Tribometer

A tribometer is an instrument that measures friction on a surface via a multitude of methods, one of which is a ball sliding on the reference surface and giving a relative friction value (called ball on disc). [cite web
url=http://www.informaworld.com/smpp/content~content=a781087543~db=all
title=Evaluation of Several Space Lubricants Using a Vacuum Four-Ball Tribometer - Tribology Transactions
publisher=www.informaworld.com
accessdate=2008-05-11
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A tribotester is the general name given to a machine or device used to perform tests and simulations of wear, friction and lubrication which are the subject of the study of tribology. Often tribotesters are extremely specific in their function and are fabricated by manufacturers who desire to test and analyze the long-term performance of their products. An example is that of orthopedic implant manufactures who have spent considerable sums of money to develop tribotesters that accurately reproduce the motions and forces that occur in human hip joints so that they can perform accelerated wear tests of their products.

However, the original method is an apparatus that includes a hanging mass and a standing mass that connects the two masses with a pulley (Refer to Diagram 1). "µ"_static , the coefficient of friction, which is a constant that is needed to determine the friction force and solve for the force needed to overcome it, can be measured through this apparatus.

The general equation for the friction force is the "F"_f = "µ"_static"N", "N" being the normal force which is equal to the weight or mg (mass x gravity) of the sitting mass.

Thus, in this apparatus, the coefficient of friction is determined primarily by two things: the tension of the rope or string connecting the two passes through the pulley and the weight of the hanging mass.

Let’s refer to the hanging mass as "m"_H and the mass on the reference surface or perhaps a flat elevated surface as "m"_T. Since the static friction is the coefficient that determines the needed force to move a certain object, there is no acceleration on the sitting object and means that the tension of the string of the sitting object "m"_T is equal to the friction force and the friction force is then equal to the weight of the hanging object because currently, the apparatus is not moving. Thus, "F"_f should equal "W", or weight.

Since "'F"_f is defined as "µ"_static"N", we must substitute for the value N, the normal force. Because the sitting object is not sinking through the surface or floating up, it is safe to assume that "N" = "mg", the weight of the object.

Therefore, the total friction force is "µ"_static"m"_T"g" = "F"_f. Now, if the friction force is equal to the weight of the hanging object, we can set the two values together.

"µ"_static"m"_T"g" = "m"_H"g" and thus, the value for "µ"_static is equal to ratio of two masses, the hanging mass over the sitting mass.

"µ"_static = "m"_H"g"/"m"_T"g"

In short, the tribometer measures the coefficient of static friction or the force needed to have a certain object move on a certain surface.

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