Oil pressure device

In an oil-pressure device which operates with an oil-pressure cylinder unit to actuate a working device, the oil cylinder unit including a cylinder, a piston movable in the cylinder, and a rod connected at and end thereof to the piston, an opposite end of the rod being connected to the working device, the rod having extension movement outwardly from and retraction movement into the cylinder responsive to imposition of oil pressure at respective larger and smaller opposite faces of the piston, the cylinder having a head side oil port communicating with the piston larger face and a rod side oil port communicating with the piston smaller face,

• an acceleration valve operable to speed up extension movement of the rod during a first part of rod travel from a fully retracted position thereof wherein there is no load imposed by the working device on the rod and until a loading is imposed by the working device on the rod, the acceleration valve including
• a counter-balance valve and a pilot check valve, a pilot pressure applied to the counter-balance valve when a pressure flow of oil is supplied to the head-side oil port acting on the counter-balance valve being insufficient to cause the counter balance valve to open from its closed condition blocking a return flow of oil from the smaller piston face side of the cylinder to an oil tank return course whereby the rod side oil port of the cylinder is maintained in communication with the head-side oil port so that the return flow of oil is diverted to the head-side oil-port increasing the oil amount at the piston large face and in consequence increase in the speed at which the rod extends at least until the working device imposes a load on the rod, on imposition of a load on the rod, pilot pressure on the counter-balance valve increases to cause opening of the counter-balance valve whereby the rod side oil port of the cylinder is placed in communication with the oil tank return course so that oil from the rod side of the cylinder can flow to said return course, and
• a slow return check valve connected between said rod side oil port and the oil tank return course and being in parallel connection with the counter-balance valve, said slow return check valve in absence of a load being imposed on said rod being operable to replenish oil to the rod side oil port by such an amount as leaks from a direction control valve to keep a rod side and a pilot side of the pilot check valve at the same pressure to prevent the pilot check valve from opening and therewith allowing unpredictable movement of the working device.