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One hydraulic design method that can minimize valve size loss is incorporating reverse-phase hydraulic cylinders into the application. This approach is most suitable under the following conditions:
1. Matching pressure and flow requirements for a “high-low” operation. Here, the hydraulic cylinder movement involves a low-pressure, high-flow stage; the second, a high-pressure, low-flow stage. A typical example is a compaction cylinder on a garbage truck. As the cylinder extends, pressure slowly increases due to the elastic properties of the material. Pressure increases rapidly at the end of the stroke. The cylinder then quickly retracts under negligible load.
2. High-volume production environments where many identical hydraulic presses operate in parallel. Typical industry examples are plastic extrusion or recycling centers.
3. Hydraulic cylinders with oversized rods. This is already necessary for many long-stroke equipment used for column strength support.
This method involves connecting a second hydraulic cylinder/press in opposite phase to the first press. This approach is more precise than the shunt method, in which a “step-down” transformer uses the energy of the high-pressure flow to generate a low-pressure flow at a higher flow rate. The two cylinders can be identical, so the flow rates in and out are the same. The principle of operation is shown below. The assumed pressure provides context.
Reverse Hydraulic Cylinder Operation
When rapid motion is required, the pump is best connected to the rod ends of both cylinders. The bore ends will freely exchange flow. When high pressure is required, the pump is best connected to the bore end, while the rod ends will freely exchange flow. To fully utilize this concept, oversized cylinder piston rods should be used. The area ratio of each cylinder will provide flow amplification for rapid motion.
Note that, although not explored further, the closed-circuit concept shown above could be implemented with a single 6-way valve to “switch” the closed-circuit pump between the two positions. Another closed-circuit design might involve separate pump + motor units for the two positions. A single frequency converter would alternate between the two. Finally, there would be a large 2-way valve between the rodless chambers of the hydraulic cylinder. This valve would open to facilitate rapid movement of the hydraulic cylinder.
Clearly, in the ideal case described above, freely connecting and disconnecting the pump is not possible. Valve connections are the solution. For the open-circuit case shown in the figure below, two 4-way valves are used.
Hydraulic Press Concept
Rapid motion is achieved by energizing the rod-side four-way valve and the port-side two-way valve. High-pressure operation occurs by energizing only the port-side four-way valve. After each work cycle, the hydraulic cylinders must be “re-phased” to ensure that each movement is completed. This “re-phasing” is accomplished by fully retracting one hydraulic cylinder while fully extending the other. Specifically, the opposing coils of the two four-way directional valves are energized while the two-way valve remains closed.
This design introduces several issues, which are addressed in the more detailed circuit shown in the figure, “Hydraulic Press Using Opposing Cylinders.” Specifically: a. Pressure buildup is always a potential hazard with oversized rod cylinders. A safety valve is required under the “A/B Rapid Stroke” valve. For maintenance reasons, it is helpful to be able to move either cylinder independently. In the above circuit, either cylinder can be extended independently by energizing the symmetrical coils of the two four-way valves. The selected cylinder will then experience full pump pressure on both sides. It will then move due to the differential area. However, it is not possible to retract either cylinder independently. This would require additional port-side dump valves, not shown in the figure below. The “Rapid Motion Enable” two-way valve must be oversized to minimize throttling losses. This is particularly important because the oil between the two cylinders has minimal cooling capacity. Specifically, a small amount of cold oil is introduced each time the high-pressure working valve is energized.
Hydraulic Press Using a Reversing Cylinder
Note that the heat exchanger is shown receiving flow from a fast-moving 4-way valve. This is a simplification for convenience. Other return lines can also feed the heat exchanger:
Several advantages of this design include: a. Minimum flow through the 4-way reversing valve enables high-low operation. Maximum flow only passes through the “fast-movement-enabling” 2-way valve, which can be appropriately oversized. A single hydraulic unit can function as two hydraulic presses. From an economical perspective, large-scale production often involves running multiple presses in parallel to achieve output. This design can reduce the total number of hydraulic units by half. The unit’s physical footprint is smaller than that of a traditional high-low design. The “fast-movement-enabling” valve acts as a pressure reducing valve. When switching from extending to retracting, energizing this valve reduces the pressure of cylinder A to that of cylinder B, and vice versa. If this pressure reduction is too abrupt, a smaller 2-way valve can be added in parallel with the existing 2-way valve.
Several disadvantages include: a. The initial cost of a hydraulic cylinder with an oversized rod is higher. However, due to column strength requirements, long-stroke hydraulic cylinder applications already require oversized rods. The “quick motion enable” two-way valve must be oversized to avoid throttling losses at high flows. Hydraulic maintenance would require taking two presses offline instead of one.
If independent retraction is required for maintenance, an additional three-way valve is required. This valve vents the bore side of either cylinder to tank. Simultaneously, the “quick extend” four-way valve pressurizes the rod side of the same cylinder to retract it. This process is repeated to retract the other cylinder.
Depending on the duty cycle frequency, an accumulator can be installed at the pump outlet to reduce the pump/motor size.
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