Introduction to Tribology and Lubrication

Tribology is the science of the mechanisms of Friction, Lubrication, and Wear of interacting surfaces that are in relative motion.

  A Tribological System (Tribosystem) transform Inputs as a type of motion, the sequence of motion, load, velocities, temperatures, and loading time; by Disturbance Variables as material and geometry properties and interactions between elements; in Outputs as force, torque, speed, motion, mechanical energy, material variables, and signal variables; and Loss Variables as friction and wear.

Friction is the resistance to movement of one body over body, the friction laws were formulated by Guillaume Amontons:

1st Law: Friction force is proportional to the applied load.

2nd Law: Friction force is independent of the apparent contact area.

3rd Law (also Coulomb's Law): Friction is independent of sliding velocity.

Generally, friction force F is the result to multiply the normal load N by the coefficient of friction m. The static coefficient of friction may be greater than the kinetic coefficient of friction. 

The microscopic mechanics that are involved in generating friction are:

1.   Adhesion.

2.   Mechanical interactions of surface asperities.

3.   Plowing of one surface by asperities on the other.

4.   Deformation and/or fracture of surface layers.

5. Interference and local plastic deformation caused by third bodies primarily agglomerated wear particles trapped between the moving surfaces.

Wear is the succession of events whereby atoms, products of chemical conversion, fragments, et al., are induced to leave the system. 

The major wear modes are:

·     Abrasive Wear: occurs whenever a solid object is loaded against particles of a material that has an equal or greater hardness.

·     Adhesive Wear: Cold-welding describes the formation of small connections whereby tiny disruptions arise during translation.

·     Corrosive & Oxidative Wear: Chemical the reaction between the worm material and a        corroding medium can be a chemical reagent, reactive lubricant, or even air.

·     Fatigue Wear: By deformations sustained by the asperities and surfaces make contact. They are accompanied by very high local stresses that are repeated a large number of times.

·     Erosive Wear: By the impact of particles of solid or liquid against the surface of an object.

·     Electrical Erosion Wear: Occurs when electric current passes between two metal surfaces through the oil or grease film. 

·     Fretting Wear: Occurs whenever short amplitude reciprocating sliding between contacting surfaces is sustained for a large number of cycles. If micro-particles are present then the name is Polishing Wear. 

·     Cavitation Wear: By the cyclic formation and the collapse of bubbles on a solid surface in contact with a fluid.

Lubrication is the process or technique employed to reduce friction between, and wear of one or both, surfaces in proximity and moving relative to each other, by interposing a substance called a Lubricant in between them. 

The main property of a lubricant is Viscosity that is defined as the internal resistance to flow of one layer of the fluid, moving in relation to an adjacent layer; Absolute Viscosity or Dynamic Viscosity (h) is the proportional factor of the shear stress in a fluid to the rate of change of velocity with respect the the thickness of the fluid film. Kinematic Viscosity (u) is the ratio of the dynamic viscosity to the density of the fluid.

Stribeck Curve, defined by Richard Stribeck, is basically a curve between Coefficient of Friction and a number defined as dynamic viscosity with relative sliding velocity per unit load. The curve defines four different forms of lubrication called the Lubrication Regimes.

 (Stribeck Curve, from Wang, J. Encyclopedia of Tribology, Springer US, York 2013)

a. Boundary Lubrication. The condition when the fluid films are negligible and there is considerable asperity contact. The mean film thickness is lower than the surface roughness; the coefficient of friction is a maximum in this area. 

b. Mixed Lubrication. The number is higher, the mean film thickness is just higher than surface roughness, so the tallest asperities of the bounding surfaces will protrude through the film and occasionally come in contact. The coefficient of friction reduces dramatically until a minimum.

c. Elastohydrodynamic Lubrication (EHL). The condition that occurs when a lubricant is introduced between surfaces that are in rolling contacts, such as ball and rolling element bearings. In this lubrication regime, the load is sufficiently high enough to produce Hertzian pressures for the surfaces to elastically deform, in those points, the lubricant film has got a Non-Newtonian behavior. The coefficient of friction is minimum in this area; the behavior is defined by the Cheng equation.

d. Hydrodynamic Lubrication. The condition when the load-carrying surfaces are separated by a relatively thick film of lubricant. This is a stable regime of lubrication and metal-to-metal contact does not occur during the steady-state operation of the bearing. The lubricant pressure is self-generated by the moving surfaces drawing the lubricant into the wedge formed by the bounding surfaces at a high enough velocity to generate the pressure to completely separate the surfaces and support the applied load. The coefficient of friction increase in this area, the behavior is defined by the Reynolds equation.

 Also the Hydrostatic Lubrication regime can be added, in which surfaces are fully separated by a lubricating film of liquid or gas forced between the surfaces by external pressure.