Pneumatic Compressors and Valves: Types & Applications
Pneumatic Compressors
Piston Compressor
This compressor sucks in air at atmospheric pressure and then compresses it. It consists of intake and exhaust valves, a piston, and a connecting rod-crank mechanism.
Admission:
The shaft rotates clockwise. The crank pulls the piston down, and the intake valve lets air in 10º after Top Dead Center (TDC) until Bottom Dead Center (BDC).
Escape:
At BDC, the intake valve closes, and as the piston ascends, it compresses the air. Under pressure, the exhaust valve opens and delivers the compressed air to the consumer.
Two-Stage Piston Compressor
Molecular motion causes a temperature rise, following the law of energy transformation. To achieve higher pressures, temperature reduction is necessary. This type of compressor includes a cooling chamber to cool the air before it enters the second compression stage.
Two-Stage, Double-Acting Piston Compressor
Compression is achieved by a reciprocating piston. The air is sucked in, compressed, cooled, and then passed to another compression stage for higher pressure and performance.
Membrane Piston Compressor
Similar in operation to the piston compressor, this type uses a suction membrane driven by a reciprocating motion. Its advantage is the absence of oil in the delivered air.
Radial Vane Compressor
An eccentric rotor with vanes rotates within a cylindrical housing. The rotating seal is maintained by centrifugal force as the vanes press against the wall. Aspiration occurs when the chamber volume is large, and compression happens as the volume decreases towards the exit. Pressures of 200 to 1000 kPa (2 to 10 bar) can be achieved with flow rates between 4 and 15 m³/min.
Screw Compressor
Suction and compression are performed by two interlocking screws. Compression occurs axially. Pressures of 1000 kPa (10 bar) and flow rates between 30 and 170 m³/min are possible.
Roots Compressor
Two rotors rotating in opposite directions enclose a volume of air between the wall and their profiles with each rotation. This air volume is then delivered at the desired pressure.
Turbo Compressor
This compressor is a three-stage turbine. Air is drawn in, and its pressure increases approximately 1.3 times in each stage. Cooling chambers are necessary between the stages.
Radial Turbocharger
Intake air is introduced axially at high speed, and compression occurs radially. This type is recommended for high flow rates.
Axial Turbocharger
This compressor operates on the principle of a fan, drawing and driving air simultaneously. Pressures are low, but flow rates can be very high.
Pneumatic Cylinders
Single-Acting Cylinder
These cylinders consist of a cylindrical tube, a bottom and front cover with a piston rod bearing, a rod, a return spring, a bearing cap, and a scraper gasket. When compressed air is applied to the rear of the piston, the rod extends. When the air is vented, the piston returns to its original position due to the spring. Due to the spring’s length, these cylinders are used for strokes up to approximately 100 mm in a single direction.
Application:
These cylinders are suitable for tasks requiring movement in one direction, such as pulling, pushing, inserting, and holding.
Single-Acting Membrane Cylinder
A rubber, plastic, or metal membrane acts as the piston in these cylinders. A clamping plate serves as the rod and is attached to the membrane. The stroke is performed by the membrane’s internal tension. Membrane cylinders are limited to very short strokes.
Application:
Stamping, riveting, and primarily holding.
Single-Acting Cylinder with Return Spring
A rubber or plastic membrane is clamped between two metal casings. The rod is fixed to the center of the membrane. The stroke is performed by the return spring, aided by the membrane’s tension. Friction occurs only in the rod’s guide bearing.
Applications:
Stretching, newspapers.
Single-Acting Cylinder, Overwhelming Membrane
This cylinder features a glass-shaped membrane. Introducing compressed air causes the membrane to expand against the cylinder’s inner wall. Like the previous type, it offers minimal friction and maximum sealing. The stroke is short, and the construction is very simple.
Double-Acting Cylinder
These cylinders consist of a tube, a rear cover, a front bearing cap, a lip seal, a scraper gasket, and a piston rod with a double lip seal. When compressed air is supplied to the rear and the front is vented, the rod extends. When air is supplied to the front, the rod retracts. At equal pressure, the piston’s force is greater during extension than retraction due to the larger surface area on the rear.
Application:
Suitable for applications requiring work in both directions. They offer longer strokes than single-acting cylinders.
