AGMA 925-A03

AGMA 925-A03 2003-MAR-13 Effect of Lubrcaton on Gear Surface Dstress

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This information sheet is designed to provide currently available tribological information pertaining to oil lubrication of industrial gears for power transmission applications. It is intended to serve as a general guideline and source of information about gear oils, their properties, and their 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 oil 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 distressmay be scuffing (adhesive wear), fatigue (micropitting and macropitting), or excessive abrasive wear (scoring). Each of these forms of surface distress may be influenced by the lubricant; the calculations are offered to help assess the potential risk involvedwith a given lubricant choice. Flow charts are included as aids to using the equations.

This information sheet is a supplement to ANSI/ AGMA 2101-C95 and ANSI/AGMA 2001-C95. It has been introduced as an aid to the gear manufacturing and user community. Accumulation of feedback datawill serve to enhance future developments and improved methods to evaluate lubricant related surface distress.

It was clear from thework on the revision of standard ANSI/AGMA 2001--C95 (ANSI/AGMA 2101--C95, metric version) that supporting information regarding lubricant properties and general tribological understanding of contacting surfaces would aid in understanding of the standard and provide the user with more tools to make an informed decision about the performance of a geared system. One of the key parameters is the estimated film thickness. This is not a trivial calculation, but one that has significant impact on overall performance of the gear pair. It is considered in performance issues such as scuffing, wear, and surface fatigue. This information sheet provides sufficient information about key lubricant parameters to enable the user to generate reasonable estimates about surface distress based on the collective knowledge available.

Blok [1] published his contact temperature equation in 1937. It went relatively unnoticed in the U.S. until Kelley [2] showed that Blok's method gave good correlation with Kelley's experimental data. Blok's equation requires an accurate coefficient of friction. Kelley found it necessary to couple the coefficient of friction to surface roughness of the gear teeth. Kelley recognized the importance of load sharing by multiple pairs of teeth and gear tooth tip relief, but he did not offer equations to account for those variables.

Dudley [3] modified Kelley's equation by adding derating factors for application, misalignment and dynamics. He emphasized the need for research on effects of tip relief, and recommended applying Blok's method to helical gears.

In 1958, Kelley [4] changed his surface roughness term slightly.

Benedict and Kelley [5] published their equation for variable coefficient of friction derived from disc tests.

The AGMAAerospaceCommittee began investigating scuffing in 1960, and Lemanski [6] published results of a computer analysis that contains data for 90 spur and helical gearsets, and formed the terms for AGMA 217.01 [7], which was published in 1965. It used Dudley's modified Blok/Kelley equation and included factors accounting for load sharing and tip relief.

TheSCORING computer program [8]was released in 1985. It incorporated all advancements made by Blok, Kelley, Dudley and AGMA 217.01. In addition, it added several improvements including:

- Helical gears were analyzed by resolving the load in the normal plane and distributing the normal load over the minimum length of the contact lines. The semi--width of the Hertzian contact band was calculated based on the normal relative radius of curvature;

- Derating factors for application, misalignment and dynamics were explicit input data;

- Options for coefficient of friction were part of input data, including a constant 0.06 (as prescribed by Kelley and AGMA 217.01), a constant under user control, and a variable coefficient based on the Benedict and Kelley equation.

SCORING and AGMA 217.01 both use the same value for the thermal contact coefficient of BM = 16.5 N/[mm·s0.5·K], and they calculate the same contact temperature for spur gears if all derating factors are set to unity.

Annex A of ANSI/AGMA 2101--C95 and ANSI/ AGMA 2001--C95 was based on SCORING and included methods for predicting risk of scuffing based on contact temperature and risk of wear based on specific film thickness.

This information sheet expands the information in annexAofANSI/AGMA2101--C95andANSI/AGMA 2001--C95 to include many aspects of gear tribology.