An apparatus 4, wherein said pocket inner surface of said retainer is formed into a shape which substantially coincides with the shape of the ball in {HEAD ~ contact} with {TAIL ~ said pocket inner surface} at the major axis position of said cam 
with
An apparatus 4 , wherein said pocket inner surface of said retainer is formed into a shape which substantially coincides with the shape of the ball in { HEAD ~ contact } with { TAIL ~ said pocket inner surface } at the major axis position of said cam
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