Abstract:
The present invention relates to a drive element such as a driving wheel for a goods-handling cart or the like, or such as a hoisting winch, the drive element comprising a wheel or a drum and an electric motor for rotating the wheel or the drum. The electric motor is a synchronous motor having a permanent-magnet rotor and a stator comprising teeth and respective individual windings on said teeth.

Description:
The present invention relates to a drive element such as a driving wheel for goods-handling carts, in particular pallet trucks or the like, or such as a hoisting winch. 
     BACKGROUND OF THE INVENTION 
     Of the various ways in which goods-handling carts can be motor-driven, the most widespread makes use of a driving wheel. 
     In most cases, the driving wheel can also be steered, being mounted to swivel about a vertical axis. 
     The swivel diameter of the wheel must be as small as possible, in particular when the driving wheel is not central, so as to maximize the distance between the wheels of the cart and increase its stability. 
     Cart manufactures often require the swivel diameter to be the same as the diameter of the wheel&#39;s tire. 
     In known driving wheels, the wheel is rotated via a stepdown gear box by an electric motor of the direct current (DC) type or of the asynchronous type, which means that the volume occupied by the motor must be located above the wheel if the requirements of cart builders are to be satisfied. 
     This gives rise to a large amount of space being occupied in the vertical direction and to a structure that is relatively complex. 
     A similar space-occupancy problem arises for hoisting winches where the motor drives a drum. 
     European patent application No. EP-A-1 001 507 discloses an electrical machine for installing in a wheel of a private vehicle, the machine having a flux-concentrating rotor with permanent magnets. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of the present invention is to propose a novel drive element such as a driving wheel or a hoisting winch having a configuration that is compact and a structure that is relatively simple. 
     In this novel drive element the electric motor for driving the wheel or the drum is a synchronous motor having permanent magnets and a stator wound on teeth. 
     The term “stator wound on teeth” is used to mean a stator in which each tooth carries its own individual winding, unlike stators where windings are interleaved and which present the drawback of greater axial size because of the relatively great length of the winding ends, due to the way in which the winding is performed. Furthermore, the number of teeth n teeth  is preferably a function of the number of pairs of poles n pairs  and of the number of phases n phases  in application of the formula n teeth =n pairs ×n phases . 
     The rotor is preferably a flux-concentrating rotor, i.e. the magnets are placed between pole pieces. Combining a flux-concentrating rotor with a stator wound on teeth makes it possible to have a machine that is relatively powerful while also being compact, thus making it possible in particular to mount the motor so that it is cantilevered-out on the end of a shaft, thereby reducing the number of ball bearings. The rotor can rotate at high speed since the magnets are protected from the pulsating flux by the pole pieces. There is therefore no need to use fragmented magnets at high speeds, as is the case with rotors whose magnets are mounted on the surface. 
     Advantageously, the magnets of the rotor are wedge-shaped when the machine is observed along the axis of rotation of the rotor, of width that tapers going away from the axis of rotation of the rotor, and the pole pieces present cutouts and are engaged by means of those cutouts on ribs of the shaft. They are thus fixed to the rotor shaft by means of complementary shapes. Co-operation between the wedge-shaped magnets and the pole pieces connected to the shaft by means of complementary shapes is particularly advantageous in that, at high speeds, the magnets can exert compression forces on the pole pieces tending to oppose splaying out of those regions of the pole pieces that are situated on either side of the ribs on which they are engaged. This makes it possible to avoid the need to engage the pole pieces on bars, unlike the structure described in U.S. Pat. No. 5,091,668. 
     In addition, the fact of placing magnets between pole pieces enables the magnet to be protected from the currents that are induced at high speeds of rotation, and thus to avoid the magnets becoming heated which could demagnetize them. 
     In the invention, the motor whose axial size is small can be placed inside a wheel, a drum, or a pulley, with the axis of the rotor preferably coinciding with the axis of rotation of the wheel, drum, or pulley. For a driving wheel, the motor no longer needs to be offset upwards and the vertical size of the driving wheel is small, thereby making additional space available to cart builders above the wheel. 
     The invention also makes it possible to simplify the structure of any gearing that might be used, which gearing can comprise a single stepdown stage, and be constituted by an epicyclic gear train, for example. 
     The stepdown ratio can be less 15, e.g. lying in the range 7 to 12, by using a permanent magnet rotor and a stator wound on teeth, making it possible to work both in attraction and in repulsion, and thus to generate greater torque than can be generated by a reluctance motor which works in attraction only. 
