## Rules Rule: It’s the law

Michael Anderson | TLT President's Report March 2018

A look at four basic principles of tribology testing.

The law of gravity was allegedly discovered when an apple fell on Sir Isaac Newton’s head.
© Can Stock Photo / DaneeShe

Throughout history, discoveries and research have led to rules being made and attributed to great people. Most notably of these might be those rules, also known as laws, from Sir Isaac Newton. After being hit on the head by a falling apple, or so the story goes, he discovered gravity. But, more important, he is remembered for his three laws of motion:

Newton’s First Law: Known as the law of inertia, an object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Newton’s Second Law: Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object). Force = mass × acceleration.
Newton’s Third Law: For every action there is an equal and opposite reaction.

These laws, introduced in the late 1600s, among other discoveries, made Newton famous. But approximately 200 years earlier, Leonardo da Vinci developed two rather simple laws of friction:

da Vinci’s First Law: The areas in contact have no effect on friction.
da Vinci’s Second Law: If the load of an object is doubled, its friction also will double.

Other scientists throughout history have embellished and added to these famous laws: Amontons, Coulomb, Stribeck, Bowden and Tabor, to mention just a few.

Motion and friction are common attributes to tribology and tribology testing. Because my background is laboratory tribology test method development, I have formulated the basic Four Rules of Tribology Testing. If you remember, in my February column I mentioned Mike’s Rule No. 3 of Tribology Testing with respect to the Stribeck curve. So I wish to share with you all four rules.

Mike’s Rule of Tribology Testing No. 1: Test machines don’t lie! Discussion: A properly calibrated test machine will report exactly what is happening at the material interface under load and in motion. It will display the exact frictional and wear properties taking place.

Mike’s Rule of Tribology Testing No. 2: All tests tell the scientist something. Discussion: Following on Rule No. 1, the friction and wear values obtained are real and, therefore, give the scientist information. It is possible that the information generated may or may not have defining value, but it will be able to direct the operator to modify the test method for obtaining more meaningful data to screen and/or rank materials for their performance. Therefore, every test always tells the operator something.

Mike’s Rule of Tribology Testing No. 3: Every test is a point on or a portion of a Stribeck curve. Discussion: The Stribeck curve (further described in my last article), shows the relation between coefficient of friction with respect to changes in viscosity (ρ), speed (ν) and load (ω) and is represented as (ρ·ν/ω). This quotient with respect to the Stribeck curve defines a lubricated condition as hydrodynamic (and elastohydrodynamic), boundary or mixed (combination of hydrodynamic and boundary).

Mike’s Rule of Tribology Testing No. 4: Test data is only meaningful and comparable when all test conditions are identified and consistent. Discussion: Tribological properties are not intrinsic. They are dependent on the test conditions under which the test is conducted. Only results obtained under identical test conditions can be used to compare the performance of different materials.

While my name won’t go down in history like Leonardo da Vinci or Sir Isaac Newton, Mike’s rules should help research scientists better understand tribology testing and the comparison of test results. Until the day comes when friction is totally understood and materials do not wear out, tribology will have a place in every industry. This is just another reminder that tribology is everywhere.

Mike Anderson is Area Manager Asia Pacific/Latin America for Falex Corp. in Sugar Grove, Ill. You can reach him at manderson@falex.com.