Pneumatic System Components Chapter 11 Pneumatic System Components Pneumatic Systems • Control Devices • Work Devices

Pneumatic systems contain components that compress and store the air that is used to increase system pressure to accomplish work. Pneumatic systems use gas under pressure to accomplish work. The main components of a pneumatic system are a power source (prime mover), an air compressor, and a receiver. See Figure 11-1.

A single large compressor or multiple small compressors can be installed depending on the needs of a facility. Facilities that use pneumatic systems have at least one compressor. A facility with a single compressor system uses a large centralized loop system. A facility with multiple compressors uses smaller decentralized loop systems that provide the entire facility with compressed air, depending on its size and needs. See Figure 11-2.

Air compressors are either nonpositive displacement or positive displacement. Air compressors are either nonpositive displacement or positive displacement. See Figure 11-3. A nonpositive-displacement air compressor is a type of dynamic air compressor that moves large volumes of air using high rotational speeds. The most common type of nonpositive-displacement air compressor is a centrifugal air compressor. A centrifugal air compressor is a type of dynamic air compressor that uses an impeller rotating at high speed to compress air. The rotation of the impeller increases air speed, and the diffuser of the compressor changes the speed of the air into pressure and flow.

A reciprocating-piston air compressor compresses air by extending and retracting a piston inside a cylinder. A reciprocating-piston air compressor consists of a crankshaft, connecting rod, piston, piston rings, cylinder, inlet valves, output valves, and unloading valves. See Figure 11-4. A reciprocating-piston air compressor operates through the following procedure: 1. The prime mover rotates the crankshaft. As the crankshaft rotates, the connecting rod moves downward and retracts the piston. 2. As the piston retracts, a vacuum is created inside the cylinder. A pressure differential is created because the pressure inside the cylinder is less than atmospheric pressure. …Complete procedural list on page 347.

Air compressor cylinders often have cooling fins to dissipate some of the heat created by friction and air compression. When a reciprocating-piston air compressor is operating, its piston extends and retracts hundreds of times per minute. The resulting friction causes heat to build up. To control and minimize heat, the compressor cylinders typically have water jackets to help reduce the buildup of heat in the compressor. Compressors that are air-cooled often have cooling fins added to dissipate some of the heat of compression. See Figure 11-5.

A multistage air compressor can be used to efficiently compress air to high levels. An intercooler is a type of pipe that is used to connect the different stages of an air compressor to cool the air that travels between the stages. An intercooler allows the heat from the first stage of compression to be dissipated before the air enters the second stage of compression. Also, both cylinders typically have cooling fins that are similar to the heat sinks on a single-stage compressor to help dissipate heat. See Figure 11-6.

In small commercial units, the prime mover is usually connected to the air compressor with V-belts pulleys. The prime mover for a multistage air compressor is typically a 3 AC electric motor. In small commercial units, the prime mover is usually connected to the air compressor with V-belts pulleys. See Figure 11-7. As the prime mover rotates, it moves the V-belts that are attached to the air compressor pulley. The V-belts must be routinely inspected for cracks and tension to ensure that they are in good operating condition. In large industrial units, the prime mover is usually connected to the air compressor by a rigid coupling or drive shaft. When using V-belt drives, the prime mover and air compressor are positioned adjacent to each other.

A rotary-vane air compressor uses sliding vanes to decrease volume as the rotor rotates. A rotary-vane air compressor consists of an offset rotor, an inlet, an outlet, sliding vanes, a cam ring, and the air compressor housing. See Figure 11-8. The airflow from a rotary-vane air compressor can be adjusted by changing the offset of the rotor and cam ring with a pressure adjustment screw. Rotary-vane air compressors can be single-stage (up to 50 psi) or multistage (50 psi to 125 psi). A single-stage rotary-vane air compressor operates through the following procedure: 1. The prime mover rotates the rotor shaft that is attached to the rotor. Complete procedural list on page 350.

A rotary-screw air compressor traps air between the screw shafts and compresses air as the volume between the screw shafts is reduced. A rotary-screw air compressor is a type of positive-displacement air compressor that uses two intermeshing screw shafts or a screw shaft with two rotors to create airflow. Rotary-screw air compressors with two intermeshing screw (male and female) shafts are the more common design. The air is forced to flow along the screw shafts and is compressed as the volume between the two shafts decreases. Airflow is positive and continuous because air is constantly pushed and forced axially along the screw shafts. See Figure 11-9.

