THE coil springs supporting crushing and screening equipment have to put up with a lot. 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. In the worst cases, the springs may break.
According to Air Springs Supply actuation and isolation engineer, Chris Webb even when functioning perfectly, the noise produced by conventional coils can be a hazard to hearing.
Webb, who has 30 years experience in his field, is developing the use of quieter and more efficient actuators and isolators, including proven rubber-and-fabric Marsh Mellow springs from Firestone.
Marsh Mellows can be used in applications such as shaker screens, crushing equipment, vibrating bins and other vibrating equipment.
While no one spring design is ideal for all applications, the company claims Marsh Mellows offer considerable advantages over both conventional coils and solid rubber springs for many tasks. They are said to 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 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 are said to provide the springs with stability and 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). Webb says this gives them great flexibility and precision.
The new springs are said to offer constant vibration isolation with changing loads. Their variable spring rate allows for a nearly constant natural frequency under the impact of such loads.
Because of their greater deflection capabilities and the load-carrying influences of the fabric reinforcement, the springs are said to be capable of carrying a greater load than solid rubber parts of the same modulus and dimensions.
The company claims 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.
The springs’ high load capability also means fewer springs may be needed in an application.
Marsh Mellow Die Springs can be compressed by up to 40% of their free height at a cycling rate of 50 cycles per minute (cpm) or less. Cycling rates of up to 300cpm can be attained.