The present invention generally relates to a transmission control device for vehicles and to a steering assembly for vehicles. More specifically, the present invention relates to a transmission control device for coordinating steering inputs with speed inputs and to a steering assembly providing increased reliability and space efficiency.
When the driver of a typical vehicle makes a turn, the vehicle responds by changing its direction. In certain vehicles, such as the conventional tractor, the steering wheel is coupled to a steering linkage which, in turn, is coupled to the front wheels. When the driver turns the steering wheel, the front wheels pivot clockwise or counterclockwise. In one type of tractor, commonly known as a zero turn radius tractor, the rear drive wheels rotate independent of one another. The driver controls both the speed and direction of this tractor by controlling the motion of the rear drive wheels relative to one another. In this case, the steering linkage is coupled to the rear drive wheels. When the driver turns the steering wheel, this causes the rear drive wheels to rotate at different rates, which causes the tractor to turn.
It has been found that, in this type of tractor, when the driver is in the process of making a turn, the radius of the turn can change even though the driver holds the steering wheel in one position. This can occur when the driver increases or decreases the ground speed while making the turn. For example, if the driver turns the steering wheel to follow along a curved driveway, and at the same time, the driver presses the foot pedal, increasing the ground speed, the tractor can slightly wander away from the curved driveway. This is because the transmission of this tractor, which controls the ratio of right rear wheel speed to the left rear wheel speed, produces a change in this ratio when the driver changes the ground speed in the midst of a turn.
In addition to this wandering disadvantage, this type of tractor also has the disadvantage of castor front wheels which do not function like the conventional automobile-type front wheels. The castor wheels, which are not linked to the steering wheel, are free to swivel in any direction. This freedom is necessary to prevent front wheel slippage when the tractor wanders as described above.
Not only are castor front wheels less familiar to automobile drivers, they tend to cause the front end of the tractor to wander when the tractor is traveling laterally along a slope or hillside. For these reasons, drivers must spend time to acquire the skill necessary for steering and operating this tractor in various driving conditions.
However, if the transmission of a zero turn radius tractor could be adapted to prevent the wandering problem described above, the tractor could use conventional automobile-type front wheels. If a conventional automobile-type steering linkage, commonly known as an Ackerman-type steering linkage, were to be used in a zero turn radius tractor, such a steering linkage would present several disadvantages. This steering linkage includes a rack and pinion gear assembly and steering arms, both of which are positioned behind the front axle of the vehicle. This type of linkage consumes valuable space which could be occupied by other parts of the vehicle. In addition, the rack and pinion gear assembly is relatively complex which leads to several disadvantages. The gear assembly can malfunction relatively frequently, require a relatively high amount of maintenance service and is relatively expensive to manufacture.
The zero turn radius tractor described above also requires a reverse travel mechanism in order for the tractor to properly respond to the driver's steering inputs while traveling in reverse. This mechanism is necessary, in part, because the steering wheel and the foot pedals are separately and independently coupled to the rear drive wheels. The reverse travel mechanism is relatively complex, including a relatively high number of mechanical and electrical parts. Therefore, the reverse travel mechanism results in a relatively significant manufacturing expense and can require maintenance, service and replacement from time to time.
Therefore, there is a need to overcome each of the disadvantages described above.