Motor-type flow divider in a priority circuit

Using a motor-type flow divider in a priority circuit like the one shown in Figure 11-16 will give unsatisfactory results. A spool-type priority flow divider sends a constant flow to one outlet as long as the pump produces at least that much flow. When pump flow increases, priority flow stays the same while the other outlet’s flow starts or increases. Flow from the priority outlet stays constant through the entire pump range.

Figure 11-24. Motor-type priority flow divider in lift-truck circuit. (Shown with engine idling.)

Figures 11-24 and 11-25 show what happens when using a motor-type flow divider in place of a spool-type priority flow divider. With the engine at idle speed, 3 gpm flows to the power steering and 7 gpm to the cylinder circuit. This circuit works well at idle — if 3 gpm is enough for the power steering. Figure 11-25 indicates what happens when engine rpm and flow increase. As pump flow increases, both the power steering and the cylinder circuits receive more fluid in the same ratio. This overspeeds the power steering while robbing oil from the cylinder circuit. (There would be little or no heat generation from this circuit, but the end result is less than satisfactory.) Motor-type flow dividers with unequal flow outlets are available in various combinations and multiple flow paths. However, the flow from each outlet changes proportionately as the inlet flow changes. This feature makes them hard to adapt to the engine-driven pumps on much off-road equipment.

Figure 11-25. Motor-type priority flow divider in lift-truck circuit. (Shown with engine speed increased.)

Motor-type flow divider speed control

There are ways to use a fixed-volume pump and motor-type flow dividers to change speeds with minimal heat generation. Figures 11-26 through 11-33 depict some of these. These circuits only give fixed preset speeds without changing hardware.

Figure 11-26. Meter-in flow-control circuit with motor-type flow divider to minimize heat generation. (Shown with cylinder extending at slow speed).

Figure 11-26 shows a 3-speed flow control circuit using a motor-type flow divider. Here the cylinder is extending slow speed. With the circuit set up as shown, it defaults to slow speed. Notice there are no flow controls. To split pump flow evenly and reduce energy loss, use a motor-type flow divider at its outlet. Each outlet of the flow divider will put out about 3 gpm.

In Figure 11-26 the cylinder is getting 3 gpm of oil and requires a pressure of 300 psi to move. Note that the pump pressure is only 100 psi. This happens because the flow divider is taking in 9 gpm and using 3 gpm to do work. The other two 3-gpm flows are going back to tank at 0 psi. While it appears these two 3-gpm flows waste energy, they are actually transferring their energy through the common to the left-hand motor. The left-hand motor becomes a pump with a 100-psi inlet and two motors driving it to 300 psi. As always in flow-divider circuits, the average of the sum of the outlets will be the inlet pressure. (300 psi + 0 psi + 0 psi = 300 psi; divide by 3 to get 100 psi.) With this system, cylinder speed slows, but the only energy loss is the inefficiency of the components used.

Figure 11-27. Meter-in flow-control circuit with motor-type flow divider to minimize heat generation. (Shown with cylinder extending at medium speed.)

To get mid speed, the directional valves shift as indicated in Figure 11-27. By energizing solenoid C2 on the right-hand 3-way valve, an extra 3 gpm goes to the cylinder to give mid speed. Note that the pump pressure rises to 200 psi as the cylinder speed doubles. There still is only hardware inefficiency to waste energy, so the system runs cool.

Figure 11-28. Meter-in flow-control circuit with motor-type flow divider to minimize heat generation. (Shown with cylinder extending at fast speed.)

To make the cylinder stroke at fast speed, shift the directional valves as shown in Figure 11-28. By energizing solenoids C1 and C2, both 3-way valves shift to send all pump flow to the cylinder. While the cylinder is in fast speed mode, pump and cylinder pressure are the same.

Figure 11-29. Meter-in flow-control circuit with motor-type flow divider to minimize heat generation. (Shown with cylinder retracting at fast speed.)

To retract the cylinder at fast speed, shift solenoid B1 along with C1 and C2, as shown in Figure 11-29. Energizing one or more solenoids in the retract mode gives different speeds that are nearly the same as when extending.

If the flow divider had more and/or unequal-size motors, selection of a combination of speeds by selecting different flow outputs is possible.

This circuit is tamper-proof. To change the preset speeds, the flow divider and/or pump must be changed.

Note: Any flow-divider circuit will intensify pressure. In Figure 11-26, if the cylinder stalled, the pressure would continue to rise. When the pump reached the relief valve setting, pressure at the cylinder would be 3000 psi. Use a second pressure-relief valve between the flow divider and the pump port of the cylinder directional valve to set a safe pressure in case of cylinder stall.