LUMP SIZE

Lump size is the largest particle dimension expected in a bulk material stream. Engineers care about it because the lump impact at the loading chute, not the average particle weight, determines the load-zone idler rating; and because oversized lumps relative to belt width cause belt distortion, spillage and chute hang-up. Material is described in two broad categories: 'sized' material, where lumps have been screened to a narrow size range (e.g. crushed stone 25–50 mm), and 'run-of-mine' or ROM material, where particle sizes range from fines up to a much larger maximum (e.g. blasted iron ore with 80 % fines and occasional 800 mm boulders).

Industry rules of thumb relate maximum lump size to belt width. For sized material, maximum lump can be up to one-third of belt width; for ROM material, the maximum lump is limited to about one-sixth of belt width (because boulders concentrated to one side would tip out of the trough). Lump size also sets the minimum drop height at the loading chute — fall distances must be limited to keep impact energy below the cushion-idler rating — and the spacing of impact idlers in the load zone, which is typically 0.3–0.6 m for severe lump-impact duty.

Standard data sheets (and the [bulk material properties](/tools/bulk-material-properties) reference) list 'typical maximum lump' alongside [bulk density](/glossary/bulk-density), angle of repose and abrasiveness. For mining ROM applications, the supplier should obtain the actual blast fragmentation distribution — published averages systematically under-represent the once-a-shift boulder that does the real belt damage.