Conventionally, a sealing device, like the one depicted in FIG. 3, that is formed with a ridge to attain a screw-seal action at a seal lip is known.
This sealing device is equipped with a seal lip 100 formed by a rubber-like elastic material (a synthetic resin material that includes rubber material and rubber-like elasticity); at an inner peripheral surface of this seal lip 100, a sealing-space-side-conical surface 102 whose large diameter increases to a degree of a sealing space (in-machine) A side, and a sealing-space-opposite-side conical surface 103 whose large diameter increases to a degree of a sealing space (out-of-machine) B side are formed, using a smallest-diameter lip-edge portion 101 as a boundary; and a plurality of first thread ridges 104 and second thread ridges 105 are formed at the sealing-space-opposite-side conical surface 103. Widths and heights of the first thread ridges 104 and the second thread ridges 105 are constant across an entire length, the ridges being disposed alternately to face opposite directions in a circumferential direction using a plurality of ridges as one set.
In other words, the sealing device constituted as described above attains shaft-sealing functionality with the lip-edge portion 101 of an inner diameter of the seal lip 100, fitted into the inner circumference surface of a housing, not shown in the drawing, closely fitted to be able to slide on an outer circumference surface of a rotating shaft 200 to inhibit oil in a sealing space A from leaking from around the shaft into an opposite sealing space B. Also, both of the first thread ridges 104 and the second thread ridges 105 attain screw-pump action in a forward direction to the rotation of the rotating shaft 200, in a rotation in either a forward or a reverse direction rotation of the rotating shaft 200. For that reason, oil that is trying to leak from the seal lip 100 at the sliding portion of the rotating shaft 200 of the sealing space A to the sealing space B opposite side is returned to the sliding portion side by forward direction screw-pump action, attaining a good sealing functionality.
Also, the sealing device depicted in FIG. 4 to suppress a drop in the screw-pump action caused by friction of the first thread ridges 104 and the second thread ridges 105, the first thread ridges 104 and the second thread ridges 105 are formed into boat-bottom-shaped projections whose widths and heights become largest at an intermediate portion in a length direction. (For example, see the Patent Document reference listed below.)