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Screw Conveyor Volumetric Capacity

Calculate absolute bulk material volumetric throughput (CFH) of industrial screw conveyors (augers) based on shaft displacement, flight geometry, and RPM.

Auger Flighting Profile

Operational Velocity

🔧 Trough Capacity Limit: You cannot run augers 100% full. Heavy/abrasive materials are restricted to 15% loading. Standard flow is 30%. Light grain peaks at 45%.

Actual Bulk Capacity

776 CFH
Cubic Feet per Hour.

Volume per Revolution

1241.7 in³
Displacement of a single pitch length.
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What is The Physics of Auger Displacement?

A screw conveyor is a continuous volumetric pump for bulk solids. However, you cannot simply calculate the volume of a 12-inch pipe to find its capacity. The complex geometry requires subtracting the physical 'void' created by the solid center shaft, because grain cannot physically exist where the steel pipe exists. Furthermore, unlike liquid pumps, gravity mathematically prevents augers from running 100% full without catastrophically surging and stalling the motor.

Mathematical Foundation

Volumetric Throughput Equation
CFHmax=0.7854×(D2−d2)×P×RPM×601728CFH_{max} = \frac{0.7854 \times (D^2 - d^2) \times P \times RPM \times 60}{1728}
CFHmaxCFH_{max}= Theoretical 100% throughput capacity in Cubic Feet per Hour.
0.78540.7854= Geometric circle approximation constant (\pi ÷ 4).
DD= Total outer diameter of the steel screw flights (inches).
dd= Outer diameter of the inner solid center shaft (inches).
PP= Distance between flights for one full 360-degree screw pitch (inches).
17281728= Dimensional constant translating Cubic Inches mathematically to Cubic Feet.

Laws & Principles

  • The Center Shaft Void Penalty: When calculating pitch volume, you cannot use the entire auger tube. The heavy steel center shaft 'd' permanently occupies massive physical space that fluid/grain cannot enter. This void must mathematically be subtracted from the outer flight 'D' geometry, or your capacity calculation will massively overestimate the machine.
  • The Trough Loading Limit: You physically cannot run horizontal screw conveyors at 100% capacity. At 100%, the material rolls completely over the top of the internal shaft, burying the hanger bearings, violently stalling the motor, and physically snapping the shaft. Heavy/abrasive materials are mathematically restricted to 15% loading. Standard granular flow is 30%. Extremely light grain peaks at 45% trough loading.
  • The Incline Penalty: The standard CFH equation mathematically assumes the screw is sitting perfectly flat (horizontal at 0 degrees). As you tilt the auger upward, gravity pulls the material backward over the tops of the flighting (fallback). At a steep 45-degree angle, an auger permanently loses exactly 70% of its mathematical volumetric capacity.

Step-by-Step Example Walkthrough

" A millwright installs a massive 12-inch outer diameter grain auger built on a heavy 3.5-inch solid center pipe. It has standard 'square' 12-inch pitch flighting and spins at precisely 60 RPM. Granular feed physically allows for a maximum 30% safe trough loading before burying the hangers. "

  1. 1. Calculate Shaft Void Delta: (12' O.D.)² - (3.5' I.D.)² = 144 - 12.25 = 131.75.
  2. 2. Map True Cross-Sectional Area: 0.7854 constant × 131.75 = 103.48 Square Inches of actual open flight face.
  3. 3. Determine Single Revolution Displacement: 103.48 Area × 12' Pitch width = 1,241.7 Cubic Inches moved per rotation.
  4. 4. Calculate Raw Output: (1,241.7 cu-in × 60 RPM × 60 Mins/Hr) ÷ 1728 feet conversion = ~2,587 Theoretical Max CFH (100% full).
  5. 5. Apply Material Guardrail: 2,587 Max CFH × 30% Allowable Trough Loading = 776 CFH.
Final Result: To definitively prevent the heavy grain from surging over the internal shaft and ripping the hanger bearings out, the 12-inch auger spinning at 60 RPM will safely deliver exactly 776 Cubic Feet per Hour of usable mechanical capacity.

Quick Answer: How much material can my auger move?

Enter your screw's Outer Flight Diameter, Inner Shaft Diameter, Pitch Length, and Motor RPM. The calculator subtracts the internal steel shaft void and processes the volumetric displacement to output the exact CFH (Cubic Feet Per Hour) capacity of the conveyor based on your specified safe loading percentage.

Core Auger Capacity Equations

Volumetric Formula (CFH)

Area_Open = 0.7854 × (Flight_Dia² - Shaft_Dia²)

Displacement_Per_Rev = Area_Open × Pitch Length

CFH_100_Percent = (Displacement_Per_Rev × RPM × 60) / 1728

CFH_Actual = CFH_100_Percent × Maximum_Loading_Percentage

Note: This formula outputs strictly Volume (CFH). To find Mass (Tons Per Hour), you must multiply the resulting CFH by the exact bulk density of your target material (lbs/ft³), then divide by 2,000.

