Hydraulic drive unit

Abstract

A hydraulic drive unit is disclosed which can prevent the occurrence of cavitation at the time of turning OFF of a hydraulic motor without greatly modifying the circuit configuration and the entire system and which, as a result, can improve the deceleration feeling of the hydraulic motor and prevent the occurrence of a low noise. More specifically, a hydraulic drive unit is disclosed which comprises a pair of main circuits ( ), ( ) each connecting an oil pressure source with a hydraulic motor (M); a counter-balancing valve ( ) disposed halfway of the main circuits ( ), ( ) so as to be capable of being changed over from one position to another, said counter-balancing valve ( ) being provided with a pair of discharge passages ( ), ( ) and a pair of return passages ( ), ( ), both of which are adapted to be opened and closed for the main circuits ( ), ( ), a pair of first check valves ( ), ( ) disposed halfway of the discharge passages ( ), ( ), a pair of orifices ( ), ( ) disposed halfway of the return passages ( ), ( ); and a pair of branch passages ( ), ( ) which are connected to the discharge passages ( ), ( ) on upstream sides of the first check valves ( ), ( ) and selectively connected to the by-pass circuit( ); return circuits ( ), ( ) each connected to the main circuits ( ), ( ) in a position therebetween; a pair of second check valves ( ), ( ) disposed halfway of the return circuits ( ), ( ); and a by-pass circuit ( ) connected to the return circuits ( ), ( ) in a position between the second check valves ( ), ( ).