Wheel drive system for independently driving right and left wheels of vehicle

A wheel drive system for independently driving right and left wheels of a vehicle is disposed between right and left wheels of the vehicle, and is comprised of a coaxial three-layer motor which includes an inner rotor, an outer rotor and a stator which are coaxially arranged; a first planetary gear reduction mechanism whose input is connected to the inner rotor, and whose output is connected to a first drive shaft of one of right and left wheels; and a second planetary gear reduction mechanism whose input is connected to the outer rotor and whose output is connected to a second drive shaft of the other of the right and left wheels. A reduction ratio of the first planetary gear reduction mechanism is greater than a reduction ratio of the second planetary gear reduction mechanism.

BACKGROUND OF THE INVENTION

The present invention relates a wheel drive system for independently driving right and left wheels of a vehicle, and more particularly to a wheel drive system which is adapted to an electric vehicle or fuel cell vehicle.

Japanese Published Patent Application No. 9-79348 discloses a pair of differential gearsets each of which comprises first, second and third rotation elements, and is connected to each of right and left wheels. An electric motor is connected to each first element, and each of right and left wheels is connected to each second element, and each brake means for restraining the rotation of each wheel is connected to each third element.

SUMMARY OF THE INVENTION

However, this system is arranged such that the two motors are attached to right and left wheels, respectively and that a motor axis of each motor is disposed parallel to a shaft of each differential device. Therefore, if the two motors are disposed at a vehicle front portion and a vehicle rear portion respectively so as to sandwich a differential device axis, the system is enlarged in the front and rear direction of the vehicle. If the two motors are disposed at a vehicle upper portion and a vehicle lower portion respectively so as to sandwich a differential device axis, the system is enlarged in the upper and lower direction of the vehicle. That is, when this system is installed in a vehicle, the system requires a large necessary space to degrade in compactness. Additionally, this system requires two parallel shaft gears and two planetary gearsets for right and left wheels. Therefore, this system has a problem that the quietness is degraded due to the increase of gear noise generation sources.

It is therefore an object of the present invention to provide a wheel drive system of independently driving right and left wheels of a vehicle, which system is capable of ensuring both of a high vehicle installation ability by improving its compactness and a quietness by suppressing gear noises.

An aspect of the present invention resides in a wheel drive system for independently driving right and left wheels of a vehicle. The wheel drive system is disposed between right and left wheels of the vehicle. The wheel drive system comprises a coaxial three-layer motor comprising an inner rotor, an outer rotor and a stator which are coaxially arranged, the coaxial three-layer motor being housed in a motor case; a first planetary gear reduction mechanism whose input is connected to the inner rotor and whose output is connected to a first drive shaft of one of right and left wheels; and a second planetary gear reduction mechanism whose input is connected to the outer rotor and whose output is connected to a second drive shaft of the other of the right and left wheels. A reduction ratio of the first planetary gear reduction mechanism is greater than a reduction ratio of the second planetary gear reduction mechanism.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, there is discussed a wheel drive system for independently driving right and left wheels of a vehicle according to the present invention, with reference to the drawings showing first through sixth embodiments thereof.

FIRST EMBODIMENT

Referring toFIGS. 1 and 2, there is discussed a first embodiment of the wheel drive system according to the present invention. As shown inFIG. 1, a fuel cell vehicle employs the wheel drive system for a vehicle of a first embodiment according to the present invention. The wheel drive system is disposed between front right and left wheels27F and37F and between rear right and left wheels27R and37R. A front wheel drive system of the wheel drive system comprises a coaxial three-layer motor10F, a first planetary gear reduction mechanism20F, a second planetary gear reduction mechanism30F, a first drive shaft26F and a second drive shaft36F. Similarly, a rear wheel drive system of the wheel drive system comprises a coaxial three-layer motor10R, a first planetary gear reduction mechanism20R, a second planetary gear reduction mechanism30R, a first drive shaft26R and a second drive shaft36R.

