Pneumatic-hydraulic booster type driving device for the cushion-knockout mechanism associated with the slide of a press-machine

A pneumatic-hydraulic booster type driving device for a cushion knockout mechanism associated with a slide of a press-machine that includes a cushion cylinder containing an oil under pressure, comprising:

• a pneumatic-hydraulic booster comprising a pneumatic cylinder of a first diameter and a pneumatic piston slidably fitted therein, said booster having a hydraulic cylinder of a second diameter smaller than said first diameter and a plunger that is connected to the piston and slidably fitted into said hydraulic cylinder, the hydraulic cylinder and the plunger cooperating to define a plunger chamber, said plunger chamber being connected in communication with a valve chamber, said valve chamber comprising a pressurized oil inlet/outlet port for flow of pressurized oil therethrough, a valve seat and an outlet chamber;
• a valve unit having a valve seat formed therein and comprising a single movable valve body and a valve closing spring disposed to bias said movable single valve body toward said valve seat to form a valve thereby, said valve unit being in communication with said plunger chamber, said valve being closed under a resultant force including a spring force of said valve closing spring and a force due to oil pressure in said valve chamber, said valve unit being formed to also comprise a back pressure cylinder for receiving pressurized oil for opening said valve by biasing said valve body away from said valve seat;
• said valve body being provided with a valve stem and and end wall of said valve chamber being provided with a guide hole opposed to said valve seat so as to support and slidably guide said valve stem of said valve body and to accommodate said valve closing spring between said guide hole and said valve body;
• said back pressure cylinder being disposed at a side of the outlet chamber opposite to the valve chamber and in communication therewith, and comprising a back pressure piston connected to the valve body and slidably fitted in said back pressure cylinder to define a back pressure cylinder chamber therewith, wherein the diameter of said back pressure piston is smaller than the diameter of said valve seat;
• and
• a knockout actuator connected to said valve unit, said knockout actuator comprising a knockout piston and an air-vent piston both being coaxial and biased by a return spring, said knockout piston having a larger diameter than said air-vent piston, said air-vent piston having an elongate output portion extending from said knockout unit in an airtight manner to reach and exert a force on the valve body when said air-vent piston is moved due to a cushion actuating pressure of pressurized oil from said cushion cylinder of said press-machine, whereby a flow area available to flow of oil to or from said plunger chamber is controlled to be larger or smaller in response to said cushion actuating pressure, the output portion of said knockout actuator being opposed to said back pressure piston to be movable to push the same in a valve opening direction.