The flexible-wall, bellows-type air cylinders are frequently overlooked for tasks for which they are ideally suited. These tasks include engineering of assemblies for high-repetition tasks in food, beverage, packaging and metal stamping plants. These air cylinders are suitable for conveyor and other materials handling applications in aggressive factory, mining and quarry environments where avoiding downtime and maximising production is important. Air cylinders are also known as air springs or air bags. Their performance has a bearing on the overall efficiency of pneumatic systems powered by pumps and compressors. Air Springs Supply discuss benefits and limitations of air springs.
In geometry, a cylinder is the rotation of a parallelogram around one of its sides. In fluid power, it is a device that extends axially when pressurised, causing motion or exerting a force. Traditionally, a fluid power cylinder has had the general geometric shape. However, it is not necessary for an axial force/motion generator to be specifically cylindrical in shape.
Most production and process engineers involved in food manufacturing, materials handling and motion control are familiar with the traditional cylinder design. It contains a piston sliding within a housing of circular cross-section connected to the work by a rod passing through one end of the device. This design necessitates several guides and seals, which align and seal the sliding surfaces. These allow a pressurised, contained column of fluid to apply force to the piston. An air spring uses none of these components to contain and channel its column of fluid. This difference is the key to its functionality.
An air spring contains its column of air in a fabric-reinforced rubber envelope or bellows. The ends are sealed by bead plates, which are crimped around the bead of the bellows. These plates contain the attachment hardware for the part, normally a blind tapped hole called a blind nut. An air fitting, generally in one bead plate, allows fluid (air) to be introduced into the chamber. The fabric in the side wall of the bellows restricts radial expansion, so pressure builds up causing axial extension. Air springs are available in a variety of styles, sporting differing components that control the shape and path of axial extension, but their basic design is the same. Each style is, in essence, a heavy-duty balloon.
In order to select the appropriate air spring, customers need to know the necessary force, the linkage motion and any special environmental concerns. Airstroke actuators from Air Springs Supply, for example, give 40kg to 40,000kg of pushing or lifting power. Offering power strokes of up to 350mm, Airstrokes are powered by basic compressor equipment found in every factory. Australian manufacturing and industrial plants use them as ram cylinders, die cushions, counterbalances, clamps, lifters, valve operators, flexible connectors, shock absorbers and isolators.
Air springs have the following Australian and international uses:
- Pallet handling equipment at a car manufacturing plant
- Web tensioning on a printing press
- Conveyor line actuation for pipe handling
- Conveyor stops and gravity gates
- Skate wheel right angle gravity transfer section on a conveyor
- High speed metal stamping
- Belt scrapers in mineral processing operations
- High frequency stress testing of materials at an aeronautical research facility
- Metal press counterbalances at a vehicle manufacturing plant
- Belt take up and roller friction brake on conveyor equipment
- Actuation of coal washing plants
Following are the benefits of air springs:
- Cost benefits: Air springs can be used instead of hydraulic systems when applying large forces. Sizes are available from fewer than 80mm to more than nearly 1000mm (3 inch to 38 inch) in diameter. The larger sizes allow force up to 40,000 kg each using only 7 bar (100psi) air pressure. Compact installation: A flexible-wall air spring is compressed to its minimum height then extended when pressure is applied. In most cases, the minimum height is less than the available stroke. As a result, air springs can be put in a compact space and extended to more than twice their starting height. Air springs are suitable for floor-mounted lifting devices.
- Side load flexibility: As an air spring has a flexible, compliant bellows wall, instead of seals or guides, the bellows follows the path of least resistance. This means users do not have to worry about side loads caused by misalignment.
- Easy attachment: As the bellows bend, bead plates do not have to remain parallel. This simplifies attachment, especially when linkage is at an angle. As long as the bellows' side walls are not over-extended or over-compressed, users can stroke through an angle without clevises.
- Constant force: The lack of seals also means lack of friction. In many cases, a constant force needs to be applied to a moving object. With traditional cylinders, the sliding seals can stick, providing a jerky motion that can damage equipment.
- Durability: Air Springs outlast cylinders in most high-speed applications. Air Springs do not require lubrication and thus have a lower system cost.
- Curtailed production losses: Air springs contain no moving parts to break, wear, leak or to cause costly disruptions of production.
- Suitability for aggressive environments: As there are no seals sliding against exposed surfaces, an air spring can often survive abrasive and corrosive environments that require special consideration when a conventional cylinder is used.
Although air springs are beneficial for several applications where they are not currently considered, there are some limitations to consider when specifying them. They are as follows:
- Air springs are single-acting: some outside force must be used to retract them to their minimum, or starting, height. Frequently, this outside force is gravity or it can be another air spring, or coil spring.
- The available stroke of an air spring is limited by the length of the side wall. This length is determined by stability concerns (length-to-diameter considerations). These design needs can frequently be met by specifying single or double or triple-convoluted air springs. However, air spring users should not stretch the wall in extension or pinch it in compression. This goes for linear as well as angular movements. Therefore, air springs are normally used for high-force and low-stroke applications.
One critical component of an air spring is rubber. This must have good elongation, flexibility and abrasion resistance, factors which limit the choice of elastomers available. These determine the temperature and chemical compatibility of the product. Temperature ranges from -20°C to more than 95°C (-65°F to 220°F) are available in most designs. Normally, users should keep air springs away from petroleum-based fluids and chemicals that attack rubber. Automotive air springs are used as suspension components in the new semi-trailers, tourist coaches and in smooth riding trains.
The air springs are suitable actuators for many materials handling applications and are also efficient isolators. An air spring not only lifts, but also isolates an object. For instance, it can lift and support a vibrating load such as a shaker or vibrator without concern for wear on components.