Further as shown inFIG. 1, a fuel cell system for applying motor drive current to coaxial three-layer motors10F and10R is arranged to extract hydrogen by reacting fuel methanol stored in a methanol fuel tank1with water in a methanol reformer2. The fuel cell system generates electricity by reacting the hydrogen with oxygen in a fuel cell3and supplies the electricity through an inverter4for converting electric power from direct current (DC) electricity to alternating current (AC) electricity to coaxial three-layer motors10F and10R to drive the motors10F and10R. Fuel cell3is of a pressure type in which fuel gas and air are pressurized and supplied into fuel cell3, and therefore fuel cell3further comprises a compressor5for generating pressurized air. Inverter4operates in response to a command from a power control unit6for controlling the operations of coaxial three-layer motors10F and10R. The fuel cell system further comprises a lithium ion secondary battery7in order to cover the insufficiency of the starting performance of fuel cell3. A fuel cell power unit8is constructed by methanol reformer2, fuel cell3and compressor5, as surrounded by a broken line inFIG. 1.

FIG. 2is a cross sectional view showing the wheel drive system of the first embodiment according to the present invention. Since the wheel drive system disposed between front right and left wheels27F and37F is the same as that disposed between rear right and left wheels27R and37R, there is explained the right and left wheel drive system by omitting the references F and R from the references.

Coaxial three-layer motor10has an appearance of one motor, but has a function of two motor/generators. This multi-axis multi-layer motor10is housed in a motor case, and comprises a stator11which is fixed to the motor case and which functions as a coiled stationary armature, an inner rotor12in which permanent magnets are imbedded and which is disposed inside the stator11, and an outer rotor13in which permanent magnets are imbedded and which is disposed outside the stator11.

In inner rotor12a plurality of permanent magnets are imbedded along an axial direction of inner rotor12. More specifically, twelve permanent magnets are imbedded in inner rotor12along the axial direction of inner rotor12and are arranged into six pairs of magnets so that each pair of the magnets performs the same polarity, and therefore inner rotor12is constructed into three pole pairs.

In outer rotor13a plurality of permanent magnets are similarly imbedded along an axial direction of outer rotor13. More specifically, twelve permanent magnets are imbedded in outer rotor13along the axial direction of outer rotor13, and the polarities of the arranged magnets are alternately changed one by one, and therefore outer rotor13is constructed into six pole pairs.

Stator11is constructed by circumferentially arranging a plurality of stator teeth which are constructed by winding coils on laminated steel plates. More specifically, eighteen coils are circumferentially arranged so as to form six phase coil three times. A compound current produced by three-phase alternating current and six-phase alternating current is applied to this coil of stator11. By this compound current, it becomes possible to independently control inner rotor12of a three-phase drive and outer rotor13of a six-phase drive.

First planetary gear reduction mechanism20receives the rotational power of inner rotor12as an input, and outputs the rotational power to first drive shaft26as an output. First planetary gear reduction mechanism20is a simple planetary gearset which comprises a sun gear21, pinions22meshed with sun gear21, a pinion carrier23of supporting pinions22, and a ring gear24meshed with pinions22. Sun gear21is connected to an output shaft of inner rotor12, ring gear24is fixed to a reducer case25, and pinion carrier23is connected to wheel27through first drive shaft26.

Second planetary gear reduction mechanism30receives the rotational power of outer rotor13as an input, and outputs the rotational power to second drive shaft36as an output. Second planetary gear reduction mechanism30is also a simple planetary gearset which comprises a sun gear31, pinions32meshed with sun gear31, a pinion carrier33of supporting pinions32, and a ring gear34meshed with pinions32. Sun gear31is connected to an output shaft of outer rotor13, ring gear34is fixed to a reducer case35, and pinion carrier33is connected to wheel37through second drive shaft36.

