Introduction to Pneumatics

Actuators

The single acting cyclinder has only one inlet. If a high pressure (i.e., pressurized air) is applied at the inlet, the cylinder will extend, acting against the spring spring force inside the cylinder and releasing the air directly at the cylinder. If a pressure lower than the atmospheric pressure is applied at the inlet, the cylinder will retract due to the spring force. The single acting cylinder retracts automatically once no high pressure is applied at the inlet anymore.

Fig. 3 FluidSIM single acting cylinder symbol

Fig. 4 The working principle of a single acting cylinder with a 3/2 way valve

The double acting cylinder on the other hand has two inlets. If a high pressure is applied at either one, the cylinder will more (either extend or retract, depending on with inlet recieves the high pressure). Unlike the single acting cylinder, the double acting cylinder will remain in its current position even after no high pressure is recieved through the inlet anymore. This is due to the absense of a spring inside. To reverse the cylinder position, pressurized air must be applied on the opposite inlet.

Fig. 5 FluidSIM double acting cylinder symbol

Fig. 6 The working principle of a double acting cylinder with a 4/2 way valve.

Note

A 5/2 valve is way more commonly used in the industry than a 4/2 valve

Directional Valves

The goal is to control the pressurized air direction to actuate the pneumatic cylinders. The most used valve is a 3/2 way directional valve.

Note

In a 3/2 way directional valve, the ‘3’ stands for the number of connectors and the ‘2’ stands for the number switching positions.

Port 1 has is to be connected with the air inlet (i.e., pressurized air source). Port 2 is considered the working port and is to be connected to a valve (e.g., a single acting cylinder). Finally, port 3 is the exhaust port, which is usually connected to an air outlet object in FluidSim.

Fig. 7 FluidSIM pneumatic circuit with a 3/2 way directional valve connected to a pressurized air source and a single acting cylinder

For a double acting cylinder, a Directional Control Valve DCV (5/2 way valve) is needed. This valve has two outputs and they are connected to the two inlets of the double acting cylinder. The valve has 3 inlets (1 inlet and 2 exhaust outputs).

Fig. 8 FluidSIM pneumatic circuit with a 5/2 way directional valve connected to a pressurized air source and a double acting cylinder

Throttle Valves

The speed is limited by the air pressure, the flowrate and the spring force (in case of a single acting cylinder). The next goal is to control the speed of the pressurized air, and thus the actuation speed of the cylinders. To do that, we use valves to limit the speed of the pressurized air.

A throttle valve limits the air flow in both directions.

A check valve completely blocks any air flow in one direction, while allowing air from with no resistance in the opposite direction.

Note

This is comparable to the diode behavior in electronics, where current is allowed to flow in only one direction and is blocked in the opposite direction.

Fig. 9 A check valve to control air flow to a single acting cylinder

A throttle check valve limits the air flow in only one direction. In the symbol, the air going from 1 to 2 is stopped by the check valve (represented as a circle in the symbol), which blocks air flow completely along that path and thus air has to go through the throttling path. In the other direction (from 2 to 1), the check valve allows the air to pass through it with no resistance.

Fig. 10 The working principle of a throttle check valve

For a double acting cylinder, it is best practice to control the air flow on the outlet path. To control the speed in both the extension and the retraction paths, a throttle check vavle is placed in both paths.