Pneumatic Systems

Pneumatic systems are defined as systems that operate using compressed air. Pneumatic systems typically include cylinders, valves, and other mechanical components that are actuated by air or gas pressure.

Compared to other energy sources, air energy is readily available, inexpensive, and allows for faster operation in compact and short spaces. Air is drawn from the atmosphere using appropriate methods, stored, pressurized, and then released back into the atmosphere. The abundance of air in the atmosphere and its low cost of acquisition ensure economical operation. Pneumatic systems are used for various tasks in nearly every industry, including clamping and releasing workpieces, moving them, and achieving linear and rotary motion.

The fundamental operating principle of pneumatic systems is to use air or gas pressure to move a piston inside a cylinder. This movement generates force by pushing or pulling the piston within the cylinder. The force is applied at an connection point at the cylinder’s output, enabling mechanical motion.

Pneumatic systems generally consist of a compressor, an air reservoir, a control valve, and a cylinder or actuator. The compressor compresses atmospheric air or gas to generate air pressure in the reservoir. The air reservoir stores the compressed air or gas and prepares it for use. The control valve regulates the flow of air pressure from the reservoir to the cylinder. The cylinder or actuator then converts air or gas pressure into mechanical motion.

Compressed air used in pneumatic systems is supplied by compressors. Air production is typically provided by a centralized pressure source and delivered to the system via pipes or hoses. This eliminates the need for individual pressure sources for each user. For mobile machines or hand tools, portable compressors are used.

In selecting a compressor, determining the facility’s air demand (capacity) is a critical factor. Choosing a compressor with lower capacity than required reduces production efficiency and shortens the compressor’s lifespan due to frequent cycling. Choosing a compressor with significantly higher capacity than needed increases investment and operating costs.

^{Compressed Air Distribution Diagram (Credit:} Electrical-Electronic Technology, MEB, 2011.⁾

For system efficiency, it is essential to distribute the compressed air produced in pneumatic systems in a way that minimizes losses. When assessing current system requirements, future growth must also be considered. The system should be designed with future expansion in mind from the outset. Potential leaks in the compressed air distribution network must be anticipated from the beginning. Otherwise, future maintenance expenses and additional system installations may result in significantly higher costs.

^{Compressed air distribution network (Credit:} Electrical-Electronic Technology, MEB, 2011.⁾

Compressed air receivers are installed at the outlet of compressors. They prevent pressure fluctuations, provide additional surface area for cooling the compressed air, and facilitate condensation and separation of moisture.

Compressed air receivers can be horizontal or vertical. The air outlet must always be located at the top of the receiver. This prevents condensed water inside the receiver from entering the system.

^{Installation configurations of compressed air receivers (Credit:} Electrical-Electronic Technology, MEB, 2011.⁾

Three methods are used for drying air.

In this drying method, air is passed through a chemical substance that forms compounds with water. This chemical substance is sodium chloride, commonly known as “salt.” Water vapor in the air reacts chemically with this substance as it passes through and separates from the air, collecting at the base of the dryer as a solution. The solution must be periodically drained. Since salt gradually depletes, it must be replenished at regular intervals. The temperature of the incoming compressed air must not exceed 30°C. A filter must be installed after the dryer to capture any salt particles that may be carried into the air stream. Oil is also removed in this process. Due to the potentially harmful effects of excessive oil on the dryer, a fine filter must be installed at the inlet.

In this method, air is passed through a substance composed of silicon dioxide (silica gel). Water vapor is absorbed by the silica gel, which exists in granular form within the dryer. No chemical reaction occurs in this process. When silica gel comes into contact with water vapor, it changes color. Once it reaches saturation, it must be regenerated. Typically, two parallel dryers are used in this method. The saturated dryer is taken offline and dried by passing hot or cold air through it. The regenerated silica gel returns to its original color. Meanwhile, the other dryer remains in operation. Over time, silica gel particles may wear down and become airborne due to air flow. Therefore, a filter at the outlet is beneficial. Silica gel contaminated or worn by oil and other impurities should be replaced every one to two years.

