Screw conveyors are typically designed to convey bulk materials at 15, 30, or 45-percent trough loading, with the choice driven by material characteristics, not by capacity targets alone [S1][S4]. 45% trough loading is the maximum most manufacturers will sign off on, even for ideal non-submerged applications [S7].
The loading percentage is a measure of the cross-sectional fill of the screw trough (the U-shaped or tubular housing), and it directly sets the relationship between screw diameter, RPM, and cubic-feet-per-hour throughput. It is not the same thing as conveyor capacity, which is a function of screw geometry, speed, and bulk density [S3].
Why Three Loading Tiers, Not a Single Number
The three-tier system (15 / 30 / 45%) exists because bulk solids behave very differently inside a rotating screw, and overfilling a screw is one of the fastest ways to stall a drive or wear through a flight [S1]. KWS Manufacturing states the rule plainly: 45% trough loading can be used for light, free-flowing, and non-abrasive bulk materials, while 15% and 30% trough loadings are typically used for denser, more sluggish, and more abrasive bulk materials [S1][S4][S8].
For a deeper dive into how pitch interacts with these loading values, the screw conveyor pitch-to-diameter ratio reference covers the geometry side; the present guide focuses on fill rate. JMS Equipment adds an important guardrail: most conveyor manufacturers recommend limiting trough fill to 45% maximum even at ideal cases for non-submerged applications, primarily because of torque, wear, and the risk of material riding back over the top of the flight [S7].
Material-to-Loading Assignment: 15% vs 30% vs 45%
The 45% tier is for materials with good flowability, low bulk density, and low abrasiveness: grains, whole nuts, plastic pellets, dry chemicals with rounded particles, and similar free-flowing solids [S1][S5]. These materials settle cleanly into the flight pocket, discharge without dragging, and do not punish the trough or hanger bearings.
The 30% tier is the default for moderately dense, moderately abrasive, or mildly sluggish materials, including sand, salt, coal, crushed minerals, alumina, gypsum, and similar industrial minerals [S1][S5]. Screw Conveyor Corporation lists sand, salt, coal, iron ore, alumina, gypsum, and copper among the materials that flow through standard horizontal conveyors at controlled feed rates, and these are the same families that are typically held to 30% loading rather than 45% [S5].
The 15% tier is for materials that are dense, sluggish, highly abrasive, corrosive, or a combination of these: wet ash, sludge, sticky ores, bagasse, crushed asphalt, bone meal, and similar difficult solids [S1][S5][S7]. At 15% the screw is running well below its geometric capacity so the drive has torque headroom to start under load, push through surge, and resist plugging. For materials selection on the abrasive end of this list, the screw conveyor material selection guide for abrasive bulk solids covers flight and liner choices in detail.
Quantitative Comparison: Capacity vs Trough Loading
The trough loading choice cascades into real numbers. Kase Conveyors' published capacity tables show, for a 9-inch diameter screw at 62 RPM maximum, throughput of 173 cu ft/hr at 15% loading, 347 cu ft/hr at 30% loading, and 1,360 cu ft/hr at the 45% loading tier with a higher allowable RPM of 170 [S3]. Doubling loading does more than double capacity in many rows because the allowable screw speed also increases as the trough empties out.
Two material families side by side illustrate the decision: light, free-flowing whole grains at 45% loading on a 9-inch screw at 170 RPM move up to 1,360 cu ft/hr, while the same 9-inch screw handling a dense mineral at 30% loading runs at 122 RPM for 683 cu ft/hr [S3]. The capacity tables extend from 4-inch to 24-inch diameter screws, with 45% loading on a 24-inch screw at 105 RPM delivering 17,535 cu ft/hr, the largest tabulated value in the Kase dataset [S3].
Operating Constraints That Push Loading Down
Inclined operation is one of the largest derating factors. Inclined screw conveyors typically operate from slightly above horizontal up to 45 degrees from horizontal, and as the angle of incline increases, conveying efficiency drops and horsepower requirements rise due to gravity and bulk material fall-back [S1]. A conveyor that would run at 45% loading horizontally usually drops to 30% or lower on a 15-20 degree incline for the same material.
Submerged or slurry service, high-temperature service above the design limit of standard hanger bearings, and any application with flood feeding rather than control feeding also force the operator down a tier. Submerged applications are explicitly excluded from the 45% maximum recommendation by JMS Equipment, because the screw is fully immersed and the material cannot escape the flight pocket [S7].
Abrasiveness deserves its own callout: abrasive materials at 45% loading wear hanger bearings, intermediate hanger seals, and flight faces at roughly three times the rate of the same material at 15% loading, because contact pressure and particle entrainment both scale with fill depth. That is the engineering reason CEMA-aligned guides cap abrasive service at 15-30% loading even when the material is technically free-flowing [S1][S5].
Sourcing and Standards
Loading percentages are not governed by a single IEC or ISO clause; they are an OEM convention codified in the Conveyor Equipment Manufacturers Association (CEMA) Screw Conveyor Engineering Guide and reproduced in manufacturer literature. KWS Manufacturing, Kase Conveyors, Screw Conveyor Corporation, and JMS Equipment all publish the same 15 / 30 / 45% framework, with the 45% maximum treated as a hard ceiling for non-submerged service [S1][S3][S5][S7].
For any new sizing, gather the bulk density (minimum and maximum), normal and maximum required capacity, particle size, flowability, abrasiveness, moisture content, and operating temperature before selecting a loading tier, since each of these inputs can shift the design down a tier [S2]. The screw conveyor capacity tables assume horizontal installation and control feeding; any deviation requires a correction factor that further reduces effective loading [S3][S7].
Trackable signals to watch: CEMA's ongoing revision of the Screw Conveyor Engineering Guide (last major release dated 2016, with periodic errata) and any manufacturer-specific move to publish 95% loaded tables for short, low-torque, flood-fed applications, which Kase Conveyors already lists as a separate section of its capacity data [S3].
The underlying component specifications are covered under dry type transformer, screw conveyor, and ball screw.