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1. A synthetic resin-made thrust sliding bearing comprising:
a synthetic resin-made upper casing including an upper annular plate portion having a circular hole in a central portion of the upper casing, a first cylindrical suspended portion formed integrally on a lower surface of said upper annular plate portion in such a manner as to be radially outwardly spaced apart a predetermined interval from an inner peripheral surface of said upper annular plate portion defining the circular hole, a cylindrical engaging suspended portion formed integrally on the lower surface of said upper annular plate portion at an outer peripheral edge thereof in such a manner as to be radially outwardly spaced apart a predetermined interval from said first cylindrical suspended portion, so as to form an upper outer annular groove in cooperation with said first cylindrical suspended portion, and an annular engaging portion formed on an inner peripheral surface of said cylindrical engaging suspended portion; a synthetic resin-made lower casing including a lower annular plate portion having in a central portion of the lower casing an insertion hole concentric with the circular hole of said upper annular plate portion, a first annular projection formed integrally on an upper surface of said lower annular plate portion on an inner peripheral side thereof, a second annular projection formed integrally on the upper surface of said lower annular plate portion in such a manner as to be radially outwardly spaced apart a predetermined interval from said first annular projection, so as to form at an inner peripheral surface of the second annular projection a lower annular recess in cooperation with an outer peripheral surface of said first annular projection and the upper surface of said lower annular plate portion, a cylindrical engaging projecting portion formed integrally on the upper surface of said lower annular plate portion at the outer peripheral edge thereof in such a manner as to be radially outwardly spaced apart a predetermined interval from said second annular projection, so as to form at an inner peripheral surface of the cylindrical engaging projecting portion a lower outer annular groove in cooperation with an outer peripheral surface of said second annular projection, and an annular engaged portion formed on an outer peripheral surface of said cylindrical engaging projecting portion; and a thrust bearing piece formed in a form of a synthetic resin-made annular disk, disposed in the lower annular recess, and having a circular hole in a central portion thereof, wherein said thrust bearing piece is disposed in the lower annular recess while maintaining an inner annular clearance between an inner peripheral surface of the thrust bearing piece defining the circular hole and an outer peripheral surface of said first annular projection and maintaining an outer annular clearance between an outer peripheral surface of the thrust bearing piece and an inner peripheral surface of said second annular projection, respectively, said thrust bearing piece being disposed in the lower annular recess such that an upper surface of the thrust bearing piece is positioned between an opening plane of the lower annular recess and the lower surface of said upper annular plate portion by being brought into sliding contact with the lower surface of said upper annular plate portion, and such that a lower surface of the thrust bearing piece is positioned by being brought into sliding contact with the upper surface of said lower annular plate portion defining a bottom surface of the lower annular recess, wherein said upper casing is combined with said lower casing by disposing said first cylindrical suspended portion in the lower outer annular groove so as to be radially superposed on said second annular projection and said cylindrical engaging projecting portion, and by causing said engaging portion to be resiliently fitted to said engaged portion, wherein if it is assumed that a radial length from an axis of said upper casing to the outer peripheral surface of said thrust bearing piece disposed in the lower annular recess is (r), and a thickness of said upper annular plate portion at an inner peripheral surface of the upper annular plate portion is (t), said upper casing has on an upper surface of the upper casing an annular flat surface having a circular outer peripheral edge whose radius (R) with the axis of said upper casing set as a center is (r)-(t), wherein said synthetic resin-made thrust sliding bearing is disposed between a lower surface of a vehicle body-side mounting member and an upper surface of an upper spring seat opposing that lower surface, such that only the annular flat surface is brought into contact with the lower surface of the vehicle body-side mounting member, wherein if it is assumed that a diameter of the outer peripheral surface of said first annular projection is (c), a diameter of the inner peripheral surface of said second annular projection is (d), a diameter of the inner peripheral surface of said thrust bearing piece is (a), a diameter of the outer peripheral surface of said thrust bearing piece is (b), a depth of the lower annular recess corresponding to a distance from the bottom surface of the lower annular recess to the opening plane of the lower annular recess is (f), and a thickness of said thrust bearing piece is (e), then a sum A of volumes of the inner annular clearance and the outer annular clearance and a volume B of that portion of said thrust bearing piece which projects upward from the opening plane of the lower annular recess are in a relationship of B>A shown by a following formula:
e(b2−a2)>f(d2−c2), wherein said upper casing further has an annular projecting portion projecting in an axial direction integrally from the upper surface of said upper annular plate portion, the upper surface of said upper casing is the upper surface of said annular projecting portion, and the annular flat surface is constituted by the upper surface of said annular projecting portion, and wherein said upper casing has on the upper surface of the upper casing a truncated conical surface portion with a descending gradient extending from an outer peripheral edge of the annular flat surface or from a radially outer portion of that outer peripheral edge to an outer peripheral surface of the upper casing. |
