Planetary gear transmission systems or components – Variable speed or direction transmission combined with... – Variable speed or direction is planetary transmission
Reexamination Certificate
2000-04-03
2002-07-02
Estremsky, Sherry (Department: 3681)
Planetary gear transmission systems or components
Variable speed or direction transmission combined with...
Variable speed or direction is planetary transmission
C475S206000, C475S203000, C180S247000, C180S249000
Reexamination Certificate
active
06413184
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power transmission system for a four-wheel drive vehicle, for distributing and transmitting a torque, as outputted from a power source through a speed change device, to front and rear wheels.
2. Related Art
The four-wheel drive vehicle is enabled to have an enhanced runability on a muddy road or a rough road by transmitting the driving force to all the front and rear wheels. Another advantage is that a running stability on the ordinary road is also enhanced by sharing the driving force among all the four front and rear wheels. Since the power transmission path is complicated, on the contrary, the power loss is increased by the friction. In order to prevent the braking phenomenon (i.e., the tight corner braking) at a turning time, on the other hand, there is needed a center differential for causing differential rotations between the front and rear wheels. Moreover, the provision of the center differential requires a limiting device for limiting the differential action of the center differential so as to prevent the so-called “off-torque” when any one of the front and rear wheels rotates idle due to a slip.
When the driving force outputted from the power source such as an engine is thus distributed and transmitted to the four front and rear wheels, various devices are necessary for eliminating the various disadvantages at the running time. When these devices are provided individually and randomly, however, the entire construction is enlarged to raise disadvantages that not only the mountability is degraded but also the weight of and the cost for the vehicle body are increased. Generally in the prior art, therefore, there have been tried technical improvements for using a device having a plurality of functions or for devising the array of various devices, as exemplified in Japanese Patent Laid-Open No. 9-123777.
In the system disclosed in this Laid-Open, a differential device (or a center differential) having a torque sensing type differential limiting function for limiting the differential torque between the front and rear wheels in accordance with the torque difference between the torque to be distributed to the front wheels and the torque to be distributed to the rear wheels is arranged coaxially with the front differential. Moreover, a direct coupling clutch (or a differential lock clutch) for integrating a member to transmit the torque to the front wheels and a member to transmit the torque to the rear wheels is also arranged coaxially with those center differential and front differential.
In the disclosed system, therefore, the torque inputted to the center differential is distributed and transmitted to the output member on the side of the front wheels and the output member on the side of the rear wheels when the direct coupling clutch is released. When a speed difference occurs between the front wheels and the rear wheels at a turning time, moreover, the center differential performs the differential action to absorb the speed difference. When the torque difference increases, moreover, the differential limiting torque rises to limit the differential rotations between the front and rear wheels.
When the direct coupling clutch is applied, on the contrary, the two output members in the center differential are integrally coupled by the direct coupling clutch so that the center differential rotates as a whole to eliminate the differential rotations between the front and rear wheels. In other words, the torque can be transmitted, even if any one of the front and rear wheels rotates idly, to the remaining wheels.
In the system disclosed in the Laid-Open, the means for the differential limit of the front and rear wheels need not be provided separately from the center differential, but the center differential, the front differential and the direct coupling clutch can be arrayed on the common line so that the system can be made compact as a whole.
When the coefficient of friction on the road surface is low as on a muddy road or a seriously undulating rough road or when the individual wheels have different coefficients of friction with the road surface, the run can be ensured by distributing the torque among the four front and rear wheels while performing the differential limit, as described above. However, even the four-wheel drive vehicle does not run exclusively on the rough road or a road having low friction coefficient but frequently runs on an excellent car road paved with asphalt to have a high friction coefficient. If the driving force is transmitted to all the four front and rear wheels even on the excellent car road, an unnecessary power loss may be caused by the friction at the power transmission system.
The four-wheel drive system of the prior art thus far described is enabled to perform the differential action by setting the center differential free or not by locking the center differential by the direct coupling clutch. With or without the differential rotations between the front and rear wheels, however, what is established is the four-wheel drive state in which the power is transmitted to the front and rear wheels, but cannot be the two-wheel drive state in which the power is transmitted to either the front wheels or the rear wheels. In the case of running on a dry paved road, therefore, the fuel economy may be deteriorated by the friction loss. In other words, the two-wheel drive state cannot be set to invite a disadvantage in the versatility.
In the prior art, on the other hand, there is known the so-called “part-time” four-wheel drive system capable of selecting the two-wheel drive state and the four-wheel drive state by the manual operation. This system is provided with a switching mechanism for coupling a transfer to transmit the power to the front wheels or the rear wheels selectively to the output member of a transmission. In this four-wheel drive system, the four-wheel drive state is established by coupling the transfer and the output member of the transmission, and the power is transmitted to only the front wheels or the rear wheels to establish the two-wheel drive state by releasing the coupled state. The switching mechanism of this kind can be added to the aforementioned system disclosed in the Laid-Open. In this case, the system may be large-sized or the characteristics of the individual drive states may change, depending upon the arrangement or assembly of the switching mechanism, so that the technical items to be considered diverge in many ways. In the prior art, however, the space efficiency or the mountability has neither been considered, nor has developed any proper array for the switching mechanism or the transfer.
Moreover, the four-wheel drive vehicle is intrinsically intended to improve the runability on a rough road or the uphill performance. In addition of the ordinary transmission for setting a gear ratio according to the vehicle speed or the accelerator opening, therefore, a speed change mechanism for switching the high-low gear ratios may be provided for especially raising the driving force. This speed change mechanism is associated with the four-wheel drive system such as the transfer or differential mechanism so that it is arranged together with the latter. In the prior art, however, neither special investigation nor technical development has been made on the arrangement of the speed change mechanism for switching the high-low gear ratios, but it is customary merely to array the speed change mechanism on the axis common to the differential mechanism or the like. This may elongate the four-wheel drive system in its entirety and deteriorate its mountability.
SUMMARY OF THE INVENTION
A main object of the invention is to provide a power transmission system for a four-wheel drive vehicle, which can be made compact.
Another object of the invention is to provide a power transmission system for a four-wheel drive vehicle, which is enabled to reduce the size by reducing the external diameter of a differential mechanism to make effective use of a surplus space ar
Estremsky Sherry
Lewis Tisha D.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Toyota Jidosha & Kabushiki Kaisha
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