Motor-type flow dividers

Motor-type flow dividers consist of two or more hydraulic motors in a common housing. All the motors share a common shaft, so they all turn at the same speed. All motors have a common inlet but separate outlets. If the motors have the same displacement, the output from each motor is nearly equal. (Some motor-type flow dividers use motors with different displacements, so each section’s output differs.) The big advantage of a motor-type flow divider over a spool-type flow divider is energy transfer between sections. A spool-type flow divider's inlet pressure is always equal to the highest outlet pressure. This means heat generation from the lower or 0 pressure outlets, because pressurized fluid goes to tank without doing any work.

In contrast, a motor-type flow divider’s inlet pressure is the average of the sum of the outlet pressures. Because there is a mechanical link between sections, excess energy transfer via this link greatly reduces heat generation. Because hydraulic motors are not 100% efficient, there still is some energy loss and heat generation in any motor-type flow divider.

Another advantage of motor-type flow dividers is their outlet options. A spool-type flow divider has only two outlets; a motor-type flow divider can have many outlets — in even or odd numbers. Most manufacturers catalog units with 6 to 8 outlets, but also will custom-build dividers to suit.

Figure 11-17. Motor-type flow divider piped to split pump flow. (Shown at rest with pump running.)

Figure 11-17 shows a motor-type flow divider splitting flow from a fixed-volume pump to separate actuators. With the cylinders at rest, all flow goes to tank through the tandem-center valves with minimal energy loss. To stroke the cylinder on the right, shift its directional valve as in Figure 11-18. Flow from the right-hand section of the motor-type flow divider sends half of the pump’s flow to the right-hand cylinder at 1500 psi. The other half of the pump’s flow goes to tank through the left valve at 0 pressure. Notice that pump pressure is approximately 750 psi instead of 1500 psi as in Figure 11-10. Pump pressure is low because most of the energy in the flow divider outlet going to tank mechanically transfers from the idling motor to the working motor. Whether one or both cylinders do work, energy going in is always equal to energy needed plus inefficiencies.

Figure 11-18. Motor-type flow divider piped to split pump flow. (Shown with right-hand cylinder extending.)

The 4-outlet motor-type flow divider in Figure 11-19 supplies four hydraulic motors. Because each motor has a different load, pressure at the motor inlets is not the same. To figure the approximate inlet pressure to the flow divider, add the outlet pressures and divide by the number of outlets. (1100 psi + 700 psi + 1250 psi + 1500 psi = 4550 psi. Divide by four outlets and 1138 psi is the pressure at the pump outlet). The 1138-psi figure is approximate due to losses in piping and the motors of the flow divider.

Figure 11-19. Motor-type flow divider piped to split pump flow into four equal parts. (Shown with pump running).

Notice the relief valve at the flow divider outlets. Because a motor-type flow divider also acts as an intensifier (See Figures 11-45 through 11-48), it is necessary to limit the pressure at each outlet. If each motor needs a different pressure, use separate relief valves at each flow divider outlet. In Figure 11-19, a set of check valves and a single relief valve sets the same pressure for each motor — and protects them from overpressure. Because the relief valve is solenoid-operated it also starts and stops all motors simultaneously.

Motor-type flow divider synchronizing two cylinders

Motor-type flow dividers work well for synchronizing actuators. Figure 11-20 shows two cylinders synchronized by a double equal-outlet, motor-type flow divider. Install the flow divider between the valve and the cylinder cap-end ports as shown. This arrangement synchronizes the extension stroke of the cylinders and provides some control for the retraction stroke (See Figures 11-22 and 11-23). Use a second flow divider at the rod-end ports for precise control on the retraction stroke when required.

Figure 11-20. Motor-type flow divider piped to synchronize two cylinders. (Shown at rest with pump running.)

As the cylinders extend, as in Figure 11-21, the flow divider splits pump flow, causing the actuators to extend at the same time. If the cylinders’ loads require different pressures, the flow divider still sends almost equal flow to each port. A motor-type flow divider has some internal bypass, causing the section with the higher outlet pressure to pass less than half flow. Therefore, use motor-type flow dividers for circuits needing only nominal synchronization. With any type of hydraulically controlled synchronization, always take the cylinders to a fixed position at one or both ends of the stroke.

Figure 11-21. Motor-type flow divider piped to synchronize two cylinders. (Shown with cylinders extending.)

Also, if pressure intensification above any of the system’s component ratings is possible, put a relief valve at the flow divider outlets. Several manufacturers supply their flow dividers with integral bypass relief valves. Set these reliefs for a safe pressure differential so intensification will not damage the cylinder. When a bypass relief valve starts relieving, the cylinder on that side stops while the opposite cylinder’s speed doubles. (If integral relief valves are not available, install external reliefs when there is a chance for actuator damage from high pressures.)

Some manufacturers pipe the integral relief valve’s outlet to tank instead of back to the flow divider inlet. This type of relief valve circuit dumps fluid to tank at a pressure low enough to keep from damaging the actuator. Using a relief valve with its outlet piped to tank causes one actuator to stop and allows the other one to continue at the same speed.

Figure 11-22. Motor-type flow divider piped to synchronize two cylinders. (Shown with cylinders retracting.)

Figure 11-22 shows how the cylinders retract under normal conditions. Flow from the pump goes to both rod-end ports and the cylinders retract together. The flow divider combines the oil from the cap-end ports and synchronization continues. However, if one cylinder binds on the retract stroke, the cylinder with less drag will run away.

Figure 11-23. Motor-type flow divider piped to synchronize two cylinders. (Left-hand cylinder shown binding.)

Figure 11-23 depicts what happens when a cylinder binds. All flow from the pump goes to the right-hand cylinder — retracting it at double speed. The right-hand motor of the flow divider turns rapidly due to the high flow. The left-hand motor of the flow divider also turns rapidly, but no oil passes through it. The left-hand motor cavitates due to this lack of fluid. After the right-hand cylinder bottoms out, pressure buildup may cause the left-hand cylinder to retract. As the left-hand cylinder retracts, the right-hand motor of the flow divider cavitates.

If the cylinders in a circuit have different return-force requirements, or are subject to binding, add a second motor-type flow divider at the cylinders’ rod-end ports. The second flow divider assures that the cylinders are synchronized on their retraction strokes also. (See Chapter 22, covering Synchronizing Circuits, for other ways to make actuators move at the same rate.)