Abstract:
The invention concerns a driving unit including a liquid cooled electric motor ( 1 ) and a planetary gear ( 3 ). One of the three main elements of the planetary gear, namely the element used as the reaction element, drives the positive displacement pump of the cooling circuit. Preferably, this element is the ring gear with an inner toothing ( 23 ) and said ring gear itself constitutes the pump rotor. The liquid circuit passes into a hollow shaft ( 11 ) of the motor rotor and through a radiator ( 31 ) incorporated in the common casing ( 5 ) containing the motor and the gear. Such a driving unit can be applied to driving a machine or a vehicle.

Description:
BACKGROUND OF THE INVENTION 
     The present invention concerns a driving unit including an electric motor and a planetary gear provided with an input shaft driven by said motor and an output shaft, said unit being provided with a cooling circuit for said motor including a positive displacement pump assuring the flow of a liquid in said circuit, the planetary gear including three main elements, namely: a sun wheel, a planet wheel carrier provided with several planet wheels, and a ring gear having an inner toothing meshed with said planet wheels, one of said three main elements being connected to the input shaft, another element being connected to the output shaft and the third acting as a reaction element. 
     DESCRIPTION OF THE RELATED ART 
     The heat generated by the electric motor has to be evacuated to avoid any risk of it overheating. In the prior art, this cooling may be assured by a cooling circuit which passes through the electric motor and wherein a coolant flows as a result of a pump located outside said electric motor. However, a unit of this type is bulky because it requires a mechanical transmission between the motor and the pump, as well as conduits for the liquid between these two elements and a radiator where the liquid is cooled. 
     For example, U.S. Pat. No. 5,127,485 discloses a driving unit of the type specified in the preamble, in the form of a compact unit driving a single wheel of a vehicle such as a golf cart. The bottom part of the casing which is common to the electric motor and the planetary gear contains oil which is pumped via an oil pump driven by a distinct electric motor, controlled as a function of the temperature of the main motor or the current passing through it, in order to cool the motor by flowing over the coils of its stator and over its rotor after having passed through a cooling radiator incorporated in the casing. Since this cooling system requires an additional motor to drive the pump and a special control unit to control the motor, it can be liable to breakdown and thus does not,guarantee sufficient cooling in all circumstances. 
     SUMMARY OF THE INVENTION 
     The object of the invention is thus to overcome the drawbacks of the prior art and to cool the electric motor using a particularly simple and compact device. 
     This object is achieved using a driving unit of the type indicated above, characterised in that the pump is driven by the planetary gear reaction element. 
     Thus, a mechanical member which is already present in the driving unit is used to drive the pump, which simplifies the construction in all circumstances. Moreover, the pump may advantageously be placed in the same casing as the planetary gear, which avoids using connecting conduits. 
     Preferably, the reaction element is the ring gear of the planetary gear and this ring gear forms at least part of the rotor of said pump. 
     Preferably the cooling circuit is provided with at least one adjustable valve, for example a flow limiter valve, controlled by an electronic control unit. It is thus possible to adjust the cooling of the motor automatically, as required by regulating the flow rate of coolant supplied by the pump. 
     Moreover, it is possible to cool the motor even when the output shaft is at rest. Indeed, in this case, the motor can be rotated by letting the reaction element rotate, which causes the coolant to flow. 
