Hydraulic pressure compensating method and device for pressure-equalizing apparatus for pressure pins

A hydraulic pressure compensating method for use with a pressing machine having a pressure-equalizing means, for compensating a hydraulic pressure value of a pressure-equalizing circuit by connecting an oil feeding means to a pressure-equalizing circuit, which is provided between a lower die and a die cushion device of said pressing machine, and which is communicated with oil chambers of a plurality of pressure-equalizing cylinders for equalizing applied pressure to a work piece during a forming operation, said hydraulic pressure compensating method comprising the steps of:

• connecting said pressure-equalizing circuit and said oil feeding means according to hydraulic pressure of said pressure-equalizing circuit and a predetermined preloading pressure at an outlet port of said oil feeding means to thereby set the hydraulic pressure of said pressure-equalizing circuit at the preloading pressure, when the hydraulic pressure of said pressure-equalizing circuit decreases after the completion of the forming operation from a peak pressure at the time of completion of the forming operation and becomes smaller than a predetermined value;
• and
• after setting the hydraulic pressure of said pressure-equalizing circuit at the preloading pressure, disconnecting said oil feeding means and said pressure-equalizing circuit, and continuing the disconnection until the hydraulic pressure of said pressure-equalizing circuit in a next forming operation decreases after the completion of the next forming operation from a peak pressure at the time of completion of the next forming operation and becomes smaller than the predetermined value once again, wherein said connecting and said disconnecting of said oil feeding means and said pressure-equalizing circuit is carried out with use of a mechanical type switching cylinder for operating a valve driving mechanism for further operating a valve to connect and disconnect, said switching cylinder:
• sensing pressure of said oil feeding means in a bottom chamber of said switching cylinder,
• sensing a pressure of said pressure-equalizing circuit in a head chamber of said switching cylinder having a smaller pressure receiving area than a receiving area of said bottom chamber, and
• driving said valve driving mechanism by reaction of said switching cylinder to said sensed pressures, so as to connect and disconnect said oil feeding means and said pressure-equalizing circuit.