When gears21,31,24and34are set such that the number of teeth of sun gear21is Z21, the number of teeth of sun gear31is Z31, the number of teeth of ring gear24is Z24and the number of ring gear34is Z34, a reduction ratio (speed reducing ratio) i20of first planetary gear reduction mechanism20and a reduction ratio (speed reducing ratio) i30of second planetary gear reduction mechanism30are given by the following expressions (1) and (2).
i20=(1+λ20)/λ20 where λ20=Z21/Z24.  (1)
i30=(1+λ30)/λ30 where λ30=Z31/Z34.  (2)

By setting the relationship between the number of teeth of sun gear21and the number of teeth of sun gear31at Z21>Z31and by setting the relationship between the number of teeth of ring gear24and the number of teeth of ring gear34at Z24>Z34, the reduction ratio i20of first planetary gear reduction mechanism20is set to be greater than the reduction ratio i30of second planetary gear reduction mechanism30.

Further, the number of teeth of ring gear24of first planetary gear reduction mechanism20and the number of teeth of ring gear34of second planetary gear reduction mechanism30have a relationship that a difference of the number Z24and the number Z34is greater than or equal to 3 (|Z24−Z34|≧3).

Subsequently there is discussed the manner of operation of the wheel drive system according to the first embodiment of the present invention.

Coaxial three-layer motor10has a merit of functioning as two motor/generators although it is appeared as one motor. However, the output characteristic of inner rotor12is a high-speed and low-torque characteristic, and the output characteristic of outer rotor13is a low-speed and high-torque characteristic.

Therefore, if the reduction ratios of planetary gear reduction mechanisms20and30for right and left wheels are set at the same ratio, the difference between the output characteristics of inner and outer rotors affect the characteristic of the drive torques of right and left wheels. For example, there may cause a difference of the right and left drive torques such that a transfer drive torque to the wheel connected to inner rotor is low and a transfer drive torque to the wheel connected to outer rotor is high.

In contrast to this, since the first embodiment according to the present invention is arranged such that the reduction ratio i20of first planetary gear reduction mechanism20is greater than the reduction ratio i30of second planetary gear reduction mechanism30, the transfer drive torque of a wheel, which is connected to inner rotor12rotatable at high speed, is largely reduced by first planetary gear reduction mechanism20so as to obtain a performance of torque increase by a large torque width. Further, the transfer drive torque of a wheel connected to outer rotor13is reduced in small degree by second planetary gear reduction mechanism30so as to obtain a performance of torque increase by a small torque width.

As a result, the torque difference of the right and left drive torques is corrected, and therefore there is obtained the equivalency between the drive torque of the wheel connected to inner rotor12and the drive torque of the wheel connected to outer rotor13.

In case of the prior art disclosed in the background of the invention, the known drive system employs two motors, and therefore the known drive system has a layout of projectedly disposing two motors around a pair of differential devices. This degrades the degree of freedom of a layout of installing the system on a vehicle and decreases a vehicle occupant space.

In contrast to this, since the wheel drive system of the first embodiment according the present invention employs a coaxial three-layer motor which appears as a motor while having a function of two independent motors, it becomes possible to achieve a high vehicle installation ability. More specifically, as shown inFIGS. 1 and 2, coaxial three-layer motor10and first and second planetary gear reduction mechanisms20and30and first and second drive shafts26and36are compactly arranged on one axial line. Therefore, the wheel drive system of the first embodiment requires a small space and has a high degree of freedom of layout in the vehicle. Therefore, the wheel drive system according to the present invention can sufficiently ensure an occupant space and other space for other devices. That is, the wheel drive system of the first embodiment according to the present invention achieves the high installation ability.

In case of the prior art disclosed in the background of the invention, the known drive system is arranged to supply each drive input of each of two motors to each of a pair of differential devices through two parallel-arranged gears, respectively. That is, both of the differential devices and the parallel arranged gears function as a gear noise generating source. Therefore, it is difficult to ensure a high quietness of this known drive system since there are many gear noise generating sources therein.

In order to ensure a high quietness of this known drive system, it is necessary to cover this known drive system with a sound-damping cover or to use a power-unit mount of performing a high vibration attenuating performance. Such an additional arrangement degrades the compactness of the drive system, and generates a demerit in cost due to the increase of the number of parts.

