The flow divider in Figure 13-15 is called a priority flow divider because it splits pump flow into a fixed controlled-flow (CF) outlet and sends excess fluid out an excess flow (EF) port. Volume orifices (drilled as specified by the purchaser) preset fluid flow out of the CF port. EF flow is any flow the pump produces over and above the controlled flow. This type flow divider is often used on vehicle power steering, where an engine-driven pump’s output may vary as rpm changes or as its flow is used for other functions. A priority flow divider assures that the power steering always has ample fluid at any engine speed or when other functions are active.

As fluid enters the valve, the path of least resistance leads through the controlled-flow-volume orifices and out port CF. If pump flow is more than the volume orifices can pass, pressure builds on the right end of the flow-control spool through the excess-flow pilot line. When pressure rises enough to overcome the bias spring and any backpressure from the steering circuit, the flow-control spool moves to the left, just enough to let excess flow exit through port EF. Excess flow changes as pump flow varies, but flow to port CF takes priority. A relief valve in port CF can be set for any pressure and has no affect on pressure at port EF. The controlled-flow relief valve is required even when maximum pressure is the same for both outlets.

Notice that controlled flow is pressure compensated. As pressure builds at port CF, it pushes back against the excess-flow pilot-pressure pilot to maintain a constant pressure drop across the volume orifices.

Priority flow dividers are also manufactured with adjustable flow for the priority port and without a relief valve for circuits that already have one. (The symbol shown is borrowed from a manufacturer's catalog because there is no standard symbol in ANSI or ISO literature.)

The flow divider in Figure 13-16 is a spool-type divider that splits flow at any predetermined rate according to the sizes of the drilled orifices. It is usually set up with identical orifice sizes for a 50-50 split. This particular design does not allow reverse flow, so bypass check valves are required when flow must return the same way it entered.

Fluid entering the Inlet port goes left and right through orifices, then out outlets 1 and 2. When either outlet encounters more backpressure than the other does, the high-pressure side forces the spool towards the low-pressure side until pressures on both sides equalize. Equal pressure drop across both orifices produces equal flow. (Most manufacturers specify flow equality at ±5%.) Pressure differences at the two outlets should be low because Inlet pressure always equals the highest outlet pressure -- which means pressure drop across the low-pressure outlet wastes energy.

Spool-type flow dividers only split flow. When more than two outlets are required, dividers must be used in series. A 50-50 split divider flowing into two more 50-50 dividers gives four equal outlets. A 66-33 divider into a 50-50 divider gives three equal outlets. The flow divider/combiner in Figure 13-17 equalizes flow in both directions. It can be used with double-acting actuators to synchronize speed in both directions of travel. The spool in this divider is made in two sections with a connecting link that allows the sections to move together in the closed condition (as shown) for combining, or be spread by Inlet pressure when they are dividing. Springs at both ends of the spool keep the sections together when pressure equalizes or is not present. Inlet orifices set nominal flow, while outlet orifices control flow to or from an actuator.

Flow to the inlet-return port goes through the inlet orifices to split into two equal parts. Pressure drop across the orifices causes the split spool to separate so the outlet orifices are working at the outer edge of the outlet-return ports. When unequal pressures on its ends shift the spool, flow is retarded to the low-pressure outlet port to keep it from receiving too much fluid. When the actuator reverses, flow into the outlet-return ports goes through the outlet orifices and on through the inlet orifices, causing the spool sections to come together. Now the outlet orifices control return flow on the inner edge of the outlet-return ports. They will retard flow from any actuator port that is trying to run ahead.

Rotary flow dividers

A rotary (motor-type) flow divider is constructed from two or more hydraulic motors — in a common housing — with a common shaft running through one set of gears on all motor sets. There is a common Inlet to all motors and separate outlets. The motors are usually gear-on-gear or gerotor design. Flow split is commonly 50-50 but many outlet flow combinations are possible by changing gear or gerotor widths.

