Reciprocating Cylinder with Automatic Venting at End of Cycle
A reciprocating cylinder drive is a very common hydraulic system. In systems where it is not necessary to hold pressure at the end of a cycle, it is desirable to unload the pump by automatically venting the relief valve, to save energy. Figures 28.5-28.8 show such a system. The system components are : A : Reservoir with Filter, B : Hydraulic pump, C, E : Check valve, D : Pilot operated relief valve, F : Two-position electro-hydraulic pilot operated Four-way Directional valve, G : Cam operated pilot valve, H : Double acting Single rod Cylinder, I : Limit Switch.
Consider the beginning of the machine cycle when the solenoid of the spring offset directional valve F is energized. Pump output is connected to the cap end of the cylinder. The vent line drawn from the directional valve output connected to the cap end of the cylinder is blocked at the cam-operated pilot valve G. Thus, vent port of the relief valve D is blocked, and the cylinder moves under full pump pressure applied to the cap end.
At the extreme end of the extension stroke, the limit switch is made on by the cylinder rod to break the solenoid circuit for the directional valve F. The directional valve now shifts to its right position and the pump gets connected to the rod end of the cylinder which now retracts. Note that the relief valve vent connection is still blocked.
Automatic Venting at End of Retraction Stroke
At the extreme end of the retraction stroke, the cam on the cylinder is operated by the rod to shift valve G. The relief valve vent port is thus connected, through E and G, to the line from the cap end of the cylinder, and to tank through the F and the inline check valve C. This vents the relief valve D and unloads the pump.
Push Button Start of Cycle
If another cycle of reciprocating motion is desired, a start button connected to the solenoid circuit is depressed to energize the solenoid, and, in turn, the directional valve shifts to direct pump output into the cap end of the cylinder. This causes the check valve in the vent line to close. Pressure again builds up and the cylinder starts extending. This releases the cam, which, under spring action, shifts and the vent port of E is again blocked at G. Thus the cycle repeats.
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