COUNTER-BALANCE VALVE

摘要

A counter balance valve is characterized in that a valve body is provided with a valve bore having pump-side first and second ports, motor-side first and second ports and an auxiliary port, a spool fitted in the valve bore to be slidable in a longitudinal direction thereof so as to establish communication between the respective ports and to block the communication therebetween, left and right springs maintaining the spool to a neutral position thereof at which the respective ports are closed, a left pressure receiving chamber acting to shift the spool by a pressure oil supplied therein to a first travelling position at which the pump-side first port and the auxiliary port are communicated with each other and the pump-side second port and the motor-side second port are also communicated with each other, and a right pressure receiving chamber acting to shift the spool by a pressure oil supplied therein to a second travelling position at which the pump-side second port and the auxiliary port are communicated with each other and the pump-side first port and the motor-side first port are also communicated with each other, in that the spool is formed with a first axial bore communicated with the left pressure receiving chamber, a second axial bore communicated with the right pressure receiving chamber, first and second small diameter bores always communicating the first and second axial bores with pump-side first and second ports, respectively, and first and second large diameter bores communicating the first and second axial bores with an outer peripheral surface of the spool, and in that the first and second large diameter bores are closed at a state that the spool is positioned to the neutral position and an intermediate position between the first and second travelling positions and the first and second large diameter bores are communicated with the auxiliary port at a state that the spool is positioned to the first and second travelling positions, respectively.