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Why I Wrote The Sheet Mechanic (And Why Calculations Aren’t Enough)

For engineers who already know the math—but still lose projects. For the last few years, I’ve been sharing technical guides here on Mechanical Design Handbook —how to size a motor, how to calculate fits, and (as you recently read) how to choose between timing belts and ball screws. But after 25 years in industrial automation, I realized something uncomfortable: Projects rarely fail because the math was wrong. They fail because: The client changed the scope three times in one week. A critical vendor lied about a shipping date (and no one verified it). The installation technician couldn’t fit a wrench into the gap we designed. University taught us the physics. It didn’t teach us the reality. That gap is why I wrote my new book, The Sheet Mechanic . This is not a textbook. It is a field manual for the messy, political, and chaotic space between the CAD model and the factory floor. It captures the systems I’ve used to survive industrial projec...
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Conveyor Motor Sizing Guide: Torque, Power, Inertia & Gearbox

Designing a conveyor system involves more than just bolting a motor to a frame. If you undersize the motor, it won't start under load due to breakaway torque. If you oversize it, you waste thousands on electricity and oversized VFDs.

In this guide, we will walk through the engineering math required to size a conveyor motor and gearbox correctly, specifically focusing on the critical "Dynamic Tension" resulting from inertia.

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1. The Physics: Effective Pull (Te)

The first step in any sizing calculation is determining the Effective Pull (Te). This is the sum of all forces resisting the motion of the belt.

The Basic Formula:

Te = Ffriction + Fgravity + Fmaterial

Where:
  • Friction: Resistance from rollers, slider beds, and skirt seals.
  • Gravity: The force required to lift the load (for inclined conveyors).
  • Material: The force required to accelerate the material from zero to belt speed.

Note: In practice, friction and material acceleration usually dominate horizontal conveyor sizing, while gravity dominates inclined applications.

Flat belt conveyor system side view, drive pulley, tail pulley, belt direction arrows, T1 tight side and T2 slack side labelled
Figure 1: Understanding belt tension (T1 vs T2) is critical for preventing belt slip.

2. Calculating Motor Power

Once you have the Effective Pull (Te) in Newtons (or lbs) and the Belt Speed (v) in m/s (or fpm), calculating the required running power is straightforward.

Metric Units (kW)

P (kW) = (Te (N) × v (m/s)) / 1000

Imperial Units (HP)

P (HP) = (Te (lbs) × v (fpm)) / 33,000

📝 Worked Example: Sizing a Bulk Conveyor

Let's consider a horizontal belt conveyor transporting bulk material with the following parameters:

  • Belt Speed (v): 1.5 m/s
  • Total Effective Pull (Te): 3,200 N
  • Startup Requirement: Full load start (High Inertia)

Step 1: Calculate Running Power
P = (3200 × 1.5) / 1000 = 4.8 kW

Step 2: Apply Service Factor
For a loaded start, we apply a service factor of 1.6 (Safety Margin).

Pmotor = 4.8 kW × 1.6 = 7.68 kW

Final Selection:
You generally have two options based on your control strategy:
1. 7.5 kW (10 HP) if using a VFD (controlled ramp).
2. 11 kW (15 HP) if using Direct-On-Line (DOL) starting to handle the immediate inrush current.

3. The Inertia Problem & VFDs

Your search data highlights a common struggle: "dynamic tension resulting from inertia." This tension spike occurs during the first few seconds of operation.

To accelerate a heavy load, the motor must overcome the Moment of Inertia (J) of the entire system. If you start a loaded conveyor "Across the Line" (Direct-On-Line), the sudden torque spike can snap belts or shear gearbox keys.

Comparison: Starting Methods

Option CapEx Cost Mechanical Stress Recommended?
DOL Starter Low Very High (Shock) ❌ No (Unless Small)
Soft Starter Medium Medium ⚠️ Acceptable
VFD (Inverter) Higher Low (Smooth Ramp) ✅ Best Practice

Recommended VFDs for Conveyor Motors

Pro Tip: Use a VFD
For high-inertia conveyors, a Variable Frequency Drive (VFD) is strongly recommended. By programming a 3-5 second acceleration ramp, you limit dynamic belt tension and protect the gearbox from shock loads.

Bonus: VFDs also reduce peak inrush current, lowering electrical infrastructure and breaker sizing requirements.
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4. Gearbox Ratio Selection

Electric motors typically run at high speeds (e.g., 1750 RPM @ 60Hz). Conveyor head pulleys run much slower (e.g., 60 RPM). You need a gear reducer to match the speed and multiply the torque.

3D render of industrial electric motor coupled to a worm gearbox speed reducer
Figure 2: A Gear Reducer is required to drop motor speed (1750 RPM) down to conveyor pulley speed (e.g. 60 RPM).

The Ratio Formula:

Ratio (i) = Motor RPM / Required Pulley RPM

Example: If you need 60 RPM at the pulley and have a 1750 RPM motor:

i = 1750 / 60 ≈ 29.1

You would select a standard 30:1 Gearbox.

Caution: Always verify the gearbox thermal rating and service factor (Class I, II, or III) specifically for conveyor duty cycles.

Recommended Tools for Maintenance

If you are troubleshooting existing conveyors, these tools are essential for diagnosing motor health.

Digital Tachometer
Measure actual belt speed to verify gearbox ratio.

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Clamp Meter (Amperage)
Check if motor is drawing over-current (High Tension).

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Frequently Asked Questions

Why does conveyor motor torque spike at startup?

Torque spikes because the motor must overcome static friction (which is higher than rolling friction) and accelerate the system's total inertia from a standstill. This requires 150% to 200% of the rated running torque.

Is a VFD required for conveyor applications?

Not always, but VFDs are strongly recommended for high-inertia or loaded-start conveyors. They allow for a controlled acceleration ramp, which prevents belt snapping and protects the gearbox gears from shock loads.

How do I size a VFD for a conveyor motor?

VFDs are sized based on the motor’s rated current and voltage. Choose a VFD with at least 10–20% higher current rating than the motor to handle dynamic startup loads. This prevents overheating or tripping during acceleration.

About the Author:
This article is written by a mechanical design engineer with hands-on experience in industrial automation, conveyor system design, and motor sizing applications.

As an Amazon Associate, I earn from qualifying purchases.

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