^{Chemical Drying Method and Physical Drying Method (Credit:} Electrical-Electronic Technology, MEB, 2011.⁾

This method is based on cooling the air to its dew point, where water vapor condenses. The incoming air is first cooled slightly in a heat exchanger and then further cooled to approximately 1.7–5°C in a refrigeration unit. Condensation occurs simultaneously with cooling, separating the water vapor. The cooled air is then used to pre-cool the incoming air. A filter is used to remove dirt and oil particles from the cooled air. This method captures 80–90% of the oil originating from the compressor. It is the most widely used drying method due to its economic efficiency.

^{Cooling Method (Credit:} Electrical-Electronic Technology, MEB, 2011.⁾

The air delivered to the point of use passes through a conditioning unit, known as an air preparation unit, consisting of a filter, pressure regulator, and lubricator, to achieve its final properties. An air preparation unit can also be briefly referred to as an FRY unit (Filter, Regulator, Lubricator).

The first component of the air preparation unit. Filters are used to separate foreign matter and water from compressed air. As air enters the filter, it passes through an inlet swirl chamber, creating a rotational motion. Solid particles and water collect on the inner wall of the housing due to centrifugal force. The air then passes through a sintered bronze filter element before exiting the filter. There are two methods for draining accumulated water from the housing. In manually drained filters, water must be drained before reaching the permissible level by opening the drain plug located at the bottom of the housing. In automatically drained filters, this process occurs automatically without human intervention.

Each pneumatic circuit has an optimal operating pressure. Excessively high pressure causes energy loss and rapid wear, while excessively low pressure prevents proper function or at least reduces efficiency. Since pressure in the compressor reservoir constantly fluctuates, a pressure-reducing valve (regulator) is required to prevent these fluctuations from affecting the system. Regardless of the inlet pressure, the outlet pressure remains constant at the value set on the regulator’s gauge. Regulators are divided into two types: vented and non-vented.

Lubricating a portion of the air used in pneumatic systems offers advantages such as minimizing wear, reducing losses due to friction, and providing protection against corrosion. Lubricators typically operate on the Venturi principle. As air passes through a narrow section, its velocity increases and pressure drops. This pressure drop creates suction from a fine oil line, causing oil to drip into the air stream. However, for dripping to occur, the air flow rate must exceed a minimum threshold; otherwise, the pressure drop is insufficient to draw oil. For proper lubrication, the lubricator’s adjustment screw is typically set to deliver 1–12 drops per 1000 liters of air. The viscosity of the oil used in lubricators at 20°C should be approximately 10–50 cSt. The distance between the conditioner and the farthest user should not exceed 5 meters; beyond this distance, the effectiveness of the sprayed oil diminishes.

^{Pneumatic System Operating Diagram (Credit:} Maktoloji⁾

Pneumatic cylinders convert compressed air energy into linear pushing or pulling motion. A pneumatic cylinder consists of front and rear caps, a cylinder barrel, a piston rod, and sealing elements.

• Single-Acting Cylinder: In this type of cylinder, compressed air acts in only one direction; thus, there is only one port for air inlet and outlet. This results in motion in only one direction. Return of the piston rod is achieved either by a spring or an external force such as the weight of the load. Sometimes the spring is mounted on the piston side to provide a pulling action. The spring stiffness is selected to ensure sufficient speed of the piston rod extension.

• Double-Acting Cylinder: In this type of cylinder, the force generated by air pressure and piston surface area moves the piston rod in both directions, enabling work in both directions. The force in each direction differs depending on the surface area exposed to pressure. There are two inlet and outlet ports on the cylinder. Double-acting cylinders are especially used when the piston rod must perform work during its return stroke. During operation, when air is supplied to the piston side, air on the piston rod side is exhausted, or vice versa.

• Cushioned Type Cylinders: When heavy loads are moved by a cylinder, cushioning is applied at the end of the stroke to prevent impact or damage. Just before the end of the stroke, a cushion pad closes the port through which air would normally escape freely. In this case, air escapes only through a very small, often adjustable, orifice. The trapped air mass compresses between the piston and the cylinder cap. On the return stroke, air passes through a check valve to continue its flow. Cushioning can be applied in one or both directions.

• Double-Rod Cylinder: In this cylinder, the piston rod is supported on both ends. This arrangement counteracts any lateral loads. Since the surface areas on both sides are identical, the forces and speeds generated are equal.

• Tandem Cylinder: This cylinder contains two double-acting cylinders connected in series within the same housing. Air is simultaneously supplied to the piston rod sides of both cylinders. This effectively doubles the surface area exposed to air pressure, thereby increasing the force on the piston rod. These cylinders are used when the piston diameter is large or when space constraints prevent installation of a larger single cylinder.

