Belt Conveyor Capacity and Drive Power per CEMA

Capacity, Belt Width, and Speed Selection

Belt conveyor capacity in tons per hour (TPH) is determined by:

TPH = Belt Cross-Section Area (ft²) × Belt Speed (ft/min) × Material Density (lb/ft³) × 60 / 2000

The cross-section area depends on belt width, trough angle, and the material's angle of surcharge (the angle at which the material piles above the belt edges). Standard trough angles are 20°, 35°, and 45°, with 35° being most common for general bulk materials.

CEMA provides capacity tables for standard belt widths (18", 24", 30", 36", 42", 48", 54", 60", 72", 84", 96") at various trough angles and surcharge angles. The material's surcharge angle is typically 5–15 degrees less than its angle of repose.

Belt speed selection depends on material type and belt width. CEMA recommended maximum belt speeds:

• Light, fine materials (grain, flour): 600–800 ft/min
• Medium materials (coal, sand): 600–1,000 ft/min for 24–36" belts, up to 1,200 ft/min for wider belts
• Heavy materials (ore, rock): 400–800 ft/min depending on lump size

Higher speeds mean smaller belt widths for the same capacity, but also more material degradation, more dust generation, and more wear at transfer points. Balance economy against operational requirements.

Drive Horsepower Components

The CEMA method calculates total drive power as the sum of several components:

HPtotal = HPempty + HPmaterial + HPlift + HPaccessories

HP_empty (empty belt friction): Power to overcome the friction of the belt, idlers, and return strand running empty. Depends on belt weight, idler spacing, conveyor length, and a friction factor (typically 0.025–0.035 for well-maintained conveyors).

HP_material (material horizontal movement): Power to move the material horizontally. Depends on tonnage, conveyor length, and a material friction factor that accounts for the flexing of the belt over idlers under load.

HP_lift (material elevation change): Power to raise (or recovered from lowering) the material through the elevation difference between tail and head. This is the straightforward gravitational component:

HPlift = TPH × lift (ft) / 1000

HP_accessories: Power absorbed by belt cleaners, skirtboard friction, trippers, and plows. Typically 3–10% of the subtotal.

Apply a drive efficiency factor (typically 0.90–0.95 for gear reducers) and select a motor with adequate capacity. Standard practice is to select the next available motor size above the calculated HP, with a minimum 10% margin.

Incline Limits and Material Properties

The maximum conveyor incline depends on the material being conveyed. Beyond the maximum angle, material slides back on the belt:

• Dry sand, gravel: 18–20° maximum
• Crushed stone, ore: 18–22° depending on lump size and moisture
• Coal: 16–18°
• Grain: 14–16°
• Wood chips: 25–27°
• Cement, powders: 20–23°

These are practical limits for smooth belts. Patterned (chevron, cleated, or rough-top) belts can increase the maximum incline by 5–15 degrees depending on the pattern and material.

Material properties needed for conveyor design:

• Bulk density (lb/ft³): Determines capacity per unit of cross-section area
• Angle of repose: Determines the surcharge angle on the belt
• Lump size: Affects belt width selection (belt must be 2–3 times the maximum lump size) and speed limits
• Abrasiveness: Affects belt selection (cover thickness and rubber grade), idler selection, and chute design
• Moisture content: Affects flowability, angle of surcharge, and tendency to adhere to the belt

Belt Tension, Takeup, and Tracking

The drive transmits power to the belt through friction between the belt and the drive pulley. The belt must have sufficient tension on the tight side (T₁) and slack side (T₂) to develop the required friction without slipping:

T1 / T2 ≤ eμθ

Where μ is the friction coefficient between belt and pulley (0.25–0.35 for bare steel, 0.35–0.45 for rubber-lagged) and θ is the wrap angle in radians (typically 180° = π radians for a single drive pulley, 210–240° for a snub-pulley arrangement).

A takeup device maintains belt tension as the belt stretches, contracts with temperature, and wears. Types include:

• Screw takeup: Manual adjustment. Simple and low-cost. Suitable for short conveyors (< 200 ft). Requires periodic adjustment.
• Gravity takeup: A weighted carriage on the return strand maintains constant tension automatically. Required for conveyors over about 200 feet or where belt stretch is significant.

Belt tracking (keeping the belt centered on the idlers) is controlled by idler alignment, loading symmetry, and training idlers. A mistracking belt is the most common conveyor operating problem and can cause edge damage, spillage, and structural damage to the conveyor frame.