What is in this article?:
- BOOK 2, CHAPTER 22: Synchronizing cylinder movement
- Double-rod end cylinders in series
- Double-pump-and-valve synchronizing circuit
- Spool-type flow-divider synchronizing circuit
- Tandem-cylinder synchronizing circuit
Spool-type flow-divider synchronizing circuit
Spool-type flow dividers split flow from a single conductor into two separate flows. The split flows may be at different rates if needed, but for cylinder synchronization, they usually are equal. Spool-type flow dividers basically consist of two pressure- compensated flow controls in one body. In this arrangement, each flow control's pressure drop modifies the opposite flow output. Because these flow controls constantly look at each other's pressure drop, they split flow relatively well. (Most manufacturers claim about ±5%, depending on the pressure differential at the outlets.)
One problem with spool-type flow dividers is that they do not allow reverse flow. Even if they did, there would be no guarantee of equal flow. A spool-type flow divider/combiner allows forward and reverse flow, and equally splits or combines the two flows. Normally a flow divider/combiner is the component of choice in cylinder synchronizing circuits. Figure 22-21 shows a spool-type flow divider/combiner synchronizing circuit. It is similar to a double-pump circuit, but only uses one pump and valve. Flow is split downstream from the single directional valve.
In Figure 22-22, the cylinders are extending. Shifting solenoid A1 on the directional valve sends oil to the flow divider, which sends half pump flow to each cylinder. Even when there is a pressure difference at the cylinders, flows are close to equal. The cylinders extend at about the same rate even with an off-center load. Each cylinder must develop enough force to lift the load above it. If one cylinder reaches its force limit and stops, the opposite cylinder tries but does not completely stop -- due to internal leakage past the flow divider spool. (Figure 22-24 shows the condition of the flow divider when cylinder (B) stalls as it retracts.)
Figure 22-23 shows the circuit after energizing solenoid B1 on the 4-way directional valve. Oil flows to the cylinder rod ends while fluid from the cylinder cap ends combines equally at the flow divider and flows on to tank. The flow divider holds back the cylinder that wants to get ahead -- thus maintaining synchronization. When the cylinders reach bottom, they re-phase automatically if the directional valve is left in the down mode long enough. Internal leakage in the flow divider spool allows the lagging cylinder to continue stroking. (Some flow-divider brands have integral bypasses that operate when the pressure differential reaches a pre-set limit.)
Because the flow divider has a common path internally, fluid can flow between the cap end ports. If the cylinders need to stop in mid-stroke, always use pilot-operated check valves (C) to prevent oil transfer. Control an overrunning load with counterbalance valve (E) between the flow divider and directional valve.
Spool-type flow dividers waste energy. Notice the gauge reading at each cylinder as it extends, PG2 shows 800 psi while PG3 reads 300 psi. In this situation, gauge PG1 at the pump reads 800 psi. The 500-psi drop across the right side of the flow divider generates heat when the cylinders extend.
Spool type flow dividers only split flow into two outputs. It would take three spool-type flow dividers to split flow four ways.
Motor-type flow divider synchronizing circuit
Motor-type flow dividers do not waste energy and are more versatile. One motor-type flow divider can splits flow from a pump and run two or more cylinders in unison. Plus, they offer multiple outlets -- up to ten or more -- and can pass unequal flows when required.
A motor-type flow divider consists of two or more hydraulic motors in one housing. The motors have a common shaft. Thus, when one motor turns, all motors turn. The motors share a common inlet but have separate outlets. Fluid from the pump enters all motors at once, rotating then in unison. If the motors are the same size, output from each section is an equal portion of inlet oil. Because a mechanical motor -- instead of an orifice -- splits flow, there is no energy loss due to different outlet pressures. Figure 22-25 shows a motor-type flow divider synchronizing two cylinders. The flow divider is installed between the directional valve and the cylinders in this circuit.
In Figure 22-26, solenoid A1 is energized to shift the 4-way directional valve. This sends oil to the flow divider, which sends equal volumes to each cylinder. The accuracy of motor-type flow dividers depends on the amount of pressure difference at the outlets. The motors have internal slippage that increases as pressure drop increases. The greater the pressure difference, the greater the flow difference and loss of synchronization.
In Figure 22-27, the cylinders are retracting. Energizing solenoid B1 on the directional valve sends oil from the pump to the cylinders' rod ends. As the cylinders retract, oil flows from the cylinders' cap ends through the flow divider to tank. The flow divider combines the cylinder flows and maintains synchronization when the cylinders travel freely.
