There are some ways to use a fixed-volume pump and motor-type flow dividers to change speeds with minimal heat generation as shown in Figures 10-56 through 10-63. These circuits will only give fixed preset speeds without changing hardware.

 Fig. 10-56. Meter-in flow-control circuit with motor-type flow divider (to minimize heat) – cylinder extending at slow speed.

Figure 10-56 shows a 3-speed flow-control circuit using a motor-type flow divider. Here the cylinder is extending at slow speed. With the circuit set up as shown, it defaults to slow speed. Notice that 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 10-56 the cylinder is receiving 3 gpm of oil and requires a pressure of 300 psi to move. Notice the pump pressure reads 100 psi. This will happen because the flow divider is taking in 9 gpm, but using only 3 gpm to do work. The other two 3-gpm flows go back to tank at 0 psi. While it appears these other flows waste energy, they are actually transferring their energy through the motor flow divider 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. In flow-divider circuits, the average of the sum of the outlets always will be the inlet pressure. In this case: (300 psi) + (0 psi) + (0 psi) = (300 psi)/3 = (100 psi) With this system, speed slows but energy loss is only the inefficiency of the parts used.

 Fig. 10-57. Meter-in flow-control circuit with motor-type flow divider (to minimize heat) – cylinder extending at middle speed.

To get mid speed, the directional valves shift as shown in Figure 10-57. By energizing solenoid C2 on the right-hand 3-way valve, an extra 3 gpm goes to the cylinder to produce mid speed. Notice that the pump pressure goes to 200 psi as the cylinder speed doubles. Still there is only hardware inefficiency to waste energy, so the system runs cool.

 Fig. 10-58. Meter-in flow-control circuit with motor-type flow divider (to minimize heat) – cylinder extending at fast speed.

To make the cylinder extend at fast speed, shift the directional valves as shown in Figure 10-58. By energizing solenoids C1 and C2, both 3-way valves shift to send all pump flow to the cylinder. Because the cylinder is at fast speed, pump and cylinder pressure are the same.

To retract the cylinder at fast speed, shift solenoids B1, C1, and C2 as shown in Figure 10-59. 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.) Notice that this circuit is tamperproof. To change the preset speeds, the flow divider and/or pump must be changed.

 Fig. 10-59. Meter-in flow-control circuit with motor-type flow divider (to minimize heat) – cylinder retracting at fast speed.

Any flow-divider circuit will intensify pressure. If the cylinder in Figure 10-56 stalled, the pressure would continue to climb. When it reached the relief valve setting, pressure at the cylinder would be 3000 psi. A second pressure-relief valve installed between the flow divider and the pump port of the main directional valve could be set at a safer pressure in case the cylinder stalls.