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1. A thrust sliding bearing comprising: a synthetic resin-made lower casing, a synthetic resin-made upper casing superposed on said lower casing, and a synthetic resin-made thrust sliding bearing piece interposed between said upper casing and said lower casing,
wherein said lower casing includes an annular base portion which has an annular upper surface and an annular lower surface, as well as a tubular small-diameter inner surface, a tubular large-diameter inner surface, and a cylindrical outer surface which are arranged between the annular upper surface and the annular lower surface and has a first annular recessed portion provided on the annular upper surface; a small-diameter hole which is defined by the tubular small-diameter inner surface of the annular base portion and is open at the annular lower surface of the annular base portion; a large-diameter hole which is defined by the tubular large-diameter inner surface of the annular base portion and is open at the annular upper surface of the annular base portion; an annular shoulder surface which is continuous from the tubular small-diameter inner surface at its annular inner peripheral end and is continuous from the tubular large-diameter inner surface at its annular outer peripheral end; an annular projecting portion which projects radially outward from a lower end portion of the cylindrical outer surface; a tubular projecting portion which is provided on an annular upper surface of the annular projecting portion and defines a second annular recessed portion by its tubular inner surface in cooperation with the cylindrical outer surface and the annular upper surface of the annular projecting portion; and an annular engaging projecting portion which projects radially outward from a tubular outer surface of the tubular projecting portion, wherein said upper casing includes an annular baseplate portion; an inner tubular suspended portion which is integrally provided on a tubular inner surface of the annular baseplate portion in such a manner as to be suspended downward from an annular upper surface of the annular baseplate portion and defines a central hole concentric with the small-diameter hole by its tubular inner surface; a thick-walled outer tubular suspended portion which is integrally provided on a tubular outer surface of the annular baseplate portion in such a manner as to be suspended from the annular upper surface of the annular baseplate portion and to extend downwardly of an annular lower end surface of the inner tubular suspended portion, and which has a cylindrical inner surface; a third annular recessed portion which is defined by an annular lower surface of the annular baseplate portion, a tubular outer surface of the inner tubular suspended portion, and the cylindrical inner surface of the outer tubular suspended portion; an inner tubular vertical wall portion and an outer tubular vertical wall portion which are provided vertically downward from an annular lower end surface of the outer tubular suspended portion; a fourth annular recessed portion which is defined by the annular lower end surface of the outer tubular suspended portion, a tubular outer surface of the inner tubular vertical wall portion, and a tubular inner surface of the outer tubular vertical wall portion; an annular engaging portion which projects downward from an annular lower end surface of the outer tubular vertical wall portion; and an annular hook portion which projects radially inward from a tubular inner surface of the annular engaging portion, and wherein said thrust sliding bearing piece is disposed in the third annular recessed portion with its annular upper surface brought into slidable contact with the annular lower surface of the annular baseplate portion defining the third annular recessed portion, and is disposed in the first annular recessed portion with its annular lower surface brought into slidable contact with an annular recessed portion bottom surface of the annular base portion defining the first annular recessed portion, such that the annular lower surface of the annular baseplate portion of said upper casing and the annular upper surface of the annular base portion of said lower casing are spaced apart from each other, said upper casing being superposed on said lower casing, such that the annular lower end surface of its inner tubular suspended portion is opposed to the annular shoulder surface with a gap therebetween, the cylindrical inner surface of its outer tubular suspended portion having a greater wall thickness than the inner tubular suspended portion concerning a radial direction is brought into slidable contact with the cylindrical outer surface of the annular base portion of said lower casing, its inner tubular vertical wall portion is disposed in the second annular recessed portion, and its annular hook portion is resiliently fitted to the annular engaging projecting portion, whereby a sealing portion based on labyrinth action is formed between the resiliently fitted portion and the slidably contacting portion of the cylindrical outer surface of the annular base portion with respect to the cylindrical inner surface of the outer tubular suspended portion by the inner tubular vertical wall portion disposed in the second annular recessed portion and the tubular projecting portion and the annular engaging projecting portion disposed in the fourth annular recessed portion.
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