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...
You sized your motor for running torque. You installed a VFD for a smooth start. But the first time someone hits the big red "Emergency Stop" button, your gearbox output shaft shears off clean.
Why? Because stopping torque demand is often 10x higher than starting torque.
In this guide, we will calculate the massive torque spikes caused by E-Stops and how to protect your conveyor from self-destruction.
Many gearboxes are sized correctly for steady-state operation but fail during emergency stop events due to transient braking torque spikes that exceed shaft and gear tooth limits.
Table of Contents
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1. The Physics: Inertia Hates Stopping
Newton's First Law states that an object in motion wants to stay in motion. When you have a conveyor belt carrying 5 tons of rock moving at 2 m/s, it has massive Kinetic Energy.
An Emergency Stop (E-Stop) forces that energy to zero in a fraction of a second. That energy has to go somewhere. If you use a mechanical brake or a locked-rotor stop, the energy goes directly into twisting the gearbox shaft.
Figure 1: While startup torque (green) is high, E-Stop torque (red) can be nearly infinite if deceleration time is zero.
2. The Formula: Calculating Braking Torque
The torque required to stop a load depends entirely on Time (t).
The Golden Formula:
Tbraking = (J × ΔN) / (9.55 × t)
Where:
- T: Torque (Nm)
- J: Total System Inertia (kg·m2)
- ΔN: Change in Speed (RPM)
- t: Time to stop (seconds)
Note: This calculated torque acts on the gearbox and shaft—not the motor nameplate rating.
The Danger of "Zero" Time
Look at the formula. Time (t) is in the denominator (bottom). As t approaches zero (an instant stop), Torque approaches Infinity.
If you slam on a mechanical brake that stops the conveyor in 0.1 seconds, the torque will be 10x to 20x the motor's rated torque. No gearbox is designed to handle that.
3. Why Service Factors Don't Save You
Engineers often say, "I selected a Class II Gearbox with a 1.4 Service Factor, so I'm safe."
Wrong.
A 1.4 Service Factor means the gearbox can handle 140% of the motor's running torque. But an E-Stop event can generate 500% to 1,000% torque. The steel shaft simply snaps before the motor even slows down.
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4. Solutions: Torque Limiters vs. Ramps
So, how do you meet safety requirements (Stop Fast) without breaking the machine?
Note: In safety-rated systems, "Category 0" stops may still require mechanical braking—making torque limiters essential for drivetrain survival.
| Solution | How It Works | Pros & Cons |
|---|---|---|
| Ramped E-Stop (VFD) | The VFD forces a fast but controlled decel (e.g., 1.0 second). | Best for Gearboxes. Protects hardware but might not meet strict "Category 0" safety rules. |
| Mechanical Brake (Disc) | Calipers clamp on the high-speed shaft. | Stops instantly, but transmits huge shock loads to the gearbox foundation. |
| Torque Limiter | A mechanical clutch slips if torque exceeds a set limit. | The "Mechanical Fuse". It sacrifices itself to save the expensive gearbox. |
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About the Author:
This article is written by a mechanical design engineer specializing in failure analysis, safety systems, and power transmission sizing.
This article is written by a mechanical design engineer specializing in failure analysis, safety systems, and power transmission sizing.
As an Amazon Associate, I earn from qualifying purchases.
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