WORKING OF HYDRAULC PRESS

Typical decompression circuit

Typical vertical ram circuit

The circuit in Figure operates a vertical single-acting hydraulic ram press with pullback cylinders for the retraction stroke. The press has a poppet-type pre-fill and gets a fast stroke from only filling the During the approach part of the stroke, atmospheric pressure pushes fluid into the large-bore ram through the pre-fill valve because there is vacuum behind the extending ram. When it contacts the work, the ram stops and the pre-fill valve closes. Pressure starts to rise and when it is high enough to open the sequence valve, pump flow goes to the pullback cylinders and the ram. Extension speed slows and tonnage increases to do the work required.
A signal that the work is complete shifts the directional control valve to send pump flow to the rod ends of the pullback cylinders and to the pilot signal of the pre-fill valve. The pre-fill valve’s pilot piston moves forward and contacts the decompression poppet. This lets trapped fluid flow out at a controlled rate.Pressure in the ram drops quickly and smoothly. When pressure is low enough, the pilot piston opens the main poppet to let fluid from the ram return to tank. When the ram loses pressure, the pullback cylinders can raise the platen and push fluid from the ram back to tank.see also

hydraulic-directional-control-valve
hydraulic-press-pre-fill-valves
hydraulics