Air springs isolate materials handling vibration

While many people know that air springs often provide the ultimate ride in stretch limos, luxury coaches and high-speed trains, many are unaware of their applications in materials handling machinery.

In fact, the same air springs that are a feature of very familiar vehicles such as heavy trucks are the ideal isolator for some less familiar applications. These range from isolation of heavy equipment for operational efficiency and OH&S benefits (conveyors, generators, compressors and vibrating screens) through to protection of delicate equipment (computers, electronic process control equipment).

What follows does not for a moment to assert that air springs are the ideal isolator for every application but it does attempt to illustrate the properties that make them outstanding for particular applications. The purpose of this article, therefore, is not to disparage any quality product,  but to look at situations where engineers may  have broader choice than they  originally believed.

Air springs are immensely strong rubber and fabric-reinforced balloons, or bellows. The shape of the bellows may vary – it  may be tubular, like the air bags used in vehicle suspensions, or it may be convoluted. These doughnut-shaped convolutions may be stacked one, two or three high (to make single, double and triple-convoluted air springs), depending on the performance required.

What makes air springs so useful in many applications is a unique property – they are the only passive isolator that operates on the principle of compressing a gas rather than deflecting a solid.

Because of this difference (compared with isolators such as coil springs and solid rubber pads) air springs are the passive isolator with the lowest natural frequency – and the lower an isolator?s natural frequency, the better the isolation effectiveness.

Air springs are the only passive isolator that can achieve a reduction in the natural frequency. With the use of an auxiliary reservoir, the natural frequency can be lowered to improve isolation effectiveness to finer degrees (which can exceed 99.9 per cent vibration isolation in demanding applications, eg isolating beds in operating theatres, weapons testing and guidance systems and expensive computers in busy industrial situations).

Airmounts (air spring isolators) are also unique for their variable spring rate. Spring rate is very useful to industrial engineers because it allows an isolator?s natural frequency to remain constant with pressure and load changes. A practical benefit of this is that it allows for the use of the same airmount at different mounting points on unevenly loaded equipment.

Because Airmounts use air as isolation media rather than a solid material, they provide less pathway for transmitting high-frequency vibration. Therefore, they reduce structurally transmitted vibration and noise transmission. Because they do not exhibit the chatter of conventional  coil springs, they are better for handling changing loads than coil springs. Any given Airmount (and they range in load carrying capacity from 40 to 40,000kg)  can easily handle a substantial change in load by simply adjusting the air pressure, whereas coil springs are designed for a very narrow operating range. Also, because of their nearly constant natural frequency, Airmounts react less violently than coil springs during start-up and  shut-down conditions, as the input  frequency changes.

A common materials handling problem 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.

Airmounts are generally used where weights exceed 100kg per mounting point.

Conveyor reliability is vital to ongoing production. When heavy material was dropped onto a conveyor involved in another application, it caused shock and vibration that threatened to destroy the conveyor structure.

Four Number 211 Airmounts, each of 9000kg load capacity, were installed under the conveyor section involved and inflated at 4.14 Bar (60psi) when the material was dropped.

The Airmounts, with a stroke of 180mm, were deflated to lower the conveyor       section after the impact was absorbed. In this application, the Airmounts acted as an isolator to raise and lower the conveyor section, as well as a shock absorber. Both roles are common (with air spring actuators being known as airstrokes).

Most industries rely on compressors to power the host of air tools and processes found in production, maintenance and materials handling facilities.

In one recent application, four Number 116 B Airmounts were fitted under a compressor, causing disturbances affecting surrounding people and processes.

The compact single-convoluted Airmounts, with a maximum load capacity of about 850kg, provided 95 per cent isolation when inflated to a design height of 115mm. Natural frequency was 168 cycles per minute.

The most frequent application of Airmounts  is in the isolation of vibrating screens.

Using Airmounts in capacities from 100 kilograms to tens of thousands, severe OH&S issues have been overcome through isolation efficiencies of 90 to 99 per cent.

In addition to overcoming problems of vibration transmission, Airmounts inflated to different pressures (and occasionally filled with different mediums, such as water or solutions instead of air) can be used to change the nature of the vibratory effect being produced by the vibrator motor.

Vibratory interference with computerised and electronic installations is common in automated industries. A recent application of Airmounts involved instrument cubicles used in a production area being subject to vibration, causing incorrect readings and inaccurate control.

After No 16 Airmounts were installed between the panels and the walls and inflated to a pressure of 0.7 to 1.7 Bar, an isolation efficiency of more than 99 per cent was achieved.

Design parameters included a disturbing frequency down to 1600 cpm, a natural frequency of 230cpm. No reservoir  was required.

While Airmounts have been extremely robust, environmental factors should be considered in evaluating their suitability for particular tasks.

The rubber part of the Airmount should be protected from contact with petroleum-based products. Any harsh chemicals should also be checked against a chemical compatibility chart to determine their reactivity with rubber materials.

Extremes of heat can also be important  because high-temperature environments can reduce the life of an Airmount. While highly cost-efficient for many applications, and generally providing the best isolation, Airmounts may not always be the lowest cost option.
When evaluating isolation needs and costs, it is important to consider all the advantages of the Airmount. If an application needs a high level of isolation and/or the ability to react to changing loads and frequencies, the Airmount will be the  best solution.

Also, as Australian states move toward more stringent workplace and environmental guidelines, this isolation capability is becoming  a major factor in machinery design.

Simon Agar is national sales manager for Air Springs Supply Pty Ltd.

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