     The teeth of the stator can advantageously receive individual coils presenting heads that are very short, thus enabling axial size to be small. The teeth of the stator then preferably lack any pole spreading, thus making it easier to install the coils on the teeth, and also to remove them should that be necessary in order to replace them. 
     The use of a stator wound on teeth is also advantageous in that it makes it possible for the motor to be implemented with a relatively small number of teeth and coils, and thus at a cost which is advantageous. The mass of the magnets used can also be relatively low, thus reducing materials costs. 
     In a particular embodiment, the drive element comprises a main casing having a tubular wall of large diameter, a tubular wall of small diameter on the same axis as the preceding wall, and a transverse wall interconnecting said tubular walls. The stator is housed in the large-diameter tubular wall which is secured to a first bearing-forming piece. The small-diameter tubular wall is secured to a second bearing-forming piece. The large and small-diameter tubular walls and the transverse wall are preferably made as a single piece. In a variant, the large-diameter tubular wall can be constituted by a fitted piece, with the small-diameter tubular wall and the transverse wall being made as a single piece. 
     In a particular embodiment, the second bearing-forming piece also constitutes a planet-carrier having at least one shaft on which there rotates at least one planet wheel, and preferably three planet wheels, each planet wheel meshing both with a gear made in or fitted to the shaft of the rotor and with a ring gear of a support. This ring can be fitted to the support or it can be made integrally therewith. For a driving wheel, a rim supporting the tire of the wheel can be fixed to said support. 
     In a particular embodiment, the drive element includes a bearing enabling the support to turn on the small-diameter tubular wall of the main casing. 
     Still in a particular embodiment, the drive element includes a cap fitted in sealed manner onto the support. The cap may serve to contain grease or oil for lubricating the gearing. 
     For a driving wheel or a winch, the drive element preferably includes an electromagnetic brake that operates on power failure and that is fitted to the first bearing-forming piece, said brake comprising a disk fixed on a fluted end of the rotor shaft. The main casing can be swivel-mounted, or in a variant it can be fixed directly to the chassis when the drive element is a driving wheel that does not need to be steerable. 
     As mentioned above, the invention enables the rotor to be coaxial with the tire of the wheel or with the drum, thereby reducing the space occupied in a vertical direction by the drive element since there is no need for the motor to be offset. 
     The invention also provides a cart fitted with at least one driving wheel as defined above, which wheel can optionally be steerable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be better understood on reading the following detailed description and on examining the accompanying drawing, in which: 
     FIG. 1 is a diagrammatic vertical axial section view of a driving wheel constituting a non-limiting embodiment of the invention; 
     FIG. 2 is a diagrammatic cross-section view showing the stator wound on teeth in isolation, together with a fragment of the rotor; 
     FIG. 3 is a partial diagrammatic view of a hoisting winch. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a driving wheel constituting an embodiment of the invention. 
     This driving wheel  1  comprises an electric motor  2  which, in accordance with the invention, is of the synchronous type, comprising a flux-concentrating permanent magnet rotor  6  and a stator  10  wound on teeth, the stator being shown in FIG.  2 . 
     Each tooth  10   a  carries an individual coil  10   b  belonging thereto. The stator  10  in the example described has eighteen teeth, while the rotor has twelve poles and the power supply is three-phase. 
     Each tooth  10   a  is deprived from pole shoes, as one can see on FIG.  2 . Each tooth may comprise notches for the mounting of a wedge, not shown, to maintain the individual coils onto the teeth. 
     The rotor  6  has transversely-magnetized permanent magnets  100  disposed between pole pieces  102 , each constituted by a stack of magnetic laminations that are identical in shape and superposed on one another. 
     When seen looking along the axis of rotation of the rotor, each magnet  10  is generally wedge-shaped, of substantially trapezoidal cross-section that tapers going towards the stator. The magnets  100  are placed radially between the pole pieces  102 . The pole pieces are not interconnected magnetically, being fixed on a non-magnetic shaft  101  by means of complementary shapes. For this purpose, the shaft has axial ribs  103  and the pole pieces have cutouts  104  of complementary shapes. The wedge-shape of the magnets  100  enables them to be held effectively and simply on the rotor. The fixing of the pole pieces onto the shaft by means of complementary shapes enables the connection between the shaft and the pole pieces to be made in such a manner as to make it possible to manufacture the shaft out of a material that is non-magnetic, such as aluminum. Other materials can be used, such as an aluminum alloy or a non-magnetic casting. 