Oil-flooded compressing mechanisms lubricate the rotors and rotor end bearings by injecting a synthetic oil bath into their internal passages. Oil-flooded compressing mechanisms lubricate the rotors and rotor end bearings by injecting a synthetic oil bath into their internal passages. The synthetic oil bath lubricates the rotors, seals the rotor clearances, and cools air temperature by absorbing the heat from compression. The synthetic oil is then separated from the compressed air, filtered, cooled, and returned to the air compressor for reuse. See Figure 11-10. Synthetic oil used in pneumatic systems must be in compliance with ISO Standard 32/46, Synthetic Compressor Oil.

Many receivers are cylindrical steel tanks that are used to store compressed air until it is needed to accomplish work. A receiver is a container that holds gas in a pneumatic system. Many receivers are cylindrical steel tanks. See Figure 11-11. After air enters an air compressor and is compressed, it can flow into a system and accomplish work, or it can flow into a receiver and be stored until needed.

A pressure switch electrically controls the energizing or de-energizing of a prime mover when a set pressure has been reached. For a 25 HP or less electric motor on a reciprocating-piston air compressor, a pressure switch is used for automatic start-stop control and the cycling of the unit. A pressure switch is a switch that electrically controls the energizing or de-energizing of a prime mover when a set pressure has been reached. A pressure switch is either located between the air compressor and the receiver or is attached directly to the receiver. A pressure switch consists of an inlet, adjustable biasing spring, electrical limit switch, and a diaphragm or piston. See Figure 11-12.

Once system pressure is high enough to overcome the biasing spring, a poppet in the safety valve opens and air from the receiver is exhausted into the atmosphere. A safety valve is a normally closed pressure control valve that is not adjustable and is used as overpressure protection on pneumatic components. The manufacturer of the valve presets the setting of the safety valve at a pressure that protects the pneumatic component it is attached to from overpressurization. Once system pressure is high enough to overcome the biasing spring, the safety valve opens and air from the receiver or any other pneumatic component is exhausted to the atmosphere. See Figure 11-13. Some safety valves include warning systems, that sound an audible alarm such as a whistle, bell, buzzer, or siren.

The different types of relief valves used in pneumatic systems are poppet, ball, and diaphragm. The different types of relief valves used in pneumatic systems are poppet, ball, and diaphragm. See Figure 11-14. Poppet relief valves and ball relief valves are installed in small pneumatic systems that are 25 HP or less. These two types of relief valves are used because they exhaust air according to how much pressure is in the receiver. Diaphragm relief valves are used in large pneumatic systems that are more than 25 HP because they can exhaust a larger volume of air than similarly sized poppet or ball relief valves. Diaphragm relief valves are typically installed in systems where there is a large volume of airflow.

Two-position, three-way directional control valves and three-position, four-way directional control valves are commonly used in pneumatic systems. Pneumatic directional control valves typically have either two or three positions. See Figure 11-15. Two-position, three-way directional control valves and three-position, four-way directional control valves are commonly used in pneumatic systems. A two-position, three-way directional control valve has a pressure port (P), a cylinder port (A), and an exhaust port (EX). A two-position, three-way directional control valve is either normally open or normally closed.

A muffler is attached to an exhaust port of a directional control valve to deaden the noise of air as it exhausts. A muffler is a device that is attached to an exhaust port of a pneumatic directional control valve to deaden the noise of air as it exhausts. All exhaust ports in a pneumatic system must have mufflers attached to them. See Figure 11-16. Many mufflers also include a method of flow control.

A needle valve is the most common type of pneumatic metering valve and is available with or without a check valve. A metering valve is a pneumatic flow control valve that controls the amount of airflow through a specific line at a given time. See Figure 11-17. A needle valve is the most common type of metering valve and is available with or without a check valve. A needle valve can be used to control the amount of airflow in one direction. It is common to use a needle valve without a check valve to control the speed of small cylinders at the exhaust port of a directional control valve.

Although there are many similarities between pneumatic and hydraulic cylinders, there are also several differences. A pneumatic cylinder is an actuator that moves in a straight line using compressed air. Pneumatic cylinders have similar parts as hydraulic cylinders, which include a cylinder barrel, a piston and rod, cap-end and rod-end ports, piston rings, cylinder seals, and end caps. However, pneumatic cylinders and hydraulic cylinders have differences in their pressure, speed, construction, and force. See Figure 11-18.

