Hydraulic apparatus

Claim 1

A hydraulic apparatus comprising: a piston for carrying a load thereon; a hydraulic cylinder for causing said piston to move vertically; a hydraulic pump for supplying operating oil via a one way valve means to said hydraulic cylinder; a normally closed valve connected between said hydraulic pump and said hydraulic cylinder; a check valve connected in parallel to said normally closed valve to permit the flow of the operating oil only toward said hydraulic cylinder; an oil tank; a normally open valve connected to a pipe passage for transmitting to said oil tank the operating oil delivered from said hydraulic pump; a control circuit for piston-raising operation for controlling said normally open valve so as to lessen the oil passage thereof in the case of raising the piston and so as to totally open said normally open valve in the case of lowering the piston; a control circuit for piston-lowering operation, for controlling said normally closed valve so as to totally close said normally closed valve in the case of raising the piston and so as to enlarge the oil passage thereof in the case of lowering the piston; a first orifice member provided on the downstream side of said normally open valve in a manner that it is connected thereto in series; a second orifice member connected between said normally closed valve and said cylinder and a check valve connected in parallel with said second orifice for permitting only the passing therethrough of the operating oil toward said hydraulic cylinder; and a second check valve for permitting the operating oil to flow into said oil tank when said hydraulic pump stops, said check valve being connected in parallel to the orifice member disposed in the downstream of said normally open valve, and pressure line means connected downstream of said pump and upstream of said one way valve means and acting to keep said second check valve closed under pump pressure.