Hydraulic drive circuit for machine tools

In a machine tool having a twin cylinder actuated member with each cylinder having a piston linked to the member to raise the member to an idle position and to drive the member downwardly through a power stroke, the improvement which comprises a hydraulic circuit for said twin cylinders including conduits supplying fluid to opposite ends of each cylinder to raise and lower the pistons, a reversing valve controlling the direction of flow of the fluid through the conduits, a positive displacement pump supplying fluid through the reversing valve to the conduits, a relief valve for said pump, an accumulator communicating with a conduit for raising the pistons, a sequence unloading valve controlled by accumulator pressure to unload the pump below a maximum pressure, a dump valve for said accumulator, and means biasing said reversing valve to a position for flowing fluid from the pump to the accumulator and to the bottoms of the cylinders for raising the pistons, and manually actuated means for shifting the reversing valve against the bias to flow fluid to the tops of the pistons and deliver a power stroke to the member whereby the member will normally be raised and held in a raised position until the reversing valve is actuated to deliver the power stroke.

In a power shear having an upstanding reciprocating blade carrier, links depending from the ends of the blade carrier, a rotatable torsion rod having levers projecting from the ends thereof pivoted to said links, and twin double acting cylinders pivoted to the ends of the levers beyond the links for swinging the levers to rotate the torsion bar and raise and lower the links for reciprocating the blade carrier, the improvement of a hydraulic circuit for operating said twin cylinders having conduits communicating with the tops and bottoms of the cylinders, a pump for flowing fluid through the conduits to raise and lower the pistons in the cylinders, a reversing valve controlling the direction of flow through the conduits, an accumulator maintaining fluid in the bottoms of the cylinders, an unloading valve for recirculating fluid from the pump whenever fluid pressure in the accumulator exceeds a predetermined maximum, and a relief valve for the pump in advance of the unloading valve to dump pump pressure when the blade carrier reaches the bottom of its stroke, whereby the blade carrier will be urged to a raised position by accumulator pressure and will be forced downwardly through a power stroke only when the reversing valve delivers fluid to the tops of the cylinders.