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Air spring alternative for industrial actuation

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Pneumatic actuators are endemic to processing and materials handling industries. Their applications range from bin hoppers, conveyors, changeover tables and pallet handling devices, through to scissors lifts, shakers, web tensioning devices and scissors lifts. Simon Agar, a pneumatics specialist with more than 15 years experience in pneumatic actuation and isolation, highlights the use of air spring alternative for industrial actuation.

Both air springs and conventional air cylinders are essentially contained columns of air that are pressurised to actuate machinery.

In the case of a conventional cylinder, a piston extends to provide force. In the case of an air spring, the bellows itself extends.

Each type of cylinder can be superior to the other in particular applications, but sometimes the familiar conventional cylinder may be specified for materials handling applications simply because it is better known than the alternative, rather than better suited.

The purpose of this article, therefore, is not to disparage any quality product, but to look at situations where engineers may find they have broader choice than they originally believed.

Whereas conventional cylinders contain their air in a rigid metal jacket, air springs contain theirs in a flexible but immensely strong rubber and fabric-reinforced bellows.

The shape of the bellows may vary, it may be tubular, like the air bags used in truck suspensions, or it may be convoluted. Depending on the stroke and performance required, these latter, doughnut-shaped, convolutions may be stacked one, two or three high (to make single, double and triple-convoluted air springs).

The ends of an air spring 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, or hydraulic medium) to be introduced into the chamber. The fabric in the side wall of the bellows restricts radial expansion, so pressure is built up, causing axial extension.

While air springs are available in a variety of styles, sporting differing components that control the shape and path of axial extension, their basic design is the same. Each style is, in essence, a heavy-duty balloon that offers 40-40,000kg of pushing or lifting power and strokes (extensions) of up to 350mm, powered by simple, basic compressor equipment found in nearly every factory.

Air springs differ substantially from conventional cylinders in that they contain no pistons, rods or static or dynamic seals. In some of the applications to follow, this means they cost less, last longer and are less expensive to operate than air cylinders.

Because friction is reduced, and there are no internal moving parts to break or wear, they are suitable for high-repetition tasks, such as actuating change gates on conveyor lines, or powering stamping presses. The flexible air springs’ ability to arc without a clevis, to bend and to tolerate uneven and fluctuating loads also means they are suitable for materials handling tasks such as tensioning webs, belt take-up on conveyors, or powering scissors lifts.

Unlike conventional cylinders, air springs may be used not only as actuators, but also as isolators. In applications such as suspending vibrating screens or motors, they isolate the offending source of vibration from the surrounding relative tranquillity. Alternatively, they may be used to isolate delicate computers of electronic materials handling componentry from ambient vibration. Because they can reliably achieve isolation efficiencies high enough to be used beneath surgical operating tables (often well exceeding 99%), they offer outstanding potential for industrial applications such as some of those to follow:

Hinged conveyor drive tables:

Conveyor lines are one of the common materials handling applications of air spring actuators (Airstrokes).

Because air springs are engineered by Firestone to be robust enough to function for hundreds of thousands of kilometres in heavy truck and bus suspensions, they are sufficiently flexible and robust to survive millions of cycles in conveyor tasks such as gravity transfer sections, actuated diverters, belt take-up, roller friction brakes, end stops and two-way switches.

Within hinged drive tables, air springs provide a number of advantages, including space savings, cost savings because of simplicity of engineering, durability because of lack of complexity and importantly flexibility. Because conveyors are widely subject to fluctuating load and directional demand, flexibility is an essential prerequisite to reliability.

Actuated hinged drive tables are used where gravity transfer stations cannot provide the speed and precision required. As material moves down a roller conveyor and over the transfer section, two air springs are inflated to raise a moving drive chain, lifting the material up and off the rollers at a right angle. The conveyor is raised in an arcing motion, requiring angular actuation.

Here, the Airstroke actuators’ short collapsed height and angular capability are advantages. Though a conventional pneumatic cylinder is limited to a stroke that is approximately equal to its collapsed height, an air spring stroke is up to four times its collapsed height, providing more motion from a smaller space. The transfer sections can be actuated through a 30-degree arc without a clevis. Air Strokes are unaffected by side loads generated by the table, and are impervious to dirt and contaminants that fall from the table.

Belt scrapers:

Belt scrapers are an unglamourous but essential component of conveyor systems. Their blades or brushes bear against the moving conveyor belt to remove material sticking to it.

Using an effective, continuous method for cleaning conveyor belts one that prevents material buildup on the belt can help extend the life and productivity of the entire belt system. By preventing material buildup, it helps keep the belt from drifting off track, prevents erratic loads on the motor, eliminates uneven wear on pulley bearings and damage to return idlers, and minimizes belt stretching.

