Importance

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Abstract

A hydraulic drive system for a machine element performing in one work cycle a sequence of motions comprising a rapid feed motion, a working stroke under load and an oppositely directed rapid-return motion. The drive element includes a hydraulic cylinder having three working surfaces A.sub.1, A.sub.2, A.sub.3. Motion control is effected by an electro-hydraulic follow-up control valve with a pre-setting device controlled by a stepping motor and a mechanical actual-value feed-back device. During rapid feed motion, the relatively small working surface A.sub.1 is subjected to the output pressure of the follow-up control valve only, while during the working stroke the pressure acts additionally on the working surface A.sub.3. The rapid-return movements of the piston of the cylinder are controlled by applying pressure to the working surface A.sub.2, or by relieving pressure from the two other working surfaces A.sub.1 and A.sub.3. A pressure-controlled reversing valve is moved by the output pressure of a pilot valve control arrangement into its operating position corresponding to the working stroke when the output pressure P.sub.A of the follow-up control valve exceeds a pre-determined threshold value P.sub.sl, and returned to its operating position corresponding to the rapid motion conditions when the output pressure P.sub.A of the follow-up control valve has dropped to a value P.sub.s2 below or at least equal to the value P.sub.s1.A.sub.1 /A.sub.L, wherein in the particular case described L.sub.L =(A.sub.1 +A.sub.3).

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First Claim