Road structure – process – or apparatus – Apparatus – Screed or drag
Reexamination Certificate
2001-03-29
2003-04-08
Will, Thomas B. (Department: 3671)
Road structure, process, or apparatus
Apparatus
Screed or drag
Reexamination Certificate
active
06543962
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
BACKGROUND OF THE INVENTION
This invention generally relates to the field of roadway paving and finishing devices. More specifically, this invention relates to a screed assembly having improved ability to control the magnitude of a change in the angle of attack of the screed plate in response to a change in the angle of the tow arms and to control the magnitude of a change in the angle of the tow arms in response to a change in the angle of attack.
Conventional paving and/or finishing assemblies typically consist of two basic units, a tractor or other vehicle capable of distributing paving material (e.g., asphalt or other bituminous materials) and a screed device. The screed device includes a screed platform, a screed plate forming the bottom surface of the platform and a means for towing the platform behind the tractor. During the process of laying down paving material, the screed platform typically is towed behind the tractor by a single pair of tow arms, one arm coupled with one side of the tractor and screed platform and the other arm coupled with the opposing side of the tractor and screed platform. In operation, the tractor tows the screed device in a forward direction with the screed plate contacting the surface to be paved or finished to level and/or smooth the surface.
As the screed device is towed behind the tractor, paving material is distributed under the screed platform. Consequently, the vertical force imparted to the screed plate is increased and the platform is vertically displaced from the existing surface by the applied paving material. The magnitude of this vertical displacement is dictated, in part, by the characteristics of the material and the displacement, in turn, determines the depth or thickness of the layer of material being laid down, i.e., the mat. Irregularities in the surface being paved or in the material properties (e.g., the viscosity or density) of the material being laid down also can affect the vertical displacement of the screed platform. Controlling the magnitude of this vertical displacement is crucial in producing high-quality mats which are of uniform thickness and texture.
The angle between the bottom surface of the screed platform (i.e., the screed plate) and the surface over which paving is being performed commonly is referred to as the “angle of attack”. This angle is a primary factor in determining the amount of material that will pass under the screed platform over a given distance and is elemental in controlling mat depth. The angle of attack will always seek an equilibrium, i.e., a substantially constant magnitude wherein the screed plate is moving in a plane essentially parallel to the grade surface. With the standard screed assembly, the magnitude of any change in the angle of attack is equal to the magnitude of any change in the tow angle. The tow angle is the angle, measured at the tow point (i.e., the point at which the tow arms are connected to the tractor), of the tow arms with respect to horizontal. The magnitude of a change in the two angles is the same, but the direction of the change is opposite, i.e., if one increases the other decreases. This result is determined by the linkage used to couple the towing vehicle and the screed platform, namely, a single pair of tow arms, one coupled with each side of the tractor and the screed platform. This linkage causes the entire screed device to act as a single lever, pivoting at the tow point.
As a standard screed platform is pulled behind the towing vehicle, the angle of attack may change intentionally or unintentionally. For instance, if the towing vehicle proceeds over an uneven surface, the angle of attack will change due to the vertical displacement of the vehicle and such changes are undesirable and unintentional. The angle of attack may also be adjusted intentionally by use of a depth crank. Such changes in the angle of attack, whether intentional or unintentional, result in mats that are either thicker or thinner, depending upon the direction of the change. For example, if the density of the paving material decreases, the vertical force imparted to the screed plate also decreases causing the screed plate to drop until the screed platform reaches an equilibrium, i.e., until the screed plate is again moving in a plane essentially parallel to the grade surface. This drop simultaneously increases the angle of attack and decreases the tow angle by the same magnitude. This permits less material to pass beneath the platform and causes the mat depth to decrease. Alternatively, if the density of the paving material increases, the vertical force imparted to the screed plate also increases causing the screed plate to rise until the screed platform reaches an equilibrium. This rise decreases the angle of attack and increases the tow angle by the same magnitude. This permits more material to pass beneath the platform and causes the mat depth to increase.
Paving over a flat, level surface with all variables held constant, a conventional screed assembly will produce a mat of constant profile. However, unevenness of the surface being paved can change the mat profile. When the plane in which the tractor is traveling changes, e.g., due to irregularities in the surface being paved, the tow angle changes accordingly. As a result, the angle of attack changes as does the thickness of the mat. Accordingly, due to irregularities in the surface being paved, evenness in the final thickness of the mat may vary. Such is not a desirable result as a smooth, even mat of uniform thickness is desired. In such circumstances, it would be desirable to have a screed assembly in which a change in the angle of attack was lesser in magnitude than the corresponding change in the tow angle rather than equal.
Circumstances also exist when it is desirable for the angle of attack to change more rapidly and/or more dramatically than the tow angle. For example, a change in the properties of the paving material, e.g., the viscosity or aggregate properties, may cause a change in the angle of attack and thus the thickness of the mat which is not desired. In such circumstances, it would be desirable to have a screed assembly in which a change in the angle of attack was greater in magnitude than the corresponding change in the tow angle rather than equal so as to compensate for the change in physical properties of the applied material.
Accordingly, there is a continuing need in the paving and finishing industry for a screed device which has improved ability to control the magnitude of the change in the angle of attack of the screed plate in response to a change in the tow angle and to control the magnitude of the change in the tow angle in response to a change in the angle of attack. Additionally, there remains a need for a screed device which may be coupled with the towing vehicle in a manner other than that typically used in the industry providing for improved sensitivity and response in the angle of attack.
SUMMARY OF THE INVENTION
Accordingly, in one of its aspects, the present invention provides a screed assembly having improved ability to control the magnitude of a change in the angle of attack of the screed plate in response to a change in the tow angle such that a change in the angle of attack is not constrained to be equal in magnitude to a change in the tow angle.
In another of its aspects, the present invention provides a screed assembly having improved ability to control the magnitude of a change in the tow angle in response to a change in the angle of attack such that the change in the tow angle is not constrained to be equal in magnitude to a change in the angle of attack.
In a further aspect, the present invention provides a screed assembly having dual sets of tow arms, one set coupled on each side of a prime mover, wherein each set includes at least two tow arms.
In another of its aspects, the invention provides a method of maintaining a more consistent mat depth despit
Addie Raymond
Koch Industries, Inc.
Stinson Morrison & Hecker LLP
Will Thomas B.
LandOfFree
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