What is Miner's Rule?
Why Gear Drive Capacity Matters
The power capacity of a gear drive plays a major role in keeping equipment running reliably and avoiding downtime. If a gear drive is undersized, it may fail early. If it is oversized, it may add unnecessary cost.
Choosing the right gear drive is not always easy. Specifications may be incomplete, and different manufacturers present options in different ways. This can make direct comparisons difficult.
When the load on the gear drive and how long that load is applied are known, a fatigue analysis can help. One of the most common methods for this type of analysis is Miner’s Rule. It allows engineers to estimate gear drive life more accurately.
What Is Miner’s Rule?
M.A. Miner introduced Miner's Rule in 1945. It is one of the most widely used methods for predicting failure caused by fatigue. The rule is based on the idea that damage builds up over time as a part experiences repeated stress. Each stress level contributes a portion of damage to the part’s total life.How Miner’s Rule Works
Miner’s Rule looks at multiple stress levels rather than just one. For each stress level:
- Si = the stress level
- Nfi = the number of cycles to failure at that stress level
- ni = the number of cycles actually applied at that stress
The damage caused by each stress level is calculated as:
Damage fraction = ni ÷ Nfi
Each damage fraction represents how much of the part’s fatigue life is used at that stress level.
When all damage fractions are added together, failure is expected to occur when the total exceeds 1.0.
Key Assumptions of Miner’s Rule
Miner’s Rule relies on several important assumptions:
- Each stress cycle at a given stress level causes the same amount of damage
- Early stress cycles are just as damaging as later ones
- The order of applied stress cycles does not matter
In simple terms, Miner’s Rule assumes that fatigue damage adds up in a linear way.
Example of Miner’s Rule in Use
Imagine a part that would fail after 100,000 cycles at a certain stress level.
If that part experiences:
3,000 cycles at that stress level
Then:
3,000 ÷ 100,000 = 0.03
This means 3% of the part’s fatigue life has been used.
If the part then sees another stress level, that stress will use up additional fatigue life. When the total reaches 100%, failure is expected.
Load Spectrums and Real Applications
In real-world applications, loads often change over time. A load spectrum shows these changes by grouping stress levels into “bins,” each with a known number of cycles.
Miner’s Rule evaluates each of these bins and adds their damage fractions together. This makes it useful for analyzing variable loads in gear drives and other mechanical systems.
Learn More About Gear Drive Solutions
For more information on gear drives, including installation manuals, videos, and technical specifications, visit the Regal Rexnord Gearboxes, Gear Drives & Gear Motors page.
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Frank C Uherek
Frank has been a Principal Engineer for the Rexnord Gear Group for over 10 years. Before his tenure at Rexnord, he spent time at Regal Rexnord, Flender Corporation and Winergy Drive Systems Corporation. In addition to his responsibilities at Rexnord, Frank is an active committee chair on both AGMA and ISO rating committees. Frank holds a Bachelor of Science in Mechanical Engineering and a Masters of Business Administration in Production and Accounting from the Illinois Institute of Technology.
Page Updated: May 8, 2026