| 摘要 |
PROBLEM TO BE SOLVED: To hold frictional braking force generated by an end spacer 17 substantially constant from beginning to end in frictional braking of a reverse rotation of a motor shaft after a thrust force reception, by preventing a reduction in the frictional braking force resulting from conformance between friction surfaces. SOLUTION: A ringlike friction portion 17c integrally protruded from a tip face 17d of the end spacer 17 is formed in a flat contact state and so as to have an effective radius (r) made larger with wear, with the result that the increase in the effective radius (r) offsets a reduction in the coefficient of friction resulting from conformance.
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