In contrast to this, the wheel drive system of the first embodiment according to the present invention does not use the parallel-arranged gears employed in the prior art, and therefore the gear noise generating source is only first and second planetary gear reduction mechanisms20and30. Therefore, it becomes possible to ensure a higher quietness of the wheel drive system by suppressing the generation of vibration noises since there are fewer gear noise generating sources.

Further, there is known a noise reduction method of a planetary gearset for properly selecting the number of teeth of a sun gear, the number of teeth of a ring gear and the number of pinion gears. Therefore, it is possible to decrease the gear noise of the planetary gearset employed in the wheel drive system of the first embodiment according to the present invention by the application of the noise reduction method of reducing the gear noise of a planetary gearset.

Further, the first embodiment according to the present invention is arranged such that a difference between the number Z24of teeth of ring gear24and the number Z34of teeth of ring gear34is greater than or equal to 3, it is possible to prevent the amplitude of a vibration generating force and the generation of beat phenomenon.

More specifically, when the vehicle normally runs straightly, right and left wheels27and37rotate at the same speed. Accordingly, carriers23and33, which are the output shafts of first and second planetary gear reduction mechanisms20and30, also rotate around ring gears24and34at the same speed, respectively. When the numbers of the respective teeth of ring gears24and34are Z24and Z34, the orders of the gear noises are Z24order and Z34order of the respective output shaft rotation speeds, and therefore the vibration generating force becomes twice as much. When the difference of the numbers of the respective teeth of ring gears24and34are1or2, there is a possibility that the beat phenomenon of the first order or second order of the rotation speed is generated. Accordingly, by setting the difference of the numbers Z24and Z34of the teeth of ring gears24and34to be greater than or equal to 3, the amplitude of the vibration generating force and the generation of the beat phenomenon are prevented. Therefore, the quietness of the wheel drive system of the first embodiment according to the present invention is improved.

Subsequently, there is discussed the advantages of the wheel drive system of the first embodiment according to the present invention. The wheel drive system of the first embodiment according to the present invention gains the following advantages (1), (2) and (3).

(1) The wheel drive system, which is disposed between the front right and left wheels27F and37F and between the rear right and left wheels27R and37R and which comprises an electric motor and a planetary gear reduction mechanism, is arranged such that a coaxial three-layer motor constructed by coaxially arranging an inner rotor12, outer rotor13and stator11is used as the motor/generators, and the planetary gear reduction mechanism is constructed by first planetary gear reduction mechanism20in which inner rotor12is an input and first drive shaft26is an output, and second planetary gear reduction mechanism30in which outer rotor13is an input and second drive shaft36is an output. Further, reduction ratio i20of first planetary gear reduction mechanism20is set to be greater than reduction ratio i30of second planetary gear reduction mechanism30. Therefore, the wheel drive system of the first embodiment according to the present invention achieves a high vehicle installation performance requiring a small necessary space and a high quietness suppressing the generation of vibration noises.

(2) The wheel drive system of the first embodiment according to the present invention is arranged such that each of first and second planetary gear reduction mechanisms20and30is constructed by a simple planetary gearset which comprises sun gear21,31, pinion carrier23,33, and ring gear24,34. Further each sun gear21,31is connected to the rotor output shaft, and each ring gear24,34is fixed to reducer case25,35. Furthermore, each pinion carrier23,33is connected to each wheel27,37through each drive shaft26,36. Therefore, it becomes possible to easily set the reduction ratio i20of first planetary gear reduction mechanism20and the reduction ratio i30of second planetary gear reduction mechanism30, respectively, in response to the output characteristic (high-speed and low-torque characteristic) of inner rotor12and the output characteristic (low-speed and high-torque characteristic) of outer rotor13since the simple planetary gearset can take a large reduction ratio.

(3) Since the difference (Z24−Z34) of the number Z24of teeth of ring gear24and the number Z34of teeth of ring gear34is set to be greater than 3, it becomes possible to suppress the amplitude of the vibration generating force and the beat phenomenon of gear noise, which are generated from both planetary gear reduction mechanisms20and30. This further improves the quietness of the wheel drive system.

