The Conveyor That Couldn’t Start Under Load
We sized a conveyor motor: 50 kg of boxes on a 2 m belt, belt speed 0.5 m/s. The running power looked like 50 W. We picked a 100 W gearmotor. On the floor, the conveyor stalled when starting with boxes on it. The motor couldn’t overcome the static friction to start. The mistake was calculating the running power but not the breakaway torque. Starting a loaded conveyor requires more torque than running it — the static friction is higher than dynamic friction. We upsized to a 250 W gearmotor with higher starting torque. The conveyor started every time. The mistake was forgetting that breakaway torque is 2–3× the running torque.
Conveyor motor sizing has two calculations: the running power (to move the belt at speed) and the breakaway torque (to start from rest under load). Miss either, and the conveyor stalls. This article runs both numbers.
Step 1: Calculate the Belt Pull (Tension)
The motor must pull the belt (and load) against friction. The belt pull (tension) is:
F_belt = μ × (m_load + m_belt) × g
Where μ is the friction coefficient of the system, m_load is the total load on the belt (kg), m_belt is the belt weight (kg), and g is 9.81.
For a slider-bed conveyor (belt slides on a steel bed), μ is high (0.3–0.5). For a roller conveyor (rollors support the belt), μ is low (0.05–0.1).
Example: Slider-Bed Conveyor
- Load: 50 kg of boxes (evenly distributed).
- Belt: 2 m × 0.4 m, weighs about 5 kg.
- Slider bed: μ = 0.4.
F_belt = 0.4 × (50 + 5) × 9.81 = 0.4 × 55 × 9.81 = 216 N.
That’s the belt pull at running speed.
Step 2: Running Power
The running power is the belt pull × belt speed.
P_running = F_belt × v
For F_belt = 216 N and v = 0.5 m/s: P_running = 216 × 0.5 = 108 W.
But this is the power at the belt. The motor loses power in the gearmotor (efficiency η). A worm gearmotor has η = 0.5–0.7. A helical gearmotor has η = 0.85–0.95. So the motor must deliver more power:
P_motor = P_running / η
For η = 0.6 (worm gear): P_motor = 108 / 0.6 = 180 W. That’s the continuous power the motor must deliver.
Step 3: Breakaway Torque (Starting)
Starting a loaded conveyor needs more torque than running. The static friction is higher than dynamic friction. The breakaway factor is 1.5–3× the running pull.
F_breakaway = F_belt × 2 (typical)
For our example: F_breakaway = 216 × 2 = 432 N. The motor must deliver this torque to start.
The gearmotor’s starting torque (breakaway torque) must exceed F_breakaway × the drive pulley radius. For a 100 mm diameter pulley (r = 0.05 m): T_breakaway = 432 × 0.05 = 21.6 N·m.
The 100 W gearmotor we picked had a rated output torque of about 10 N·m. It couldn’t deliver 21.6 N·m. That’s why it stalled. The 250 W gearmotor had 25 N·m — enough.
| Conveyor Type | Friction μ | Breakaway Factor | Typical Gearmotor |
|---|---|---|---|
| Slider-bed (belt on steel) | 0.3–0.5 | 2–3× | 0.18–0.4 kW |
| Roller bed (belt on rollers) | 0.05–0.1 | 1.5–2× | 0.09–0.25 kW |
| Live roller conveyor (lineshaft) | 0.05–0.08 | 1.5× | 0.18–0.55 kW |
| Modular plastic belt | 0.2–0.3 | 2× | 0.25–0.75 kW |
Step 4: Gearmotor Speed (RPM)
The belt speed determines the drive pulley RPM.
n_pulley = v / (π × D) × 60
Where D is the drive pulley diameter (m). For v = 0.5 m/s and D = 0.1 m: n_pulley = 0.5 / (π × 0.1) × 60 = 0.5 / 0.314 × 60 = 95 RPM.
A standard AC motor runs at 1,400 RPM (4-pole). The gearmotor ratio i = 1,400 / 95 ≈ 15:1. Pick a 15:1 gearmotor. The output speed is about 93 RPM — close enough.
Step 5: Check the Gearmotor’s Rated Torque
The gearmotor datasheet lists the rated torque (N·m) and the output RPM. Check two things:
- Rated torque: The continuous torque the gearmotor delivers. Must exceed T_running = F_belt × r. With margin.
- Starting torque: The breakaway torque (typically 2–3× rated). Must exceed T_breakaway.
For our example: T_running = 216 × 0.05 = 10.8 N·m. T_breakaway = 21.6 N·m. A gearmotor rated 15 N·m continuous with 30 N·m starting torque handles it.
The conveyor motor workflow: 1) Calculate belt pull: F = μ × (m_load + m_belt) × g. 2) Running power: P = F × v. Divide by gearmotor efficiency. 3) Breakaway: multiply F by 2×. 4) Required torque: T = F × pulley radius. 5) Pick a gearmotor with continuous torque > T_running and starting torque > T_breakaway. 6) Gear ratio = motor RPM / pulley RPM.
Incline and Decline Conveyors
If the conveyor slopes up or down, add the gravity component.
- Incline: The motor must lift the load. F_gravity = m × g × sin(angle). Add this to the belt pull. A 10° incline adds m × g × 0.17 to the pull.
- Decline: The load pulls the belt downhill. The motor must hold it back (regenerative). Add a brake to prevent runaway.
For an incline conveyor, size the motor for the gravity component (not just friction). A 100 kg load on a 10° incline needs 100 × 9.81 × 0.17 = 167 N extra. That doubles the belt pull.
Accumulating vs. Transport Conveyors
Two conveyor types need different motor sizing.
- Transport (continuous): The load moves continuously. Size for the running pull. Standard.
- Accumulating (stops and goes): The load stops on the belt (the belt slides under it). The pull is higher (the belt rubs against the stopped boxes). Size for the accumulation pull (1.5–2× the transport pull).
An accumulating conveyor with 50 boxes on a 2 m bed has all the load sliding against the belt. The pull is higher than a transport conveyor with the same load. Account for this.
A Conveyor Sizing Checklist
- What is the total load on the belt? (kg)
- What is the conveyor type? (Slider bed, roller, modular?)
- What is the friction coefficient μ?
- Is the conveyor inclined? (Add gravity component.)
- Belt pull: F = μ × m × g (plus incline).
- Belt speed v? (m/s)
- Running power: P = F × v / η.
- Breakaway factor: 2× for slider bed.
- Required torque: T = F_breakaway × pulley radius.
- Gearmotor output RPM = v / (πD) × 60.
- Gear ratio = motor RPM / output RPM.
- Does the gearmotor’s starting torque exceed T_breakaway?
The Bottom Line
Conveyor motor sizing isn’t the running power. It’s the belt pull (friction × weight), the running power (pull × speed), and the breakaway torque (2× the running pull). The conveyor that stalled under load wasn’t undersized for running — it was undersized for starting. Size the gearmotor’s starting torque for breakaway, not just the continuous rating. Account for inclines (gravity) and accumulation (higher friction). The conveyor that starts reliably every time wasn’t the biggest motor — it was sized for breakaway, not just running.
