Proportional valves in resistive-load circuits
The circuit in Figure 14-16 arranges a proportional valve and a pressure-compensated pump to cover all the situations in the previous section. Acceleration and deceleration are fully adjustable through a broad range with this circuit. When the load, speed, or pressure changes, it is easy to change the control parameters to match the new situation. Normally an electronic dashpot changes shifting speed of the spool between zero and five seconds. To make up for changes in fluid viscosity, pressure, or load, decelerate to a minimum creep speed and finally close the valve completely via the end-of-stroke limit switch.
Proportional valves for running-way loads
Loaded cylinders that are vertically mounted usually run away or over-run pump flow in one direction. When an on/off directional valve shifts, the cylinder free falls. Free fall is a safety hazard that can cause tool or machine damage.
The counterbalance valve in Figure 14-17 controls an over-running cylinder. The valve allows flow from the run-away end of the cylinder as fast as the pump supplies the opposite end. When the cylinder strokes in the opposite direction, the load is resistive. Control acceleration and deceleration with any of the resistive-load circuits in the previous section when using a counterbalance valve.
Proportional directional valves control inlet and outlet flow so that there is pressure at both ends of an actuator when it moves. A counterbalance valve often needs an external drain when used with a proportional directional valve. Without an external drain, pressure at the outlet of the counterbalance valve adds to the spring setting that keeps the valve from opening. Notice that the circuit in Figure 14-18 shows the external drain line on the counterbalance valve. With this circuit the cylinder stops smoothly when the proportional directional valve centers rapidly, as in an emergency stop.
Proportional directional valves control running-away loads because most spool designs control flow to and from the actuator. If the actuator is a hydraulic motor or a double rod-end cylinder, volume at the inlet and outlet is the same. As the proportional valve shifts to move the actuator, restricted flow from the opposite side controls acceleration, deceleration, and maximum speed.
However, the majority of cylinders have a single rod, making the volume leaving the rod end less than what enters the cap end. The volume difference is almost 50% when using a 2:1 rod cylinder. In these cylinders, the rod area equals half the piston area. (Some manufacturers offer proportional valves with spools that only allow approximately half flow through the rod port. These valves work well with a 2:1 rod cylinder.)
Two problems can occur when using a standard spool-type proportional valve with single-rod-end cylinders and running-away loads. Figure 14-19 shows the cylinder running away from the pump, causing cavitation in the cylinder’s cap end. The cylinder runs away because the proportional directional valve’s meter-out function lets out more oil than it allows in at the cap end. Because the cap end does not stay full, it will pause when it meets a load while the pump fills the cap-end void. When a cylinder runs ahead of the pump, use an anti-cavitation check valve to allow fluid from the tank into the cylinder’s cap end. This circuit works for applications with the over-running load at the cylinder’s rod end.
With an over-running load at the cap end of the cylinder, the pump tries to force the cylinder to move faster than fluid can leave it. The excess fluid retards the cylinder’s motion. The circuit works, but the pump wastes energy because it is at full pressure unnecessarily. The circuit in Figure 14-20 shows an external pilot-operated pressure control valve teed into the cap-end line to provide a path for excess fluid to flow directly to tank. Giving the extra oil a second path reduces rod-end pressure and wasted energy. In effect, this is a meter-in circuit for a running-away load.