Double-Acting Cylinder with Double Internal Damping
These cylinders are necessary when moving large masses with double-acting cylinders. In addition to the standard components, they include a cylinder head with non-return valves, an adjustable throttle, and a buffer piston. Before reaching the end position, the piston interrupts the direct air outlet to the buffer. This creates an air cushion due to overpressure in the remaining cylinder space, converting kinetic energy into pressure as the air can only escape through a small section. When the airflow reverses, the air flows freely through the non-return valve, and the piston extends again with full force and speed.
Compact Double-Acting Cylinder
The cylinder tube and bottom cover are integrated. The piston is guided in the tube by plastic rings. This cylinder’s advantage is its small size compared to conventional cylinders.
Double-Acting Cylinder Suitable for Contactless Sensing
A permanent magnet is embedded in the piston rod, whose magnetic field activates proximity switches. One or more proximity switches can be mounted on a guide bar within the cylinder, depending on the stroke length. These contactless switches can detect the cylinder’s end or intermediate positions.
Double-Acting Cylinder with Double Rod
This design can withstand greater transverse bending moments than standard double-acting cylinders because the rod is doubly supported. Both piston surfaces are equal, resulting in equal forces. When space is limited, cams can be attached to the rod ends for control and signaling.
Rotating Cylinder (Vane Type)
This type of cylinder can achieve rotary movements up to 300°. However, they are less common in pneumatics due to sealing difficulties and the limited torque they can produce relative to their size.
Rotating Cylinder (Rack and Pinion Type)
In this design, the piston rod is a rack that meshes with a gear wheel, converting linear motion into rotary motion. The rotation angle depends on the piston stroke and the sprocket radius. The available torque on the output shaft depends on the piston surface area, pressure, and sprocket radius.
Usage:
Tube bending, gate operation, etc.
Rotating Cylinder (Dual Rack and Pinion Type)
The cylinder pistons are connected by a common rack. A sprocket engages with both racks. When compressed air is introduced into one chamber, the piston moves, and the force is transmitted through the rack and gear. Introducing compressed air into the opposite chamber reverses the wheel’s rotation. Using two units doubles the torque.
Disadvantage:
Small backlash compensation.
Telescopic Cylinder
This cylinder consists of cylindrical tubes and piston rods. During extension, the inner piston moves first, followed by the subsequent rods or tubes from the inside out. The retraction of the telescopic rods is done by external forces. The application force is determined by the surface area of the smallest piston.
Application:
Situations requiring a long lifting stroke with a relatively short cylinder structure (e.g., platform lifts).
Vane Motor
This motor consists of a rotor cylinder and two end caps with bearings. The rotor has slots in which vanes slide. The rotor is eccentrically mounted to the cylinder axis. The vanes are pressed against the cylinder’s inner wall, forming working chambers of varying sizes. Introducing compressed air into the lower chamber generates torque due to surface and radial forces. For rotational motion, the chamber expands as the air expands. Vane motors operate at relatively high speeds, are reversible, and cover a wide power range.
Advantages:
- Simple construction
- Low weight per unit power
- Overload protection
- Continuously adjustable
Radial Piston Engine
The main components are radially arranged cylinders, connecting rods, a crankshaft, an air distributor valve, and bearings for synchronous operation. The control valve is powered by a fixed sequence of two pistons that perform the power stroke. Using five cylinders ensures uniform rotation. High starting torque is a characteristic feature of radial piston engines.
Pneumatic Control Valves
2/2 Control Valve, Normally Closed, Ball Seat
A spring presses the ball against the seat, closing the airflow from P to A. When the cam is depressed, the ball is lifted from its seat, overcoming the spring force and the pressure on the ball. The simple structure allows for a compact design.
3/2 Control Valve, Normally Closed, Ball Seat
The spring-loaded ball prevents airflow from P to A. A is connected to the atmosphere (R) through the cam’s internal bore. When the cam is actuated, it first closes the passage between A and R, then the ball allows airflow from P to A. Reversing the cam closes P to A first and then opens A to R. The valve operates without interference during air supply and exhaust.
3/2 Control Valve, Normally Closed, Flat Seat
This valve features a plate pressed against the seat by a spring. Compressed air pressure provides additional sealing force. Poppet valves are characterized by a large cross-sectional area, short strokes, and insensitivity to impurities (resulting in a long service life). The 3/2 valve closes the passage from P and connects A to R. When the cam is depressed, it first closes A to R and then opens P to A.
Application:
Control of single-acting circuits and as a signal element for actuating pilot-operated valves.
