Hydraulic Presses Machines

Presses action are typically used for molding, shaping, shearing, and many other operations.

parallel hydraulic presses

Parallel hydraulic presses

To understand the hydraulic pressing calculation, let us set an example of a 30-in. bore and 10-in. stroke hydraulic press cylinder. The system needs to complete the operation in 30 s.

The flow rate required to complete an individual press is 61 GPM which requires a high capacity line. Presses along the line are closing under the control of individual operators, meaning that several presses can be closing simultaneously. 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 disadvantage of the parallel circuit design is the usage of high pump capacity to move the fluid from the reservoir to the individual presses.

large press hydraulic machine

Large press hydraulic machine

The figure above shows a press hydraulic circuit which avoids the pumping of fluid back and forth from the reservoir.

The hydraulic press circuit consists of three hydraulic cylinders; the main cylinder is the larger cylinder located in the middle while the two other cylinders are referred to be side cylinders or kicker cylinders. The primary function of the side cylinder 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 above 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.

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