In addition to ensuring that scrapers are made from the correct material (not too hard, so as to damage the belts, not too soft, so as to be ineffective on material being conveyed) it is important to pay special attention to the style of actuation.

While in some instances metal springs, flexible blocks or torsion bars will satisfactorily position the scraper adjacent to the belt surface, other applications increasingly require more sophisticated, flexible and continuously supple arrangements. These can be provided by flexible pneumatic actuators, such as Airstroke® air spring actuators.

Web tensioning:

Conveyors or rolls of paper or fabric, moving across a number of rollers at high speed require constant tension to prevent slack, breakage, or damage. Here, long-stroke, rolling sleeve air springs offer advantages. The friction-free actuators provide consistent tension over their entire stroke.

Conventional cylinders may suffer from seal and bearing ‘sticktion’ and therefore do not operate as smoothly, providing inconsistent tension.

In addition, air springs are more tolerant of shock loads then conventional pneumatic cylinders. Shock to the web or roll can damage the cylinder or, as a result of the cylinder’s ability to absorb shock, damage the web.

Spool lifters:

Safety issues are paramount when loading and unloading the giant rolls and spools commonly employed by industries involved in processing and packaging using paper rolls, metal coils and cable and wire products.

Not only do the actuators of the loading and unloading mechanisms have to cope with huge variations in weight from full to empty, but also they have to deliver precise control and reliability in a role where failure could halt entire production lines.

Airstroke actuators that have proved their reliability over millions of cycles of service in Australasia were used by Thermoplastic Engineering Corporation in the USA, manufacturer of extrusion and cabling machinery and equipment for the wire and cable industry.

To help ensure employee safety Thermosplastics Engineering put Model 116-1 Airstrokes to work in the company’s Single Twist Flyer Arm Cabler, a machine that twists wire while winding it onto an enormous spool.

An empty reel weighs about 180 kg, but a full reel weighs more than 2720kg. The machine involved had a base about 30cm off the floor, so moving the reels into and out of the cabler was a daunting task.

In order to safely move Thermoplastic Engineering’s spools into place, an engineering team led by the company’s head of engineering support, Pat Wilkins devised a two-part ramp system for raising the unloaded spool up to the precise height of the cabler. The machine’s operator rolls the spool onto the ramp, up and over a raised “stop”, which keeps it in place.

When the operator enacts a nearby hand switch, two Model 116-1 Airstroke actuators fill to 5.5 bar (80psi) to raise the spool to a height where the operator can safely roll it into the cabler.

After the cabling process is complete, the operator deflates the actuators to lower the spool. The raised stop prevents the now-loaded 2720kg reel from rolling away as the ramp is lowered safely.

The compact single-convoluted Airstrokes involved have a stroke of more than 10cm, double the deflated Airstrokes' height of 5cm. This low deflated (collapsed) height makes them easy to fit into production lines, including conveyor lines.

Wilkins said that the compact Airstrokes were the best solution for this application because they are located in a place that is difficult to access for maintenance.

Scissors-lift tables:

Frequently in materials handling operations scissors-lift tables move loads from one height to another, while maintaining a specific attitude, regardless of load centre.

Because of static friction, conventional cylinders to not provide smooth motion, particularly at low pressures. A certain weight is required to overcome static friction, so the table lowers in jerky increments instead of a smooth stroke.

Air Springs are not affected by static friction, so they provide smooth operation at any pressure.

Some scissors-lift tables are constructed with steel springs, with different packages requiring different springs. Air Spring pressures, meanwhile, can be adjusted to accommodate a variety of package weights within specified limits.

Bin hopper isolator:

In addition to being versatile actuators, air springs can also function as long-life isolators. As Australian states move toward stringent workplace and environmental guidelines, this isolation capability is becoming a major factor in machinery design.

A common materials handling isolation application is isolation of bin vibration. This type of vibration is typically used to maintain a homogeneous mix or flow of material inside a hopper. It must be isolated from bin supports so as to prevent structural fatigue.

Solid rubber isolators or steel springs can be used, but they must be tuned to one specific load and a single height.

Air springs provide a high degree of isolation compared with other methods, where they are used down to disturbing frequencies of 3-4 Hz. Isolator inflation can be changed to compensate for different loads or heights without compromising isolation efficiency.

Air Spring isolators (Airmounts) are generally used where weights exceed 100 kg per mounting point.

Conclusion:

This article has shown some applications where air springs are advantageous. For a broader assessment of the features and benefits of Air Springs, visit the website of Air Springs Supply .

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