3/2 Control Valve, Normally Open, Flat Seat
In the resting position, the passage from P to A is open, and A is connected to the exhaust R. When the lever is pressed, the first plate closes P to A, and the second plate, connected by a shaft, opens A to R.
3/2 Control Valve, Pneumatically Operated
This normally closed valve is actuated by compressed air (Z). The pressures P and Z keep the piston in a locked position. The control piston’s surface area must be sized to ensure reliable switching at pressures equal to P and Z.
3/2 Control Valve, Pneumatically Operated (Membrane)
This valve operates using a membrane. Its large surface area allows switching with a Z pressure of 120 kPa (1.2 bar) and a supply pressure of 600 kPa (6 bar). By swapping P and R, the valve can be used as normally open.
4/2 Control Valve, Pneumatically Operated
This valve, actuated by compressed air, has two control pistons. The left piston opens A to R, and the right piston allows P to B. The pistons are driven by compressed air supplied to the membranes through Z. Switching occurs after venting Z and the return of the pistons. The membranes, due to their internal pressure, return to their original positions.
3/2 Valve, Solenoid Operated
Solenoids are used for valve actuation when the control signal comes from an electrical element, such as limit switches, pushbuttons, timers, or programmers. They are particularly useful for long control distances. Without coil excitation, the core, under spring force, closes the connection to P, and A is vented through R. Energizing the solenoid draws the armature inward, closing R and connecting P to A. The valve is not interference-free.
3/2 Control Valve, Roller Actuated, Pilot Operated
The actuating force required for seat valve control increases with pressure. This force can be reduced by using smaller pilot valves. During operation, the small valve opens the connection from P to the piston’s membrane, closing R and connecting P to A. The force on the roller actuator is 1.8 N (180p) at a pressure of 600 kPa (6 bar). The valve can be made normally open by rotating the pilot head 180°, with R becoming the inlet and P the exhaust.
4/2 Control Valve, Roller Actuated, Pilot Operated
In this 4/2 valve, the membranes are driven by a 3/2 pilot valve. The actuating force is similar to the previous example, ranging from 10 N (100p) at a pressure of 800 kPa (8 bar).
4/2 Control Valve, Solenoid Actuated, Pilot Operated
This 4/2 valve is controlled by a solenoid-operated 3/2 valve. The main pistons are driven by compressed air when the solenoid is energized. Pilot operation allows for rapid actuation with relatively small solenoids.
5/2 Control Valve, Pneumatic (Membrane)
In this membrane seat valve, all seat connections are normally closed. The valve is alternately switched by the Z and Y inputs. The control pistons, due to the membrane tension, maintain their position until a counter-signal is received. The valve has memory functionality.
5/2 Control Valve, Sliding
These valves are characterized by the transverse movement of the slide relative to the valve body. They can be actuated by various means. The actuating force only needs to overcome the friction between the pistons and the body. In pneumatic pilot valves, the pilot pressure can be lower than the working pressure, but the stroke is longer than that of seat valves.
Different Types of Sliding
The sealing of the slide within the valve body can be achieved through precise adjustment. The clearance is between 0.002 and 0.004 mm to minimize leakage losses. Instead of expensive adjustments, O-rings are often used for sealing, reducing the risk of internal wear. Small holes are arranged in the shape of the output port.
4/2 Control Valve, Side Cursor
This valve is switched by applying pressure pulses to the Z and Y ports, which actuate a double control piston. The work ports A and B are connected by a flat cursor to the exhaust R. When worn, the cursor automatically resets under the influence of an integrated spring and the applied pressure, ensuring a long service life. A pressure pulse to either Z or Y is sufficient for switching.
4/2 Control Valve, Electrical Impulse, Pilot Operated
This valve is switched by a flat cursor controlled by integrated solenoid-operated 3/2 valves. When the solenoid is energized, the pilot air performs the switching, and the valve remains in that position until the next pulse is received.
3/2 Control Valve, Sliding (Spring Return)
This spring-return valve can be used as a signal element for pulse-operated valves.
Inactive: P closed, A vented to R.
Active: P connected to A, R closed.
3/2 Control Valve, Sliding (Manual)
This simple manual valve is used as a main shut-off and venting valve in pneumatic systems. Moving the outer sleeve to the left connects P to A through an annular channel. Returning the sleeve to its original position closes P and connects A to R, venting the system.
4/3 Control Valve, Disc
These valves are typically manually or pedal operated. In the middle position, both work channels are vented. The piston of a double-acting cylinder without air supply will remain in a fixed position, allowing for external manipulation.