IMPACT IDLER

An impact idler is a specialized carrying idler installed under the loading chute of a conveyor, where lumps of material fall onto the belt with high kinetic energy. Instead of bare steel rolls, an impact idler has a series of thick rubber cushion rings (typically 12–25 mm radial thickness, shore A 60–75 hardness) mounted along the shaft. The rubber compresses on each impact, distributing the load over a longer time interval and across a wider belt area, which dramatically reduces peak stress on the belt cover and carcass.

Impact idlers are typically spaced more closely than standard carrying idlers — often at one-third the normal idler pitch — over a distance of 3–6 metres downstream of the chute. Under heavy loading or large lump duty, garland (catenary) impact idlers are sometimes used: three or five rolls suspended from chains so the entire assembly can articulate downward and absorb the worst impacts. Behind the impact zone, standard troughing idlers resume.

Engineers select impact idlers by lump size, drop height, material density and belt speed, then check the resulting load against the published CEMA class rating of the impact idler. Bearing housings are often heavier (Class D, E or F equivalent) than the surrounding carrying idlers because the dynamic load factor at the load point can reach 2–3× the static load.