SECOND EMBODIMENT

Referring toFIG. 3, there is discussed a second embodiment of the wheel drive system according to the present invention. The second embodiment is basically the same as the first embodiment except that first planetary gear reduction mechanism20is constructed by a compound planetary gearset.

More specifically, as shown inFIG. 3, first planetary gear reduction mechanism20is constructed by a compound planetary gearset which comprises stepped pinions each of which is constructed by integrally connecting a large-diameter pinion22L meshed with a sun gear21and a small-diameter pinion22S meshed with a ring gear24. Second planetary gear reduction mechanism30employed in the second embodiment is the same as that of the first embodiment. That is, second planetary gear reduction mechanism30of the second embodiment is constructed by a simple planetary gearset which comprises a sun gear31, a pinion carrier33and a ring gear34. Ring gears24and34are fixed to reducer cases25and35, respectively. Pinion carriers23and33are connected to wheels27and37through drive shafts26and36, respectively. Since the other constructions of the second embodiment are the same as those of the first embodiment, the other elements are denoted by the same references of the first embodiment, and the explanation thereof is omitted herein.

Subsequently, there is discussed the manner of operation of the wheel drive system of the second embodiment according to the present invention. When gears21,24,22L and22S is set such the number of teeth of sun gear21is Z21, the number of teeth of ring gear24is Z24, the number of teeth of large diameter pinion22L is Z22L and the number of teeth of small diameter pinion22S is Z22S, the reduction ratio i20of first planetary gear reduction mechanism20having stepped pinions22is obtained from the following expression (3).
i20=1+i0 wherei0=(Z22L·Z24)/(Z22S·Z21).   (3)
As is apparent from the expression (3), first planetary gear reduction mechanism20of the second embodiment according to the present invention can obtain a large reduction ratio as compared with that of the first embodiment while maintaining the size thereof.

Subsequently there is discussed the advantage of the second embodiment. The wheel drive system of the second embodiment according to the present invention can gain the following advantage (4) in addition to the advantages (1) and (3) of the first embodiment.

(4) The second embodiment according to the present invention is arranged such that first planetary gear reduction mechanism20is constructed by a compound planetary gearset which comprises stepped pinions22each of which is constructed by integrally connecting large-diameter pinion22L meshed with sun gear21and small-diameter pinion22S meshed with ring gear24and that second planetary gear reduction mechanism30of the second embodiment is constructed by a simple planetary gearset which constructed by sun gear31, pinion carrier33and ring gear34and that sun gears21and31are connected to the respective rotor output shafts, ring gears24and34are fixed to the respective reducer cases25and35, and pinion carriers23and33are connected to wheels27and37through drive shafts26and36, respectively. Therefore, first planetary gear reduction mechanism20can obtain the further large reduction ratio i20as compared with that of the first embodiment. This first planetary gear reduction mechanism20is effectively applicable to a high-speed type motor.

THIRD EMBODIMENT

There is discussed a third embodiment of the wheel drive system according to the present invention with reference toFIG. 4. The third embodiment is arranged such that both of first and second planetary gear reduction mechanisms20and30are constructed by compound planetary gearsets each of which comprises stepped pinions22,23.

More specifically, as shown inFIG. 4, each of first and second planetary gear reduction mechanisms20and30is constructed by a compound planetary gearset which comprises stepped pinions22,32each of which is constructed by integrating large-diameter pinion22L,32L meshed with sun gear21,31and small-diameter pinion22S,32S meshed with ring gear24,34. Sun gears21and31are connected to the output shafts, respectively. Ring gears24and34are fixed to reducer cases25and35, respectively. Pinion carriers23and33are connected to wheels27and37through drive shafts26and36, respectively. Since the other constructions of the third embodiment are the same as those of the first embodiment, the other elements are denoted by the same reference of the first embodiment, and the explanation thereof is omitted herein.

