PTFE (polytetrafluoroethylene) is used as a material for packings for sealing an annular gap between a shaft and a housing that move relative to each other. PTFE can be used under high temperatures and high pressures, and has excellent characteristics such as high chemical resistance and excellent sliding properties. On the other hand, PTFE has a shortcoming that it is prone to plastic deformation (settling) particularly when used in high temperature environments. If settling occurs in a sealing lip of the packing, it leads to deterioration of sealing properties.
A technique for maintaining the sealing lip in tight contact with an outer circumferential surface of a shaft or an inner circumferential surface of a shaft hole of a housing is known (see Patent Literatures 1 and 2), wherein a metal spring member is mounted in an annular mounting groove formed in the packing.
However, PTFE has a higher linear expansion coefficient than that of metal materials commonly used as the housing material. In high temperature environments, therefore, the sealing lip on an outer circumferential side of the packing (outer circumferential lip) tends to deform so as to expand radially outward, so that it is firmly pressed against the inner circumferential surface of the shaft hole of the housing. As a result, a large settling occurs in the outer circumferential lip, hence, as a countermeasure for settling, it is not sufficient to simply mount a spring member.
Conventionally, in order to prevent the metal spring member from coming off, an annular protrusion that protrudes radially inward is provided at the distal end of the outer circumferential lip. The outer circumferential lip thus has a relatively high rigidity.