FIG. 1 , the locking plate  34  is in a position in which the coil spring  38  is compressed and the teeth  40  are disengaged from the cylindrical wall  16 . 
   In the space between the locking plate  34  and the end wall  14  there are three axially spaced gear wheels, namely a first gear wheel  44 . 1 , a second gear wheel  44 . 2 , and a third gear wheel  44 . 3 . The gear wheels  44 . 1  to  44 . 3  are rotatable and also axially displaceable on the cylindrical section  28  of the axle. 
   The first gear wheel  44 . 1  has gear teeth  46  formed in one of the faces thereof, i.e. on that side of the gear wheel which faces the end wall  14 . On the opposite side thereof it has a single notch  48  and a pair of pins  50 , one on each opposite side of the notch. The gear wheel is of a strong, non-magnetic material. Nylon 6 is a suitable material. In one of the faces thereof there is a sector-shaped cavity  52  in which there is accommodated a block of steel or other ferro-magnetic material (not shown in the drawings). 
   The second gear wheel  44 . 2  is of the same construction as the first gear wheel  44 . 1 . The third gear wheel  44 . 3  differs from the gear wheels  44 . 1  and  44 . 2  in that it has gear teeth  54  on the periphery thereof instead of in one of the axially facing sides thereof, in that it does not have the notch  48  and pins  50 , and in that it is provided, on that side thereof which faces the locking plate  34 , with three circumferentially spaced locking pins  56 . 
   In the locking plate  34  there are three locking holes  58 , whose arrangement is such that when the gear wheel  44 . 3  is in a predetermined rotational position with respect to the locking plate, the locking pins  56  can enter into the locking holes, thus allowing the locking plate to move axially towards the gear wheel  44 . 3 . 
   The end wall  14  has a circular ridge  60  which protrudes axially into the space between the end walls  14  and  18 . In this ridge there is a single notch  62 , and it has a pair of pins  64  on each opposite side of the notch. The arrangement of the notch  62  and the pins  64  is similar to the notch  48  and pins  50  of the gear wheels  44 . 1  and  44 . 2 . 
   The locking plate  34  carries three circumferentially spaced lifter pins  66 . Each of the lifter pins  66  has a cylindrical shaft  68  and a head  70  at one end of the cylindrical part. The cylindrical shaft  68  is a free sliding fit in a bore  72  in the locking plate  34 , the head  70  being on that side of the locking plate which faces the end wall  18 . 
   On that side thereof which is in the space between the locking plate and the end wall  14  each lifter pin  66  carries a series of three elements. In the case of one of the lifter pins  66 , these elements are in the form of pinion gears  74 . 1 ,  74 . 2 , and  74 . 3 , each of which is rotatable on the cylindrical shaft  68  and is located axially with respect to the pin by means of a circlip  76 . The circlips engage with grooves  78  in the cylindrical part. Each of the pinion gears  74 . 1  to  74 . 3  has a central, disc-shape part  80 . On one side of the disc-shaped part  80 , in the axial face thereof, there are teeth  82  and  84 . The teeth  82  alternate with the teeth  84 , the teeth  82  being axially longer than the teeth  84 . The teeth  82  and  84  are arranged to cooperate with the notch  48  and pins  50  (in the case of the pinion gears  74 . 2  and  74 . 3 ) and with the notch  62  and pins  64  (in the case of the pinion gear  74 . 1 ). On the other side of the disc-shaped part  80  there are gear teeth  86 , these being arranged to mesh with the gear teeth  46  (in the case of the pinion gears  74 . 1  and  74 . 2 ) and with the gear teeth  54  (in the case of the pinion gear  74 . 3 ). 
   The elements on the other two lifter pins  66  are in the form of lifter discs  88  which, like in the case of the pinion gears  74 . 1  to  74 . 3 , are axially slidable and rotatable on the respective pins, and are held in position by means of circlips  76 . 
   Operation of the wheel will now be described. 
