Control a Cylinder's Direction With DCV

In the previous Trainer Lab #4, you had to change the hose in order for the cylinder to extend and retract. Obviously, this is not how a cylinder's direction is controlled in the real world. To change the direction of the double-acting cylinder without moving hoses, we need a switch that controls the direction of flow. In pneumatics, this switch is called a Directional Control Valve (DCV). Almost every pneumatic circuit uses one or more pneumatic DCVs.

For our trainers, we use different configurations of DCV, and we operate them using different methods: Manually, Electronically, or Pilot Operated. You will have a chance in the future Trainer Labs to use all of the methods of operation.

Here is a picture of one of the valves we will use with our trainers.

This valve is a 5-Port, 4-Way, 2-Position Directional Control Valve. It consists of the following components:

- Valve Body - The valve body provides the passageway for the sliding spool. It has five ports that provide flow paths for air drilled into it. Pneumatic DCV bodies are often made of cast aluminum because of the low pressures.
- Spool - The spool is the "switch" of the valve. It slides back and forth in the body to channel the flow through to specific ports. The spool shown in the following illustration has grooves. These grooves, when the spool is shifted, can allow air to flow past the o-ring seals inside the body.
- Operator - The operator is the mechanism that moves the spool from one position to another. Examples of operators that are used to shift pneumatic DCVs include the lever, manual palm button, solenoid (electrical operator), and pneumatic operator.
- Springs - A 3-position valve usually has a main spring on either side of the spool. When the operator is not energized, the main spring is used to position it in the middle.

Here is a cut-away diagram that shows the insides of a DCV.

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

The "P" is for the pressure or inlet port. "EA" and "Ea", which can also be labeled "R" and "S" depending on the manufacturer of the DCV, are used to label the exhaust ports. There are also two actuator ports labeled "A" and "B". These are usually connected to the lines going to the cylinder or motor. They allow air to flow to and from the actuator.

Directional control valves are manufactured in many styles and sizes with a wide variety of options. One major classification of directional control valves is by the number of flow paths or "ways" available for air to flow through a particular valve. Pneumatic DCVs are available as one-way, two-way, three-way, four-way, and five-way types. Besides classifying valves by the number of ways, a valve can also be classified by the number of positions to which it can be adjusted. A position determines which ports are connected to each other. Industrial pneumatic DCVs are most often supplied as either 2-position or 3-position types. The type we will explore in this LAP is a 3-position type. As a shortcut, DCV ways and positions are commonly stated together by using two numbers separated by a slash, such as 3/2, 4/3, 5/3, etc. The first number is the number of ways and the second is the number of positions. As an example, a 3/2 DCV is a 3-way, 2-position directional control valve.

The schematic symbols for directional control valves use what is called a flow envelope to show the state of the flow paths for each valve spool position. These flow paths can be shown as either open or closed by the envelope, as shown in the illustration above.

Review/Examples

This is a lot to consume at once, so let's review what all this means. To do this, let's look at some schematics of different types of valves and what they can do.

Note: In the fluid power industry, the terms "open" and "closed" mean just the opposite of the meaning in the electrical industry. An electrical switch passes electricity when it is closed. A fluid power valve passes fluid when it is open.

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

Okay, here are some valve configurations. Let's go through them one at a time and explain what they are.

Example 1

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

This is a Manually Operated (handle on the left) Spring Return (spring on the right in normal position, which normal position is always shown in the schematic, holding the valve in the P to A position) 2-Way (ways are indicated by arrows or blocked ports). This means that there are two paths the air can go. For the 1st path, P (pressure) goes to A (work port) when the valve is in normal position. For the 2nd path, P is blocked and in A, pressure is blocked coming back. The two positions are indicated by squares. Number of squares equal the number of positions on a normally open DCV. So, all together, this is a Manually Operated, Spring Return, 2-Way, 2-Position, Normally Open DCV.

This type of valve would be used to control the flow of air in one direction and allows for you to turn flow off and on.

Example 2

Here is an example of another valve.

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

This is an example of a 5-Way, Spring Return, Manually Operated, 2-Position Valve. In industry, you may also hear it called a 5-Ported, Spring Return, Manually Operated, 2-Position Valve. This is the type of valve you would use to control a double-acting cylinder in different applications. It is important to recognize that with this type of valve, any time there is pressure, the cylinder is going to do something. In this application, in normal condition, the cylinder will extend as soon as there is pressure in the system. P goes to B, A goes to Exhaust port E (b), and Exhaust Port E (a) is blocked. If you push the handle, the valve would switch, and P would go to A, B would go to Exhaust Port E (a), and Exhaust Port E (b) would be blocked.

Example 3

Here is another valve configuration:

Parker Hannifin Corporation. (1980). Industrial Pneumatic Technology. Parker Hannifin Corp. DOI: 1557690154

This type of valve is a 5-Way, 3-Position, Manually Operated, Spring-Centered, Closed-Centered. This type of valve would be used if you wanted to be able to stop a cylinder anywhere in the stroke. For example, if you had some sort of lifting device and the cylinder was lifting a load that you wanted stopped at a certain height, the normal condition of this valve was closed at the center position. With springs on both sides, when the lever is not pushed, the valve "springs" to the center position and is closed. Thus, no air can travel anywhere and the cylinder stops at whatever position it is at in the stroke.

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