Land vehicles – Wheeled – Running gear
Reexamination Certificate
1998-05-15
2001-02-20
Swann, J. J. (Department: 3611)
Land vehicles
Wheeled
Running gear
C180S209000, C280S043170, C280S405100
Reexamination Certificate
active
06189901
ABSTRACT:
BACKGROUND
1. The Field of the Invention
This invention relates to vehicle equipment, and, more particularly, to auxiliary trailing axles for trucks.
2. The Background Art
Highway construction and maintenance is a matter of substantial concern to local, state, and federal governments. Road construction has always been an expensive proposition. Roads constructed using modern knowledge, methods, and technology have greatly improved the load-bearing capacity of vehicles traveling over those roads.
Specific limitations exist on loading of axles. It is well established that bridges are designed to carry specific weights. However, in actual bridge design, several additional, localized factors exist. For example, a bridge may have a surfacing material such as concrete or asphalt. These may be designed in various compositions to support various loads and provide predictable durability. However, underlying a bridge or road surface is a structure of specific members each designed for supporting a particular maximum force or load.
Bridges in various parts of a roadway system have varying weight-carrying capacities. A truck having weight over some number of axles, must also have those axles distributed across a suitable length of the bridge in order to distribute the load of the truck properly over individual structural members of the bridge.
Thinking in terms of a truck, not as a truck, but as a series of axles, each bearing a load, one sees another important factor in the mutual design criteria between vehicles and roadways (e.g. bridges). That is, axles cannot be separated from the truck. The truck has a length; therefore, axles cannot be completely separated from each other. Therefore, all of the axles of the truck will pass over the bridge together. The truck has to distribute axles with some maximum length.
Moreover, road construction does leave all streets, highways, roads, and the like with specific limitations on sustainable loads and the like. For example, just as building construction must start far below the surface level of the earth in order to support a foundation, a road bed is deeply laid for many roads. Above a road bed are laid various types and grades of materials. Ultimately, a surface material is provided on which vehicles roll directly.
A fundamental engineering principal is involved in the concept of maximum stress and principal stress directions. In solid materials of uniform, isotropic, properties, principal stresses are compression tension, and shear. At any location, principal stresses may be axial or may be resolved axially. Accordingly, once stresses have been resolved along orthogonal axes, all loads may be represented as either tension or compression. However, in a material subjected to any combination of tension, compression, or both, along principal axes, shear stress is induced at an angle with respect to the principal stresses.
Therefore, many years of stress analysis have developed by reliance on a host of methods. Nevertheless, in terms of understanding the principal stress planes, Mohr's circle is an engineering construct useful for explaining the directions and magnitudes of principal stresses.
In accordance with these concepts of principal stresses, as known in the art, certain design approaches may be used to minimize stress or support stress as needed. One important principal is St. Venant's principal. St. Venant's principal may be thought of in terms of principal stress. Accordingly, whenever stress is localized, by a load on a solid structure, the load will be distributed on an angle corresponding to the angle of the principal shear planes.
Therefore, one may think of a road bed as a pile of rocks of varying sizes and qualities. Nevertheless, each individual particle in a road bed or a road surface material experiences stress from a load according to its comparative distance from the point of application of the load. Therefore, as a practical matter, treating the load as a major force component on the road, and assuming isotropic properties, one may imagine loads being distributed at an angle of 45 degrees away from the direct or normal load applied to a road surface. Accordingly, two feet below a road surface, a load may be distributed two feet away from the point of application of the load in each horizontal direction, assuming that the load is normal to the surface. In reality, road materials are not solid. Thus, each particular solid particle is, or may be, heavily, locally loaded.
Also, competing considerations exist in road construction. For example, a layer of aggregate does not support tension. It only supports compression. However, rocks neighboring a loaded rock can restrain the loaded rock from moving. Thus, the concept of principal stress is a useful concept in understanding the damage that may be done to a road.
One reason why trucks, cars, and vehicles in general rely on pneumatic tires is to improve the ability of the vehicle to absorb shocks from the roughness of a surface. Nevertheless, another purpose of rubber tires is to distribute the load of the vehicle over a surface area of a road surfacing material. Tire pressures relate directly to the distortion of a tire in order to present a certain amount of area onto a road for supporting the weight of the vehicle.
For example, a four thousand pound vehicle having a total of fifty square inches of tire surface to the road must have a tire pressure of approximately twenty pounds per square inch in order to support the load. To support the same load or weight of a vehicle at forty pounds per square inch only twenty-five square inches of tire tread must be in contact with the road. Thus, local pressure on a road surface may be controlled, to a certain extent, by the inherit limits on tire pressures.
However, further down through a road bed away from a tire running on a road surface, the total force of a tire has been integrated by St. Venant's principal. One may note that two axles, close together will produce more load in a road bed than the same two axles, carrying the same two loads, but spaced a substantial difference apart, with respect to the thickness of the road bed.
Thus, one may see that axle location may be very important, as is the net, local force presented on a bridge or a road bed by an axle. In this context, an axle may be used to refer to the axle itself, or to the axle and tires as they represent force application to a road bed from a vehicle supported thereby.
Trucks today may be manufactured to have tandem axles spaced a comparatively long distance apart, as compared with trucks of previous years. Also, trucks now carry auxiliary axles that can be engaged for distributing a load along a different length of the truck. For example, long truck bodies or trailers may have wheels located nearer the front end, rather than leaving the entire weight distributed between a front axle and a rear axle or between a tractor and a pair of closely spaced tandem axles at the rear.
Auxiliary axles are added to concrete mixer trucks to accommodate limitations on bridge weights. Also, auxiliary axles may be added to accommodate the large differential load between an empty truck and a loaded truck. Thus, auxiliary axles may be engaged for a limited time, only while a vehicle is loaded and is traveling on a road. At a work site, a truck may not need auxiliary axles as a support for the vehicle itself, and may disengage them.
Thus, heavily loaded trucks having changes in load actually applied thereto, may need auxiliary axles. Those axles need to be distributed along a maximum length, and may need to be distributed along the vehicle itself To protect roadways, to satisfy bridge weight limitations, and to support substantial loads, auxiliary axles may be used in vehicle construction.
Auxiliary axles themselves present various problems. One may think of the problems as difficulties that auxiliary axles do not solve. In some instances auxiliary axles introduce new problems of their own to vehicle construction, legal compliance of vehicles with road and bridge limits, o
Larcom Larry
Smith Fred P.
Dunn David R.
F. S. New Products Inc.
Pate & Pierce & Baird
Swann J. J.
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