Planetary gear transmission systems or components – Differential planetary gearing – Spur gear differential
Reexamination Certificate
1999-11-24
2001-07-03
Estremsky, Sherry (Department: 3681)
Planetary gear transmission systems or components
Differential planetary gearing
Spur gear differential
C475S089000, C475S199000, C180S249000
Reexamination Certificate
active
06254506
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to motor vehicle transmissions of the kind in which a differential assembly has an input for connection to an engine, first and second outputs for transmitting drive from the input to respective road wheels or groups of road wheels and a speed dependent resistance device for applying a torque reaction in response to relative rotation between the outputs, the torque reaction increasing with the rotational speed of the relative rotation. Transmissions of this kind are generally known, e.g. from GB-A-1 475 141 which shows a simple 3-element epicyclic differential assembly and a viscous coupling acting as the speed dependent resistance device. Furthermore, DE-A-4 113 963 shows a viscous coupling acting between the outputs of a bevel gear differential, a dog clutch providing selection between a controlled mode and a locked mode.
BACKGROUND OF THE PRESENT INVENTION
The requirements of such motor vehicle transmissions vary according to vehicle operating conditions. In general, the resistance device is required to provide a low resistance at high vehicle speeds and a high resistance at low vehicle speeds. Hence the degree of resistance is usually a compromise between these requirements.
OBJECT OF THE PRESENT INVENTION
It is an object of the invention to provide a motor vehicle transmission of the kind referred to which allows the above requirements to be met more readily.
SUMMARY OF THE PRESENT INVENTION
According to the invention there is provided a motor vehicle transmission including a differential assembly having an input for connection to an engine, first and second outputs for transmitting drive from the input to respective road wheels or groups of road wheels and a speed dependent resistance device for applying a torque reaction in response to relative rotation between the outputs, the torque reaction increasing with the rotational speed of the relative rotation, characterised in that the speed dependent resistance device is operable to selectively couple the first output to the second output or to the input whereby the differential assembly is provided with at least two operating modes, a low resistance mode providing a low resistance to the relative rotation and a high resistance mode providing a high resistance to the relative rotation.
The differential assembly may also have a free mode of operation in which there is no resistance to relative rotation between the outputs. This is useful for high speed conditions. Furthermore, the differential assembly may also have a locked mode of operation in which relative rotation between the outputs is prevented. This is useful for extreme slippery conditions.
Conveniently the low resistance mode is obtained when the resistance device is connected between the first output and the input whilst the high resistance mode may be obtained when the resistance device is connected between the first output and the second output.
The differential assembly may further comprise a sleeve which is moveable between positions in which said operating modes are selected, in which case the resistance device may have two rotary members, relative rotation of which provides the torque reaction, one rotary member being coupled to the first output and the sleeve being coupled to but axially slidable relative to the other rotary member. Conveniently, the sleeve may be selectively connected to the differential input to effect the low resistance mode and, to effect the high resistance mode, the sleeve may be selectively connected to the second differential output. To effect the free mode the sleeve may be selectively disconnected from both the second differential output and the differential input and, to effect the locked mode, the sleeve may be selectively connected to the differential input and the second differential output.
The differential assembly may comprise an epicyclic gear train having an annulus gear which acts as the input, a sun gear which acts as the first output, a carrier which acts as the second output, a first set of planet gears, each rotatably supported by the carrier and in intermeshing engagement with the annulus gear, and a second set of planet gears each rotatably supported by the carrier and in intermeshing engagement both with a respective planet gear of the first set and with the sun gear.
Preferably, the speed sensitive device is a viscous coupling.
The transmission may further comprise a range change gearbox having a high range for normal use and a low range for use in difficult conditions and a range change mechanism to effect a change between the high and low ranges, in which case the operation of the differential assembly may be operatively linked to the range change mechanism such that when the range change gearbox is in the high range the differential assembly is operable in the low resistance mode and when the range change gearbox is in the low range the differential assembly is operable in the high resistance mode.
The transmission may include a main gearbox providing a range of forward gear ratios and a gear shift mechanism to select different ratios in the main gearbox, operation of the differential assembly being operatively linked to the gear shift mechanism such that when the range change gearbox is in the high range and the main gearbox is in a high ratio the differential assembly is in the free mode. Alternatively or additionally, the operation of the differential assembly may be operatively linked to the gear shift mechanism such that when the range change gearbox is in the low range and the main-gearbox is in a low ratio the differential assembly is in the locked mode.
Where the transmission includes a main gearbox providing a range of forward gear ratios and a gear shift mechanism to select different ratios in the main gearbox, the operation of the differential assembly may be operatively linked to the gear shift mechanism such that when the main gearbox is in a low ratio the differential assembly is in the high resistance mode and when the main gearbox is in a high ratio the differential assembly is in the low resistance mode. Such an arrangement is useful where there is no range change gearbox. In such a case, the operation of the differential assembly may be operatively linked to the gear shift mechanism such that when the main gearbox is in its highest ratio the differential assembly is in the free mode. Alternatively or additionally, the operation of the differential assembly may be operatively linked to the gear shift mechanism such that when the main gearbox is in its lowest ratio the differential assembly is in the locked mode.
The transmission may further comprise control means operative to change the differential assembly between the low and the high resistance modes, the control means being sensitive to vehicle road speed to put the differential assembly in the low resistance mode when the vehicle is travelling above a high/low threshold road speed and to put the differential assembly in the high resistance mode when the vehicle is travelling below the high/low threshold road speed. In such a case, the control means may be operative to put the differential assembly in the free mode when the vehicle is travelling above a low/free threshold road speed higher than the high/low threshold road speed and may be operative to put the differential assembly in the locked mode when the vehicle is travelling below a high/locked threshold road speed lower than the high/low threshold road speed. Selection of the free mode according to road speed may be usefully combined with the other methods of selection so that, for example, selection dependent on selection of a high ratio in the main gearbox or dependent on both selection of a high ratio in the main gearbox and selection of a high ratio in the range change gearbox is conditional on the vehicle travelling above the low/free threshold road speed.
REFERENCES:
patent: 4074591 (1978-02-01), Dick
patent: 4526063 (1985-07-01), Oster
patent: 4677875 (1987-07-01), Batchelor
patent: 4699237 (1987-10-01), Matsumoto
patent: 478107
Davis & Bujold P.L.L.C.
Estremsky Sherry
Rover Group Limited
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