What is in this article?:
- BOOK 2, CHAPTER 10: Flow control circuits
- Types of flow-control circuits
- 3-speed meter-in circuit
- Meter-in flow control of a running-away load
- When meter-in circuits are necessary
- Action of a meter-in air circuit with a varying load
- Meter-out flow controls
- Three-speed meter-out circuit
- Meter-out pneumatic circuit with a variable load
- Bleed-off or bypass flow controls
- Three-speed bleed-off circuit
- Different locations for flow controls
- Heat generation in hydraulic flow-control circuits
- Motor-type flow-divider speed control
- Another motor-type flow-divider speed control
- Controlling speed of hydraulic motors
- Three-port flow control
Three-speed bleed-off circuit
Figure 10-44 shows a 3-speed bleed-off circuit, controlled by energizing different solenoids. To get additional speeds, add more blocked-center directional valves and flow modules like DV01. The limiting factor would be the number of stations on the manifold. (Using a bar manifold and modular valves eliminates many fittings and possible leaks.) Use this circuit with fixed-volume hydraulic pumps only.
Fig. 10.44. Three-speed bleed-off flow-control circuit using modular valves on bar manifold – at rest with pump running.
To extend the cylinder at fast speed, as in Figure 10-45, shift solenoid A2 on directional valve DV02. All pump output flows to the cylinder to produce fast speed. This condition always is the highest speed. Adding a bleed-off modular flow control under DV02 will allow adjustment of fast speed. Use of a primary adjustment at DV02 would slow both other speeds.
Fig. 10.45. Three-speed bleed-off flow-control circuit using modular valves on bar manifold – cylinder extending at full speed.
Energizing solenoid B1 on directional valve DV01, as in Figure 10-46, directs a portion of pump flow to tank through the module underneath it. The cylinder speed varies, but it can only be slower than fast speed. This is middle speed. Either solenoid A1 or B1 could produce middle speed while the opposite solenoid would produce slow speed.
Fig. 10.46. Three-speed bleed-off flow-control circuit using modular valves on bar manifold -- cylinder extending at middle speed.
Actuating solenoid A1 on directional valve DV01, as in Figure 10-47, bleeds oil to tank through the other flow control, resulting in slow speed.
Fig. 10.47. Three-speed bleed-off flow-control circuit using modular valves on bar manifold -- cylinder extending at slow speed.
The cylinder can retract at fast speed or at any of the same slower flow settings as above. Energizing solenoid B2 on directional valve DV02 sends pump flow through the directional valve directly to the cylinder for fast speed. The middle and slow speeds give the same flow rate as extend. Cylinder speed during these reduced flows is somewhat faster due to the decreased area on the rod end.
Note: Speed change with a bleed-off circuit is very smooth. The cylinder decelerates smoothly, although a slight jerk or shock may be evident when changing from slow to a faster speed.