Motor-type flow dividers
A motor 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 (3/4-in. ports).