Isolating vibration

WHILE air springs have proven extremely robust, environmental factors should be considered when evaluating their suitability for particular tasks. *Simon Agar writes

While air springs have proven extremely robust, environmental factors should be considered when evaluating their suitability for particular tasks. *Simon Agar writes

Anyone who has ever ridden in air-suspended luxury vehicles will have seat-of-the-pants knowledge of the quality of air isolation.

However, while many people know that air springs often provide the ultimate ride in stretch limos, luxury coaches and high-speed trains, not so many are aware of their extensive applications in materials handling machinery.

In fact, the same air springs that are usually hidden away in the chassis of even very familiar vehicles such as heavy trucks are now being discovered as the ideal isolator for use in some less familiar applications.

These range from isolating of heavy equipment for operational efficiency and occupational health and safety benefits, (conveyors, generators, compressors and vibrating screens) through to protecting of delicate equipment (computers, electronic process control equipment).

What follows does not for a moment attempt to assert that air springs are the ideal isolator for every application — no isolator is that — 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 find they have broader choice than they originally believed.

Air Springs are in essence 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 latter, 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: air springs 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.

Furthermore, air springs (or Airmounts as our Firestone products are titled) 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 even finer degrees (which can exceed 99.9% vibration isolation in demanding applications, such as isolating beds in operating theatres, weapons testing and guidance and expensive computers — sometimes even entire computer rooms – in busy industrial situations).

Airmounts are also unique in that they have a variable spring rate. This quality is very useful to industrial engineers in that it allows the isolator’s natural frequency to remain nearly constant with changes in pressure and load.

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 the isolation media rather than a solid material, they provide less of a pathway for transmitting high frequency vibration.

Therefore, they reduce structurally transmitted vibration and, as a result, reduce noise transmission.

Because they do not exhibit the chatter that conventional coil springs do, they are better suited for handling changing loads or overload conditions than coil springs.

Any given Airmount (and they range in load carrying capacity from 40-40,000 kg) 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.

In addition, because of their nearly constant natural frequency, Airmounts react in a much less violent manner than coil springs during start-up and shutdown conditions, as the input frequency changes.

Bin hopper isolator

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 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.

Conveyor isolator and elevator

Conveyor reliability is vital to ongoing production in many industries.

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

The Airmounts, with a stroke of 180 mm (7 in), 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, and as a shock absorber; both roles are common (with air spring actuators being known as Airstrokes).

Compressor isolation

Nearly all 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 No 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% isolation when inflated to a design height of 115mm (4.5in). Natural frequency was 2.8 Hz (168cpm).

Vibrating screens

One of the most frequent applications of Airmounts is in isolating vibrating screens, which are widely employed in industries to process chemicals, minerals and food and beverage ingredients, for example.

Using Airmounts in capacities from a few hundred kilograms to tens of thousands, severe occupational health and safety issues have been overcome through isolation efficiencies of 90-99%.

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.

Control panel isolation

Vibratory interference with computerised and electronic installations is encountered throughout 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 installing No 16 Airmounts between the panels and the walls and inflating them to a pressure of .7 to 1.7 Bar, an isolation efficiency of more than 99% was achieved.

Design parameters included a disturbing frequency down to 26.6 Hz (1600 cycles per minute), a natural frequency of 3.8 Hz (230 CPM). No reservoir was required.

Conclusion

While Airmounts have proven to be extremely robust even in heavy industrial applications ranging from mine sites to metal stamping and pressing, environmental factors should be considered when evaluating their suitability for particular tasks.

The rubber part of the Airmount should be protected from contact with petroleum-based products.

In addition, any harsh chemicals should be checked against our chemical compatibility chart to determine their reactivity with rubber materials.

Extremes of heat can also be an important factor to consider, because high temperature environments can reduce the life of an Airmount. (Versions are available with higher temperature resistance) 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 most likely be the best solution.

In addition, 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 the National Sales Manager for Air Springs Supply Pty Ltd. He is a pneumatics specialist with 15 years experience in pneumatic actuation and isolation.

Air Springs Supply Pty Ltd

sales@airsprings.com.au

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