A relief valve that decelerates an actuator
Figures 18-8 through 18-14 show normally closed, solenoid-operated relief valve B used to rapidly extend, then decelerate a free-falling cylinder. Deceleration takes place when the cylinder makes a limit switch that deenergizes the solenoid on relief valve B . Relief pressure should be set 150 to 200 psi higher than the pressure required to raise the cylinder. Any higher relief pressure shortens the deceleration stroke and increases shock.
Figure 18-8 shows a cylinder with its rod port piped to tank through normally closed solenoid-operated relief valve B . Prefill valve F allows the cap end of the cylinder to fill during rapid advance. (See Chapter 7 for an explanation of the prefill valve’s function.) Check valve C at the rod port keeps cylinder flow from going to tank through directional valve A.
To extend the cylinder, energize solenoid A1 on directional valve A to pass oil to the cylinder’s cap end, as in Figure 18-9. Also energize solenoid C1 on relief valve B , venting it to tank and allowing the cylinder to fall freely. As the cylinder falls, the cap end fills from the pump and from tank directly through prefill valve F.
As the cylinder extends, high flow leaving the cylinder’s rod end goes to tank. Just before the rod contacts the work, a limit switch deenergizes solenoid C1 on relief valve B , Figure 18-10. As valve B tries to close, pressure increases in the cylinder’s rod end, keeping the valve partially open. Backpressure from relief valve B quickly and smoothly slows cylinder descent. The cylinder continues to slow while the relief valve shuts. The cylinder does not completely stop because the pump forces it to extend after free fall.
After deceleration, relief valve B acts as a counterbalance valve, as in Figure 18-11, so the load cannot run away. The cylinder extends at pressing speed to the work. This part of the stroke should be as short as possible to save time. Prefill valve F closes as the cylinder decelerates and allows pressure to build in the cap end. The slowdown is smooth and controlled — without shock or bouncing. This circuit decelerates the cylinder when commanded by an electrical signal at any point in its stroke.
Figure 18-12 shows the circuit while the cylinder is pressing. When the cylinder contacts the work, energize solenoid C1 on relief valve B again. Energizing the solenoid on the relief valve lets oil from the cylinder’s rod end flow to tank at minimal pressure. This allows the weight of the platen and tooling to add to the pressing force because they are no longer counterbalanced. Pressure increases in the cylinder’s cap end to perform the work.
Deenergizing solenoid A1 on directional valve A lets it center and decompress the cylinder, Figure 18-13 shows directional valve A centered, blocking the cylinder’s cap-end port and unloading the pump. At the same time, a signal to single-solenoid valve E in the cap-end line shifts it open. Trapped pressurized oil in the cylinder’s cap end flows to tank through an orifice, thus lowering pressure without shock. Pressure switch D indicates when pressure is low enough to shift valve A to retract the cylinder. (See Chapter 7 for an explanation of a decompression circuit. A decompression circuit keeps the cylinder from rapidly losing pressure and shocking the system.)
To retract the cylinder, energize solenoid B1 on directional valve A to send oil to the cylinder’s rod end, as in Figure 18-14. Oil from the pump starts to retract the cylinder. Pilot oil opens prefill valve F to tank. Oil from the cylinder’s cap end flows to tank through the prefill valve and the main directional control valve. The cylinder retracts rapidly at low pressure.