A cylinder cushion sometimes includes a needle valve with a check valve in the end cap at the rod end, cap end, or both. When the rod and piston of a pneumatic cylinder extend or retract, they are stopped at the end of their stroke by one of the end caps. The rod-end end cap stops the rod during extension and the cap-end end cap stops it during retraction. Abruptly stopping the rod during high-speed operation can damage the end caps. To prevent this damage, a cylinder cushion is often installed. A common cylinder cushion design is a tapered plug attached to the piston or rod that fills an exit hole on either end of the cylinder. A cylinder cushion sometimes includes a needle valve with a check valve in the end cap at the rod end, cap end, or both. See Figure 11-19.

A single-acting, spring-return cylinder returns the rod to its retracted position when airflow is exhausted. Single-acting cylinders are pneumatic actuators that use compressed air for the linear movement of a piston in one direction and a spring to return it to its original position once the air is removed. The two most common types of single-acting cylinders are spring-return and spring-extend cylinders. A single-acting, spring-return cylinder holds the rod in a retracted position with spring pressure when no airflow is applied. When enough airflow is applied, it overcomes spring pressure and extends the rod. When air is allowed to exhaust, the spring returns the rod to its retracted position. See Figure 11-20.

A double-acting cylinder uses compressed air to move a piston in both directions. A double-acting cylinder is a pneumatic cylinder that uses compressed air to move the piston in both directions. Double-acting cylinders are able to produce force in both directions by applying pressure to either side of the piston. Double-acting cylinders are available with a variety of lengths, diameters, and mounting methods. See Figure 11-21.

Double-rod cylinders are used when there is a load on each end of the rod. A double-rod cylinder is a pneumatic cylinder that has one piston and a rod that protrudes from both ends of the cylinder barrel. The control of a double-rod cylinder is the same as with a double-acting cylinder. However, a double-rod cylinder can provide equal piston speed and force in both directions, allowing it to operate differently than a double-acting cylinder. See Figure 11-22.

A rodless cylinder has a cartridge attached to the piston. A rodless cylinder is a pneumatic cylinder that has a cartridge attached to a piston that slides back and forth within the cylinder barrel. The piston usually has a magnetic band around its diameter to allow for proper placement. See Figure 11-23. Rodless cylind- ers are typically used where space is limited, such as in sliding doors that are in confined spaces or in production manufacturing lines that include paint spra- ying, food placement, or parts-washing operations.

A diaphragm cylinder uses a plastic, metal, or rubber diaphragm to extend or retract the rod. A diaphragm cylinder is a pneumatic cylinder that uses a plastic, metal, or rubber diaphragm to extend the rod. Most diaphragm cylinders apply compressed air to the diaphragm to extend the rod and spring pressure to retract it. The advantage of a diaphragm cylinder is that there is minimal friction, and no lubrication is required. See Figure 11-24.

The most common type of rotary cylinder has a rack-and-pinion mechanism. Rotary cylinders are often used for moving parts along an assembly line. The most commonly used rotary cylinders in pneumatic applications have rack-and-pinion mechanisms. See Figure 11-25. Rotary cylinders that rotate a specific amount of degrees for each movement or step are commonly used in equipment such as automated machinery.

Air motors can be vane, radial piston, or axial piston. An air motor is a pneumatic device that uses airflow to create rotating mechanical energy. Air motors produce high operating speeds up to 15,000 rpm. The speed of an air motor depends on airflow and the load attached to the motor. An air motor can produce more speed than a hydraulic motor but not as much torque. Air motors can be vane, radial piston, or axial piston. See Figure 11-26.

The two most common types of vacuum cups are flat and bellow vacuum cups. There are many different designs, sizes, materials, and manufacturers of vacuum cups. The two most common types of vacuum cups used with pneumatic systems are flat and bellow vacuum cups. See Figure 11-27. A flat vacuum cup is used to move objects vertically or horizontally. It can also be attached to a rotary cylinder to move or lift heavier or fragile objects a specific number of degrees. For example, if a packaging system needs to place a piece of cardboard between the two layers of an object, a flat vacuum cup is used to lift the piece of cardboard from its stack and place it between the layers of the object.