Patent ID: 7704055

Claim:
A hydraulic system for the operation of hydraulic screw-tensioning appliances, the hydraulic system comprising: a high-pressure system; the high-pressure system including; a high-pressure pump delivering a hydraulic fluid at a constant high pressure, settable by means of a settable pressure valve to a first high pressure line connected to the high pressure pump and directly to a hydraulic screw tensioning appliance; a high-pressure space; a second high-pressure line connecting the first high-pressure line to the high-pressure space; a directional seat valve arranged in the high-pressure space and movable between an open position and a closed position, a bore cooperating with the directional seat valve; and a constantly pressureless high-pressure backflow line connected to the bore, wherein the directional seat valve is constantly and directly acted upon in a closing direction toward the closed position by high pressure from the high-pressure pump thereby closing the bore and interrupting a connection between the high-pressure space and the constantly pressureless high-pressure backflow line, a low-pressure control circuit, hydraulically separate from said high-pressure system, the low-pressure control circuit including: a low-pressure pump delivering a hydraulic fluid at a constant low pressure, settable by means a pressure valve to a low-pressure line connected to the low-pressure pump; a control piston connected to the low-pressure line; a randomly controllable switching valve connected to the low-pressure pump constantly making a connection to a constantly pressureless low-pressure backflow line as long as the randomly controllable switching valve is in an open position, wherein the randomly controllable switching valve is operable to be closed thereby selectively breaking the connection between the low-pressure line and the constantly pressureless low-pressure backflow line, and the control piston acts upon the directional seat valve to displace the directional seat valve in an opening direction toward the open position to make a connection between the first high-pressure line and the constantly pressureless high-pressure backflow line via the second high-pressure line and the high-pressure space when the control piston is acted upon in a controlled way by low-pressure from the low-pressure pump via the low-pressure line to displace the directional seat valve in the opening direction based on operation of the randomly controllable switching valve toward the closed position, and a motor operable to drive the high-pressure pump and the low-pressure pump.