Cam 1 rotates about fixed axis A and has spiral groove a. Slider 5 reciprocates in fixed guides B-B and carries rocker arm 2 to which the stud shafts of two rollers, 3 and 4, are pivoted. The rollers alternately engage groove a (see view facing arrow E). When the cam turns 1,5 revolutions, one of the rollers, for example 3, runs up the inclined surface at the outer end of spiral groove a (see Section x-x) and is thereby pushed out of engagement with the groove. At the same time, roller 4 is swivelled into engagement with the inner end of groove a, and slide 5 is reversed.
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