1. Field of the Invention
This invention relates generally to a differential and more particularly to a differential wherein a coupling torque is variable and a DC motor is utilized to vary the coupling torque.
2. Description of the Prior Art
In an all-terrain vehicle or utility vehicle, the differential is used to distribute power to the front wheels, while having the ability to allow for a difference in rotational speed at each wheel from each other and the rear wheels for smooth, low-effort steering. One of the traditional methods in practice is to employ engagement dogs (or splines, etc.) to control left to right differentiation. A limitation with this type of system is the inability to engage or disengage “on the fly”. This type of engagement is “all or nothing”; it cannot be modulated like a clutch that is capable of slipping. It is also implicated that it cannot effectively be employed on an “automatic” system that engages only when wheel slip has been detected. A driver using this type of system therefore must anticipate driving conditions, and choose between greater traction capability or steering effort and handling.
The Polaris/Hilliard system attempts to overcome some of these obstacles with over-running clutches. The disadvantages of this type of system include: The system must operate with a front to rear ratio of ˜0.83:1, depending on the vehicle. This is due to the need to prevent engagement on surfaces of good traction during turns. This ratio difference results in sudden engagement under some circumstances, as well as a loss of ultimate traction, as the front and rear fight each other with different rotational rates when the system is engaged. Additionally, the front wheels cannot be used for engine braking, severely limiting the system's capability in steep downhill terrain.
Another method utilized by Honda employs a differential mechanism that uses differential cams and a roller clutch to engage/disengage the FWD. The Honda system as a whole is, however, very different in two specific manners: 1) It does not have computerized automatic engagement of the FWD, and 2) it utilizes a fundamentally different mechanism for the left/right power distribution. This part of the system cannot be automatically or manually controlled and will not supply significant power to a tractive wheel if the other wheel is completely non-tractive (in the air or on ice). Also, the tuning is fundamental to the design and cannot accept user or computer input.
Another traditional automotive method is to employ a Torsen® style limited slip device to act between the left and right tires. This type of the system cannot be automatically or manually controlled and will not supply significant power to a tractive wheel if the other wheel is completely non-tractive (in the air or on ice). Also, the tuning is fundamental to the design and cannot accept user or computer input.
Another traditional method is to employ a “limited slip” mechanism between the left and the right wheels. Since ATV's don't have power steering to overcome the resulting increase in steering effort, steering effort becomes unacceptably high. Also, the amount of engine torque that can be transmitted to only one wheel is severely limited, reducing off-road capability.
Another traditional automatic method uses the silicon viscous technology to apply linear force to a clutch pack in response to differences in speed between the left and the right front wheels. A limitation of this method is that it cannot be tuned as a function of vehicle speed and therefore compromises high-speed handling with low-speed capability. In addition, this type of system cannot be disabled, even in two-wheel drive mode.
An automatic method in current practice uses fly-weights that spin in accordance with the difference in lift and right wheel speed. At some preset speed, the fly-weights cause the engagement of a locking mechanism. This type of system is well known for its dangerous handle-bar jerk and poor handling upon engagement when used on a FWD.
In designing an all-terrain vehicle or utility vehicle, several characteristics are sought to be optimized. These include the requirements that the differential is narrow; light; provides for a low-steering effort; has predictable dynamics to minimize unexpected handlebar motion, unexpected braking effects, and sudden loss of capabilities during changing riding conditions; and true four-wheel drive, wherein one wheel with poor traction must not prevent significant engine torque delivery to the other wheel.
The present invention addresses the limitations of the prior art and provides for a new differential.