Coil springs supporting crushing and screening equipment associated with conveyor lines have to withstand harsh conditions.
In addition to functioning reliably in often wet and corrosive environments, they also have to cope with rapidly changing loads and sudden severe impacts when dealing with conveyor inputs or outputs.
Not surprisingly, coil springs 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. Consequently, these springs may not perform to their original specifications.
Coil springs can also break causing downtime in addition to the problem of maintenance workers having to get into some of the most inaccessible areas of industrial equipment.
According to actuation and isolation specialist Simon Agar, the row produced by conventional coils even when they are functioning perfectly can be a hazard to hearing.
Mr Agar, who has 20 years of experience in his field with Air Springs Supply is developing the use of quieter and more efficient actuators and isolators including proven rubber-and-fabric Marsh Mellow springs from Firestone.
Marsh Mellows rubber-and-fabric springs are in use worldwide in shaker screens, crushing equipment, vibrating bins and various vibrating equipment that are employed for materials handling, mineral processing and primary product processing.
While no one spring design is ideal for all applications, Marsh Mellows springs offer considerable advantages over both conventional coils and solid rubber springs for many tasks.
Key advantages include silent operation with more spring travel and greater load capacities than conventional coil springs. Marsh Mellows springs also outperform all-rubber alternatives in many areas such as load capacity and compactness 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 are combined in different ways to meet specific load and performance requirements. This gives them great flexibility and precision, says Mr Agar, whose experience spans several hundred applications of both solid air springs and hollow air springs throughout Australia.
One of the big advantages of Marsh Mellows springs 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.
Greater deflection capabilities of Marsh Mellows springs and the load-carrying influences of the fabric reinforcement allow them to carry higher loads.
Their ability to maintain a cylindrical shape while supporting greater loads means a smaller overall size can be used compared with an all-rubber spring 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.
Additionally, 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
Marsh Mellow springs’ flexibility also helps eliminate downtime and potential damage to machinery. When a coil spring fails, it will often crack, allowing fragments of the coil to damage equipment. This problem is eliminated with the rubber construction of Marsh Mellows springs.
Additionally, Marsh Mellow springs exhibit exceptionally high overload characteristics and usually do not fail catastrophically, offering some support even during failure.
Marsh Mellow rubber springs reduce structurally transmitted noise caused by vibration. Their operation is quiet unlike steel springs, which often suffer coil chatter.
Marsh mellows have no moving parts. No maintenance or lubrication is required to maintain their performance.