Meter-out flow control circuits
Figure 13-8 shows a schematic drawing of a meter-out flow control circuit that restricts fluid as it leaves an actuator port. Meter-out circuits work well with both hydraulic and pneumatic actuators. Cylinder-mounting attitude is not important because outlet flow is restricted and an actuator cannot run away. Meter-out flow controls work on resistive loads or running away loads because the actuator can never move faster than the fluid leaving it allows.
The left-hand circuit in Figure 13-8 is shown at rest with the pump running. Notice how the check valves in the flow controls allow fluid to bypass the orifices and freely enter the cylinder. As fluid leaves the cylinder, it is forced through the orifices at a set rate. The only gauge showing pressure is PG3 because the load on the cylinder rod is inducing pressure at the valve’s blocked port.
The right-hand circuit shows conditions when the cylinder is extending. The directional control valve shifts to straight arrows and pump flow bypasses the upper flow control to go to the cylinder cap end. Fluid leaving the cylinder rod end is held back before it goes to tank -- even with an external load trying to move it. The cylinder extends at a reduced speed in both hydraulic and pneumatic circuits until it meets a resistance it can’t overcome or it bottoms out. With the non-compensated valve shown, speed can vary as pressure fluctuates or viscosity changes in a hydraulic system. (There are no pressure-compensated flow controls for pneumatic circuits.)
While the cylinder is in motion, gauges PG1 and PG2 read the relief valve or pump compensator setting. Gauge PG4 reads tank backpressure. Gauge PG3 reads load-induced pressure plus the pressure from cap-area-to-rod-area intensification. This intensified pressure could be 1.2 to 2 times the cap-end pressure, or higher, depending on the rod size.
Meter-out flow controls work equally well in pneumatic circuits when the load is constant. Changing loads can cause the actuator to stop and/or lunge under certain circumstances. (For a more extensive coverage of flow control circuits and situations that can arise with them, see our second e-book entitled "Fluid Power Circuits Explained," which will be launched on hydraulicspneumatics.com in the coming months.
Bleed-off flow control circuits
Bleed-off flow control circuits are found only in hydraulic systems and normally only in those with fixed-volume pumps. There is little or no advantage to using this type flow control with pressure-compensated pumps. Figure 13-9 shows a bleed-off circuit at rest with the pump running. A needle valve’s inlet is teed into a line going to the cylinder and its outlet is connected to tank. The circuit only works with one actuator moving at a time because all pump flow goes to the presently operating function. Like a meter-in circuit, it only works with resistive loads because it controls fluid into the actuator. The main plus for this type speed control is it saves energy while using a fixed-volume pump with low-pressure travel forces.
When the directional valve in Figure 13-9 shifts, all pump flow passes through it and toward the actuator. On the way to the actuator, part of the flow is bled off to tank, so the actuator does not reach full speed. Pressure at PG1 only rises to whatever it takes to move the actuator and its load, so excess flow goes to tank at low pressure. (When using a fixed-volume pump and a meter-in or meter-out circuit, excess flow also goes to tank, but at relief valve pressure.) Many circuits only perform work at the end of stroke so this flow control system saves energy while the actuator moves to and from the work position, yet still gives good speed control.
Some words of caution:
- Pressure in the actuator during traverse time must be higher than the pressure in the path to tank, so fluid will flow to tank.
- Because pressure may change during traverse time (especially when the actuator contacts the workpiece), use a pressure-compensated needle valve so flow to tank remains constant.
- Even with a pressure-compensated needle valve, actuator speed will be inconsistent. Pump and/or actuator efficiency allows bypass that directly affects flow to the actuator not bleed-off to tank.