What is in this article?:
- Book 2, Chapter 10: 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
To control the speed of an actuator, most designers use flow controls. Air circuits normally need controlled flow because the plant air compressor is greatly oversized for almost any given circuit. Hydraulic circuits usually have a dedicated power source sized to meet the cycle time so flow restrictors are unnecessary.
Flow controls always generate some heat in hydraulic circuits, so consider some other method of controlling actuator speed where possible. The circuit examples in this chapter explain the types of flow-control systems and how to apply them.
Figures 10-1 and 10-2 show symbols for fixed orifices, rudimentary components that will control flow. A fixed orifice can be a simple restriction in a line or a factory-preset control with pressure compensation and a bypass. Their low cost and the fact that they are tamper-proof are two main reasons for using fixed orifices.
Use the needle valve shown in Figure 10-3 when control of fluid flow in both directions is necessary. Add the check valve arrangement shown in Figure 10-4 when a needle valve needs pressure compensation in both directions. These check valves, sometime referred to as bridge rectifiers, force fluid to flow through the needle valve in the same direction regardless of actuator movement. (Remember, pressure compensation only works in one direction of flow.)
When talking about flow-control hardware, some manufacturers use different terminology. Normally the term flow control refers to an adjustable needle valve with an integral bypass, as pictured in Figure 10-5. This type of flow control meters flow in one direction and allows free flow in the opposite direction. However, some companies identify the flow control in Figure 10-5 as a throttle valve. These companies say a flow control must have a bypass and be pressure-compensated as shown in Figure 10-6.
When a hydraulic actuator needs accurate speed control, use a pressure-compensated flow control. System pressure fluctuations or load changes will cause actuator velocity to change. Regardless of the cause of the pressure differences, flow across the orifice will change unless the flow control is pressure compensated. Only use a pressure-compensated valve when very accurate speed control is needed because its cost is as much as six times that of a non-compensated valve.
Types of flow-control circuits
There are three types of flow control circuits from which to choose. They are: meter-in, meter-out, and bleed-off (or bypass). Air and hydraulic systems use meter-in and meter-out circuits, while only hydraulic circuits use bleed-off types. Each control has certain advantages in particular situations.
Figure 10-7 shows a meter-in flow-control circuit for a cylinder. Notice that a bypass check valve forces fluid through an adjustable orifice just before it enters the actuator. Figure 10-8 shows the circuit while the cylinder is extending – with the pressures and flows indicated. With a meter-in circuit, fluid enters the actuator at a controlled rate. If the actuator has a resistive load, movement will be smooth and steady. This is because hydraulic fluid is almost incompressible.
In pneumatic systems, cylinder movement may be jerky because air is compressible. As air flows into a cylinder, as depicted in Figure 10-9, pressure increases slowly until it generates the breakaway force needed to start the load moving. Because the subsequent force needed to keep the load moving is always less than the breakaway force, the air in the cylinder actually expands. The expanding air increases the cylinder speed, causing it to lunge forward. The piston moves faster than the incoming air can fill the cylinder, pressure drops to less than it takes to keep the cylinder moving and it stops. Then pressure starts to build again to overcome breakaway force and the process repeats. This lunging movement can continue to the end of the stroke. A meter-out circuit is the best control to avoid air-cylinder lunging.
Figure 10-7 shows the components in a meter-in flow-control circuit. Notice that a bypass check valve forces fluid through an adjustable orifice just before it enters the actuator.
Figure 10-8 shows an extending hydraulic cylinder and indicates the pressures and flows in various parts of the circuit. With a meter-in circuit, fluid enters the actuator at a controlled rate. If the actuator has a resistive load, movement will be smooth and steady with a hydraulic circuit. This is because oil is almost non-compressible.
In the case of an air system, pressure builds slowly and cylinder movement may be jerky. This jerky movement comes from compressibility of the air. As air enters the cylinder, Figure 10-9, pressure builds slowly until it generates the breakaway force to start the piston moving. Because moving force is always less than breakaway force, air in the cylinder expands. The expanding air speeds up cylinder movement, causing it to lunge forward. This increased speed moves the piston faster than the incoming air can fill the space behind it, so pressure drops to less than it takes to keep it moving and the cylinder stops. After the cylinder stops, pressure starts to build again to develop breakaway force and the process repeats. This lunging movement can continue to the end of the stroke. A meter-out circuit is the best control for an air cylinder.
If the actuator has an overrunning load, such as in Figure 10-10, a meter-in flow control will not work. When the directional valve shifts, the vertical load on the cylinder rod makes it extend. Because fluid cannot enter the cylinder’s cap end fast enough, a vacuum void forms there. The cylinder then free falls, regardless of the setting of the meter-in flow adjustment. The pump will continue to supply metered fluid to the cap end of the cylinder and will eventually fill the vacuum void. After the vacuum void fills, the cylinder can produce full force.