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When crank 1 rotates about fixed axis A, motion is imparled to roller 2 through connecting link 3. Spring 4 holds roller 2 in contact with the working surfaces of fixed plate cam 5. This gives rod 6 a turning slotted motion about axis B and reciprocating motion with respect to slotted cross-head 7. Roller d of ratchet wheel 8 slides a long the slot in cross-head 7. In each revolution of crank 1, its lug b engages and turns ratchet wheel 8 through an angle corresponding to one tooth, gradually raising and lowering axis B. This varies the motion of rod 6 derived from cam 5.