   When the wheel is in the normal, unlocked condition, as illustrated in  FIG. 1 , the locking pins  56  are out of register with the locking holes  58 . The coil spring  38  urges the locking plate  34  against the ends of the locking pins  56 , which in turn urges the gear wheels  44 . 1 ,  44 . 2 , and  44 . 3  axially towards the end wall  14 , with the gear wheel  44 . 1  abutting on the bushing  26 . 1 . Under these conditions the teeth  40  of the locking plate are not entered in the recesses  42 , so that the assembly comprising the casing  12 , the end wall  18 , and the tyre  20  is free to rotate with respect to the axle  22 . 
   For each revolution of the wheel (i.e. the assembly of casing  12 , end wall  18 , and tyre  20 ) engagement of the pins  64  with the teeth  84  of the pinion gear  74 . 1  advances the pinion gear  74 . 1  through an angle which corresponds to the pitch of the teeth  84 . Rotation of the pinion gear  74 . 1  in turn advances the first gear wheel  44 . 1 . The notch  62 , as it passes the pinion gear  74 . 1 , accommodates the corresponding tooth  82 , to permit rotation of the pinion gear. Once the notch  62  has moved past the pinion gear the teeth  82  slide along the smooth outer periphery of the ridge  60 , and this prevents the pinion gear from rotating. 
   Likewise, for each revolution of the first gear wheel  44 . 1 , engagement of the pins  50  with the teeth  84  of the pinion gear  74 . 2  advances the pinion gear  74 . 2 , and this in turn advances the second gear wheel  44 . 2 . The second gear wheel  44 . 2  in a like manner advances the third gearwheel  44 . 3 . It will be appreciated that the drive train between the casing  12  and the third gearwheel  44 . 3  provides a very large reduction ratio. 
   Rotation of the gearwheel  44 . 3  will eventually bring the locking pins  56  into register with the locking holes  58 . When this has happened, the locking plate  34  will be able to move in the direction of the end wall  14 , causing the teeth  40  to enter into the recesses  42  and so lock the wheel. The parts will now be in the positions illustrated in FIG.  2 . 
   To unlock the wheel, two sufficiently strong magnets are required. The first magnet is applied to one side of the wheel, i.e. the side of the end wall  18 . This will attract the locking plate  34  (which, as mentioned earlier, is of a ferro-magnetic material) and displace it towards the end wall  18 , against the bias of the coil spring  38 , and so moves the teeth  40  out of the recesses  42 . 
   Simultaneously, the locking plate  34  displaces the lifter pins  66  towards the end wall  18 . This will have the effect of lifting the gear wheels  44 . 1 ,  44 . 2 , and  44 . 3  slightly away from one another and away from the end wall  14 . As a consequence of this, the teeth  82  of the pinion gear  74 . 1  move axially out of contact with the smooth outer circumference of the ridge  60 , and those of the pinion gears  74 . 2  and  74 . 3  move axially out of contact with the smooth outer circumference of the gear wheels  44 . 1  and  44 . 2  respectively. This will allow the pinion gears  74 . 1  to  74 . 3  and hence the gear wheels  44 . 1 ,  44 . 2 , and  44 . 3  to rotate freely and independently of one another. The various parts will now be in the positions illustrated in FIG.  3 . 
   The second magnet is applied to the circumference of the wheel. This will cause the gear wheels  44 . 1 ,  44 . 2 , and  44 . 3  to rotate until the ferro-magnetic parts which are in the cavities  52  are at their closest position, rotationally, to the second magnet. This has the effect of resetting the wheel, with the locking pins  56  again being out of register with the locking holes  58 . The two magnets can now be removed, causing the parts to return to the positions illustrated in  FIG. 1 , under action of the coil spring  38 . The shopping cart of which the wheel  10  forms part will now be able to travel for a predetermined distance, whereupon it will lock again. 
   The second magnet can be mounted in a fixed position. For example, it may be mounted below a surface across which the cart can travel. This will ensure that the gear wheels  44 . 1  to  44 . 3  will always return to the same rotational positions when the locking mechanism is reset. 
   The locking holes  58  may be drilled holes. The angular position at which the locking holes  58  are drilled will determine the distance for which the cart can travel after the locking mechanism has been reset, before the wheel locks.