present invention are shown in fig7 17 in fig7 inner annular projections 20a which are provided on the inner periphery of reinforcing disc 20 constitute inner spacers outer annular projections 20b on the outer periphery of reinforcing disc 20 constitute outer spacers the inner rims of discs 10a 10b are bonded to inner annular projections 20a the outer rims of discs 10a 10b are held in contact with outer annular projections 20b annular outer rim support member 21 is provided which has a pair of brims 21b outer rim support member 21 surrounds the periphery of discs 10a 10b and reinforcing disc 20 and brims 21b are bonded to the outer rims of discs 10a 10b since the diameter of reinforcing disc 20 is slightly smaller than that of discs 10a 10b annular gap g exists between the periphery of reinforcing disc 20 and the inner surface of outer rim support member 21 gap g of discs 10a 10b permits movement in the radial direction relative to member 21 in response to contraction or expansion in fig8 inner annular projections 22a which are provided on the inner periphery of reinforcing disc 22 constitute inner spacers a pair of annular rings 18a 18b made of elastic.