Bidirectional Hydrodynamic Polytetrafluoroethylene Seal

Abstract

A bidirectional hydrodynamic polytetrafluoroethylene shaft seal and a method for manufacture thereof. An annular polytetrafluoroethylene sealing member has a radially outer annular planar portion and a radially inner annular shaft-sealing non-planar portion inclined to the planar portion. An annular metal case clamps the planar portion in place, and the inner portion has a shaft-engaging side with an inner edge and a pair of spaced-apart coaxial projecting circular ribs. A first circular rib lies adjacent to the inner edge, and a second circular rib lies farther from the inner edge. A series of curved segmental ribs, each having a pair of ends spaced toward the first rib from the second rib and curved in between its ends toward the first rib, extend in to and intersect the first rib. To make the seal, a flat annular wafer of polytetrafluoroethylene of constant thickness is coined at an annular inner portion adjacent the inner periphery to provide the first and second circular ribs and a series of straight segmental ribs, each of which extends tangential to the inner side of the inner rib, thereby intersecting the inner rib, the crest of the straight ribs projecting out further than the crest of the inner rib. Then the annular inner portion is formed into a generally frustoconical shape, whereby the straight ribs appear as curved ribs. The frustoconical portion is further flexed on installation on a shaft to be generally cylindrical from at least the outer rib to the inner periphery. This invention relates to a shaft seal made from polytetrafluoroethylene as the sealing element and so constructed as to be bidirectionally hydrodynamic. Molded, elastomeric bidirectionally hydrodynamic seals are quite common. However, there has been a total lack of success in the manufacture of hydrodynamic seals having a polytetrafluoroethylene sealing element, principally because of the difficulty in working with this material. Until recently the only known method of obtaining a raised flute formation on a polytetrafluoroethylene hydrodynamic seal was to mold it. Recently, it has been found possible to cold form hydrodynamic features into polytetrafluoroethylene sealing elements, as shown in application Ser. No. 426,373, now U.S. Pat. No 3,857,156, assigned to the assignee of the present invention. This new method has made possible the forming of close dimensional tolerance in the design features of such seals and has made possible manufacture of the seal of the present invention. This is not to say the present invention could not be constructed as a molded polytetrafluoroethylene seal element, but only that it is believed that consistently good results are assured with the new method of manufacture. Thus, attempts to make hydrodynamic seals of polytetrafluoroethylene were until recently unsuccessful and have become successful only as a result of work involving the present inventors, employing processes which involve either machining a spiral groove or other hydrodynamic structure into the seal or by coining the structure into the seal. The aforesaid U.S. Pat. No 3,857,156 does the latter, and experience has shown that this is a practical method of manufacture of hydrodynamic polytetrafluoroethylene seals. In other words, a flat washer is cold-formed with the aid of a die under considerable pressure into a shape enabling hydrodynamic action. Up to now, however, such hydrodynamic structures gave hydrodynamic action in only one direction of shaft rotation. Another type of polytetrafluoroethylene seal which has substantially the same foot pattern and functions hydrodynamically in accordance with the same sealing principle is shown in U.S. Pat. No. 3,801,114, granted Apr. 2, 1974, wherein bidirectional hydrodynamic action is achieved by employing three washers or annular wafers of polytetrafluoroethylene for each seal which washers are arranged concentrically side by side. The washers constitute a static seal lip, a set of hydrodynamic flutes, and a second but discontinuous seal lip, respectively. Such a seal performs well, but only when the respective washers are assembled absolutely concentrically, and this may be difficult to achieve on a production basis. An object of the present invention is to provide a shaft seal in which the sealing element is polytetrafluoroethylene and in which that sealing element is so made as to be bidirectionally hydrodynamic. It is an important feature of the invention that the finished shaft seal employs only a single annular wafer or washer of polytetrafluoroethylene and not a plurality of such washers or wafers and yet is bidirectionally hydrodynamic. The shaft seal of the present invention incorporates a metal case which provides a cylindrical bore-engaging wall and which acts to clamp in place the polytetrafluoroethylene sealing member. The sealing member itself is annular with a radially outer annular planar portion that is clamped into the case a radially inner annular shaft-sealing