When you must to split a single hydraulic line into two or more identical flow paths, a tee or several tees can be the first solution. However, if the resistance in all the branches is not identical, flow can vary greatly in each path. Adding flow controls at the tee outlets makes it possible to change resistance and equalize flow in each branch, but as the machine operates, work resistance changes often require constant flow modifications. A device called a flow divider splits flow and compensates for pressure differences in most cases. A flow divider can split flow equally, unequally, and into more than two paths. One design maintains a constant flow for one outlet and directs any excess flow to a second outlet.
Figure 11-1 pictures the ISO symbol for a flow-dividing valve. While the ISO symbol shows the function of the valve, it does not indicate which design it is. Fluid entering the flow divider splits and passes to both outlets equally. Figure 11-2 shows the symbol for a spool-type flow-divider and gives a better indication of the valve’s operation. Note that a spool-type flow divider will not allow reverse flow. When using a spool-type flow divider to synchronize cylinders, add check valves to pass reverse flow. However, when the cylinders reverse, there is no synchronization with a spool-type flow divider.
Figure 11-3 shows a divider/combiner that synchronizes actuators in both directions of travel. It splits pump flow to the actuators and also assures that equal reverse flow returns from both cylinder ports.
Figure 11-4 pictures a flow divider with bypass relief valves that allow a lagging cylinder to complete its stroke. Reverse-flow check valves allow free flow around the divider spool while the actuator returns.
Figures 11-5 and 11-6 show a priority flow-divider symbol. Port CF (controlled flow) of this flow divider always has the same flow when the pump is producing that flow or more. Excess pump flow goes through port EF (excess flow) to tank — or to another circuit.
Figures 11-7 and 11-8 show motor-type flow-divider symbols (as drawn by the manufacturers). This type flow divider is more efficient in most circuits. Motor-type flow dividers also work well in flow- and/or pressure-intensification circuits. They are available with multiple outlet ports and/or unequal flows.
Spool-type flow dividers
Spool-type flow dividers split flow through pressure-compensated fixed orifices. The pressure-compensation feature ensures near-equal flow through the orifices — even when inlet and/or outlet pressures fluctuate.
Spool-type flow dividers can split flow equally or unequally, according to the orifice sizes. Always use spool-type flow dividers at or near their rated flow. Because most designs use fixed orifices, equality of flow is poor when used below their rated flow. If flow exceeds the rating of the valve, high pressure drop causes poor performance and fluid heating.
The dividing accuracy of spool-type flow dividers can be as close as ±5%, depending on the pressure difference at the outlet ports.
Figure 11-9 shows a spool-type flow divider splitting pump flow equally. With this circuit, flow to each directional valve is nearly equal, even with one cylinder working at high pressure while the other cylinder is at low pressure or stopped by a centered valve.
In Figure 11-10, fluid from port 1 flows to tank through the directional valve while fluid from port 2 drives a cylinder. Pressure at port 1 is 0 psi while pressure at port 2 is 1500 psi. Under these conditions, pressure at the flow divider inlet also is 1500 psi. Pressure at the inlet of a spool-type flow divider is always equal to the highest-pressure outlet. This condition generates a lot of heat because pressurized oil leaving port 1 is not doing work. It is best to use a spool-type flow divider in circuits where both outlet ports are at or near the same pressure. The higher the pressure variation, the greater the energy wasted as heat with spool-type flow dividers. When outlet pressures continuously vary by more than 300 to 500 psi, it is best to use a motor-type flow divider.
When splitting flow into more than two paths, add another spool-type flow divider to each outlet of the first divider. Figure 11-11 shows a synchronizing circuit for four unidirectional hydraulic motors. Flow split equally by the first spool-type flow divider goes to two more spool-type flow dividers. The second pair of spool-type flow dividers split the half flow from the first spool-type flow divider, and sends equal flow to the four motors.
When using spool-type flow dividers for equal flow, the total number of dividers must be an odd number. If used in any even combination, flow will not be equal from all outlets -- unless the first divider has unequal flow from its outlets.
To get three equal outputs with spool-type flow dividers use one with unequal outputs, say 33.3% and 66.7%. Send flow from the 33.3% side to power the first actuator. Send flow from the 66.7% side to an equal-flow divider. Flows from the equal flow divider outlets is now 33.3% of total pump flow, so all three outputs are the same.
Notice that these circuits cannot handle reverse flow. Reverse flow through a spool-type flow divider will lock up one actuator when return pressure differs at the outlet ports.
Also notice that each outlet of a flow divider can have a different pressure. Figure 11-9 shows outlet 1 with a relief valve set at 1500 psi, and outlet 2 set at 2000 psi. (If both cylinders operate at the same pressure, substitute a single relief valve at the pump.) However, if both cylinders are moving and one of them stalls at 2000 psi, both cylinders will stop. The relief valve arrangement in Figure 11-11 allows any motor needing more than 2000 psi to stop while all other motors continue turning.