You selected the right AGMA Class gearbox . You calculated the belt tension perfectly. But the moment you hit "Start," the belt snaps or the gearbox makes a terrifying clunk. The culprit is likely your Starting Method . In conveyor systems, the starting torque profile matters more than steady-state power. Note: We previously discussed VFDs as Energy Savers for pumps and fans. For conveyors, however, the goal is not lowering your electric bill—it is preventing your gearbox from exploding. Table of Contents 1. The Physics of Shock Loads 2. Why Soft Starters Stall Conveyors 3. The VFD Torque Advantage 4. Comparison: Cost vs. Protection 5. Final Verdict Advertisement 1. The Physics of Shock Loads When an AC induction motor starts Direct-On-Line (DOL), it draws 600% to 800% of its rated current (Inrush Current). More importantly, it produces a sudden spike known as Locked-Rotor Torqu...
Figure 1: Inclined Conveyor forces. Starting torque must overcome both gravity and static friction.
Many belt conveyors that run perfectly at steady speed still fail during startup. Typical problems include motor tripping, gearbox shock, belt slip, or excessive mechanical stress.
This article explains how to calculate the required starting torque and acceleration power for a belt conveyor, using practical engineering methods suitable for industrial design.
To complement this article, you should also read our related guide on How to Calculate Motor Power and Torque for a Belt Conveyor , which covers steady-state running conditions.
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1. Why Starting Torque Matters
Motor power calculated for steady-state operation is often not sufficient for startup. At startup, the conveyor must overcome:
- Static friction (stiction)
- Inertia of belt, material load, rollers, and pulleys
- Gearbox and bearing losses
- Full load acceleration
Most belt conveyor failures occur during startup, not during normal running.
2. Running Torque vs Starting Torque
Running torque is the torque required to maintain constant speed once the conveyor is already moving.
Starting torque is the torque required to:
- Break static friction (breakaway torque)
- Accelerate all moving and rotating masses
Figure 2: The "Breakaway Torque" required to start motion is significantly higher than the running torque.
In industrial practice:
Starting torque = 1.5 to 2.5 × Running torque
The exact ratio depends on conveyor length, load condition, friction level, and drive method.
3. Conveyor Mass and Inertia Components
During startup, the motor must accelerate:
- Material load on the belt
- Belt mass
- Drive and return pulleys
- Carry idlers and return rollers (rotational inertia)
For practical engineering calculations, all these effects are combined into an Equivalent Total Moving Mass (meq).
A safe and commonly used industrial approximation is:
meq ≈ 1.1 × mtotal
The additional 10% accounts for the rotational inertia of rollers and pulleys. This approach is conservative and suitable for most industrial belt conveyors.
4. Step 1 – Calculate Running Force
For a horizontal belt conveyor, the steady-state running force is:
Frun = μ × mtotal × g
Where:
- μ = coefficient of friction (typically 0.02–0.05 for roller conveyors)
- g = 9.81 m/s2
This force represents the dynamic friction during normal operation.
5. Step 2 – Calculate Acceleration Force
Acceleration force is required to increase belt speed from zero to its operating speed.
First, calculate acceleration:
a = v / tstart
Where:
- v = belt speed (m/s)
- tstart = startup (acceleration) time (s)
Then calculate acceleration force:
Facc = meq × a
6. Step 3 – Total Starting Force
The total force required during the acceleration phase is:
Fstart = Frun + Facc
Engineering note: At the instant of startup (t = 0), static friction must be overcome. Breakaway force is typically about 2 × Frun.
If acceleration is very slow (low Facc), static friction may govern motor sizing. Always ensure Fstart exceeds breakaway friction.
7. Step 4 – Starting Torque at Drive Pulley
Torque at the drive pulley during startup:
Tstart,pulley = Fstart × (D / 2)
Where D is the drive pulley diameter.
Figure 3: Torque is the rotational force required to turn the pulley against the belt resistance.
8. Step 5 – Required Motor Starting Torque
If a gearbox is used:
Tstart,motor = Tstart,pulley ÷ (i × Î·g)
Where:
- i = gearbox ratio
- ηg = gearbox efficiency (typically 0.94–0.97)
9. Step 6 – Acceleration Power
Instantaneous power required during acceleration:
Pacc = Fstart × v
The selected motor must be capable of delivering both the running power and this acceleration power.
10. Worked Example – Industrial Conveyor Startup
Given:
- Belt speed = 1.2 m/s
- Startup time = 3 s
- Drive pulley diameter = 0.30 m
- Material load = 400 kg
- Belt mass = 100 kg
- Coefficient of friction μ = 0.03
- Gearbox ratio i = 25 : 1
- Gearbox efficiency ηg = 0.95
Total mass:
mtotal = 400 + 100 = 500 kg
Equivalent mass (including inertia):
meq = 1.1 × 500 = 550 kg
Running force:
Frun = 0.03 × 500 × 9.81 = 147 N
Acceleration:
a = 1.2 ÷ 3 = 0.4 m/s2
Acceleration force:
Facc = 550 × 0.4 = 220 N
Total starting force:
Fstart = 147 + 220 = 367 N
Starting pulley torque:
Tstart,pulley = 367 × (0.30 ÷ 2) = 55.1 Nm
Motor starting torque:
Tstart,motor = 55.1 ÷ (25 × 0.95) = 2.32 Nm
Design recommendation:
Select a motor and VFD capable of delivering at least 200% rated torque for the required startup time.
Select a motor and VFD capable of delivering at least 200% rated torque for the required startup time.
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11. Practical Engineering Notes
11.1 DOL (Direct-On-Line) vs VFD Start
- DOL start: 200–300% torque applied instantly, high mechanical shock
- VFD start: Controlled acceleration, reduced stress and belt slip
VFDs are strongly recommended for loaded industrial conveyors.
11.2 Long and Inclined Conveyors
Long or inclined conveyors may require:
- Longer acceleration times (5–10 s)
- Soft starters or fluid couplings
- Multiple drive stations
12. Conclusion
Correct calculation of starting torque and acceleration power is essential for reliable belt conveyor operation.
By accounting for friction, inertia, and acceleration time, engineers can prevent startup failures and confidently select motors, gearboxes, and drives.
For steady-state power and torque fundamentals, refer to: How to Calculate Motor Power and Torque for a Belt Conveyor .
Next Step: Conveyor Acceleration Analysis
Motor power calculations cover steady-state operation only. To ensure safe startup and avoid excessive belt tension or gearbox stress, acceleration behavior must also be checked.
Read Next: How to Calculate Belt Conveyor Acceleration Time (Step-by-Step) »
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