     Finally, according to a variant of the invention, the pump, the adjustable valve and the electronic control unit form a regulating means acting on the reaction element to regulate the speed and/or torque of the output shaft. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood upon reading the following description of a preferred embodiment of the invention, given by way of non limiting illustration with reference to the annexed drawings in which: 
     FIG. 1 is a radial cross-section of a driving unit according to the invention, including an electric motor, a planetary gear and a hydraulic circuit passing into these two components, the Figure being formed of two half cross-sections along the line A 1 —A 1  of FIG.  2  and line A 2 —A 2  of FIG. 3, 
     FIG. 2 is a longitudinal cross-section along the line B—B of FIG. 1, 
     FIG. 3 is a longitudinal cross-section of half of the unit along the line C—C of FIG. 1, and 
     FIG. 4 is a general diagram of the unit according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The driving unit shown in FIGS. 1 to  3  includes a motor  1  and a planetary gear  3  in a common casing  5  in two parts. The motor described here is an electric motor, but the invention could also be applied to an internal combustion engine. In a conventional manner, electric motor  1  includes a stator  7  and a rotor  9  provided with a hollow shaft  11 . The shaft rotates about a central stationary tube  12 . The assembly of these elements is housed in a motor frame  13  on which rotor shaft  11  rests via bearings  15 . In the following description this shaft  11  is called the “input shaft”. 
     Also in a conventional manner, planetary gear  3  is formed of a central sun wheel  17 , a planet wheel carrier  19  provided with several planet wheels  21  (three as is seen more clearly in FIG. 1) and a ring gear  23  (i.e. an exterior planetary wheel) provided with teeth on its inner surface  24 . The planet wheels  21  are mounted on planet wheel carrier  19  via shafts  25  and mesh simultaneously with sun wheel  17  and the toothing of ring gear  23 . Planet wheel carrier  19  is extended by a shaft  27  called the “output shaft”, intended to rotate a driven element (not shown in the Figures) and which can be an element of a machine or a vehicle wheel, for example. 
     Electric motor  1 , its frame  13  and planetary gear  3  are mounted in cylindrical casing  5 , which is made in several parts to allow the different elements to be assembled. More precisely, this casing  5  is formed of a circular bottom  29  which appears on the left of FIG. 2, an annular central portion  31  provided with fins and forming a radiator and a portion  33  opposite bottom  29 , which protects more particularly planetary gear  3  and is called hereinafter the transmission casing  33 . This transmission casing  33  has a generally conical shape and its portion of smaller diameter defines an opening  35  for the passage of output shaft  27 . The end of planet wheel carrier  19  which is connected to output shaft  27  rests on a bearing  37  housed in this opening  35 . Moreover, the other end of the planet wheel carrier rests on another bearing  39  which itself rests on frame  13  of the electric motor. 
     As illustrated in FIG. 2, in addition to its inner toothed face, ring gear  23  has two opposite plane lateral faces, called respectively front lateral face  41  (i.e. the face visible in FIG. 1) and back lateral face  43 . Further, in the region located opposite said back lateral face  43 , electric motor frame  13  has an annular plane contact surface  45 . In a symmetrical manner, in the region located opposite front lateral face  41  of the ring gear, transmission casing  33  has an annular plane contact surface  47 . These two contact surfaces  45 ,  47  contribute to guiding ring gear  23  and are separated from each other by a distance which allows said ring gear to rotate, but guaranteeing good sealing therewith. 
     With reference now to FIG. 1, it can be seen that transmission casing  33  has a cylindrical inner surface  49  and an outer wall provided with several orifices  51  intended to accommodate screws securing it to motor frame  13 . Ring gear  23  has a smooth outer surface  53 , opposite to its toothed inner surface  24 . While the profile of inner surface  24  is circular, the profile of outer surface  53  is not circular, in this case oval. In other words, the radial thickness  e  of ring gear  23  is not constant. As a result there are two symmetrical and diametrically opposite chambers  54  between surfaces  49  and  53 . These chambers are delimited laterally by aforementioned contact surfaces  45 ,  47 . In a circumferential direction, they are delimited by two stationary segments  59  which slide radially into casing  33  and are pressed against surface  53  of the rotor by springs  61 . By sliding against these segments, non circular surface  53  causes the volume of chambers  54  to vary. Thus, according to an important feature of the invention, ring gear  23  and planetary gear casing  33  form a positive displacement pump  60  (FIG.  4 ), ring gear  23  forming the rotor of said pump. According to a variant which is not shown here, ring gear  23  can constitute only part of the rotor, in the sense that two crescent shaped parts can be fitted onto a conventional ring gear (i.e. a circular ring gear), available on the market, to obtain the final oval or similar shape. 