Subsequently, there is discussed the manner of operation of the wheel drive system of the third embodiment according to the present invention. Since first and second planetary gear reduction mechanisms20and30are constructed by compound planetary gearsets respectively having stepped pinions22and32, it becomes possible that both of first and second planetary gear reduction mechanisms20and30of the third embodiment can have further large reduction ratios i20and i30as compared with both planetary gear reduction mechanisms20and30of the first embodiments and second planetary gear reduction mechanism20of the second embodiment.

Subsequently there is discussed the advantage of the third embodiment. The wheel drive system of the third embodiment according to the present invention can gain the following advantage (5) in addition to the advantages (1) and (3) of the first embodiment.

(5) The third embodiment according to the present invention is arranged such that first and second planetary gear reduction mechanisms20and30are constructed respectively by compound planetary gearsets which respectively comprise stepped pinions22each of which is constructed by integrally connecting large-diameter pinion22L meshed with sun gear21and small-diameter pinion22S meshed with ring gear24. Therefore, first and second planetary gear reduction mechanisms20and30can obtain the large reduction ratios i20and i30. These first and second planetary gear reduction mechanisms20and30are effectively applicable to a high-speed type motor.

FOURTH EMBODIMENT

There is discussed a fourth embodiment of the wheel drive system according to the present invention with reference toFIG. 5. The fourth embodiment is arranged such that first planetary gear reduction mechanism20is constructed by a compound planetary gearset which is arranged such that a ring gear functions as an output.

More specifically, as shown inFIG. 5, first planetary gear reduction mechanism20is constructed by a compound planetary gearset. The compound planetary gearset comprises stepped pinions22each of which is constructed by integrating large-diameter pinion22L meshed with a sun gear21and a first ring gear24L and a small-diameter pinion22S meshed with a second ring gear24S. Sun gear21is connected to an output shaft of inner rotor12, first ring gear24L is fixed to reducer case25, and second ring gear24S is connected to wheel27through a first drive shaft26. Since the construction of second planetary gear reduction mechanism30of the fourth embodiment is the same as that of second planetary gear reduction mechanism30of the third embodiment and the other constructions of the fourth embodiment are the same as those of the first embodiment, the other elements are denoted by the same reference of the first embodiment, and the explanation thereof is omitted herein.

Subsequently, there is discussed the manner of operation of the wheel drive system of the fourth embodiment according to the present invention. When gears24L,24S,22L and22S are set such that the number of teeth of first ring gear24L is Z24L, the number of teeth of second ring gear24S is Z24S, the number of teeth of large-diameter pinion22L is Z22L and the number of teeth of small-diameter pinion22S is Z22S, the reduction ratio i20of first planetary gear reduction mechanism20is expressed by the following expression (4).
i20=(io′+io′)/(io′−1) whereio=Z24L/Z21>1,io′=(Z22L·Z24S)/(Z24L·Z22S)>1, andio″=io·io′>1.   (4)
As is apparent from the expression (4), the reduction ratio i20of first planetary gear reduction mechanism20of the fourth embodiment can be set at a further large reduction ratio as compared with first planetary gear reduction mechanism20of the second or third embodiment.

Subsequently, there is discussed the advantage of the fourth embodiment according to the present invention. The wheel drive system of the fourth embodiment according to the present invention can gain the following advantage (6) in addition to the advantages (1) and (3) of the first embodiment.

(6) The fourth embodiment according to the present invention is arranged such that first planetary gear reduction mechanism20is constructed by a compound planetary gearset which comprises stepped pinions22, each of which is constructed by integrally connecting large-diameter pinion22L meshed with sun gear21and first ring gear24L and small-diameter pinion22S meshed with second ring gear24S. Sun gear21is connected to an output shaft of inner rotor12, first ring gear24L is fixed to reducer case25, and second ring gear24S is connected to wheel27through first drive shaft26. Therefore, first planetary gear reduction mechanism20can have the large reduction ratio i20which is larger than those of first planetary gear reduction mechanisms20of the first, second and third embodiments. This first planetary gear reduction mechanisms20of the fourth embodiment is effectively applicable to a high-speed type motor.

