JohdotaA pneumatic cylinder, or air cylinder, makes a straight-line movement: the piston and piston rod move when compressed air is fed to one side of the piston and the other side is let out to exhaust.
Double-acting and single-acting
A double-acting cylinder has two ports, and compressed air moves it in both directions. The extending force is larger than the retracting force, because the area on the piston rod side is smaller. For example, a 32 mm ISO cylinder at 6 bar gives about 480 N out and 415 N in.
A single-acting cylinder pushes with compressed air and returns by spring force. The spring force is small: for example, the spring of a 32 mm single-acting cylinder returns it with a force of 30 N, while compressed air at 6 bar pushes with about 440 N. A single-acting cylinder is controlled with a 3/2 valve.
Control with a 5/2 valve
The ports of a double-acting cylinder are connected to working ports 2 and 4 of a 5/2 valve. With a spring-return valve the cylinder has a rest position: when the coil has no voltage, the air goes to port 2, and when the coil is energised, to port 4. Which end of the cylinder each port is connected to decides whether the rest position is retracted or extended. A valve with two coils leaves the cylinder in the position it was last driven to.
Position sensors
For the position sensors, the cylinder piston has a magnet. Position sensors are fitted to the side of the cylinder, usually in a slot, and they detect the magnet through the cylinder body. When the piston is at a sensor, the sensor output is on and its indicator light is lit. This gives the PLC the end position, and the step sequence moves on only when the movement is complete.
- Reed sensor. The magnet closes a contact (a reed switch) inside the sensor. A reed sensor works on both AC and DC, and no leakage current flows through it when it is open.
- Electronic sensor. Senses the magnetic field without a contact. It is often a three-wire sensor with a PNP output, but two-wire types also exist.
A two-wire sensor gets its supply through the same wires that it uses to switch the input. An electronic two-wire sensor therefore has a voltage drop of a few volts, and a small leakage current even when it is open. If two such sensors are wired in series, the voltage at the input may stay too low.
Speed control
Cylinder speed is set with a one-way flow control valve, which can be screwed straight into the cylinder port. Air flows freely into the cylinder through the check valve, and the air leaving the cylinder is throttled with an adjusting screw (meter-out). Each port gets its own valve, so the speed in each direction can be set separately.
In your region
Showing notes for
United States, Canada
NFPA interchangeable air cylinders are built to the ANSI/NFPA T3.6.7 mounting dimensions, with bores in inches, such as 1.5" and 2". This NFPA standard is about fluid power, not machine wiring like NFPA 79.
Pressure is given in psi. One supplier gives about 60–90 psi at the point of use, from a plant supply of about 100–130 psi.
Source: SMC: NCA1 NFPA interchangeable air cylinders · AutomationDirect: pneumatic system design considerations
UK & Ireland, Europe, Australia & New Zealand, Other IEC countries
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Frequently asked questions
How does a double-acting cylinder work?
A double-acting cylinder has two ports, and compressed air moves it in both directions: air is fed to one side of the piston while the other side is let out to exhaust. The extending force is larger than the retracting force, because the area on the piston rod side is smaller.
How do you control a double-acting pneumatic cylinder?
Connect its two ports to working ports 2 and 4 of a 5/2 solenoid valve. With a spring-return valve the air goes to port 2 when the coil has no voltage and to port 4 when it is energised, so the cylinder has a rest position. A valve with two coils leaves the cylinder in the position it was last driven to.
What is the difference between a single-acting and a double-acting cylinder?
A double-acting cylinder is moved by compressed air in both directions and is controlled with a 5/2 valve. A single-acting cylinder pushes with compressed air and returns by spring force, which is small. It is controlled with a 3/2 valve.
How does a reed switch on a pneumatic cylinder work?
The piston has a magnet, and the reed sensor is fitted to the side of the cylinder, usually in a slot. When the piston is at the sensor, the magnet closes a contact inside it and the sensor tells the PLC that the cylinder is at that end position. A reed sensor works on both AC and DC and has no leakage current when it is open.
How do you control the speed of a pneumatic cylinder?
Fit a one-way flow control valve in each cylinder port. Air flows freely into the cylinder through the check valve, and the air leaving the cylinder is throttled with the adjusting screw (meter-out). With a valve in each port, the speed in each direction can be set separately.
Sources
- Festo DSBC-32-80-PPVA-N3: double-acting ISO 15552 cylinder (data sheet 1376425)
- Festo ESNU-32-25-P-A: single-acting cylinder, spring return (data sheet 196377)
- Festo SME-8M-DS-24V-K-2,5-OE: reed position sensor (data sheet 543862)
- Festo SMT-8M-A-PS-24V-E-2,5-OE: electronic PNP position sensor (data sheet 574335)
- SMC: C96 series ISO cylinders, position sensors and how to connect them to a PLC, catalogue
- Festo GRLA-1/8-QS-8-D: one-way flow control valve for throttling exhaust air (data sheet 193145)
- SMC: SY series 5-port solenoid valves, catalogue (ports 1(P), 2(B), 4(A), 3(EB), 5(EA))
- SMC: NCA1 NFPA interchangeable air cylinders
- AutomationDirect: pneumatic system design considerations