1. Field of the Invention
This invention is broadly concerned with a motor drive apparatus for moving a vehicle and is intended particularly to be used in an electric vehicle which employs wheel motors for movement thereof.
2. Description of the Related Art
A conventional electric vehicle has a battery and an electric motor, wherein the electric motor is adapted to change electric power stored in the battery into a rotational output to rotate driving wheels. A relatively high motor speed is appropriately reduced by a deceleration means such as a differential gear unit between the electric motor and the driving wheels to control the travelling speed of the vehicle.
Another type electric vehicle employs wheel motors inside of respective fenders, with each wheel motor driving one vehicle drive wheel. In this type electric vehicle, a planetary gear unit is generally used as a deceleration means to effectively utilize the inside space of the driving wheel. One type of planetary gear unit conventionally used for this type electric vehicle is oriented relative to a motor drive apparatus case secured to a vehicle body as described and taught by the present applicants in U.S. Pat. No. 5,156,579, issued Oct. 20, 1992 to Wakuta et al., which is entitled "LUBRICATING DEVICE FOR A VEHICLE MOTOR" and in U.S. Pat. No. 5,163,528, issued Nov. 17, 1992 to Kawamoto et al., which is entitled "WHEEL MOTOR PROVIDED WITH A REDUCTION GEAR".
More specifically, a magneto stator of the electric motor is fixed to the motor drive apparatus case and a corresponding rotor is arranged inside of the magneto stator to rotate relative to the motor drive apparatus case. The rotor of the electric motor is connected with a sun gear of the planetary gear unit via an output axle thereof.
Referring now to other components of the planetary gear unit, a ring gear is fixed to the motor drive apparatus case and a carrier having at least one pinion meshing both with the sun gear and the ring gear is connected to an axle of a vehicle drive wheel. The axle for the vehicle drive wheel is rotatably supported at a central portion thereof by bearings in the motor drive apparatus case.
Accordingly, the motor speed is reduced by the above-mentioned planetary gear unit and then transmitted to the vehicle drive wheel at a desired rotary speed to move the vehicle.
It is known that the clearance between the magneto stator and the rotor defining the electric motor should be narrowly and carefully maintained to obtain stable operation of the electric motor. Accordingly, the electric motor must have both longitudinal end portions of the output axle of the rotor rigidly supported to obtain stable operation.
However, the structure described in the prior application entitled "MOTOR DRIVE APPARATUS WITH DECELERATION UNIT AND AN ELECTRIC VEHICLE USING THE SAME" suffers from a disadvantage in terms of stable operation. Specifically, while one end portion of the output axle for the rotor is rotatably supported by the case via a bearing assembly, the other end portion thereof is not supported by the case but, rather, connects to the sun gear which engages the ring gear secured to the case through the pinion. Therefore, upon acceleration of the motor speed up to the travelling speed of the vehicle, the pinion meshing with the sun gear also accelerates, which causes an undesirable vibration of the output axle of the rotor at the end unsupported by the case, so that the clearance between the magneto stator and the rotor cannot be narrowly and carefully maintained.
Furthermore, when the electric vehicle goes around a bend at high speed, the output axle for the inside driving wheel is urged by centrifugal force outwardly of the planetary gear unit and, at the same time, that of the outside vehicle driving wheel is oppositely affected, i.e. urged toward the planetary gear unit. When the electric vehicle rattles on a bumpy road with hard shock to the suspension assembly provided for the driving wheel, the output axles of the driving wheels may be damaged.
But, as already mentioned, the output axle of the driving wheel is only supported, at the center portion thereof, in the bearing assembly relative to the motor drive apparatus case, so that the output axle of the driving wheel tends to be easily vibrated by any external forces acting axially and/or radially thereon. As a result, it is likely that the pinion attached to the carrier will be irregularly urged toward the sun gear and the ring gear with an unstable force to cause a biased wear and/or nicking of the pinion, sun and ring gears and unusual noise will emanate from the planetary gear unit.
One solution providing a narrow clearance between the magneto stator and the rotor is a rotor having a hollow portion and an output axle which is stably supported, at both longitudinal ends thereof, on the motor drive apparatus case via bearing assemblies. The planetary gear unit is mounted in the hollow portion of the rotor. However, the bearing assemblies which rotatably support the rotor should be large in size because the planetary gear unit is provided in the hollow space of the rotor. As the peripheral velocity of each bearing supporting the rotor becomes high, friction-loss within the motor may increase. Because the sun gear in such a planetary gear unit is supported by additional bearing assemblies, the result is an increase in the number of bearing assemblies in the motor drive apparatus, which is costly. This structure still involves the disadvantages of biased wear and nicking of the pinion, sun and ring gears or emanates unusual noise from the planetary gear unit.
It is an object of the present invention to provide a motor drive apparatus consisting of minimum components to avoid biased wear and nicking of the pinion, sun and ring gears, to avoid unusual noise from the planetary gear unit and to decrease the friction-loss of the motor.