Presses are used for molding, shaping, shearing, and many other operations. Some of the older manufacturing plants have lines of presses connected in parallel as shown in Fig. 7.9.
To give perspective to the problem, let us assume that the press cylinder has a 30-in. bore and 10-in. stroke. It needs to close in 30 s to achieve the desired cycle time.
The flow rate required to close an individual press is 61 GPM; thus, a highcapacity line is required. Presses along the line are closing under the control of individual operators, meaning that several presses can be closing simultaneously. (Some of the older press lines have 10, 20, or maybe more presses on a single circuit.) The reader can readily visualize that the flow dynamics in the main supply and return lines are hopelessly complex. Press operation is erratic; sometimes a press closes in 30 s, and sometimes it takes 60 s. When two or more presses are closing simultaneously, flow takes the path of least resistance, so it goes to the press with the smallest pressure drop first. After this press is closed, the flow completes the closing of the other presses.
Another significant disadvantage of the parallel circuit design is the volume of fluid that must be moved from the reservoir to the individual presses. This flow requires a high pump capacity and thus a high energy input.
A design that avoids the pumping of fluid back and forth from the reservoir is shown in Fig. 7.10. A manual DCV is shown, but typically the press is controlled by a solenoid-actuated DCV.
The main press cylinder is the large center cylinder in Fig. 7.10. The two other cylinders will be referred to as side cylinders. They are sometimes referred to as kicker cylinders. Their primary function is to raise and lower the platen. The main press cylinder supplies most of the force needed once the platen contacts the work piece.
The circuit in Fig. 7.10 works as follows. When the operator shifts the DCV, flow goes to the two side cylinders, and they extend. Flow does not go to the press cylinder, because the sequence valve remains closed. As the press cylinder descends, the resulting negative pressure in the cap end pulls fluid from the reservoir into the press cylinder. (Often, the reservoir is above the press so that gravity helps to fill the large cylinder.) When the platen contacts the workpiece, the pressure builds, and the sequence valve opens. Now system pressure is applied to the press cylinder, and full force (side cylinders + press cylinder) is applied to the work piece.
When the DCV is shifted for retraction, the line to the sequence valve is connected to the reservoir. There is no pressure to hold the sequence valve open; consequently, it closes. Flow from the press cylinder cannot go back through the sequence valve; it must go through the check valve into the reservoir. The key to operation of this circuit is the pilot-operated check valve.