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
Opposing trim cylinders with synchronous movement
Two cutter heads, driven by opposing cylinders, must meet near perfectly in center regardless of differing loads. If the cylinders are out of phase, parts can be damaged and the profiled cutters may break.
An original circuit used two pumps and directional valves to synchronize the cylinders. The machine was useable but parts often were under par and cutting tools had to be changed frequently.
The schematic diagram in Figure 23-10 shows the new circuit now on the machine. Now the cutters move almost perfectly in unison, and produce a better part with no tooling damage.
A single 30-gpm pump replaced two 21-gpm pumps with a cycle time decrease of 1.5 seconds.
Clamp cylinder (C) has an oversize rod and uses a regeneration circuit for fast, low-volume extension. Counterbalance valve (E) keeps the vertical clamp cylinder from running away. Sequence valve (J) assures that the part stays tightly clamped as trim cylinders (G) and (H) advance to the work.
Both directional valves (A) and (B) have pump-to-tank center conditions, so the pump unloads until they shift.
Check valve (N) prevents backflow to the clamp cylinder's rod end while the trim cylinders work.
Check valve (M) provides backpressure to supply pilot oil to both solenoid pilot-operated directional valves and makeup fluid to the tandem cylinders.
Two check valves (L) allow fluid into the double-rod end cylinder's trapped area to make up for leakage. Pressure here also keeps the seals energized to reduce leaks.
Normally closed 2-way directional valve (K) re-synchronizes the trim cylinders if they get out of phase. It operates automatically any time limit switches (F) do not make simultaneously.
Clamp cylinder (C) extending in regeneration — Energizing solenoid A1 on directional valve (A) starts clamp cylinder (C) forward, as seen in Figure 23-11. Oil from the rod end of cylinder (C) regenerates to the cap end and almost doubles its speed. (Because the cylinder needs minimum force to move to the part, a regeneration circuit such as this works well and uses a smaller pump.)
Set counterbalance valve (E) high enough to force rod end oil to regenerate through sequence valve (0) until the cylinder meets the work.
Clamp cylinder (C) advances at fast speed until it contacts the work. At work contact, pressure increases and opens counterbalance valve (E). This action drops backpressure on the rod end of cylinder (C), giving full force to clamp the part. When valve (E) opens, sequence valve (O) closes, shutting off the regeneration path.
Pressure must climb to 750 psi before oil can pass sequence valve (J) and flow on to trim cylinder valve (B). Sequence valve (J) makes sure the part has ample clamp force while the trim cylinders advance to the work. Pressure in clamp cylinder (C) never drops below 750 psi if sequence valve (J) is set correctly.
Cylinder (C) extended at full force, cylinders (G) and (H) extending in synchronization — After clamping the part, energize solenoid A2 on directional valve (B) as shown in Figure 23-12. Sequence valve (J) maintains 750 psi at the clamp cylinder but allows pump flow to go to the trim cylinders.
Trim cylinders (G) and (H) now advance in synchronization. They stroke together regardless of load difference until both cylinders stall at relief pressure. The reason trim cylinders (G) and (H) stay together is that oil from each of the double-rod end cylinders transfers to its opposing cylinder's opposite end. If one cylinder moves an inch, it is only because the opposing cylinder also moved that distance. If one cylinder needs to develop more force, energy from the opposing cylinder transfers to it. Neither cylinder stalls until resistance against it is greater than both cylinders can overcome.
In case of leakage from the double-rod end cylinders, backpressure check valve (M) forces pump fluid into the trapped area through check valves (L).
Except for piston or rod seal bypass, the trim cylinders always stay synchronized. In case they do get out of phase, normally closed 2-way directional valve (K) allows them to re-synchronize.
All cylinders retracting — After trimming the part, the valves shift to the positions shown in the schematic diagram in Figure 23-13. Energizing solenoids B1 and B2 on directional valves (A) and (B) directs pump flow to the rod end of cylinder (C). Pump flow goes around the bypass check in counterbalance valve (E) to the cylinder's rod end. Sequence valve (O) is held closed by its light spring and pressure from retracting cylinder (C).
Oil from the cap end of cylinder (C) goes through directional valve (A), check valve (N), and directional valve (B) to the rod ends of cylinders (G) and (H). The pump only has to retract cylinder (C). Return flow from cylinder (C) retracts the trim cylinders. This is possible because cylinder (C) has more than enough oil to fill the rod ends of the trim cylinders at the low force required to retract them.
All three cylinders continue retracting quickly because cylinder (C) has an oversize rod. When trim cylinders (G) and (H) bottom out, solenoid B2 on directional valve (B) deenergizes so clamp cylinder (C) can complete its retraction stroke.
When cylinder (C) bottoms out, the limit switch indicates the end of the cycle and deenergizes solenoid B1 on directional valve (A). Now the pump unloads because the circuit is back to the "At rest with pump running" condition.
Cylinders (G) and (H) stay synchronized because they must both make their limit switches (F) before directional valve (B) centers.
Cylinders (G) and (H) re-synchronizing — During each cycle, trim cylinders (G) and (H) return to make limit switches (F). If one limit switch makes first, that indicates the cylinders are out of synchronization. The schematic diagram in Figure 23-14 shows how the cylinders re-synchronize if this happens.
In the diagram, trim cylinder (H) is late making its limit switch. When this condition occurs, the control circuit automatically energizes solenoid C1 on normally closed directional valve (K). Because all cylinders are in the retract mode, pressure on the rod end of cylinder (H) keeps it retracting. It can move by itself because open directional valve (K) provides a flow path from back to front of the double-rod end cylinders during this part of the cycle.
When both limit switches (F) make, solenoid C1 on directional valve (K) deenergizes. Solenoid B2 on directional valve (B) deenergizes, letting it center, and allowing clamp cylinder (C) to keep retracting.
Because re-synchronizing is automatic, there is never a build up of stroke error, which could allow damage to cutters or parts.