Machine element or mechanism – Control lever and linkage systems – Hand operated
Reexamination Certificate
1999-12-21
2001-08-14
Fenstermacher, David (Department: 3682)
Machine element or mechanism
Control lever and linkage systems
Hand operated
C074S3880PS, C280S090000
Reexamination Certificate
active
06272947
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to vehicle steering systems and more particularly to a centering device for controlling the steering system of a large motor vehicle, such as a tandem truck, a motor home, a bus, or a specialty vehicle with steered rear wheels such as a fire engine or a long tandem bed vehicle for hauling large special objects of unusual shape. A steering shaft is controlled so that a center steering position is selected and maintained for two or more steerable wheels of the vehicle in spite of spurious steering inputs, such as those caused by variable crosswinds, crowned or slanted roadways, or other factors tending to adversely affect vehicle steering by the driver.
BACKGROUND OF THE INVENTION
The steering systems of motor vehicles and the like are designed primarily for driver control. In these systems, the steering force required on the steering wheel and the ratio between steering wheel movement and movement of the steerable ground wheels depend upon the characteristics of the particular vehicle and the conditions under which it will usually be operated. A wide variety of extraneous forces can act on a vehicle steering system and spurious steering inputs caused by these forces must be dealt with satisfactorily in order to provide stable and controllable steering of a vehicle. As vehicle speed increases, the effects of any spurious steering inputs are magnified, making it necessary for the driver to exercise more precise and careful driving control.
The effects of any spurious steering inputs also are magnified as the size and weight of the vehicle increases. A particularly unstable vehicle is one of the tandem type where one or more trailer sections are towed by a cabin or engine section having front steerable wheels, and the aft wheels of the last trailer section also are steerable, such as is the case with the ladder section of a fire engine and with the last section of tandem trailer beds of specialty vehicles for carrying large and/or heavy equipment or other objects of unusual shape.
In other words, in such specialty vehicles both the front pair of wheels and the rear pair of wheels are steerable. In the case of fire engines, the ladder section often has a second steering wheel requiring a second operator. However, in the case of other specialty vehicles, it is preferable that both the front steerable wheels and the rear steerable wheels be controlled by a single steering wheel and operator in the vehicle cab, an arrangement that can be particularly unstable because of the length of the steering train and of the number of steering shafts and linkages involved.
One drawback of prior art steering systems is that spurious inputs transmitted from the roadway through the steerable wheels affect substantially the entire steering assembly and the only stabilizing resistance is provided by the driver's manipulation of the steering wheel. The negative action of the steerable wheels is caused by spurious steering inputs from crosswinds, slanted or crowned roads, bad road surfaces, and other adverse dynamic steering forces. As described further below, inherent geometric steering characteristics also may be responsible for spurious steering inputs. The transmission of these various inputs between the steerable wheels and the steering wheel causes steering system oscillations and vehicle wandering that require constant corrections and therefore produce driver fatigue.
The transmission of these spurious inputs between the steerable wheels and the steering wheel causes the interconnecting components of the steering system to repeatedly oscillate between states of tension and compression. Such oscillations cause wear and slack in ball joints and other connections and have long been considered a primary source of stress fatigue which can lead to premature failure of various steering system components. Mechanical slack due to worn parts can also be a cause of steering system oscillations and vehicle wandering that require constant corrections and therefore contribute to driver fatigue.
Vehicles with steering systems having positive caster generally track relatively straight ahead and resist steering inputs away from center, including those of the driver, provided that the roadway they are travelling on is smooth and is not slanted or crowned. Such positive caster is provided by positive caster offset, which is also known as mechanical trail. Caster offset is the distance from the ground intersection point of a pivot line drawn through the pivot axis of a steerable wheel to a contact point at the center of the area over which the wheel contacts the ground. The pivot axis of a steerable wheel of a motor vehicle is usually provided by a “king pin”. Because the contact point of a steerable wheel with positive caster trails the pivot line point of the wheel, side forces cause the wheel to turn in the direction that the force is being applied. A good example of this is the way in which the castered wheels on the front of a shopping cart are easily turned in the direction of applied force.
The adverse effects described below are some of the negative aspects of achieving steering stability with positive caster offset. Because of the side force applied by gravitational pull on a slanted or crowned highway, positive caster offset causes a motor vehicle to freely turn to the low side, creating a steering wheel pull that requires counteractive steering input from the driver to keep the vehicle from leaving the highway. The amount of driving fatigue that is directly caused by positive caster offset under these conditions may be appreciated by considering the many millions of miles driven by truck drivers and other motorists each day on crowned or slanted highways.
Another fatiguing driving condition that may be encountered by a motorist is that of controlling a crosswind steering input. The amount of adverse steering input caused by crosswinds is directly related to the amount of positive caster offset, which is a classic example of having to balance a benefit with a detriment. The small amount of stability gained from castering the steerable wheels on a non-windy day may be paid for many times over when driving in a crosswind because of the destabilizing effect of the crosswind when combined with positive caster offset. Positive caster offset also allows steering inputs from rutted and other imperfect roadway surfaces to steer back against the driver and thereby cause road wander, which is a universal driving complaint, particularly by driver's of heavy vehicles such as trucks and motor homes.
For the lack of a more advanced method, steerable wheel castering has been accepted by the industry as a low-cost method of achieving steerable wheel returnability. Thus, large, heavy over-the-road vehicles are presently provided with generous amounts of positive caster. Not much thought has been given to the self-defeating side effects of steerable wheel castering. Instead, the lack of directional stability is blamed on the size and weight of the vehicle.
As the size and weight of over-the-road vehicles increases, the need for directional stability becomes more important. Learning to drive a heavy vehicle means learning to control the back steer caused by the adverse side effects of steerable wheel castering. The failure of the industry to recognize the critical need to provide directional stability by replacing steerable wheel castering with another method of achieving steerable wheel returnability may go down in history as one of the longest enduring heavy vehicle design oversights.
The lack of directional stability is fundamentally the reason that heavy vehicle driving is much more stressful than it otherwise needs to be. Keeping a heavy vehicle, that is lacking in directional stability, tracking straight and under control for extended periods of time is a major cause of driving fatigue and related accident potential. The failure of numerous driver fatigue and alertness studies to consider the contribution made by “driving” fatigue in the overall evaluation is ind
Connolly Bove Lodge & Hutz
Fenstermacher David
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