What is in this article?:
- BOOK 2, CHAPTER 23: Three fluid power circuits
- High-efficiency circuit operates increment feeder for cardboard sheets
- Opposing trim cylinders with synchronous movement
High-efficiency circuit operates 100-ton trim press
Figure 23-1 is a schematic circuit diagram for a 100-ton trim press, shown at rest with the pump running. This press has a 24-in. total stroke with a 1.0-in. high-tonnage stroke. It completes a dry cycle in about 10 seconds.
Pump PUMP01 is rated at 13 gpm. Its compensator is set at approximately 3000 psi. Output from the pump flows through flow meter FM01 to directional valve DV01, which has ports "P," "B," and "T" connected in center position. From there, fluid flows to backpressure check valve CK02 and 3-micron filter RF01. Pressure at gauge PG01 holds between 75 and 100 psi with the press at rest, providing ample pilot pressure for directional valve DV01.
Port "A" on directional valve DV01 connects to the cap ends of two 3.25-in. bore double-acting cylinders CYL01 and the extend side of double-rod end cylinder CYL02. The rod sides of these cylinders connect to port "B" on the directional valve.
Directional valve DV02 supplies pilot pressure to sequence valve SEQ01 and two pilot-operated check valves POCK01 for cylinder regeneration during fast advance. Directional valve DV02 also decompresses the cylinders before retracting.
Externally piloted sequence valve SEQ01 connects the cap and rod ports of the 3.25-in. bore cylinders in a regeneration circuit for a fast advance stroke.
Check valves POCK01 allow oil to flow from one end of the double-rod end cylinder to the other during fast advance and retract. Shuttle valve SV01 directs pilot pressure to the pilot-operated check valves from two sources.
Counterbalance valve CB01 holds the platen in position at rest, and keeps it from running away during the high-tonnage advance stroke. (Because it has an external pilot, set it one time for all platen and tooling weights.) External piloting also eliminates all cylinder rod-end backpressure during the high-tonnage portion of the cycle.
Check valve CK01 allows oil out of double-rod end cylinder CYL02 during the high-tonnage stroke, but prevents flow from entering it as the platen retracts.
Pressure switch PS01 ends the pressing cycle at a set tonnage. After the pressing cycle is completed, pressure switch PS02 keeps directional valve DV01 from shifting until the cylinders decompress.
Platen extending rapidly — Energizing solenoid A1 on directional valve DV01 and solenoid A2 on directional valve DV02 moves the cylinders to the positions shown in Figure 23-2.
Pump flow goes to the cap ends of cylinders CYL01 and they begin to extend. At the same time, pilot pressure opens sequence valve SEQ01 and pilot-operated check valves POCK01. Oil now regenerates from the rod ends of the 3.25-in.-diameter cylinders through SEQ01 to their cap ends for high speed at low volume. Oil in the bottom of CYL02 transfers to the top, keeping it full, ready for high-tonnage pressing.
The pressure setting of counterbalance valve CB01 is high enough to make sure oil regenerates during this portion of the cycle, but low enough not to waste energy after regeneration is disabled. (About 400 psi works well on presses in this type of service.)
The platen extends at approximately 7.5 ips for about 3.1 sec. to produce 23 in. of travel.
Platen at full tonnage — A limit switch or encoder indicates that the platen is in close proximity to the part and the circuit shifts as shown in Figure 23-3.
Solenoid A1 on directional valve DV01 stays energized, but solenoid A2 on DV02 deenergizes.
Now sequence valve SEQ01 and pilot-operated check valves POCK01 close. The 3.25-in.-diameter cylinders no longer regenerate, so forward speed slows.
The double-rod end cylinder can still transfer fluid through the pilot-operated check valves so the platen does not slow to pressing speed until it contacts the work. Counterbalance valve CB01 opens during the slow-down part of the cycle, letting fluid go to tank, but not allowing the platen to run away.
At contact with the work, pilot-operated check valves POCK01 close, so pump flow goes to all three cylinders. Forward speed slows to approximately 0.6 ips, so it takes about 1.75 sec. to travel 1.0 in. Force during this portion of the stroke is more than 237,000 lb at 3000 psi.
Externally piloted counterbalance valve CB01 opens fully during the trimming part of the cycle to eliminate all backpressure on the cylinders.
During part trim, oil from the bottom of the double-rod end cylinder flows to tank through check valve CK01 and counterbalance valve CB01 along with fluid from the other two cylinders.
When pressure reaches the setting on pressure switch PS01, directional valve DV01 deenergizes to unload the pump and set up for decompression.
Cylinders decompressing — At the end of the high-pressure trim cycle, there may be a lot of stored energy in the cylinders and piping. (Also, if the tie rods on the press stretch, more energy is stored.) If directional valve DV01 shifts immediately to retract the cylinders, stored energy would send oil to tank with enough velocity and force to damage pipes and valves. In a short time, shock damage to plumbing and components could make the machine inefficient and messy.
To avoid this, after trimming the part, the circuit changes as shown in Figure 23-4. Directional valve DV01 centers to unload the pump and block oil in the cylinders' cap ends. Energizing solenoid B2 on directional valve DV02 opens a controlled flow path from the cap end of the cylinders to tank. Stored energy dissipates rapidly through this path to tank without decompression shock.
This adds time to the overall cycle, but greatly increases machine life and reliability. (Always figure in the added time of decompression when designing a circuit with stored energy.) Decompression can take from 0.25 to 2.00 sec., depending on fluid volume and pressure. Offset the added cycle time with a faster cylinder stroke.
When pressure in the cap-end line of the cylinders reaches approximately 300 psi, pressure switch PS02 energizes solenoid B1 of directional valve DV01 to retract the platen.
Platen retracting at high speed — After the cylinders decompress, solenoid B1 on directional valve DV01 energizes as shown in Figure 23-5. Pump flow goes to the rod ends of 3.25-in.-diameter cylinders CYL01 through the bypass check valve on counterbalance valve CB01 to retract the platen.
Check valve CK01 blocks pump flow from the bottom of double-rod end cylinder CYL02 during the retract cycle.
Pilot oil from the cylinder rod end line opens pilot-operated check valves POCK01 through shuttle valve SV01. This action opens a path for oil transfer from top to bottom of the double-rod end cylinder.
The platen retracts at a rate of approximately 4.75 ips, and travels 24 in. in about 5 sec. Oil in the double-rod end cylinder transfers from top to bottom through pilot-operated check valves POCK01 to keep it full for the next cycle.
It appears the double-rod end cylinder keeps the same fluid in it all the time -- and might overheat. In actual operation, this cylinder gets some fresh oil from the pump and sends an equal amount to tank during each cycle. If the high-force stroke of the cycle is 0.5 in., cylinder volume is replaced completely every 48 cycles.
The reason for a circuit design such as shown here is to reduce pump displacement as well as valve and piping size while maintaining a fast cycle. A conventional cylinder regeneration and hi-lo pump circuit requires approximately 70 gpm to meet the cycle time of this circuit.
Another fast-cycle option is a prefill valve and a cylinder with a 9.0-in. oversize rod. This circuit could meet or exceed the above cycle time. A prefill circuit lets the platen lower from its own weight, so the extend part of the stroke could be faster.
This schematic diagram uses several of the basic circuits shown in other parts of this manual. Refer to the appropriate sections for a refresher on different valve functions and use.