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Answers to Readers' Questions on Physical Science:

Coefficient of Friction Tables

The following 12 comments and questions have been sent in. They are listed according to date.

List of first 10 items

• Wants friction factorfor steel on steel and lithium grease
• Friction table seems incorrect
• What types of friction aren't used much?
• Determining the kinetic coefficient friction for wood
• Wants to know how measurements were made
• Testing methods for bronze bushing friction
• Coefficient of friction for brass on brass
• This helped me a lot for my research project
• Why doesn't the coefficient of friction have units?
• Friction is rarely constant

Next 10

Wants friction factorfor steel on steel and lithium grease

Question

October 29, 2009

Friction factor for steel sprayed with lithium grease on steel surface.

Does anyone have a friction factor for steel on steel coated with lithium grease?

Paul Salvian
Western Engineering Ltd.

Paul - Canada

18593

Answer

I haven't seen any numbers for that combination. If somebody has such statistics, I can pass them on to you.

Typically, it is best to set up a small experiment and measure the coefficient of friction under the conditions that you use the materials. Often, values you see in charts do not apply to specific situations.

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Friction table seems incorrect

Question

April 13, 2009

When reading through your page on friction I noticed that the coefficients for aluminum-on-aluminum are 1.05 to 1.35 (Static) and 1.40 (Dynamic).

Just curious, how can this be? I always thought that the static coefficient has to be higher than the dynamic. How would this manifest itself physically ? Would the force required to keep an object moving be somehow greater than the force required to start it moving? This seems contradictory.

I'm working on an analysis of two aluminum mating parts, in which friction plays a prominent role. Any clarification you can give me will be very helpful. Thanks !

Dave - USA

17595

Answer

It certainly doesn't make sense that static friction should be less than dynamic or kinetic friction. Major engineering tables on the coefficient of friction seem to have the same values for aluminum-on-aluminum. My guess is that it is a typo that has been replicated everywhere from a single experiment. I'm going to delete that item from the table, since it is obviously incorrect.

A major problem in many coefficient of friction tables is that there is usually no indication concerning the environmental conditions of the experiment or who made the measurement. For example, the aluminum measurement could have been done under different conditions and even with different pieces of metal.

The best bet is to perform some simple experiments to determine the friction on your specific parts. Then you can be sure of the values.

Best wishes in your analysis.

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What types of friction aren't used much?

Question

November 18, 2007

Hi. I have a science question about friction that I can't figure out. It is "What types of friction aren't used much? a. static b. Kinetic C. Fr and Fn or d. Deformation and molecular. Thanks so much for making this site, it has helped me on so much of my physical science that I don't understand.

Danielle - USA

14801

Answer

I'm not sure what Fr and Fn refer to. Sometimes Fr is short for the force of friction. They should be defined.

Deformation friction is used in all automobile tires, but molecular friction is not really used much.

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Determining the kinetic coefficient friction for wood

Question

March 28, 2007

What is the method of determining the kinetic coefficient friction for wood on wood?

iffat - Canada

13446

Answer

The coefficient of friction of wood on wood can vary on the quality of the surfaces. Using the two types of wood in question, you can lay one piece on the floor and the other on top. Use a spring scale to measure the force while sliding the one piece over the other. Adding a weight on top can increase the friction and make it easier to take measurements.

Slowly pull the top weight with the spring scale until they start sliding. Then keep the rate of sliding constant while to take your reading.

This of course is a very basic way of taking the measurement, but it is a start. You can make it more sophisticated with digital scales and such.

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Wants to know how measurements were made

Question

October 30, 2006

Dear Sir/Madam,

I have visited your web page on sliding friction at http://www.school-for-champions.com/science/frictioncoeff.htm , and the results are very useful, however i am interested in exactly how you got the results, was this conducted in a laboratory with specific apparatus? - was it a true sliding test or a reciprocating test? - and depending on that, at what speed was the sliding going on, and also what geometry was the contact?

thank you very much for your help,

Peter

Peter - UK

12273

Answer

As stated in the resources for this page, most of the coefficient of friction values were taken from the Engineer's Handbook site. Unfortunately, as with many coefficient of friction listings, there is no indication of what apparatus or equipment was used to establish the values.

I did issue some complaints in the article about the lack of information concerning the environment and testing conditions.

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Testing methods for bronze bushing friction

Question

July 6, 2006

Dear Sir / Medam, we go through your www. and get very best infromation. We are interested to know the testing method for testing bushing. we have verious types of busing made form bronze metals. we like to test them for best performance among them ie.life of bushing. Is there perfect method of testing which is helpful for us ? Thank you. Kind Regards.

bharat - India

11436

Answer

A good resource on experimental methods of testing bushing friction is at:
http://www.lubedev.com/articles/tester.html

That should give you some ideas on how to improve your busing performance.

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Coefficient of friction for brass on brass

Question

May 24, 2006

could you give me the coefficient of friction for brass on brass please

andrew - UK

11180

Answer

Sorry, we only have data on the items listed.

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This helped me a lot for my research project

Question

May 4, 2006

This helped me alot for my research project. I wanted to know the writers email adress so i could put him on my researcher spread sheet. If you are allowed to give me Ron Kurtus's email it would help me alot.

- USA

11017

Answer

You can use this email address. It is probably better to list the Web address:
http://www.school-for-champions.com/science/frictioncoeff.htm

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Why doesn't the coefficient of friction have units?

Question

February 1, 2006

I noticed that the coefficient of friction never has any units (Like g, kg, m/s). Why is it just a number without units?

Allie - USA

9886

Answer

A coefficient is a number of a proportional relationship. The friction force is proportional to the normal force and that number is the coefficient of friction.

That may not be too clear, but it is the best way I can describe it.

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Friction is rarely constant

Question

January 17, 2006

I disagree with the answer to question number 3 on your website. Friction is rarely constant, even under identical test conditions, therefore the best answer is C.

To qualify my answer - I am the former Chirman of the industry Advisory Board at the Center for Advanced Friction Studies, Southern Illinois University.

Matt - USA

9714

Answer

Thanks for pointing that out. Since the experimental conditions for the measurements seem ill-defined, I wanted to point that out in the quiz. The fact that the coefficient of friction has a tolerance range is a topic that probably should be explained in another lesson. I changed the wording in #c so not to cause confusion with the variation in friction situation.

I appreciate your feedback on the quiz question.

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