Bucket elevator capacity is governed by the simple product of three terms: bucket volume (V), a fill factor (ψ, typically ≤ 0.65 for swinging pendulum chains and close to water-level for centrifugal steel buckets), and the linear throughput of buckets per hour set by belt speed and bucket pitch [S2][S4].
In OEM catalogs, that math is tabulated as a lookup: Martin lists maximum CFH against bucket width, projection, depth, spacing, and chain pitch across centrifugal, continuous, super-capacity, and mill-duty series [S1]. Floveyor publishes a single-line envelope, capacity 2–400 m³/hr with bucket spacing 75–305 mm (3–12 in) and chain pitch 102–305 mm [S6]. TPI pegs centrifugal AC-bucket chain units at 15,000 ft³/hr and ACS saddlebag units at 12,000 ft³/hr, with continuous-discharge chains reaching 25,000 ft³/hr at slower speeds [S5].
Capacity Equation: Pitch, Speed, and Fill Factor
The standard bucket elevator throughput equation, P = (V × ψ × v) / P_pitch × 3600, ties pitch directly to capacity: halving the spacing nearly doubles the buckets per metre and lifts volumetric output proportionally, holding speed and bucket volume constant [S2]. Gough's worked example uses 15 m/min belt speed, 0.25 m/bucket pitch (4 buckets/m), and 0.005 m³ working volume, giving 60 buckets/min, 0.3 m³/min, and 18 m³/hr [S4].
The 0.65 fill factor is the working-volume ceiling for pendulum-hung chain buckets, used to suppress spillage and uncontrolled swing, while centrifugal steel buckets are usually rated to water-level capacity for the same nominal width [S3][S4]. Chain pitch is a hard constraint: it sets the minimum spacing physically mountable on a given sprocket, and continuous-discharge and chain units generally will not tolerate the operator-side spacing adjustments that belt units do [S3].
Centrifugal vs Continuous vs Super-Capacity: Spacing Rules by Class
Centrifugal discharge units mount buckets at wide intervals because the bucket must dig, accelerate, and throw the load, so the published minimum spacing is bucket projection plus 50 mm (or projection + 2 in on CC-style buckets) [S6][S7]. Common TPI centrifugal bucket sizes run 6×4 up to 20×10 in., with 1.5 in. as the rule-of-thumb maximum lump, and head-pulley tip speeds in the high range needed to fling material clear of the casing [S5].
Continuous-discharge units are the opposite geometry: buckets are packed tight, often overlapping by the next bucket's back, so the back of the leading bucket deflects material into the discharge chute at low speed [S3]. TPI lists continuous sizes from 8×5 up to 24×12 in., with the Super-Capacity variant adding two-strand chain mounting and bucket sizes up to 36×12 in. for higher lift centres [S5]. The trade-off is mechanical: continuous units carry more dead weight per metre of chain, so head-shaft bearings and drive torque scale up versus an equivalent-capacity centrifugal belt unit, a point covered in the bucket elevator head pulley lagging trade-offs reference.
Casing, Bucket, and Lump-Size Geometry
FEECO's sizing rule of thumb is to add 2 in. of lateral clearance per side of the bucket and enough vertical casing height to clear the projection plus discharge trajectory, so a 16×8 in. bucket typically lands inside a 20×48 in. casing [S3]. Martin's catalog table pairs bucket width with casing width, head and boot sprocket pitch diameters, and maximum lump size, so the geometry is fully coupled: changing bucket width forces a casing and sprocket re-size [S1].
Lump size is not a free parameter. Centrifugal units are sized for dry, free-flowing material under 1.5 in. lump, with the exact limit scaling with bucket width, while continuous units take larger lumps because the buckets are fed by direct loading rather than digging from a boot pile [S5]. When a process needs to lift cohesive or easily-degraded material, the choice flips: potash almost always goes continuous, sand almost always goes centrifugal, irrespective of throughput target [S3].
Belt vs Chain Mounting: How Mounting Reshapes the Spacing Decision
Belt-mounted elevators allow significant bucket-spacing adjustment, are quieter, smoother, and tolerate more corrosion and abrasion with lighter hanging weight, but they slip if lagging is wet or worn and are less tolerant of boot flooding [S5]. Chain-mounted elevators have fixed spacing locked to chain pitch, positive engagement with no slip, higher temperature tolerance, but more vibration, larger head shafts for the same capacity, and harder maintenance access [S1][S5].
For a given capacity target, picking belt lets the designer trade spacing against speed within a wider envelope, while chain locks the pitch to the sprocket and pushes the variable toward bucket volume and speed. The bucket elevator head pulley lagging trade-offs piece covers the traction side of that same decision for belt units, where lagging choice determines whether the wide-spacing centrifugal case is even tractable at start-up.
Decision Matrix: Which Configuration Matches the Duty
For dry, free-flowing, non-fragile material under 1.5 in. lump and capacities up to 15,000 ft³/hr, a centrifugal belt or chain unit with AA/AC buckets and spacing at projection + 50 mm is the cost-default choice [S1][S5][S6]. For fragile, easily aerated, or larger-lump material at 15,000–25,000 ft³/hr, continuous-discharge chain with MF/HF/SC buckets, overlapping or tight per-projection spacing, and direct loading, is the right call [S3][S5].
For severe-duty cement, rock, fertilizer, lime, gypsum, coal, and fine-ore service at high lift centres, mill-duty centrifugal chain with AC buckets, or super-capacity continuous chain with SC buckets between two strands, scales to the higher shaft centres and capacities; TPI notes super-capacity SC buckets between twin chains handle 15,000 ft³/hr and above [S1][S5]. Chain pitch stays in the 102–305 mm window across Floveyor's published range, with bucket spacing sliding inside that envelope according to bucket projection and discharge style [S6].
Selection Pitfalls and Sourcing Standards
The most common sizing error is mixing water-level and struck-volume ratings when scaling a competitor's bucket to a new casing, since today's standard is water-level but some catalogs still publish struck [S3]. A second pitfall is using the 0.65 working fill factor for chain pendulum buckets but water-level fill for steel centrifugal buckets, which overstates chain capacity by a wide margin at the same nominal bucket volume [S4].
For verification, FEECO, Gough, Floveyor, and Martin publish worked numbers and lookup tables against bucket projection, chain pitch, and maximum CFH; TPI's catalog is the cleanest side-by-side for centrifugal AC (15,000 ft³/hr) versus ACS saddlebag (12,000 ft³/hr) versus continuous (25,000 ft³/hr) at the same nominal bucket size [S1][S3][S4][S5][S6]. When the duty falls between catalog rows, the safe move is to step up to the next casing width and re-check sprocket pitch diameter against the published head-sprocket table rather than over-speed a smaller casing [S1].
Two signals to watch over the next sourcing cycle: Chinese NE-series plate-chain elevators continue to adopt functionally equivalent AA, AC, MF, HF, and SC bucket profiles under different model codes, so cross-border spare-parts qualification needs explicit profile-mapping rather than model-code matching [S2]; and the bucket-elevator category, covered in the bucket elevator encyclopedia entry, now overlaps more with construction machinery and equipment catalog filters on aggregate and cement-plant RFQs, so datasheets that publish full pitch, spacing, and CFH tables, not just nominal bucket width, will be the ones that shortlist cleanly.
The underlying component specifications are covered under lamps and light fittings.