     Each individual coil  10   b  is constituted by winding electric wires around a winding axis, which wires can be rectangular in cross-section, having their long sides parallel to the winding axis. In a variant, circular section wires could be used. 
     The coils  10   b  are excited so as to produce a rotating field in order to drive the rotor. 
     The driving wheel comprises a main casing  3  which can be swivel-mounted on an element  5  secured to the chassis of the cart by means of a hollow shaft  12  and a ball bearing  4  suitable for pivoting about a vertical axis Z-Z′. 
     In a variant that is not shown, the main casing  3  is fixed in non-swivel manner to the vehicle chassis, and the wheel is not used for steering. 
     The main casing  3  has a large-diameter tubular wall  7  and a small-diameter tubular wall  8  interconnected by a transverse wall  9 . 
     In the example shown, these three walls are made as a single piece, but in a variant they could be made differently. 
     The motor stator  10  is received inside the large-diameter tubular wall  7 , the transverse wall  9  presenting an annular groove  11  for receiving the projecting portions of the stator windings. 
     The rotor shaft  13  has an end  14  that is fluted and an end  15  that is provided both with a gearwheel cut into it and with a circularly cylindrical stub axle  16 . In a variant, the gear could be fitted onto the shaft. 
     The large-diameter tubular wall  7  is fixed to a bearing-forming endpiece or cheekplate  18  which supports a ball bearing  19  within which the fluted end portion  14  rotates. 
     The small-diameter tubular wall  8  of the main casing  3  is fixed to the planet-carrier  20  by means of screws  41  and is coupled to the planet-carrier  20  by any suitable means, e.g. pins  22 . In a variant, the coupling between the tubular wall  8  and the planet-carrier  20  can be achieved by means of complementary shapes, by making teeth at the end of the tubular wall and a housing of complementary shape in the planet-carrier. 
     The planet-carrier  20  carries a ball bearing  41  within which the stub axle  16  of the rotor rotates. 
     The planet-carrier supports pins  24  parallel to the axis of rotation X-X′ of the rotor, and each of these pins carries a planet wheel  25 . 
     In the example described, the gearing has three planet wheels  25  which mesh firstly with the gear  15  cut into the shaft  13  of the rotor, and secondly with a ring  27   a  belonging to a support  27 . In a variant, the ring  27   a  could be constituted by a fitted element. 
     A ball bearing  28  is placed between the small-diameter tubular wall  8  and the support  27  to enable the support to rotate about the axis X-X′. 
     A rim  30  supporting a tire  31  is fixed to the support  27  by means of screws  32 . 
     A cap  33  is fixed in sealed manner to the support  27 . 
     In the embodiment described, the support  27  has a circularly cylindrical surface at its periphery with an annular groove formed therein to receive an O-ring  34 , against which the cap  33  bears. Sealing could be obtained in some other way. 
     At its side opposite from the epicyclic gear train, as described above, the wheel  1  has a brake  36  that is fixed to the bearing-serving piece  18  and that acts in the event of a power failure. 
     The brake comprises a disk  37  provided with a fluted hub  38  which engages on the fluted end  40  of the rotor, and also provided with brake linings, in conventional manner. 
     The brake also comprises moving equipment  40  suitable for pressing against the disk  37  when no power is applied to the motor. 
     Cables  45  for powering and controlling the electric motor extend through the hollow shaft  12  that is swivel-mounted to the chassis. In a variant, these cables could be disposed otherwise. 
     The above-described driving wheel  1  occupies an axial space that is smaller than the outside diameter of a new tire, or that is close thereto, thus making it possible to satisfy the requirements of cart manufacturers. 
     The motor is housed inside the wheel, without being offset above the tire, thus making additional space available in the cart. 
     Naturally, the invention is not limited to the embodiment described above. 
     In particular, as mentioned above, the main casing need not be swivel-mounted to the chassis of the cart. 
     It is also possible to omit the stepdown gearing, in which case the motor would drive the wheel directly. 
     It is also possible to omit the brake, it being possible to perform braking in some other way, e.g. using the motor. 
     Naturally the gearing could be modified and in particular it is possible to use some other number of planet wheels. 
     The invention also applies to a hoisting winch, in which case the motor drives a winch or a pulley. 
     For a hoisting winch, the rim  30  and tire  31  are replaced by a winch  30 ′ as shown in FIG. 3, on which a cable C can be wound. The main casing  3  can be modified to be not swivel-mounted.