FIFTH EMBODIMENT

There is discussed a fifth embodiment of the wheel drive system according to the present invention with reference toFIG. 6. The fifth embodiment is arranged such that second planetary gear reduction mechanism30is constructed by a compound planetary gearset which is arranged such that a ring gear34functions as an output.

More specifically, as shown inFIG. 6, second planetary gear reduction mechanism30is constructed by a compound planetary gearset which comprises stepped pinions32each of which is constructed by integrating large-diameter pinion32L meshed with sun gear31and small-diameter pinions32S meshed with ring gear34, pinion carrier33is fixed to motor case15, and ring gear34is connected to wheel37through second drive shaft36. Since the construction of first planetary gear reduction mechanism20of the fifth embodiment is the same as that of first planetary gear reduction mechanism20of the second or third embodiment and the other constructions of the fifth embodiment are the same as those of the first embodiment, the other elements are denoted by the same reference of the first embodiment, and the explanation thereof is omitted herein.

Subsequently, there is discussed the manner of operation of the wheel drive system of the fifth embodiment according to the present invention. Since second planetary gear reduction mechanism30of the fifth embodiment is arranged such that pinion carrier33is fixed to motor case15and that ring gear34is connected to wheel37through drive shaft36, the output of outer rotor13is inversely rotated and transmitted to wheel37. Accordingly, it is necessary to rotate inner and outer rotors12and13inversely with each other in order to rotate right and left wheels27and37in the same direction. By this arrangement of the fifth embodiment, the reaction force of inner rotor12applied to stator11has a direction opposite to the direction of the reaction force of outer rotor13, and therefore the force applied to stator11is reduced.

Second planetary gear reduction mechanism30discussed in the first through fourth embodiments shown inFIGS. 2 through 5may be arranged such that the output shaft of outer rotor13is connected to sun gear31, carrier33is fixed to motor case15, and ring gear34is connected to wheel37through drive shaft36as discussed in the fifth embodiment, although they are not shown in the drawings. Such modified arrangements of the first through fourth embodiments also have the advantages obtained by the fifth embodiment.

For example, in case that the idea of second planetary gearset reduction mechanism30of the fifth embodiment is applied to second planetary gearset reduction mechanisms (simple planetary gearsets)30shown inFIGS. 2 and 3as discussed above, when gears31and34thereof are set such that the number of teeth of sun gear31is Z31and the number of teeth of ring gear34is Z34, the reduction ratio i30of such arranged second planetary gear reduction mechanism30using the idea of the fifth embodiment is given by the following expression (5).
i30=−(1/λ30) where λ30=Z31/Z34.   (5)

On the other hand, when gears31,34,32L and32S of the fifth embodiment are set such that the number of teeth of sun gear31is Z31, the number of teeth of ring gear34is Z34, the number of teeth of large-diameter pinion32L is Z32L and the number of teeth of small-diameter pinion32S is Z32S, the reduction ratio i30of second planetary gear reduction mechanism30of the fifth embodiment is given by the following expression (6).
i30=−iowhereio=(Z32L·Z34)/(Z32S·Z31).   (6)

From the comparison of the expressions (5) and (6) with the expressions (2) and (3), it has been found that second planetary gear reduction mechanism30of the fifth embodiment can further decrease the reduction ratio i30as compared with that of second planetary gear reduction mechanism30employed in the first, second, third or fourth embodiment. Therefore, the fifth embodiment gains the advantage that it becomes easy to have a difference between the reduction ratio i30of second planetary gear reduction mechanism30relative to the reduction ratio i20of first planetary gear reduction mechanism20.

Subsequently, there is discussed the advantage of the fifth embodiment according to the present invention. The wheel drive system of the fifth embodiment according to the present invention can gain the following advantage (7) in addition to the advantages (1) and (3) of the first embodiment.

