Patent ID: 6539712
Filing Date: 2003-04-01
Classification: E02F,F15B

Abstract:
A hydraulic drive unit comprising:a pair of main circuits (1), (2) each selectively connecting an oil pressure source with a hydraulic motor (M); a counter-balancing valve (3) disposed halfway of the main circuits (1), (2) so as to be capable of being changed over from one position to another, said counter-balancing valve (3) being provided with: a pair of discharge passages (10), (11) and a pair of return passages (12a), (12b), both of which are adapted to be opened and closed for the main circuits (1), (2); a pair of first check valves (13), (14), each disposed halfway of a respective one, of the discharge passages (10), (11); a pair of orifices (28), (29) each selectively disposed halfway of a respective one of the return passages (12a), (12b); and a pair of branch passages (21), (22), each connected to a respective one of the discharge passages (10), (11) on upstream sides of the first check valves (13), (14) and selectively connected to a by-pass circuit(27); return circuits (23), (24) each connected to a respective one of the main circuits (1), (2) and connected to each other; a pair of second check valves (25), (26), each disposed halfway of a respective one of the return circuits (23), (24); and the pass circuit (27) connected to the return circuits (23), (24), where they are connected to each other.