Spool-type flow divider/combiners
Spool-type flow dividers only allow flow in one direction. From the symbol in Figure 11-2, it is plain that reverse flow would lock up one of the cylinders. The cylinder that needs less resistance actually gets more. In a circuit where flow must go both ways, use a check valve to bypass the flow divider.
Figure 11-12 shows spool-type flow dividers in a circuit that synchronizes two cylinders. As the cylinders extend, the flow divider splits the flow and cylinder speed is nearly the same. When the cylinders retract, bypass check valves allow fluid to go around the divider. There is no synchronization from the cap-end flow divider at this time. A second flow divider with bypass check valves on the rod-end ports (as shown) is necessary for identical movement while retracting. As depicted in Figure 11-4, some flow dividers come with integral bypass check valves. Integral bypass check valves save piping time, have fewer leaks, and are more compact.
Because flow dividers are not 100% accurate, one of the cylinders may lag. Because there is internal leakage past the spool, any flow divider will let the lagging cylinder continue its travel. Because of the bypass leakage, the speed of the lagging cylinder while it is going to the end of its stroke is very slow. Integral relief valves (as shown in Figure 11-4) allow the lagging cylinder to catch up quickly. Set these relief valves between 50 and 150 psi. Once the pressure difference across the valve reaches this pressure range, fluid bypasses the restricted spool to quickly re-phase the cylinders.
In Figure 11-13, a single flow divider/combiner synchronizes cylinders in both directions of travel. Here a flow divider/combiner replaces the flow divider and check valves in Figure 11-12. Because there is no ANSI symbol for the flow divider/combiner, add bi-directional arrows to the one-way flow-divider symbol. This more-detailed symbol helps to clarify the valve’s action. Bi-directional arrows show the divider/combiner function. These detailed symbols come from manufacturers’ catalogs and represent their interpretation of their valve’s function.
As the cylinders extend, the divider/combiner splits the flow to keep cylinder speeds nearly the same. When the cylinders retract, the divider/combiner shifts internally and equalizes return flow also.
A flow divider/combiner wastes energy the same as a standard flow divider. In essence these devices are infinitely variable pressure-compensated flow control pairs. Any flow control will cause heat because it is a restriction.
Flow dividers or flow divider/combiners are not designed to control running-away loads. For the circuits in Figures 11-12 and 11-13, a counterbalance valve in the line between the directional valve and the flow divider may be necessary if the loads can run away.
Spool-type priority flow dividers
Figure 11-14 shows a typical spool-type priority flow divider circuit. A priority flow divider maintains constant flow from the controlled flow (CF) port. Any additional flow passes out the excess flow (EF) port. The non-standard symbol in the Figure is one typically found in manufacturers’ catalogs. The controlled flow may be fixed or adjustable, according to the circuit needs. The excess flow may be sent to tank or to another circuit as required. (When there is pressure at the excess flow port, make sure the valve design can handle it.)
Some priority flow dividers are more like 3-port flow controls and cannot stand backpressure at the EF port. Use these flow dividers for bleed-off flow controlling only. With a bleed-off type priority flow divider, pressure at EF port causes flow at the CF port to fluctuate.
In Figure 11-14, a fixed-orifice priority flow divider is used on a vehicle with power steering and hydraulic actuators. This is the standard circuit for a forklift truck using a fixed-volume pump. The power-steering circuit needs 7 gpm and pump flow at idle is a minimum of 10 gpm. The actuators need as much as 15 gpm for maximum speed.
When the vehicle is operating, the power steering circuit will always have at least 7 gpm. When the mast or tilt cylinders need fluid, excess pump flow operates them. Because there is little excess flow at idle, the mast and tilt cylinder's speeds are slow at this time.
The circuit in Figures 11-15 and 11-16 controls the speed of a hydraulic cylinder powered by a fixed-volume pump. The adjustable controlled-flow port of the priority flow divider connects to the cylinder valve, with the excess-flow port piped to tank. This arrangement controls cylinder speed and keeps heat build up low because the pressure in this circuit is only slightly higher than the cylinder needs.
Most priority flow dividers are pressure compensating so the priority flow remains constant even when pressure changes occur. As long as there is enough pump output, the controlled flow is constant. Excess flow changes as pump volume varies.
A priority flow divider wastes energy just like any spool-type divider. The inlet pressure to the divider is the same as the highest outlet pressure. When either outlet port is pressurized, the port with little or no pressure is wasting energy and generating heat.