Lubricated Sliding Friction

by Ron Kurtus

Engineering laboratories have measured the coefficient of friction for a number of materials that are lubricated by grease and have tabulated the results. These coefficient of friction values apply only to sliding hard surfaces against each other. Since various experimental parameters such as the type of grease are not listed, considerations should be made in using these tabulated values because they may not directly relate to your application.

Questions you may have include:

• What considerations must be made in using such tables?
• What type of grease is used?
• What are the coefficient values?

This lesson will answer those questions. Useful tool: Units Conversion

Considerations

The information available on the various coefficients of friction provide a starting point on the subject, but unfortunately the values lack a good scientific basis.

Condition of materials

Effective scientific measurements typically state the exact condition of materials and that of the surrounding environment. Although lubrication with grease may negate the surface roughness factor of the materials, it is still good scientific practice to document the roughness. Another needed factor is the physical condition of the materials.

Test setup

Describing the test setup is important in order to allow others to duplicate and verify the measurements. With friction, the normal force can be a factor in the case of hard materials that may deform slightly under high pressure. That value should be part of the test description.

Testing environment

It is also important to state the testing environment. Although it is unlikely measurements were made under extreme temperature and humidity conditions that affect the coefficient, at the very least those items should be noted.

Greased materials

The static and kinetic coefficients of friction were measured for a variety of materials where "grease" was used as a lubricant. A major problem in using these readings is that we do not know what type of grease was used. Lubrication can vary considerably, depending on the material.

Another factor is the thickness of the lubricating lever of grease. It is assumed a thin coat of grease it used. A real thick coat of grease could greatly change the coefficient values.

Table

The values in the following table show that measurements on most materials seemed to be either static or kinetic. The reference source of the values listed does not indicate why both types of friction were not measured.

You can use these values for the coefficient as a guide, but it is best to perform some tests and measurements in your specific application to get good values.

Lubricated Coefficient of Sliding Friction

Kinetic

Cast Iron Cast Iron - 0.07
Cast Iron Oak - 0.08
Copper Mild Steel - 0.18
Glass Glass 0.1 - 0.6 0.09 - 0.12
Nickel Nickel 0.28 0.12
Nickel Mild Steel - 0.18
Oak Oak (cross grain) - 0.07
Steel (mild) Cast Iron 0.18 0.13
Steel Copper Lead Alloy 0.16 0.15
Steel (mild) Phos. Bros - 0.17
Steel (mild) Steel (mild) - 0.09 - 0.19
Steel (hard) Steel (hard) 0.05 - 0.11 0.03 - 0.12
Teflon Steel 0.04 0.04
Teflon Teflon 0.04 0.04

Summary

Coefficient of friction have been measured for a number of hard, greased materials. A problem occurs with the values in the table concerning the type of grease used. Considerations should be made in using this tabulated material, since it may not directly relate to your application.

Resources and references

Ron Kurtus' Credentials

Websites

Friction Resources - Extensive list

Engineering Library - Coefficient of Friction

RoyMech (UK) - Friction Factors

Books

(Notice: The School for Champions may earn commissions from book purchases)

Friction and Lubrication in Mechanical Design (Mechanical Engineering Series) by Ali Seireg; Marcel Dekker Pub. (1998) \$199.95

Physics of Sliding Friction (NATO Science Series E:) by B.N. Persson, E. Tosatti; Springer Pub. (1996) \$358.00

Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. I will try to get back to you as soon as possible.

Students and researchers

www.school-for-champions.com/science/
friction_sliding_lubricated.htm

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