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Double-rod end cylinders in series
Figure 22-7 shows a fairly accurate way of synchronizing cylinders using double-rod end cylinders piped in series. Oil from the directional valve extends the first cylinder, the first cylinder's top port supplies oil to extend the second cylinder, and the second cylinder's top port connects to the other port of the directional valve. In this arrangement, oil trapped between the cylinders must have a means of replenishing or draining. As this circuit operates, cylinder seal leakage either depletes the trapped volume or adds to it. Either situation alters synchronization adversely.
Figure 22-7. Synchronizing circuit with double-rod end cylinders in series flow -- at rest with pump running.
In Figure 22-7, when 2-position, spring-centered, single-solenoid, tandem-center leveling valve D is deenergized, it allows oil to flow from cylinder (A) to cylinder (E). The valve is deenergized while the cylinders extend and retract to do work. (Figure 22-10 shows how the cylinders are leveled at the end of a cycle.)
Energizing solenoid A1 of the main directional valve, as in Figure 22-8, sends oil to cylinder (A) , causing it to extend. Oil from the opposite end of cylinder (A) flows through leveling valve (D) to the push end of cylinder (E). Oil from the opposite end of cylinder (E) flows to tank through the main directional valve. When the trapped volume is completely full and if all seals do not leak, the cylinders synchronize nearly perfectly, regardless of load position.
To retract the cylinders, energize solenoid B1 of the main directional valve as in Figure 22-9. This sends oil to the retract side of cylinder (E). Oil from the opposite end of cylinder (E) flows through leveling valve (D) to the top of cylinder (A). Oil from the opposite end of cylinder (A)flows to tank through the counterbalance valve and main directional valve.
Figure 22-10 shows how the cylinders maintain synchronization as they cycle. When the platen nears bottom, it contacts limit switches B and F. If the switches make simultaneously, no leveling occurs. If one limit switch makes before the other, the cylinders obviously are out of synchronization, so solenoid C1 on the leveling valve energizes. With solenoids B1 and C1 energized, pump oil flows to the retract sides of cylinders (A) and (E), forcing them to retract fully. Cylinders (A) and (E) can retract because the extend sides of both cylinders have a direct path to tank. When both limit switches make, the leveling valve and retract solenoids deenergize. (This leveling circuit also works for horizontally mounted cylinders.)
With series cylinder synchronizing, load placement is not important. The cylinders stay level regardless of load position or weight. The only things a heavy off-center load might cause are more seal leakage, or oil volume changes due to compressibility.
It is important to note that, because the cylinders are in series, they each have to be able to lift the total load. No matter the load placement, or the number of cylinders in series, each one must be capable of lifting the entire load. At the same time only one cylinder's volume is considered when calculating pump flow.
Other ways to use cylinders in series
To save cost, reduce potential leakage at the extra rod seals, and eliminate space needed for the second rod, use the circuit in Figure 22-11. The cylinders in this circuit oppose one another, so one extends while the other retracts. This is one way to synchronize single-rod cylinders in a series circuit. Connecting identical rod end volumes together allows series synchronization the same as double-rod end cylinders. Space for the top cylinder could be a problem on some machines so the circuit in Figure 22-12, although more expensive, works equally well. (Use the same tandem-center valve makeup circuit as seen in Chapter 21, figures 7-10 to level the cylinders after each stroke.)
Mounting is more conventional using three single-rod cylinders piped as in Figure 22-12. The only purpose of cylinder (B) is to connect equal areas. This design is still less expensive than two double-rod cylinders and it has one less leak source. This circuit requires make up valves that allow cylinder (C) to retract, cylinder (A) to retract without cavitation, and cylinder (B) to stroke if the other two do not reach home position simultaneously.
Figures 22-13 through 14 show how to attain reasonable synchronization with a set of equalizing flow controls on single-rod end cylinders in series. The cylinders are extending in Figure 22-13. Oil from the directional valve goes through needle valve (C) to the cap end of cylinder (B), thus controlling its speed. At the same time, some bleed oil from the directional valve goes through needle valve (D) to the cap end of cylinder (A). Set needle valve (D) to make up for lower oil volume as it transfers from the rod end of cylinder (B) to the cap end of cylinder (A). Without needle valve (D), cylinder (A) would lag every cycle and be out of synchronization. Changing flow at needle valve (C) means readjusting needle valve (D) also. Both needle valves work best if they are pressure compensated. This is a problem in this circuit because there is bi-directional flow. Refer to Chapter 10, Figure 10-4 to see a pressure-compensated needle valve piped for bi-directional flow.
To retract the cylinders, the directional valve shifts as in Figure 22-14, porting oil to the rod end of cylinder (A). As cylinder (A) retracts, oil from its cap end transfers to the rod end of cylinder (B). Excess oil volume from cylinder (A) goes directly to tank through needle valve (D). Needle valve (C) controls the up and down speeds of the platen.
Each cylinder in a series circuit must be powerful enough to lift the entire load. When load position changes, it affects synchronization due to the resulting change in pressure drop across needle valve (D). An off-center load that is too heavy for one cylinder to lift still allows oil transfer through needle valve (D), throwing the platen out of synchronization. Add pilot-operated check valves (E)if the cylinders must stop in mid stroke. Without these pilot-operated checks, oil transfer through needle valve (D) allows the cylinders to drift.