[170] Exercise: Hydraulic crane (speed reduction)
Press tools of varying
weights are inserted into a press by means of a hydraulic crane.
A double acting cylinder raises and lowers the load. During the
commissioning of the hydraulic crane, it has become apparent that
the advance speed of the piston rod is too high. The solutions
have been proposed to reduce this speed; a circuit with exhaust
flow control and a circuit with a back-pressure valve. A suitable
solution must be selected and reasons stated for this choice.
Since the second solution is not capable of operating in this
form, this must be rectified by correcting the circuit
diagram.
If the solution with exhaust flow control is selected, it must be borne in mind that the cylinder, flow control valve and connectors must be suitable for this intensified pressure. The solution chosen is the circuit with the back-pressure valve; in this case, the load is clamped hydraulically and a pressure-intensification effect does not occur, since the pressure can be adjustable by means of the pressure relief valve as appropriate to the load. A non-return valve must be installed to provide a by-pass on the return stroke.
Inlet flow control cannot be used to control a tractive load; the load forces oil out of the piston-rod chamber faster than oil can flow into the piston chamber. Vacuum is created and air will escape.