Classifying – separating – and assorting solids – Sifting – Elements
Reexamination Certificate
2002-06-18
2004-05-18
Walsh, Donald P. (Department: 3653)
Classifying, separating, and assorting solids
Sifting
Elements
C209S408000, C209S397000, C209S404000, C209S400000, C209S315000
Reexamination Certificate
active
06736271
ABSTRACT:
BACKGROUND OF THE INVENTION
Machines have been used to screen material for many years. The word “material” is necessarily general, because screening apparatus are used in applications from wastewater treatment and food manufacturing to the processing of aggregates and metals. Regardless of the type of material to be screened, though, separation of material based upon relative size is a valuable undertaking.
Since the predominant consideration in screening material relates to the sizes of particles to be separated, machines have been devised to screen material on an increasing scale and with greater efficiency. In the most generic sense, a screening machine consists of a hopper for receiving material and a screen at or near the lower portion of the hopper. The screen normally will have a number of openings therein for allowing material particles up to a certain size to pass through the screen. Particles that are too large to pass through any screen opening are moved out of the hopper using a method such as gravitational direction of the larger particles toward an open end of the hopper.
In order to screen with greatest efficiency, most screening machines are designed to oscillate rapidly to encourage a phenomenon called “stratification.” When a container of various-sized materials is shaken, the smaller particles tend to move toward the bottom of the container, while the larger ones remain toward the top of the container. If a hole of a size roughly equivalent to the size of the smaller particles is made in the bottom of the container, the smaller particles will likely pass through the hole during shaking, having become “stratified” on the bottom of the container. In this manner, stratification increases the efficiency of the separation process.
Modern screening machines are typically rectangular boxes, usually sloped, with one or more levels of screen surface with which the material interacts. Each level of screen surface is supported by a frame to which the screen surface is fastened, bolted, or otherwise secured in order to support the screen surface against the load of material that is placed into the rectangular box and shaken. Although the screening machine may be horizontally-oriented, many screening machines are inclined at an angle between 15 and 30 degrees, with the material being fed into the upper end of the machine and being allowed to flow to the bottom end of the rectangular box, which is usually open-ended. In this manner, material which fails to pass through the screen may be collected and possibly re-screened.
In a screen machine such as the above, rapidly vibrating the screening machine as material is being processed in the machine actually causes the material to assume a fluid-like characteristic as it moves across the screen surface, which enhances the stratification properties of the machine. A common range of rates for vibrating the screen machine is 600 to 3600 RPM. The vibration of the machine is achieved by various methods, the most common of which involves having one or more weighted shafts connected or integral to the machine that, when rotated, throws the material in the machine away from the screen surface, allowing the smaller particles to come in contact with the screen surface and sift through.
Originally, screen media was made of woven wire cloth or perforated steel plate. Such media wears out rapidly when handling abrasive particles, so more wear-resistant materials such as rubber or polyurethane have been used to significantly reduce the frequency and cost of maintenance on screens. Panels made of these materials, though, will stretch until they break, so cable or fabric reinforcement has been molded into such screens to strengthen them. The reinforcement is tensioned between the side walls of the screen machine, thereby providing beneficial additional structural strength to the wear materials. Examples of such designs are seen in U.S. Pat. Nos. 4,819,809 and 4,857,176. Significantly, however, both of those patents do not innovate to the extent of the subject matter herein.
U.S. Pat. No. 4,819,809 teaches a vibratory screen having a flexible molded polyurethane body having screen openings therein, and many aramid fibers reinforcing the panel, with the entire screen being able to be tensioned in place on a vibratory screening machine. The patented screen is coated with polyurethane before tensioning of the screen, whereas the screen panel of the invention is tensioned before the panel is coated with resilient coating, resulting in greater versatility for the screen panel of the invention. Moreover, the screen panel of the instant invention involves a selection of materials that is broader than that contemplated by the patent.
U.S. Pat. No. 4,857,176 teaches a substantially similar vibratory screen to that of U.S. Pat. No. 4,819,809. However, the vibratory screen of U.S. Pat. No. 4,857,176 teaches a vibratory screen for use with an arched-bed screening machine, and as noted elsewhere herein, the screen panel of the invention is made to be installed on a flat-bed screening machine, a more efficient type of screening machine due to the fact that material being screened does not tend to work its way down the slope to the sides of the machine. Arched-bed screening machines of this type thus fail to maximize the screening area, because the material being screened moves away from the top of the arch, minimizing use of that part of the screen, and gravitates toward the lower portions of the screen, where a greater percentage of the load ensures that some material that would otherwise be screened is blocked by other material and fails to find a passage through the screen.
The screen surface of a typical screening machine is made of one or more removable screen panels attached to the frame in any of a number of ways. The screen surface may be arched in the center, being supported by an arched support frame. In this configuration, screen panels are stretched from wall to wall across the screen so that as the edges of a panel are drawn outward, the center of the panel is pulled down against the crest of the arch. The tighter the screen surface is drawn against the supporting frame, the more the panel or panels are kept from flogging, prolonging the life of the screen.
In known screening machines, the arch in the screen support frame is desirable to properly tension the panels, but the arch has an unfortunate effect of causing material riding on the screen surface to move toward the side walls of the machine, leaving the crest of the arch without material on it, thereby effectively reducing usable screen area. To address this and other deficiencies, several manufacturers introduced screen media systems incorporating a flat support frame (i.e., a “flat-bed” machine) with a number of smaller screen media modules mechanically fastened to it. These systems provide for easier maintenance and panel change-outs compared with tensioned systems, and they allow screen operators to fine-tune the separation they are achieving. For example, different panels on the same flat-bed machine can have various-sized openings, screening at different granularities on the same screening machine.
An important shortcoming of modular screen panels in known flat bed systems is that the panels are not tensioned. Instead, each screen module has rigid internal structural elements that assist in keeping the module anchored to the support frame, and also aid the screen bed to resist sagging or breaking under the load of material being screened. However, structural elements within the screen modules can significantly reduce the area that can be perforated, thereby reducing the efficiency of the screening machine. Furthermore, in portions of the module that lack structural elements, the load must be supported solely by the rubber or urethane. The result is an overly thick perforated section, with large bar widths between openings, and a correspondingly low number of openings in the panels.
The design deficiencies of flat-bed modular screen panels are magnified when trying to screen fine ma
Beavers Lucian Wayne
Miller Jonathan R
Waddey & Patterson
Walsh Donald P.
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