The cutaway view and symbol in Figure 13-18 pictures a 2-outlet 50-50 split gear-motor-type flow divider. (There is no ISO or ANSI symbol for a motor flow divider so the one shown in the figure is from a supplier’s catalog.) One gear from each motor set is keyed to the common shaft, so both motors must turn at the same rate. If one motor stalls, they both stop because of the common-shaft arrangement. Due to internal clearances in the motor elements, there is some bypass flow that does not turn the motors. As a result, the outlet flows are not always exactly equal . . . especially at high outlet-pressure differences.

From Figure 13-18, it should be obvious that this flow divider does not have a priority side like a spool-type flow divider does. Thus, when Inlet flow changes, it is always split equally. The main advantage of motor-type over spool-type flow dividers is there is less wasted energy when the outlets are not at or near the same pressure. If pressure at the right outlet was 1500 psi and pressure at the left outlet was 300 psi, pressure at the inlet would be 900 psi. Pressure at the inlet is always the average of the sum of the outlets.

This feature can be an asset or a problem. If one outlet meets resistance while the other is flowing to tank, an inlet pressure of 2000 psi can result in the pressurized outlet intensifying to 4000 psi. If pressure that high cannot be tolerated, a relief valve must be installed at the outlets. On the other hand, intensification can allow a 1000-psi system to produce 2000 psi to perform work -- similar to a hi-lo pump circuit. Note that while pressure doubles, flow is halved through the high-pressure outlet.

Looking at Figure 13-18, it appears the motor flow divider is also a combiner. This is partially true. The circuit in Figure 13-19 shows a motor flow divider synchronizing two hydraulic motors. As the motors turn in right-hand rotation, they stay almost perfectly synchronized. Pressure to each motor may vary but flow from each flow-divider outlet remains near constant. If the directional control valve shifts to turn the motors in left-hand rotation, the flow divider may get equal flow and the hydraulic motors may stay synchronized. However, if one hydraulic motor meets more resistance than it can overcome and stalls, all pump flow goes to the running hydraulic motor. The second motor then turns twice as fast. During this scenario, one flow-divider motor overspeeds while the opposite one cavitates. The only way to make sure both hydraulic motors stay synchronized in both directions of rotation is to install motor flow dividers at both valve ports.

Spool and motor flow dividers work reasonably well to synchronize circuits with hydraulic motors and cylinders. However, because both devices do not divide flow perfectly, the actuators they control will not stay perfectly synchronized. A high-pressure difference at the divider's outlets is the worst problem; it can allow a 5 to 10% lag in actuator position. This means that synchronizing circuits using flow dividers often require some type of re-synchronizing valving to realign the actuators more exactly when they stop at home position. (Due to internal bypass, actuators with short cycles may re-synchronize themselves because the error is small.)

Another design consideration is the intensification of pressure at the outlets of a motor flow divider. The circuit in Figure 13-20 has two cylinders that are synchronized by a motor flow divider. Because this circuit operates at 2000 psi, it is possible that pressure at one cylinder could reach as much as 4000 psi due to intensification. Intensification occurs when one cylinder is lightly loaded or has no load and the other one is loaded heavily. In Figure 13-19, the load is shifted to one side of the platen -- making the right-hand cylinder do all the work. Inlet pressure is at 2000 psi and the cylinders are stalled. Pressure at the lightly loaded left-hand cylinder is 250 psi, so pressure at the right-hand cylinder is 3750 psi. The intensification is due to energy transfer through the motors in the flow divider. Because inlet pressure for both motors is 2000 psi, the unused 1750 psi from the left side is transmitted through the common shaft and drives the opposite motor to 3750 psi. (For other flow-divider circuits. see the author’s book, “Fluid Power Circuits Explained,” available through the same outlet for this manual.)

Most flow control functions are available as modular or sandwich valves that mount between directional control valves and a subplate. Figure 13-21 shows most of the common configurations presently offered by fluid power suppliers. Although the symbols show non-compensated flow controls, most configurations also are available with pressure-compensated flow controls. Where a needle valve is shown, a flow control with bypass may actually be installed. This is not a problem because there is never a reason for flow reversal. Figure 13-21 also shows two modular flow dividers that are available from one supplier. These modules are usually available in all valve sizes up to D08 (¾-in. ports).