(7) The fifth embodiment according to the present invention is arranged such that second planetary gear reduction mechanism30is constructed by a compound planetary gear gearset which comprises stepped pinions32each of which is constructed by integrally connecting large-diameter pinion32L meshed with sun gear31and small-diameter pinion32S meshed with ring gear34, and that sun gear31is connected to an output shaft of outer rotor13, pinion carrier33is fixed to motor case15, and ring gear34is connected to wheel37through second drive shaft36. Therefore, second planetary gear reduction mechanism30can reduce the force applied to stator11since it is necessary to rotate inner and outer rotors12and13in the opposite directions due to the arrangement of the fifth embodiment. Further, the fifth embodiment gains the advantage that it becomes easy to make a difference between the reduction ratio i20of first planetary gear reduction mechanism20for inner rotor12and the reduction ratio i30of second planetary gear reduction mechanism30for outer rotor13.

SIXTH EMBODIMENT

There is discussed a sixth embodiment of the wheel drive system according to the present invention with reference toFIG. 7. The sixth embodiment is arranged such that brake mechanisms are provided between first planetary gear reduction mechanism20and reducer case25and between second planetary gear reduction mechanism30and reducer case35, respectively.

More specifically, as shown inFIG. 7, brake mechanisms28and38are respectively disposed so that ring gears24and34are releasable from reducer cases25and35. In other words, brake mechanisms28and38are added to the structure of the third embodiment so as to engage and disengage ring gears24and34, respectively. Brake mechanisms28and38are of a hydraulic and multi-plate type. Herein, in case that carrier33is fixed to reducer case35as is similar to the fifth embodiment shown inFIG. 6, a brake mechanism is disposed between carrier33and motor case15although it is not shown. Since the other constructions of the sixth embodiment are the same as those of the third embodiment, the other elements are denoted by the same reference of the third embodiment, and the explanation thereof is omitted herein.

Subsequently, there is discussed the advantages of the sixth embodiment. The wheel drive system of the sixth embodiment according to the present invention can gain the following advantage (8) in addition to the advantages gained by the first through fifth embodiments.

(8) The sixth embodiment according to the present invention is arranged such that brake mechanisms28and38are provided so as to engage and disengage ring gears24and34of first and second planetary gear reduction mechanisms20and30relative to reducer cases25and35, respectively. Therefore, by putting brake mechanisms28and38in the disengaged state when it is not necessary to utilize the regenerative braking, it becomes possible to suppress heat producing of both planetary gear reduction mechanisms20and30. This improves the durability of both planetary gear reduction mechanisms20and30.

Although the invention has been described above by reference to the first to sixth embodiments of the wheel drive system according to the invention, concrete construction of the system is not limited to these embodiments. The change and addition in design may be allowed as far as they do not deviate from the invention defined by the claims.

For example, although the wheel drive system according to the present invention has been shown in the first to sixth embodiments, an optimum construction may be selected according to the required characteristics of a vehicle and an output characteristics of a motor. Although the coaxial three-layer motor has been shown and described such that an inner rotor is disposed inside of a stator and an outer rotor is disposed outside the stator, it may be arranged such that the stator is disposed at an innermost position and the inner and outer rotors are disposed outside the stator, or such that the stator is disposed at an outermost position and the inner and outer rotors are disposed inside the stator.

Although the first to sixth embodiments have been shown and described such that the wheel drive system according to the present invention is adapted to a fuel cell vehicle, it will be understood that the wheel drive system according to the present invention may be adapted to an electric vehicle. Further, although the first to sixth embodiments have been shown and described such that the wheel drive system according to the present invention is installed between the front right and left wheels and between the rear right and left wheels, the wheel drive system may be installed only between the front right and left wheels or only between the rear right and left wheels. Furthermore, the wheel drive system according to the present invention may be adapted to a motor four wheel drive vehicle in which front wheels or rear wheels are driven by an internal combustion engine, and the other of the rear wheels or the front wheels are driven by a motor.

This application is based on Japanese Patent Application No. 2003-353200 filed on Oct. 14, 2003 in Japan. The entire contents of this Japanese Patent Application are incorporated herein by reference.