BOOK 2, CHAPTER 10: Flow control circuits

Controlling speed of hydraulic motors

Figures 10-64 through 10-67 show schematic diagrams of flow-control circuits for hydraulic motors. The circuits in Figures 10-64 and 10-66 are the best for this purpose. In both of these circuits the motor only has pressure on one port. This arrangement provides the motors’ internal leakage with a path to tank at minimal backpressure. High backpressure in a motor can cause wear on the motor shaft seal and leakage. Motors with external case drains eliminate high case backpressure at both ports, but motor drain flow increases.

For a running-away load, the schematic diagram shown in Figure 10-65 is best. When using a meter-out circuit, specify a high-pressure shaft seal due to backpressure at the motor outlet port. A case drain to let internal leakage go directly to tank can take the place of the high-pressure shaft seal, and is preferred in most cases.

Fig. 10-64. Meter-in flow-control circuit for hydraulic motor.

The bleed-off circuit in Figure 10-66 will generate the least heat of the first three. The only problem is that the motor will slow a little as pressure increases -- due to pump and motor inefficiency. The pressure-compensated flow control will maintain a constant bypass of fluid as pressure climbs. However, pump output will decrease and motor bypass will increase, causing the motor to slow. Use high-efficiency components to keep this speed change minimal.

Fig. 10-65. Meter-in flow-control circuit for hydraulic motor. (Requires high-pressure seals and case drain, as shown.)

The circuit in Figure 10-67 is the most efficient of the four in this section. With a variable-volume pump, pressure is always just what the load requires, backpressure is negligible, and heat generation is minimal. To obtain very accurate control, use a servo-controlled pump with electronic feedback from the motor.

Any of the flow-control circuits for cylinders works well with motors. The main difference is dealing with leakage. Most cylinders have dynamic seals that do not leak. Also, there are no areas to trap internal leakage in a cylinder.

Fig. 10-66. Bleed-off or bypass circuit for hydraulic motor.