News Releases

September 2008

Screen out infuriating noise and breakages with sweetly supple spring alternatives

The coil springs supporting crushing and screening equipment have to put up with a lot in life.

Not only are they expected to function reliably in often wet and corrosive environments, but also they have to cope with rapidly changing loads and sudden severe impacts.

Not surprisingly, many suffer trauma as a result. One problem is noisy coil chatter as loads change. Under constant battering, coils may also suffer spring set, or permanent compression below their original design height. This means they do not perform to their original specifications.

In the worst cases, they break. Then there is not only downtime to consider, but also maintenance workers have to get right into some of the most inaccessible entrails of industrial equipment.

"And even when they are functioning perfectly, the row produced by conventional coils can be a hazard to hearing - not to mention sanity," says actuation and isolation specifier Simon Agar.

Mr Agar, who has 15 years experience in his field with Air Springs Supply Pty Ltd, is developing the use of quieter and more efficient actuators and isolators, including proven rubber-and-fabric Marsh Mellow® springs from Firestone.

Marsh Mellows are in use world wide in applications such as shaker screens, crushing equipment, vibrating bins and other vibrating equipment used in applications including food processing, manufacturing, materials handling, mineral processing and primary product processing.

While no one spring design is ideal for all applications, Marsh Mellows offer considerable advantages over both conventional coils and solid rubber springs for many tasks, he says. They operate silently with more spring travel and greater load capacities than conventional coil springs, while outperforming all-rubber alternatives in many areas, such as greater load capacity and more compact size for comparable tasks.

The cylindrical Marsh Mellow springs are constructed of a solid rubber core with a hollow centre and several plies of fabric-reinforced rubber as an outer cover. The plies provide the springs with stability as well as a consistent cylindrical shape.

The spring's components (rubber, bias plies, size of centre) are combined in different ways to meet specific load and performance requirements (such as those of crushing and screening equipment). This gives them great flexibility and precision, says Mr Agar, whose experience spans several hundred applications of both solid and hollow (bellows-type) air springs throughout Australia over more than 20 years. (Details of applications can be researched on Air Springs' website, www.airsprings.com.au.)

Vibration Isolation

One of the big advantages of Marsh Mellows is that they offer constant vibration isolation with changing loads. Their variable spring rate allows for a nearly constant natural frequency under the impact of such loads.

And because of the Marsh Mellows' greater deflection capabilities - and the load-carrying influences of the fabric reinforcement - it can carry a greater load when compared to a solid rubber part of the same modulus and dimensions.

Their ability to maintain a cylindrical shape while supporting greater loads means a smaller overall size can be used compared with an all-rubber springs of identical load capacity. This is important when considering an application with a small design envelope.

Marsh Mellow springs' high load capability also means fewer springs may be needed in an application, resulting in less overall cost.

Marsh Mellow Die Springs can be compressed by up to 40 per cent of their free height at a cycling rate of 50 cycles per minute (cpm) or less. Cycling rates of up to 300 cpm can be attained.

Also, the lateral spring rate of a Marsh Mellow spring can be less than the vertical spring rate, resulting in a lower lateral natural frequency.

The low natural frequencies of Marsh Mellow springs mean they provide excellent isolation of forced frequencies in the range of 800-1200 cycles per minute (13-20 Hz).

Increased stability, reduced downtime

Rubber is an incompressible fluid which will flow to the path of least resistance. This means that as a Marsh Mellow's height compresses, the fabric-reinforced rubber plies pantograph and the diameter grows. This supports the rubber core laterally, even at 30-40 per cent compression.

Marsh Mellow springs' flexibility also helps eliminate downtime and potential damage to machinery. When a coil springs fails, it will often crack, allowing fragments of the coil to damage equipment. This problem is eliminated with the rubber construction of the Marsh Mellows.

Additionally, Marsh Mellow springs exhibit exceptionally high overload characteristics and usually do not fail catastrophically, offering some support even during failure.

Marsh Mellow springs reduce structurally transmitted noise caused by vibration. Their operation is quiet, unlike steel springs, which often suffer coil chatter and readily transit high frequency structural noise.

Marsh mellows have no moving parts. No maintenance or lubrication is required to maintain their performance.

Naturally different types of springs have different performance characteristics suited to particular tasks. These should always be discussed with suppliers having wide experience in the particular area of application being considered.

For more information about Air Springs Supply's national distribution and technical support network, please contact Air Springs Supply Pty Ltd, 10 Angas St, Meadowbank, Sydney 2114, ph (02) 9807 4077, fax (02) 9807 6979, sales@airsprings.com.au