The assembly is composed of body case, seal ring, sealing flange, stretchable rubber bag, ring of clamping collar and clamping collar. Being setup inside the body case, the stretchable rubber bag with open end is toward outside. One end of the bag is in closed integrative form, and another end of the bag is in flanging plane form. The ring of clamping collar and clamping collar are setup on inner side cylindroid of the said flanging plane. Inner annular plane of the sealing flange impacts flanging plane of the bag so as to form a sealing surface. Comparing with prior art, the invention omits seal structure in one end in order to reduce probability of oil leakage. Seal structure in impacted flanging plane, which is easy to be installed, is adopted in another end instead of spring steel wire ring so as to avoid oil leakage caused by fatigue deformation of spring steel wire ring and aging cracks of rubber.