BisonConvey
ENGINEERING TOOLS

CEMA IDLER CLASS SELECTOR

Recommended CEMA idler class and roll diameter

Select the CEMA idler class (A through E2) and the corresponding roll diameter for a troughed belt conveyor. The recommendation is based on belt width as the primary CEMA criterion, then auto-bumps a class for high speeds (>3.5 m/s), large lump sizes (>200 mm), or heavy-duty wide-belt combinations.

Belt

Material

Recommended idler
CEMA class
C
Per CEMA classification A–E
Roll diameter
127 – 152
mm
Nearest standard belt width
1200
mm
Selection logic
  • Width ≤ 500 mm → Class A (rolls ⌀89 mm)
  • 500 – 800 mm → Class B (rolls ⌀102 – 127 mm)
  • 800 – 1200 mm → Class C (rolls ⌀127 – 152 mm)
  • 1200 – 1600 mm → Class D (rolls ⌀152 – 178 mm)
  • 1600 – 2000 mm → Class E1 (rolls ⌀178 mm)
  • > 2000 mm or heavy-duty wide+dense → Class E2 (rolls ⌀178 – 219 mm)
  • Speed > 3.5 m/s OR lump > 200 mm → bump one class

Need a verified idler specification with bearings and seals?

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How CEMA idler classes work

CEMA (Conveyor Equipment Manufacturers Association) classifies idlers into six load capacities — A, B, C, D, E1, E2 — based on the combined static and impact load on the rolls. Class A is the lightest duty (narrow belts, packages); Class E2 is the heaviest (very wide belts, heavy ore in mining).

Belt width is the dominant criterion because it sets the static carrying load. Wider belts and denser materials need bigger rolls with heavier bearings to hit the same L10 life. Speed and lump size act as modifiers: high speed multiplies bearing cycles, and large lumps generate impact loading at loading zones.

After choosing a class, the actual roll diameter follows. Higher classes use larger rolls (⌀89 to ⌀219 mm) with deeper bearing seats. Loading-zone idlers always step up by one class regardless of the standard carrying-idler class — the impact load is the design driver there.

CEMA application chart

Indicative belt-width / material-density combinations and the resulting CEMA class. Speed > 3.5 m/s and lump > 200 mm push one class higher.

Belt widthMaterial densityClassRoll ⌀
≤ 500 mmAnyA89 mm
650 – 800 mm< 1.6 t/m³B102 – 127 mm
800 – 1200 mm< 2.0 t/m³C127 – 152 mm
1200 – 1600 mm1.0 – 2.5 t/m³D152 – 178 mm
1600 – 2000 mmAnyE1178 mm
2000 mm +AnyE2178 – 219 mm

Common pitfalls

  • Using the same idler class along the entire conveyor. Loading zones always need one class higher than carrying idlers — impact load is the design driver there.
  • Forgetting that return idlers can usually drop one class below the carry-side because they only support the empty belt.
  • Sizing for steady-state speed only. Many mining belts spend hours per day in slow-speed startup or maintenance crawl — verify L10 bearing life at the slowest steady speed too.
  • Mixing CEMA class with shell wall thickness. A Class D shell of insufficient thickness will dent under impact even if the bearings are correct.
  • Ignoring temperature. Below 0 °C, standard greases stiffen — specify low-temperature grease or step up one class to keep startup torque margin.

When to consult an engineer

This selector gives a first-pass CEMA class. Real installations also need shell thickness, bearing seal selection (labyrinth vs triple-lip), seat coating for corrosive service, and impact-idler stiffness at loading zones. For idler quotes covering full mining belts or upgrades on existing conveyors, talk to a BisonConvey engineer.

Get an idler specification

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