Effect of Tribology and Lubrication on Gear Surface Distress

AGMA 925-B22

AGMA 925-B22


AGMA 925-B22 covers lubricant-related damage modes in gear teeth. Various methods of gear surface distress are included, such as scuffing and wear, and micro- and macropitting.

This document contains additional information about lubricant viscometric data, Dudley’s regimes of lubrication theory, surface roughness measurements, EHD theory, FZG lubricant test rigs, Gaussian theory, flow charts, and example calculations.



The purpose of AGMA 925-A03 is to provide the user with information pertinent to the lubrication of industrial metal gears for power transmission applications. It is intended that this document serve as a general guideline and source of information about conventional lubricants, their properties, and their general tribological behavior in gear contacts. This information sheet was developed to supplement ANSI/AGMA Standards 2101-D04 and 2001-D04. It has been introduced as an aid to the gear manufacturing and user community. Accumulation of feedback data will serve to enhance future developments and improved methods to evaluate lubricant related wear risks.

It was clear from the work initiated on the revision of AGMA Standards 2001-D04 and 2101-D04 (metric version) that supporting information regarding lubricant properties and general tribological knowledge of contacting surfaces would aid in the understanding of these standards. The information would also provide the user with more tools to help make a more informed decision about the performance of a geared system. This information sheet provides sufficient information about the key lubricant parameters to enable the user to generate reasonable estimates about scuffing and wear based on the collective knowledge of theory available for these modes at this time.

In 1937, Harmon Blok published his theory about the relationship between contact temperature and scuffing. This went largely unnoticed in the U.S. until the early 1950s when Bruce Kelley showed that Blok’s method and theories correlated well with experimental data, he had generated on scuffing of gear teeth. The Blok flash temperature theory began to receive serious consideration as a predictor of scuffing in gears. The methodology and theories continued to evolve through the 1950s with notable contributions from Dudley, Kelley and Benedict in the areas of application rating factors, surface roughness effects and coefficient of friction. The 1960s saw the evolution of gear calculations and understanding continue with computer analysis and factors addressing load sharing and tip relief issues. The AGMA Aerospace Committee began using all the available information to produce high quality products and help meet its long-term goal of manned space flight. R. Errichello introduced the SCORING+ computer program in 1985, which included all of the advancements made by Blok, Kelley, Dudley and the Aerospace Committee to that time. It became the basis for Annex A of ANSI/AGMA 2101-D04 and 2001-D04 which helped predict the risk of scuffing and wear. In the 1990s, this annex formed the basis for AGMA’s contribution to ISO 13989-1.

Just as many others took the original Blok theories and expanded them, the Tribology Subcommittee of the Helical Gear Rating Committee has attempted to expand the original Annex A of ANSI/AGMA 2001-D04 and 2101-D04. Specifically, the subcommittee targeted the effect lubrication may have on gear surface distress. As discussions evolved, it became clear that this should be a standalone document which will hopefully serve many other gear types. This should be considered a work in progress as more is learned about the theories and understanding of the various parameters and how they affect the life of the gear. Some of these principles are also mentioned in ISO/TR 13989-1.

AGMA 925-A03 was approved by the AGMA Technical Division Executive Committee on March 13, 2003.

AGMA 925-B22 replaces AGMA 925-A03. It provides more information on surface roughness and gear mesh lubrication. The calculations for gear mesh temperature have been pulled out of the scuffing calculations and placed into a new Clause 8 to assist with surface distress calculations for micropitting and scuffing. The central film thickness has been updated to include a thermal reduction factor as originally proposed by Gupta. Thermal Properties of Steels, given in Annex L, have been expanded using data from the AISI Steel Handbook, 9th edition. The discussion of micropitting in 11.2 now includes a description and parameters for risk evaluation. Additional changes include clarifying diagrams and corrections from requests for interpretation that have been submitted to the AGMA Helical Gear Rating Committee. 

The first draft of AGMA 925-B22 was made in September 2014. It was approved by the AGMA Technical Division Executive Committee in April 2022.



This information sheet is designed to provide currently available tribological information pertaining to lubrication of industrial gears for power transmission applications. It is intended to serve as a general guideline and source of information about gear lubricants and the influences of their properties, gear tooth surface roughness, and pressure distribution in the contact region on the general tribological behavior in gear contacts. Manufacturers and end-users are encouraged, however, to work with their lubricant suppliers to address specific concerns or special issues that may not be covered here (such as greases).

The equations provided herein allow the user to calculate specific lubricant film thickness and instantaneous contact (flash) temperature for gears in service. These two parameters are considered critical in defining areas of operation that may lead to unwanted surface distress. Surface distress may be scuffing (adhesive wear), fatigue (micropitting and macropitting), or excessive abrasive wear. Each of these forms of surface distress may be influenced by the lubricant. The calculations are offered to help assess the potential risk involved with a given lubricant choice. Flow charts are included as aids to using the equations.

This information sheet is a supplement to ANSI/AGMA 2101. It has been introduced as an aid to the gear manufacturing and user community. Accumulation of feedback data will serve to enhance future developments and improved methods to evaluate lubricant related surface distress.


The following standards contain provisions which are referenced in the text of this information sheet. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this document are encouraged to investigate the possibility of applying the most recent editions of the standards indicated.

ANSI/AGMA 2101, Fundamental Rating Factors and Calculation Methods for Involute Spur and Helical Gear Teeth (Metric Edition)

ANSI/AGMA 1010, Appearance of Gear Teeth – Terminology of Wear and Failure

ISO 4287, Geometrical Product Specifications (GPS) —Surface texture: Profile method —Terms, definitions and surface texture parameters

ANSI/AGMA 6123, Design Manual for Enclosed Epicyclic Gear Drives

Pages: 113

ISBN: 978-1-64353-116-8

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