Real-World Scenarios

✓ The Half-Pitch Feed Hack

A processing plant had a massive hopper feeding directly into a standard 12-inch screw conveyor. The sheer weight of the 20-foot tall pile of grain violently forced the screw to run at 100% capacity instantly, completely burying the first hanger bearing and snapping the drive shaft within two days. The millwright pulled the screw out and replaced the first 2 feet of flighting under the hopper with "Half-Pitch" flighting. (The gap between blades was 6 inches instead of 12 inches). Because half-pitch physically moves half the volume per revolution, the screw strictly metered itself to 50% capacity, permanently saving the downstream hanger bearings from being buried.

✗ The Over-Speed Limit Fracture

A quarry needed to move 20% more gravel, so the operator simply swapped the motor pulley to spin the heavy 16-inch auger at a blinding 120 RPM instead of its rated 60 RPM. He assumed capacity scaled linearly. It didn't. The extreme centrifugal force at 120 RPM caught the heavy gravel and violently threw it outward against the steel U-trough wall instead of pushing it forward. The material acted like a brake pad, destroying the motor, while the abrasive gravel wore directly through the 1/4-inch steel trough in less than a month. Screw conveyors cannot exceed standard speed limits.

Maximum Allowable Trough Loading Guidelines

Loading % Material Characteristics Common Examples Max Recommended Speed
15% Max Extremely abrasive, sluggish, or heavy chunks. Sand, crushed rocks, cement clinker. Low (< 50 RPM)
30% Max Moderately abrasive, granular, free-flowing. Dry sand, ash, raw sugar, salt. Medium (< 90 RPM)
45% Max Light, non-abrasive, highly free-flowing. Dry grains, seeds, plastic pellets. Normal (< 120 RPM)
100% ✗ Never run horizontally 100% full. GUARANTEED FAILURE N/A

Note: A screw running at 15% loading means the material only comes up roughly 1/4 of the way up the side of the trough. The material must absolutely never touch the hanger bearings suspended above the shaft.

Pro Tips & Common Mistakes

Do This

  • ✓Upgrade the Pipe Diameter for long spans. A standard 12-inch auger uses a 3-inch center pipe. If the screw spans 20 feet without a middle hanger bearing, the heavy 3-inch pipe will sag in the middle, scraping the flighting violently against the trough. Upgrading strictly to a 4-inch center pipe dramatically stiffens the screw, allowing massive spans without sagging, slightly reducing CFH capacity but permanently saving the machine.
  • ✓Use a Variable Frequency Drive (VFD). Volumetric throughput is perfectly linear to RPM. If you calculate you need exactly 500 CFH, design the screw mathematically for 600 CFH at 60 RPM, then use a VFD to slightly slow the motor down down to exactly 50 RPM in the field. This gives you perfectly dialed control.

Avoid This

  • ✗Never assume vertical augers use the same math. A horizontal screw pushes material cleanly. A vertical screw strictly relies on extreme RPM and centrifugal force to throw the grain against the tubular wall, creating enough friction to defeat gravity. Vertical augers require special flighting, extreme RPM, and massively more horsepower per CFH.
  • ✗Don't ignore the Incline Penalty. If you calculate an auger can do 1,000 CFH laying flat on the ground, tilting it up a mere 15 degrees will immediately drop actual capacity to roughly 700 CFH because the granular material cascades violently backward over the center shaft during transit.

Frequently Asked Questions

Why can't I run a screw conveyor 100% full?

Unlike a liquid pipe, bulk material relies on gravity. If a trough is 100% full, the material rolls wildly over the top of the center shaft. It will permanently bury the intermediate hanger bearings suspended above the shaft, grinding the bearings into dust and violently stalling the motor due to the immense packing friction.

What is standard auger pitch?

Standard pitch is exactly equal to the outer diameter. A 12-inch auger typically has a 12-inch pitch (distance between the blades). Short pitch (e.g., 6-inch pitch on a 12-inch auger) is used exclusively under hoppers to chokefeed the conveyor, or on steep inclines to prevent material fallback.

Does spinning the auger faster linearly increase capacity forever?

No. As you approach very high RPMs (above 120 RPM for heavy solids), centrifugal force overcomes gravity. Instead of pushing the material forward, the flights simply throw the material violently against the outer trough wall, spinning it in a circle (cascading). Capacity plummets and wear rates skyrocket.

How do I calculate Tons per Hour (TPH) from this calculator?

CFH is pure spatial volume. To find physical weight (TPH), you must multiply the calculator's CFH output by your material's specific Bulk Density (lbs per cubic foot), then divide the result by exactly 2,000 to convert pounds into Tons.

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