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How to select a pneumatic solenoid valve for a single acting cylinder?

Summary

A single acting pneumatic cylinder is a linear actuator and realizes a working stroke by filling the cylinder with compressed air. The return stroke is usually accomplished by a spring. The cylinder has one connection port that is used either to fill or vent the cylinder. To control the cylinder, a 3/2-way valve is used. 3/2-way means three ports and two positions: one port connects to the source of compressed air, one port is required as exhaust and the third port connects to the cylinder. The valve has two positions: filling or venting the cylinder. The required valve size can be calculated once the cylinder and application properties are known.

Basic principle of single acting cylinders

A pneumatic cylinder is a linear actuator that works with compressed air. The cylinder’s main parts are the piston, the piston rod, the cylinder tube, gaskets and seals. Single acting cylinders also have a spring inside the cylinder. A single acting cylinder works with compressed air to actuate the piston in one direction and spring force to return to the base position. Work can be performed in the air driven direction. The cylinder has one port that is used to both supply and vent compressed air.

Two types of single acting cylinders exist: spring return and spring extended. The most common type is the spring return cylinder. In the spring return cylinder, the spring is located between the front end of the cylinder and the piston (around the piston rod). In this design, the piston rod extends when compressed air is supplied to the cylinder. As soon as the air supply is cut off, the piston rod retracts by spring force. The spring extended cylinder works the other way around. The piston retracts when compressed air is supplied. When the air supply is switched off, the spring pushes the rod out. In the spring extended cylinders, the spring is located between the piston and the rear end of the cylinder.

Symbols of single acting cylinders

Symbols of pneumatic cylinders according to ISO1219-1; External force return (left), spring return (middle) and spring extend (right)

There are some advantages to use single acting cylinders over double acting cylinders: less tubing, less use of compressed air, and less wiring is needed for the system. Single acting cylinders also have disadvantages: the spring takes up space and limits the working stroke of the cylinder. Furthermore, the spring force reduced the pneumatic force and limits the resulting force of the cylinder.

section view of a spring return and spring extend pneumatic cylinder

Schematical section view of a spring extend (top) and spring return (bottom) single acting pneumatic cylinder.

The single acting cylinders can be specified by the following key parameters:

  • Stroke
  • Bore size
  • Rod diameter
  • Spring force
  • System pressure

The stroke is the distance between the end and the base position (length of the movement). The bore size is the diameter of the piston. To choose the right cylinder, please follow the sizing method of the cylinder’s manufacturer. The single-acting cylinder is normally controlled by a three-port valve, for example a pneumatic solenoid valve. One port connects to the source of compressed air, the second port is used to supply/vent air to the cylinder and the third port is an exhaust port.

Basic principle of pneumatic solenoid valves

Operating principle

Pneumatic solenoid valves are used to control the flow direction of compressed air. A moving part inside the valve blocks or opens the ports of the valve. The moving part is called spool or piston. The movement of the spool can be controlled in two ways: direct operation, or indirect operation.

Placeholder for valve picture sg. like this:

With direct operation, the spool is directly actuated by the solenoid. Direct operated valves are independent on the system pressure, and can therefore be used for low pressures or vacuum.

In case of indirect operation, the spool is not directly actuated by the solenoid. The valve makes use of the system pressure to move the spool. Hereto, an additional pilot valve is used. The pilot valve is a small direct actuated 3/2 way valve. The pilot valve provides compressed air to a small air cylinder inside the valve. The compressed air in this cylinder exerts a force on the piston and actuates the spool to switch the valve.. This way, a relatively small solenoid can be used to switch the valve. These valves are referred to as internally piloted, and require an inlet pressure to switch. Therefore, these valves cannot be used for vacuum applications, unless the pilot valve is operated with an external source of compressed air (externally piloted).

Pneumatic Solenoid Valve Types

Several type of pneumatic solenoid valve is available. The valve function is always depends on the application. The most common valve functions in pneumatic systems are the followings:

symbols of 3/2-way solenoid valves

Symbols for 3/2-way solenoid valves; Normally Open & mono-stable (left), Normally Closed & mono-stable (center) and Normally Closed & bi-stable (right).

The 3/2 way valve can be Normally Open (NO) or Normally Closed (NC). A Normally open (valve lets the air flow from port 1 to port 2, when it is not actuated. If the solenoid is energized, the valve switches and the air is vented from port 2 to port 3. A normally closed valve works the opposite way. When the solenoid is de-energized, the air is vented from port 2 to port 3. As soon as the solenoid is energized, the valve switches and compressed air can flow from port 1 to 2. Normally closed valves are the most common. NO/NC valves also exist, these valves can be used both ways (NO/NC).

The 3/2-way solenoid valves can be mono- or bi-stable. Mono-stable valves are often spring return and work like a doorbell: they remain switched/actuated while the solenoid energized. The bi-stable version often has two solenoids and works like a light switch; it is switched with a pulse of one solenoid, and switched back with a pulse of the other solenoid.

symbolic representation of a spring return cylinder controlled by a NC mono-stable 3/2-way solenoid valve

Symbolic representation of a spring return cylinder that is controlled by a mono-stable NC 3/2-way solenoid valve. In the rest state of the solenoid valve (left), the cylinder retracts by spring force and the air can escape through the exhaust. In the energized state of the valve (right), the cylinder is filled with air and extends.

