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When the electro-hydraulic directional valve 6 is switched to the left position, the pressure in the upper chamber of cylinder 15 has not yet been released, and the pressure is very high. The upper position of directional valve 10 is open, and the oil from main pump 1 returns to the oil tank through valve 6 left position and valve 11. At this time, the main pump 1 is running under low pressure, which is not enough to open the main valve core of the hydraulic control check valve 14. However, it can open the unloading small valve core in 14. The high-pressure oil in the upper chamber of the main cylinder is discharged back to the charging tank 13 through the opening of this unloading small valve core, and the pressure gradually decreases. This process continues until the pressure in the upper chamber of the main cylinder drops to a lower value. The directional valve 10 closes, and the oil supply pressure of pump 1 increases, pushing open the main valve core of the hydraulic control check valve 14. At this time, the pressure oil from pump 1 enters the lower chamber of the master cylinder through valve 6 on the left and hydraulic control check valve 7; And the oil in the upper chamber of the master cylinder is drawn through valve 14 and filled in tank 13 to achieve rapid return of the master cylinder.
When the hydraulic press is working, a direct acting relief valve is added at the pump outlet to prevent the machine from being stuck by debris or workpieces. This is because the output pressure oil increases with the working time of the pump, and the hydraulic oil cannot reach the hydraulic cylinder, which protects the safety of the hydraulic pump and hydraulic components. It acts as a safety valve. When the pump pressure reaches the conduction pressure of the relief valve, the relief valve opens and the hydraulic oil flows back to the oil tank, which can protect safety. In hydraulic systems, the relief valve is placed near the hydraulic pump to increase the stability of the hydraulic system and improve the machining accuracy of parts.