• Multi-Position Cylinders: These consist of at least two double-acting cylinders mounted sequentially within the same housing. One cylinder’s piston rod is fixed to a pivot, and work is performed by the other cylinder’s piston rod. In some applications, the number of cylinders is increased to achieve up to 12 positions.

• Rotary Cylinder: In this variant of the double-acting cylinder, the piston rod has a gear profile at its end. This drives a gear wheel, converting linear motion in both directions into rotary motion. Common angular displacements are 45°, 90°, 180°, 270°, and 720°. These cylinders are used for pipe bending, rotating workpieces, controlling air conditioning units, and operating valves.

^{Types of Pneumatic Cylinders (Credit:} Electrical-Electronic Technology, MEB, 2011.⁾

Air motors convert compressed air energy into rotary motion. Their most important features are their compact size relative to their power output and their ability to be easily controlled over a wide speed range due to their favorable torque characteristics. These motors can operate under harsh conditions such as heat, moisture, dirt, and vibration, and can be loaded to a complete stop without damage. They do not emit toxic gases and pose no explosion risk. Due to their adaptability, air motors are used across a wide range of applications, from large cranes on oil drilling platforms to small motorized air screwdrivers.

• Radial Piston Motors: In piston motors, the energy of air is converted into mechanical rotary motion via pistons and a crankshaft. A large number of pistons are required for smooth operation. The motor’s power depends on the inlet pressure, number of pistons, piston surface area, and piston speed.

• Axial Piston Motors: The operating principle of axial piston motors is similar. For smooth operation and balanced torque distribution, two pistons are pressurized simultaneously. These air motors can be configured to rotate either clockwise or counterclockwise, with speeds of approximately 5000 rpm. They are used in applications requiring heavy loads, such as cranes, concrete breaking, and drilling.

• Vane (Lobe) Air Motors: They are preferred due to their simple construction and low weight. The sliding vane type is most commonly used. A rotor is mounted eccentrically within a cylindrical chamber. Vanes are fitted into slots on the rotor. During operation, centrifugal force pushes the vanes against the inner wall of the cylinder, creating a seal. These motors operate between 3000 and 9000 rpm and can rotate in both directions.

• Gear Air Motors: Rotational motion is achieved through two meshing gears subjected to air pressure. One gear is connected to the output shaft. They are manufactured in spur gear, helical gear, and double helical gear configurations. They are used in applications requiring high power. Due to these characteristics, they are used in mining, conveyor belt systems, drilling and breaking tools, petrochemical industries, and iron and steel industries for cranes, overhead bridges, mixers, and starting systems of large diesel engines.

• Turbine-Type Motors: These operate on the reverse principle of axial compressors. They are used in applications requiring low power and have very high rotational speeds, reaching up to 500,000 rpm. They are used for diamond cutting, grinding, and dental milling.

^{Types of Pneumatic Motors (Credit:} Electrical-Electronic Technology, MEB, 2011.⁾

Pneumatic control circuits consist of a signal element, a control element, and an actuating element. The signal and control elements govern the motion of the actuating element. In pneumatic applications, these are referred to as valves. A valve controls the pressure, flow rate, direction, and start-stop conditions of a fluid from a hydraulic pump or pressurized tank. Based on their operating principles, valves are classified into three groups: directional control valves, flow control valves, and pressure control valves.

Squares are used to represent valves symbolically. These squares convey information about the valve’s function, not its construction.

Directional control valves are manufactured in two construction types: seat-type valves and slide-type valves.

• Seat-Type Valve: In this type of valve, the flow passage is sealed by a ball, disc, plate, or plug. A gasket is used for sealing. Since there are no interacting internal components, these valves have a long service life. They are not sensitive to dirt or dust. The force required to actuate the valve varies according to the stiffness of the internal spring.

• Ball-Type Seat Valve: The valve body has two connection ports and two positions, making it a 2/2 valve. Initially, the ball seals the flow passage due to the spring below it. Pressurized air from the P line cannot reach the A line. When the valve’s actuating pin is pressed, the ball is pushed downward, connecting the P and A lines. As long as the pin is pressed, the P line remains open to the A line. When the actuating force is removed, the spring returns the ball to its original position, closing the P line.

