Gearbox Service Factor: AGMA Class I, II & III Guide

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

• 1. What is Service Factor?
• 2. AGMA Classes (I, II, III) Explained
• 3. The Hidden Killer: Thermal Rating
• 4. Worked Example: Selecting a Gearbox

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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

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:

1. Mechanical Rating: How much torque the steel gears can handle before snapping.
2. 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
P_required = Motor Power × S.F.
P_required = 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)

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About the Author:
This article is written by a mechanical design engineer specializing in power transmission, gearbox selection, and industrial maintenance.

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