CEMA IDLER CLASS

The CEMA (Conveyor Equipment Manufacturers Association) idler classification, published in CEMA Standard No. 502 and described in 'Belt Conveyors for Bulk Materials' (the CEMA orange book), is the dominant idler rating system in North America and is widely referenced internationally. It groups idler rolls into six classes — A, B, C, D, E and F — by combination of shell wall thickness, bearing size, seal arrangement and resulting maximum allowable bearing load.

The classes scale roughly as follows. Class A: light duty, package conveyors, belt width up to 30 in, 4 in rolls. Class B: light duty bulk handling, belt 18–36 in, 4–5 in rolls. Class C: medium duty, belt 18–42 in, 5 in rolls. Class D: heavy duty, belt 24–60 in, 6 in rolls, common in quarries and aggregates. Class E: very heavy duty, belt 36–96 in, 6–7 in rolls, mining and overland conveyors. Class F: extra heavy duty, belt 36–96 in, 7–8 in rolls, large primary and overland systems. Each class has a published rated load that varies with belt speed and roll spacing — selection charts in the CEMA orange book translate calculated idler load into the appropriate class.

European and ISO 1537 idler ratings use a different nomenclature (e.g. 89 mm, 108 mm, 133 mm, 159 mm, 194 mm roll diameter with their own load tables), but the underlying engineering — calculating idler load from belt mass + material mass + idler spacing × impact factor, then matching to a load-rated roll — is the same.