It is often said that we understand lubrication, but do know the mechanism that allows lubrications to help reduce friction?
This tip will look at the different lubricant regimes that impact friction and the lubricants used to reduce friction.
Before we discuss reducing friction, let’s understand what it is. Friction is a force that resists one surface from sliding or rolling over another. Factors affecting friction include:
Friction is expressed as a coefficient (µ), proportional to the sliding and rolling forces (F) over the load (P) placed,
µ = F(Force) / P(Load)
The coefficient of friction ranges from 0 to 1. The higher the value, the greater the frictional forces.
A lubricant is designed to help reduce friction or the heat generated from metal to metal contact. This contact results in a number of conditions which generate different types of fluid films. These include:
Boundary lubrication conditions: This occurs at start-up, low speed or high loads and may impact bearings, cylinders and gears. During boundary conditions, two mating surfaces begin to move against each other and are in contact. During transition from static to dynamic, the surface asperities of the metals touch, resulting in adhesive wear. Oil film thickness ranges from 0 to 2.0 micron. At 2 microns, the coefficient of friction is at its highest and this is when up to 70% of normal wear occurs.
Hydrodynamic lubrication regime: Occurs when the oil film is sufficient to completely separate the two mating surfaces that are geometrically similar. Under hydrodynamic condition, there is no risk of opposing asperities coming into contact and the co-efficient of friction drops to a near low point. The oil film is between 2 and 100 micron and any friction encountered is within the lubricant itself.
This occurs in all machinery that has a journal and a shaft, most prevalent at normal operating speeds. The hydrodynamic lubrication phase can be maintained through the rated life of the component assuming temperature, speed and load does not change.
Mixed lubrication film mode: Falls between boundary and hydro-dynamic conditions. It contains characteristics of both, as there are regions of no metal-to-metal contact and areas of some asperity interaction.
Elasto-hydrodynamic film phase: Results from two non-conforming surfaces coming into rolling contact under high load and very high contact pressure. Thinks gears and roller element bearings. The surfaces actually distort or deform under load, and the oil trapped between the mating surfaces. The thin oil film increases in viscosity to the point of almost becoming a solid film, molding to the deformed surface to provide lubrication.
Lubricants and friction
The use of a lubricant to reduce friction is necessary to minimize the wear that may occur. This involves a fully formulated oil of the correct viscosity with carefully selected additives.
I hope this week’s tip was helpful, and please let me know if you have any questions. Or, if you found this useful, please “Like” this article!
Excellent information on adhesive wear to consider. Thanks, Rick Russo.
When metal to metal contact occur, high temperatures occur, which instantaneously weld the contacting metals together. As the sliding motion continues, those welds break apart. The pit or spall that results is called adhesive wear because the two opposing surfaces were bound together.
What is adhesive wear? Can you give an example of where it might occur? Thank you.