     The pump can rotate in both directions, depending on the reaction torque direction. The two chambers  54  are connected respectively to suction and discharge orifices  55  and  57 . In FIG. 1, ring gear  23  is supposed to rotate in the direction of arrow F 1  and the suction and discharge orifices are as shown. If however, the motor rotates in the other direction (the vehicle reverses), the reaction torque causes ring gear  23  to rotate in the opposite direction (arrow F 2 ), suction orifices  55  then become discharge orifices  57  and vice versa, the coolant being pumped in the other direction. 
     Finally, it will be noted that ring gear  23  constituting the pump rotor does not need to be centred precisely in the planetary gear casing, since segments  59  pressed against it by springs  61  allow it to have a radial play with respect to surface  49  of casing  33 . 
     The general structure of the driving unit is described with reference to the diagram of FIG. 4, which shows electric motor  1 , gear  3  and coolant pump  60 , as well as circuit  62  for the coolant which is advantageously water. This circuit begins at the discharge orifices of pump  60  and forms a closed loop to return to the suction orifices of the pump. This circuit is symmetrical and includes a pair of adjustable flow rate limiter valves  63 , each located downstream of each discharge orifice. A branch pipe parallel to each valve  63  includes an anti-backflow valve  65 . This circuit  62  is symmetrical so as to allow the pump to rotate in both directions. Electronic unit  67  can contain a programme which automatically controls the flow rate of coolant, as well as the pump discharge pressure and thus the reaction torque exerted by the coolant on the planetary gear. 
     Adjustable valves  63  are automatically controlled by an electric control unit  67  which can receive signals from sensors indicating for example the rotational speeds of input and output shafts  11  and  27 , the temperature of motor  1  etc.. The cooling circuit also passes through radiator  31  and electric motor  1  before returning to pump  60  passing through anti-backflow valve  65 . Further, the circuit can be connected to a liquid tank or to a hydraulic accumulator  73 . 
     The coolant circuit through the motor will now be described in more detail with reference to FIGS. 1 to  3 . 
     As illustrated in the cross-section of FIG. 2, when the coolant originating from discharge orifice  57  has passed through valve  63  shown in FIG. 4, it penetrates, via a conduit  75 , a first annular channel  77  which extends over the entire periphery of the motor, then the coolant passes through a group of cooling conduits  81  of the stator to join a second annular channel  79  similar to channel  77 , at the other end of the motor. As illustrated in FIG. 1, cooling conduits  81  are arranged substantially over the entire circumference of the stator. They can have a cross-section of any shape. Then, the coolant leaves second annular channel  79  and penetrates, via a radial conduit  83 , central tube  12 , where it comes out to pass between tube  12  and the inner surface of the hollow motor shaft, where it takes heat from the rotor. The coolant then passes into another radial conduit  85  and penetrates, via an orifice  86 , radiator  31 . The radiator includes a collector channel  87  connected to another collector channel  89  via a group of cooling tubes  91  passing through the fins of radiator  31  over the whole of its length. Finally, the coolant leaves collector channel  89  to join suction orifice  55 . The two channels  87  and  89  each extend over a little less than half the circumference of the motor, as can be seen in FIG. 1, where it can also be seen that tubes  91  of radiator  31  have a circular cross-section. 
     It should be noted that radiator  31  could be designed differently in accordance with requirements, for example it could be arranged on one side of the motor only in order to be well exposed to a current of air. This current of air could be generated by a cooling fan and guided by a sheet metal cover covering the radiator. 
     In FIG. 2 it can be observed that contact surfaces  47 ,  45  respectively of motor frame  13  and of transmission casing  33  are provided with grooves  93 . Moreover, motor frame  13  has a bore  95  connecting grooves  93  to the entry of annular channel  77 . Since the coolant is preferably water, grooves  93  allow water leaks, which could otherwise penetrate the planetary gear, to be recovered.