If one cylinder binds up and stops traveling, as in Figure 22-28, all oil from the pump goes to the free-moving cylinder. The flow divider section that is not getting oil from the stopped cylinder continues to turn and cavitate, causing the free cylinder to retract at twice speed. When there is a chance of cylinder binding, install a motor-type flow divider at both ends of the cylinders. A flow divider on the rod end forces the binding cylinder to synchronize or stalls them both.
The internal slip of motor-type flow dividers is usually sufficient to level the cylinders. Another option is integral relief valves that allow fluid to bypass a motor at a predetermined adjustable pressure.
As mentioned, an advantage of motor-type flow dividers is that they waste little energy. Notice the gauge values in Figure 22-26. The left cylinder requires 900 psi, while the right cylinder only needs 300 psi. With those conditions, the inlet pressure to a spool-type flow divider has to be 900 psi. With a motor-type flow divider, the inlet pressure only has to be 600 psi. Because the motor-type flow divider has a mechanical link through a common shaft, energy transfer between sections lowers the required pressure at the inlet.
Another advantage is that motor-type flow dividers with two, three, even ten or more outlets are common. Instead of stacking 2-outlet spool-type dividers, use only one multiple-outlet motor-type flow divider for many circuits.
One caution: motor-type flow dividers will intensify outlet pressure as they operate. (See Chapter 11 for an explanation of motor-type flow divider intensification.) With a 2-outlet equal-flow divider, if relief valve pressure is over half the maximum rated pressure of any component it feeds, install a relief valve at each outlet. The outlet relief valves protect the cylinders, valves, and lines from excess pressure.
Master-and-slave cylinder synchronizing circuit
Figures 22-29 through 32 show one of the most accurate ways to hydraulically synchronize cylinders. Figure 22-29 shows the circuit at rest. Cylinder (C) -- mechanically linked to two cylinders (D) -- provides the driving force. The (D) cylinders have the same bore, stroke, and rod diameter as working cylinders (A) and (B). One cylinder (D) connects to cylinder (A), while the other cylinder (D) connects to cylinder (B). In case of external leakage, makeup check valves (H) let oil into the dead areas of cylinders (A), (B), and(D) at low pressure. A 75-psi backpressure check valve in the tank line gives sufficient pressure to make sure the trapped oil volume stays full. Leveling valves (J) through (M) retract the cylinders to home position when they get out of phase. Limit switches (F) and (G) indicate cylinder home positions and operate the leveling valves when the cylinders get out of synchronization. Counterbalance valve (E) stops the cylinders from running away while they retract.
Force from cylinder (C) is enough to do the whole operation by itself. This cylinder produces all the force and passes it on to slave cylinder (D), then to the working cylinders (A) and (B).
The location of the load on the platen affects synchronization only slightly. Energy transfer from the master/slave linkage moves the same volume of oil regardless of pressure. Cylinder (A)operates at twice pressure with the load above it as with a centered load. To protect the cylinders from overpressure, set the relief valve for no more than half the cylinder pressure rating.
Figure 22-30 shows solenoid A1on the 4-way directional valve shifted to extend cylinder (C). Cylinder (C) pushes cylinders (D), and oil from the cap ends of cylinders (D)flows equally to the cap ends of cylinders (A)and (B). Oil from the rod ends of cylinders (A)and (B)returns to the rod sides of cylinders (D). Cylinders (A)and (B) extend in unison if cylinder (C) has enough power to do the job. If one working cylinder stalls, both stop.
To retract the working cylinders, energize solenoid B1 on the 4-way directional valve as in Figure 22-31. Cylinder (C) then retracts and pulls both slave cylinders (D) back, forcing working cylinders (A) and(B) to retract also.
If the working cylinders get out of synchronization, the circuit diagram in Figure 22-32 shows how they level. While solenoid B1 on the 4-way directional valve stays shifted, energize solenoids A2 through A5 on directional valves (J) through (M). This directs pump oil to the rod sides of cylinders (A), (B) and(C), and to the cap sides of both (D) cylinders. At the same time, oil from the cap sides of cylinders (A), (B) and(C) and the rod sides of both cylinders (D) flows to tank. In this condition, the pump forces all cylinders to their home positions, ready for the next cycle.
This circuit is an accurate but expensive way to synchronize cylinders. One advantage is that the master and slave cylinder can be located remotely, to leave the work area less cluttered. Also, energy transfer minimizes the required cylinder size and still handles off-center loads.