portion which is non-planar and is in fact inclined to the planar portion. This inner portion, before installation, is generally frustoconical in shape but after installation most of it becomes generally cylindrical in shape, hugging the shaft. The invention is characterized by the inner portion having a shaft-engaging side with an inner edge and having a pair of spaced-apart concentric projecting circular ribs. In other words, there is a first circular rib which is close to the inner edge of the sealing wafer and a second circular rib spaced away from the first rib and farther from the inner edge. Between the two circular ribs in a series of curved segmental ribs each having a pair of ends that are spaced away from the second rib in the direction of the first rib.. Each of these curved ribs is curved toward the first rib, extending into it and actually intersecting it, being substantially tangential to the inner side of the first rib, that is, the side closest to the inner edge. In order to obtain this structure and to do so in a manner which is practical, the polytetrafluoroethylene element is first made as an annular wafer or washer from flat material. It may be cut out from a flat sheet or it may be machined from a tubular billet. In either event, the starting wafer is flat and of constant thickness. In the method of the invention the inner annular portion of this wafer is then coined while the wafer is still flat and planar, to provide an inner circular rib adjacent the inner periphery and an outer circular rib that is concentric with the inner rib but spaced away from it in the direction of the air side of the seal. The outer rib is of lesser diameter when mounted on the shaft to be sealed than the inner rib since, as explained below, it must function as the static seal lip. The coining operation also provides a series of straight segmental ribs extending tangentially to the inner side of the inner rib, thereby intersecting the inner rib, and the ends of the segmental ribs lie near to but spaced away from the outer rib. The annulus of the segmental ribs is, as seen when mounted on the shaft to be sealed, of greater inside diameter than the outer circular rib but less inside diameter than the inner circular rib. The diametral size relationship between the inner circular rib and segmented ribs is such that the latter lifts the former slightly off the shaft at the midpoint of the latter to form an oil passageway for each direction. As explained before, this particular structure and the diametral size relationships important to its hydrodynamic sealing function are made commercially possible by the preferred method of manufacture, which is cold forming by use of coining dies. Thus, coining dies (blank-pierce-coin, or pierce-and-coin) are provided with grooves for coining the two circular ribs by simply cutting circular grooves into the die on a lathe, while the grooves for coining the series of straight rib segments are provided by cutting straight grooves into the die. Both operations are rather easily done by a skilled machinist. The grooves cut for the segmented members are deeper than the grooves cut for the oil side rib but shallower than those cut for the air side rib. After the coining step, the annular inner portion of the wafer is formed into a generally frustoconical shape. This forming causes the formerly straight ribs to appear as curved ribs. The polytetrafluoroethylene element may be clamped into the case at any time after the coining operation, that is, either before or after forming the frustoconical shape. When the finished seal is installed in a bore and when a shaft is installed, the sizes of the shaft relative to the sealing element are such that much of the frustoconical portion is then forced into what is substantially a cylindrical shape, at least in the area including the two circular ribs. The hydrodynamical operation of this seal is as mentioned above substantially that described in U.S. Pat. No. 3,801,114, and we refer to it as "preferential flow" or "plowing", as opposed to an entirely different hydrodynamic sealing principle which is well known in the art and can be referred to as the "pressure gradient" principle. A seal functioning in accordance with the last named principle is shown and described in U.S. Pat. No. 3,347,554. So it is seen that by a combination of the coining-in of a certain shape on to a flat washer and the forming of that flat washer so that the coined part becomes frustoconical and therefore easily flexed into a cylindrical shaft-engaging shape, bidirectional hydrodynamic action is obtained in such a manner and that bidirectional hydrodynamic action has been obtained on a single-element polytetrafluoroethylene seal. The outer circular rib acts as a limiting wall and helps to retain leaked oil near the segmented ribs. Other objects and advantages of the invention will appear from the following description of a preferred embodiment.

Patent Drawings (2 sheets)

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