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 correctly. You calculated your belt tension. Yet, six months later, the gearbox overheats and leaks oil, or the output shaft shears off. Why?
The answer is almost always the Service Factor (S.F.).
Selecting a gearbox based solely on motor horsepower is the most common mistake in mechanical design. In this guide, we will break down the AGMA Standards for service factors to ensure your gear reducer survives the real-world shocks of operation.
Table of Contents
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1. What is Service Factor?
The Service Factor is a multiplier applied to the motor's power to account for shock loads, continuous operation, and external stresses.
The Golden Rule of Selection:
Gearbox Mechanical Rating ≥ Motor HP × Service Factor
Gearbox Mechanical Rating ≥ Motor HP × Service Factor
This is why gearbox catalogs list multiple power ratings—selecting the wrong service factor is the fastest way to destroy a new gearbox.
If you have a 10 HP motor and a Service Factor of 1.4, you typically cannot buy a "10 HP Gearbox." You need a gearbox mechanically rated for at least 14 HP.
2. AGMA Classes (I, II, III) Explained
The American Gear Manufacturers Association (AGMA) categorizes loads into three main classes. Choosing the wrong one is why conveyors fail.
| AGMA Class | Service Factor | Load Type | Typical Application |
|---|---|---|---|
| Class I | 1.0 | Uniform Load | Fans, Centrifugal Pumps, Liquid Agitators |
| Class II | 1.4 | Moderate Shock | Belt Conveyors (Standard), Lifts, Concrete Mixers |
| Class III | 2.0 | Heavy Shock | Rock Crushers, Reciprocating Compressors, Shakers |
Note for Conveyor Designers: Never use Class I (1.0) for a conveyor belt unless it is uniformly loaded and runs less than 3 hours a day. The standard for most industrial conveyors is Class II (1.4).
Figure 1: Shock loads from conveyors can strip gear teeth if the Service Factor is too low.
3. The Hidden Killer: Thermal Rating
A gearbox has two ratings:
- Mechanical Rating: How much torque the steel gears can handle before snapping.
- Thermal Rating: How much power the gearbox can transmit continuously before the oil overheats and breaks down.
In worm gears (common in conveyors), friction generates significant heat. A gearbox might be mechanically strong enough (10 HP), but thermally limited (only 7 HP). If you run it 24/7, it will cook the seals and leak.
This is why many "correctly sized" gearboxes fail in continuous-duty conveyor applications.
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4. Worked Example: Selecting a Gearbox
Let's finish the design from our previous Conveyor Motor Sizing Guide and Belt Tension Guide.
📝 Engineering Scenario
- Selected Motor: 7.5 kW (10 HP)
- Application: Bulk Material Conveyor
- Duty Cycle: 16 hours/day (Continuous)
- Target Speed: 60 RPM (Output)
Step 1: Determine Service Factor
Since this is a conveyor running >10 hours/day, we look at the AGMA table: Class II (1.4 S.F.) is required.
Step 2: Calculate Required Mechanical Rating
Prequired = Motor Power × S.F.
Prequired = 10 HP × 1.4 = 14 HP
Selection Verdict:
You must select a gearbox with a catalog rating of at least 14 HP (or 11 kW). Do not buy a "10 HP rated" gearbox, or it will fail prematurely under these loads.
Recommended Maintenance Tools
To prevent gearbox failure, check oil temperatures and vibration regularly.
- Infrared Thermometer Gun (Check for overheating > 80°C)
- Vibration Meter Pen (Check for bearing wear)
About the Author:
This article is written by a mechanical design engineer specializing in power transmission, gearbox selection, and industrial maintenance.
This article is written by a mechanical design engineer specializing in power transmission, gearbox selection, and industrial maintenance.
As an Amazon Associate, I earn from qualifying purchases.
⚡ Industrial Automation Guide
Designing heavy-duty machinery? Ensure your calculations match the real-world load:
- Motor Sizing: Calculate Motor Power & Torque (Step-by-Step)
- Conveyors: Conveyor Belt Tension (T1 & T2 Calculations)
- Drive Systems: Drive Selection: Belts vs Screws
- Project Management: The Sheet Mechanic (Book)
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