COUNTER-BALANCE VALVE

摘要

A counter-balance valve having the construction wherein a valve hole having first and second pump-side ports, first and second motor-side ports and an auxiliary port, a spool fitted to the valve hole slidably in the transverse direction and establishing or cutting off communication between ports, right and left springs for holding the spool at a neutral position at which communication between ports is cut off, a left pressure receiving chamber for moving the spool to a first travelling position at which the first pump-side port communicates with the auxiliary port and the second pump-side port communicates with the second motor-side port by a pressure oil supplied, and a right pressure receiving chamber for moving the spool to a second travelling position at which the second pump-side port communicates with the auxiliary port and the first pump-side port communicates with the first motor-side port by the pressure oil supplied, are provided to a valve main body, first shaft hole communicating with the left pressure receiving chamber, a second shaft hole communicating with the right pressure receiving chamber, first and second small diameter holes allowing normally the first and second shaft holes to communicate with the first and second pump-side ports and first and second large diameter holes for allowing the first and second shaft holes to communicate with the outer peripheral surface of the spool are formed on the spool, respectively, the first and second large diameter holes are closed when the spool exists at the neutral position and at the position between the neutral position and the first and second travelling positions, and the first and second large diameter holes communicate with the auxiliary port when the spool exists at the first and second travelling positions.