• Disc-Type Seat Valve: The flow passage is sealed by a sealing element mounted on a disc. Initially, with no actuating force on the pin, the P line is open to the A line; thus, this valve is normally open. When the pin is pressed, the sealing element beneath the disc moves downward, closing the P pressure line. Air in the A line is exhausted through a vent next to the pin. When the actuating force is removed, the valve returns to its normal position.

• Slide-Type Valve: In this type of valve, connections to each line are made via a control piston (slide). Sealing is achieved using O-rings placed on grooves of the slide. Sometimes a bronze bushing is inserted into the valve body to allow the O-rings to move within it. This ensures good sealing and reduces the force required to move the slide.

^{Directional Control Valves (Credit:} Electrical-Electronic Technology, MEB, 2011.⁾

Pressure control valves are rarely used in pneumatic systems. A safety valve releases air to the atmosphere when the pressure reaches a preset value. They are used on air tanks.

• Check Valve: This valve allows flow in one direction only and prevents reverse flow. It is also known as a non-return valve. It is manufactured as either a ball-type or flap-type valve. Initially, a spring pushes the flap toward the flow passage. When air pressure from the left overcomes the spring force, the element moves right, opening the passage. When air is supplied from the right, the air pressure and spring force together close the flow passage.

^{Pressure Control Valves (Credit:} Electrical-Electronic Technology, MEB, 2011.⁾

• Speed Control Valve: This component contains a check valve and controls the speed of the actuating element by restricting flow in one direction. In the restricted direction, fluid is forced through an adjustable orifice. In the opposite direction, the check valve opens, allowing unrestricted flow.

• Flow Restriction Valve: This valve is used when speed control is required at any point in the stroke of single-acting or double-acting cylinders. In double-acting cylinders, it can be used for cushioning when controlling large masses. For this purpose, a cam mechanism mounted on the piston rod presses against the valve’s roller, pushing a conical adjustment slide downward. This achieves the desired speed setting. Adjustment is possible by changing the cam’s contact length or the roller’s contact length with the slide. Due to the internal check valve, speed control is possible in only one direction.

• Quick Exhaust Valve: Used to increase speed in cylinders. In some applications, no work is performed during the cylinder’s return stroke, and reducing this dead time is desired. The quick exhaust valve is mounted close to the cylinder. This allows air from the cylinder to exhaust through the quick exhaust valve rather than through the directional control valve. The valve has three connection ports: P, A, and R. The A port connects to the cylinder. The R port is open to the atmosphere. When pressurized air is supplied through the P port, the internal movable sealing element closes the R port, filling the cylinder. During return, pressure in the A port moves the sealing element to close the P port, allowing air in the cylinder to rapidly exhaust through the R port.

• OR Valve: This valve is used in pneumatic circuits for logical operations. It has three air connection ports: X and Y are inlet ports, and A is the outlet port. By design, air supplied from either X or Y results in an output at A. Thus, air signal from either inlet port is sufficient to produce an output at A.

• AND Valve: This logic valve also has three air connection ports: X and Y as inlets, and A as the outlet. However, an output at A is obtained only when air is supplied simultaneously to both X and Y. If air is present only at X, the internal movable element disconnects X from A, preventing output at A. Only when air is also supplied to Y will an output be obtained at A. The same applies if air is supplied only to Y.

^{Flow Control Valves (Credit:} Electrical-Electronic Technology, MEB, 2011.⁾

• The source of pneumatic energy, air, is available in unlimited quantities in the atmosphere.
• Air is clean and does not pollute the environment.
• Speed and force can be adjusted to suit the application.
• Compressed air can be transported over long distances.
• High speeds can be achieved with pneumatic systems.
• There is no risk of fire from compressed air.

• Because air is compressible, it is not always possible to achieve the desired piston speed.
• Operating pressure is typically 6–7 bar, limiting the generation of large forces.
• Air is exhausted to the atmosphere after use, resulting in air consumption.
• When mufflers are not installed on exhaust lines, noisy operation occurs.

Pneumatic systems, widely used in industrial applications, are continuously updated and improved to enhance efficiency. The primary sectors using these systems are automation and robotics. Other sectors include food processing, textiles, cleaning, painting, filling, and packaging machines, assembly lines, and construction machinery.