I'm currently working on ball-on-disk tribological test using different oils.
Is there a direct relation between the coefficient of friction and the lubrication regime of the coupled surfaces?
Is the resulting coefficint of friction related to the materials properties or the lubricant properties? Or both of them?
Thank you to all that would help me.
Stribeck (Stachowiak and Batchelor, 2001) demonstrated that the coefficient of friction is directly proportional to the viscosity of the lubricant and the difference in speed of the contact surfaces; and inversely proportional to the pressure which is exerted on the contact surfaces. The figure in the attached paper shows the Stribeck curve which is the friction coefficient against the bearing number: μω/p (lubricant viscosity (η), speed (ω) and contact pressure (p)). There are three regimes as defined in the Stribeck diagram: boundary lubrication, mixed lubrication and hydrodynamic lubrication.
Thank you very much for your kind reply, Dr. Bachir Achour. Has the contact pressure to be calculated by the Hertzian contact theory?
Your question spans quite a big field, but I will try to answer succinctly.
There is no simple formula relationship between the coefficient of friction and the lubrication regime. You could look at the Stribeck curve and see the trend for coefficient of friction as the contact moves through the regimes, but this is more indicative than conclusive. To calculate it, you would probably need numerical simulations of the lubricant and surface load carrying capacities.
One approach would be to evaluate the ratio of the film thickness relative to the composite surface roughness. As a rough guide, where this ratio is large the regime is probably hydrodynamic, between 3 and 1.5 is probably mixed and less than 1.5 is probably boundary.
Please also bear in mind that that regime of lubrication will vary in the contact, as the surfaces are non-conformal.
The coefficient of friction that you measure can be dependent on a lot of factors. The lubricant viscosity, surface roughness and material hardness are probably the main factors, but the importance of these will change depending on what regime of lubrication you are in. Any additive within the lubricant, temperature, pressure, directionality of the surface roughness and slide-to-roll ratio may also be important factors.
There is a nice paper here that might help; https://www.researchgate.net/publication/335573001_Modelling_Transitions_in_Regimes_of_Lubrication_for_Rough_Surface_Contact
Dear Enrico Baroni,
In the bearing number μω/p also called the Hersey number P is the normal load per length of the tribological contact. Hersey's original formula uses the rotational speed (revolutions per unit time) for ω and the load per projected area (i.e. the product of a journal bearing's length and diameter) for P.
Alternatively, the Hersey number is the dimensionless number obtained from the velocity (m/s) times the dynamic viscosity (Pa∙s = N∙s/m2), divided by the load per unit length of bearing (N/m).
Viscosity and load are generally given parameters, it is useless to calculate them.
P is the applied load. Is based on the forces that will be applied to the bearing over its lifetime. It is a function of the force and contact area of the bearing material.
Regards
To a first approximation the Hertzian contact theory may be used for the pressure calculation in ball-on-disk contact.
- Badges
- Science topic
- Similar topics
- Psychology