Transfer case with four-wheel underdrive operating mode

Details Transfer case with four-wheel underdrive operating mode Transfer case with four-wheel underdrive operating mode

1999-11-15

2001-03-20

Wright, Dirk (Department: 3681)

Planetary gear transmission systems or components

Variable speed or direction transmission combined with...

Variable speed or direction is planetary transmission

Reexamination Certificate

active

06203465

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates generally to transfer cases for four-wheel drive vehicles and more specifically to a transfer case having a center differential which is configured to selectively provide a four-wheel underdrive operating mode.
The obvious benefits of improved traction and vehicle control achieved by four-wheel drive systems in adverse driving conditions such as snow, freezing rain, ice and even water has been known and appreciated by vehicle designers for many years. A not so obvious benefit relates to the use of four-wheel drive when towing a trailer carrying a boat, snowmobiles and the like. Here, too, the improved traction provides improved driver control and stability in adverse driving conditions. When trailer towing is viewed as a specific operational mode, additional features and requirements may be added to the list of vehicle design criteria. For example, when the vehicle weight is augmented by several hundred or several thousand pounds, a slightly higher overall drive ratio provides improved torque and thus improved acceleration.
In contrast, when the vehicle is utilized in two-wheel drive the presumption exists that neither the enhanced traction nor improved acceleration would be beneficial or necessary inasmuch as either the vehicle is not towing a trailer, the road conditions are good or both.
The present invention is directed a transfer case having a center differential which provides direct two-wheel drive and an underdrive four-wheel drive operating mode which improves vehicle performance.
SUMMARY OF THE INVENTION
A transfer case for a four-wheel drive vehicle includes a center, planetary gear differential which provides a direct output in two-wheel drive and a slightly reduced speed output in four-wheel drive. In two-wheel drive, the outputs of the center differential are locked together by a clutch collar and drive torque is provided to the primary drive line only. In four-wheel drive, the clutch collar unlocks the differential and couples the output from a sun gear to a chain drive sprocket in the secondary drive line. The center differential is driven through the carrier, operates as an open differential and is preferably configured with a 65% primary (rear), 35% secondary (front) drive line torque split. The ring (outer) gear drives the primary drive line and the sun (center) gear drives the secondary drive line through cooperating chain sprockets and a chain. The chain sprockets are of unequal size and effect a speed reduction between the drive and driven sprockets. This speed reduction is nominally the same as the speed reduction from the input to the primary output achieved through the center differential. Thus, in four-wheel drive mode, the drive ratio through the drive lines is raised and the speed reduced, improving the acceleration and pulling power of the vehicle. In two wheel drive, more favorable fuel economy is provided with the lower (direct) drive ratio. With the open center differential, skid control and torque distribution may be achieved through the vehicle's antilock or anti-skid braking system.
It is thus an object of the present invention to provide a transfer case having both direct drive and underdrive modes of operation.
It is a further object of the present invention to provide a transfer case having a center differential which may be locked out to provide direct two-wheel drive and unlocked to provide four-wheel drive at a reduced speed (underdrive) mode.
It is a still further object of the present invention to provide a transfer case having an open center differential wherein skid control is provided by, for example, an anti-lock braking system incorporated in the motor vehicle which is independent of the transfer case.
It is a still further object of the present invention to provide a motor vehicle transfer case wherein an open center differential and unequal chain sprockets provide drive torque to primary and secondary drive lines in a four-wheel underdrive mode to improve vehicle performance.
Further objects and advantages of the present invention will become apparent by reference to the following description and appended drawing wherein like reference numbers refer to the same component, element or feature.


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patent: 5078660 (1992-01-01), Williams et al.
patent: 5106351 (1992-04-01), Williams et al.
patent: 5499951 (1996-03-01), Showalter
patent: 5984821 (1999-11-01), Showalter

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