To control a single acting cylinder, a 3/2 valve is used. The 3/2 valve has three ports and two positions. The ports are IN (1 or P), OUT (2 or A) and EXHAUST (3 or R). The valve has two positions: one to pressurize the cylinder (air flows from port 1 to 2, port 3 is closed), and the other to vent air from the cylinder to the exhaust (air flows from port 2 to 3, port 1 is closed). Various fittings can be used to connect to the IN and OUT ports. A muffler can be installed in the exhaust port to reduce the acoustic noise. The valve’s footprint (mounting holes) is specified by the manufacturer or standards, like NAMUR or ISO standard. The valves also can be mounted on base or manifold.

Valve sizing

In order to specify the appropriate size of valve size, the pneumatic system’s air consumption and required air flow has to be calculated. Boyle-Charles’s law (pV=nRT) can be used for the calculation. In case of a single acting cylinder, the cylinder’s volume, the tubing length, the operation frequency, and the system’s loss is relevant for the calculation.

Air consumption per cycle =

$$Q\left [ dm^{3} \right ]=\left ( A\cdot L\cdot \frac{p+0.1}{0.1\cdot b} +a\cdot l\cdot \frac{p}{0.1}\right )\cdot \frac{293}{T}\cdot 10^{-6}$$

Air consumption per minute =

$$Qc\left [ \frac{dm^{3}}{min} \right ]=Q\cdot N$$

Flow rate during working stroke =

$$Qc\left [ \frac{dm^{3}}{min} \right ]=\frac{Q}{t}\cdot 60$$

In which:

  • A: pressure receiving area [mm2 ]
  • a: pipe inner section area [mm2 ]
  • p: supply pressure [MPa] (1MPa = 10bar)
  • N: operation frequency [cycle/min]
  • L: cylinder stroke [mm]
  • l: piping length [mm]
  • t: total stroke time [s]
  • T: temperature [K] (K = °C + 273.15)

The above formulas are valid for A.N.R. conditions. The symbol A.N.R. is a French abbreviation for “conditions de l’atmosphère normale de rèfèrence”, which means “standard reference atmospheric conditions” (20℃, 1013mbar, humidity (relative) 65%).

schematic drawing of pneumatic circuit of single acting cylinder with a 3/2-way valve

Pneumatic circuit with a single acting spring return cylinder and a NC 3/2-way valve.

Example

In this example, a pneumatic system contains a single acting spring return cylinder, with the following parameters:

  • System pressure = 0,5 MPa (5 bar)
  • Temperature = 293K (20°C)
  • Cylinder stroke (L) = 50 mm
  • Bore size / Piston diameter = 40 mm

To connect the cylinder with the 3/2 valve, the following tube is used:

  • Piping length (l) = 2 m
  • Pipe inner diameter (d) = 4 mm

The required operation frequency (N) is 50 cycles/min.

The pressure receiving area can be calculated from the bore size:

$$A=\frac{\pi \cdot D^{2}}{4}=\frac{\pi \cdot 40^{2}}{4}=1256.4 mm^{2}$$

The pipe inner area is:

$$a=\frac{\pi \cdot d^{2}}{4}=\frac{\pi \cdot 4^{2}}{4}=12.57 mm^{2}$$

Now, the air consumption at the time of extrusion (Q) can be calculated. In this case, this is also the air consumption for one cycle:

$$Q=\left ( 1256.64\cdot 50\cdot \frac{0.5+0.1}{0.1}+12.57\cdot 2000\cdot \frac{0.5}{0.1} \right )\cdot \frac{293}{293}\cdot 10^{-6}=0.5dm^{3}$$

The air consumption/minute is:

$$Qm=0.5\cdot 50=25\left [ \frac{dm^{3}}{min} \right ]$$

The total stroke time (t) is 0,6 s.

The required air flow:

$$Qw=\frac{0.5}{0.6}\cdot 60=50\left [ \frac{dm^{3}}{min} \right ]$$

When the air consumption and the required air flow are calculated, the valve can be selected. The valves data sheets have to be checked to find the appropriate valves. All data sheets contain information about the flow rate of the valves (see the picture below). The flow rate of the selected valve needs to be higher than the system’s required air flow. Please keep in mind, all pneumatic system have some air and pressure loss!

Flow diagram

Example of a flow rate diagram with nominal flow rate (L/min) against inlet pressure and pressure loss.

In the example, the valve air flow has to be above 50 dm3/min while the system pressure is 0,5 MPa. The flow rate in dm3/min is equal to l/min. The valve’s data sheet contains the data dm3/min, or l/min. The MPa value also can be changed to bar, so 0,5 MPa is equal to 5 bar pressure. With the help of the flow rate diagram can be found that the smallest valve (type PS-32AS-AM5) is large enough for this system. At 5 bar, the valve is able to deliver about 300 l/min compressed air.

The selected valve is able to actuate the single acting cylinder, but the cylinder’s operation frequency (piston speed) can be higher than the required, as the valve’s flow rate is not the same as the calculated required flow rate. The piston’s speed can be adjusted by a throttle valve that is mounted to the exhaust port of the solenoid valve. The valve’s material, the environment (dust, water or other chemical droplets, medium temperature, environment temperature), IP level, minimum-maximum pressure and voltage are other important factors for selection. The correct valve size itself is not enough to build a proper pneumatic system.

Additional Information

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