The pressure holding time of the four column hydraulic press is controlled by the time relay in the electrical control circuit and can be adjusted within 0min-24min. During pressure maintaining, there is no oil activity in the system except that the hydraulic pump is unloaded at a lower pressure. Hydraulic pump - sequence valve - Commercial cylinder reversing valve - lower hydraulic cylinder reversing valve - oil tank quick return time relay when the time delay expires, the pressure holding accommodation, the solenoid is energized, the right position of the pilot valve is involved in the system, and the pressure relief valve enables the reversing valve of the upper hydraulic cylinder to be connected to the system in the right position. At this time, the hydraulic control check valve is opened and the upper hydraulic cylinder returns quickly. When the upper slider rises to the stop and hits the home travel switch, the electromagnet is powered off, and the pilot valve and the directional valve of the upper hydraulic cylinder are in the middle position. At this time, the upper hydraulic cylinder stops. The hydraulic pump is unloaded at a lower pressure.  
 
When the sliding block of the four column hydraulic press moves down to a certain direction and depresses the travel switch SQ2, 2 sends out the power signal, ya6 is powered off, and the pump oil enters the upper chamber of the master cylinder through the pzbz channel at the left position of valve 8. The charging valve closes automatically under the effect of two layers of pressure oil (also greater than one atmospheric pressure) and tension spring in the charging valve. At this moment, only the hydraulic pump supplies a certain flow of pressure oil to the upper chamber of the master cylinder, In addition, the upper cavity area of the master cylinder is large, so the slider moves down slowly and pressurizes. After touching the workpiece, the pressure rises. Pressure holding delay. When the pressure in the system rises to the air effect of the pressure relay, the electrical signal is announced, the electromagnet controlled by the four column hydraulic press is powered off, and the pilot valve and the upper cylinder directional valve are in the middle position. At this moment, the system enters pressure maintaining.