A four ball test has three balls stationary and the top ball rotates. What type of contact does the fourth ball have with respect to the other three balls?
The contacts could be considered as point contacts. At the end of the test you can measure the mean wear spot diameter on the three lower balls and consider it in your computations of the real area of the contact.
Of course, point contact. If we discuus friction kinematics of 4-ball test -- it is rotating, circular, with three points in contact.
Philippe. Scheme of two bodies contact discussed in their static conditions. In the movement it is influence on the geometry of friction trace.
Considering the bodies as rigid, at any instance the ball will have point contact on macro-scale. But, it is known that no body is completely rigid and under the load elastic deformation takes place on micro-scale. The area of contact taking place can be found from Hertzian equation. Hence, the fourth ball is having Hertzian contact at 3 points.
More than types of contacts what you have a in a “Four Ball Machine” test is different types of wear. Those happened during the increment of contact among the surfaces area by wear and incremented load. The “Four Ball Machine” performance wear rate vs. time, quasi static friction & continuous stribeck curves.
The four balls Machine produce a rotate wear friction under rotational motion. A ball tip is loaded onto other 3 balls with a lubricant as test sample with a precise weight and at a specific position from the center of rotation. For each oil sample Friction coefficient is accurately measured by the deflection of a load.
So more than contact you have different type of wear in one test:
. Adhesive wear
. Abrasive wear (Particles of metal among the contacts surfaces)
. Fatigue
. Corrosive wear in 4 ball machine will depend of the additive package.
Dear Carlos! Shubrajit is interested in the type of contact but no wear mechanism.
Buyanovskii: I mentioned wear because in 4 ball machine test only have been worked with contact Herzt as "type of contact"... So far I didn't see (yet) nothing to validate various type of contact or validate contact models at the macroscopic scale in a 4 ball machine test. At theory level I just lay on Lineal Elasticity...The fundamental "linearizing" assumptions of linear elasticity are: infinitesimal strains or "small" deformations (or strains) and linear relationships between the components of stress and strain...
I like to lay on "Theory of Elasticity of Timoshenko" for analyzing 4 ball machine conditions. If some one already proved various type of contact in a 4 ball machine test, I really will like to know it.
https://cdn.preterhuman.net/texts/science_and_technology/physics/Mechanics/Elasticity/Theory%20of%20Elasticity%20-%20TIMOSHENKO.pdf for more information's in the subject.
I. A. Buyanovskii....I really thanks you and appreciate your input. Also I read with avidly and interest your previous job on Nanostructured Carbon Coating. I will humble inquiry for your guidance and experience with Nanocomposites. We are working with different process to obtain Nanocomposites from Natural Fibers (Cellulose). If you have any advise or possible assistance, would you like to pointe us in the right directions? I will be really thankful.. My bests regards.
Dear Carlos, thank you for your interest in my work. I think your research in the field of nanocomposites based on cellulose up to date. Unfortunately, I never investigated such materials. However, if you have specific questions, I will try to answer them.
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