Textiles: spinning – twisting – and twining – Strand structure – Covered or wrapped
Patent
1996-07-18
1998-09-08
Stryjewski, William
Textiles: spinning, twisting, and twining
Strand structure
Covered or wrapped
57 3, 57 6, 57234, 87 1, 87 5, 87 13, 156172, D02G 336
Patent
active
058028285
DESCRIPTION:
BRIEF SUMMARY
The present invention refers to braided packings used for sealing pump or valve shafts, which, in turn, are used in a wide variety of industrial environments and applications.
In the manufacturing of such packings, the conventional asbestos yarns have been recently substituted for many types of synthetic yarns or filaments.
The yarns or filaments, to be appropriated for such applications, must have at least the following properties: their braiding and also to resist to fluid pressure and possible abrasion within the stuffing box; fluids, as well as those generated by friction of the rotating pump or valve shaft; and
Many high performance packings are manufactured today from polytetrafluoroethylene (PTFE)-based yarns. These yarns can be obtained by using a matrix spinning process or by cutting a "peeling" or a thin layer of material from a round sintered PTFE billet, which had its surface previously "scratched", so as to produce uniform threads or ribs thereon.
This process is described in U.S. Pat. No. 4,025,598 granted to Sasshofer et al and assigned to Chemigfaser Lenzing Aktiengesellschaft of Austria.
Another process for the manufacturing of PTFE filaments is described in U.S. Pat. No. 3,953,566 granted to Gore and in its continuation-in-part, U.S. Pat. No. 3,962,153. The yarns made by the processes of those patents are usually known as "expanded PTFE yarns" and the products resulting from those patents can still be combined with graphite, which enhances the PTFE anti-friction properties and improves its thermal conductivity.
The graphite can be added to the yarn either during the preparation of the PTFE mixture, before the extrusion step, or by coating the semi-finished yarn, as taught in U.S. Pat. No. 4,256,806 granted to Snyder.
DE-A-23 26 826 discloses a yarn in which filaments of different materials, such as of polytetrafluorethylene and aromatic polyamid, are combined providing a composite yarn. The different filaments are preferably joined without torsion. None of the filaments provides a cover or an external layer to the others. The thus formed composite yarn can itself be coated with a dispersion of particles of polytetrafluorethylene. On the one hand, a dispersion of particles does not generate a coating with uniform thickness around the composite yarn which results in variable mechanical and chemical properties along the yarn. On the other hand, it does not have a good mechanical resistance, since it does not result in a structured material as the filaments which form the composite yarn.
Similarly, EP-A-0 282 284 discloses a yarn which is composed of fibers of poly(phenylene sulfide) and of polytetrafluorethylene. This composite yarn is further impregnated with a dispersed organic material, e.g., polytetrafluorethylene or graphite, and/or an inorganic material. This also results in a coating with non-uniform properties and relative poor mechanical resistance.
PTFE yarns impregnated or coated with graphite are much superior to pure PTFE yarns, in that they can be used in the manufacturing of packings for shafts rotating with shaft surface speeds of around 25 m/s, when compared to the speeds in the range of 12 m/s of the packings made of pure PTFE.
PTFE packings are not recommended in applications where very high pressures exist in the stuffing box; in these applications the PTFE packings, specially those from expanded yarns, have been known by their characteristic of "extruding" through the gap between the shaft and the pumps housing and/or bushing.
In this case, the braiding machines have been using yarns made of aramide fiber, which are known by their great mechanical strength and resistance to high temperatures. The chemical, sealing and anti-friction properties of the aramide yarns are improved by impregnating them with PTFE dispersion.
This has been achieved, conventionally, by impregnating or coating those yarns with PTFE dispersion, in a ratio of 20% to 100% in weight of PTFE, relative to the weight of the yarn. This process has partly achieved the intended objectives but still have lef
REFERENCES:
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patent: 3844195 (1974-10-01), Rhodes et al.
patent: 3953566 (1976-04-01), Gore
patent: 3962153 (1976-06-01), Gore
patent: 4025598 (1977-05-01), Sasshofer et al.
patent: 4163825 (1979-08-01), Wimmer
patent: 4256806 (1981-03-01), Snyder
patent: 4502364 (1985-03-01), Zucker et al.
patent: 4559862 (1985-12-01), Case et al.
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Patent Abstracts of Japan, vol. 018 No. 380, (M-1639), 18 Jul. 1994 & JP,A, 06 101764 (Nippon Valqua Ind. Ltd) 12 Apr. 1994.
Manegro Administracao E Participacoes Ltda
Stryjewski William
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