Wear: How And Why

From The AMSOIL Dealer Magazine February 1995

Wear is the loss of material with subsequent change in clearance between surfaces moving relative to each other. Continued wear results in component malfunction.


Tribology is the study of friction, lubrication and wear. Most tribological research focuses on lubrication for it is the simplest, most available and least expensive wear control method available.

Tribologists have identified three fundamental lubrication regimes, two of which, hydrodynamic  lubrication and boundary lubrication, play major roles in the lubrication of automotive engine bearings, which are sliding bearings. The third regime, elastohydrodynamic, occurs most frequently in rolling bearing applications.

Hydrodynamic lubrication

Hydrodynamic lubrication (HDL) occurs when moving surfaces are separated completely by a lubricant film. Because HDL prevents surface contact, it minimizes friction, noise and wear. Well-designed, well-maintained HDL components often enjoy long service lives. Piston rings and journal bearings, such as the crankshaft, are HDL components.

Journal bearings are cylindrical shells with a shaft extending through the cavity. During shutdown, the shaft rests on the bottom of the bearing. During operation, the shaft, supported by the lubricant, rotates in the bearing.

During startup, as the shaft begins to rotate, it travels slowly along the bottom of the bearing. The movement of the shaft draws lubricant into the crescent-shaped space between the shaft and the bearing. The lubricant wedge builds pressure as the shaft gains speed. Hydrodynamic pressure lifts the shaft toward the middle of the bearing and lubricant surrounds the shaft. The geometry of the shaft and bearing, the shaft’s velocity, the lubricant’s viscosity and the load borne by the system all affect the formation and maintenance of the lubricant wedge.

In contrast, piston rings tilt as they slide along the cylinder bore. Their tilting creates the lubricant wedge that provides the hydrodynamic lift and protection of the piston and cylinder.

Lubricant viscosity

Lubricant viscosity is crucially important to the development and maintenance of the protective lubricant wedge. Lubricants with excessive viscosity cause excessive heat. The energy required to overcome the lubricant’s resistance to flow (its viscosity) is converted in the lubricant to heat, which damages components and accelerates lubricant breakdown. Lubricants with insufficient viscosity cannot develop adequate hydrodynamic pressure to fully separate moving surfaces, leading to wear.

Because the lubricant wedge depends on high speed shaft rotation, HDL breaks down in low speed operations, such as engine startup. During low speed operations, engine journal bearings depend on boundary lubrication for protection against wear.

Boundary lubrication

Boundary lubrication (BL) occurs when lubricant pressure is insufficient to completely separate the surfaces of slowly moving elements under heavy load. Journal bearings find protection against excessive wear during periods of BL from a coating of anti-wear additives. The coating on the shaft rubs the coating on the bearing, protecting the metal surfaces from rubbing one another while the hydrodynamic lubricant wedge builds during low speed operations.

BL offers protection to other systems at times in their operation when they are not protected by hydrodynamic or elastohydrodynamic films: gears, during the sliding part of tooth engagement; hypoid differentials, which, due to their severe geometry and extreme loads, never enjoy full film protection; and cams and followers, whose geometry and load preclude HDL formation, even during normal operation.

Like engine journal bearings during low speed operations, cams and followers are protected by anti-wear additives; unlike journal bearings, cams and followers depend on anti-wear additives throughout normal operations. Gears, hypoid differentials and other systems exposed to high loads are often protected by extreme pressure additives which work by chemically interacting with the metal surfaces to form protective coatings.

What Allows AMSOIL Synthetic Motor Oils To Provide The Best Protection, Helping Engines Wear Very Little?

Viscosity Index

Viscosity Index describes a lubricant’s loss of viscosity as a result of increases in temperature. Long hydrocarbon chain molecules give lubricants higher viscosity than short chain molecules do. Because
synthetic lubricant molecules are uniform in size, each molecule having the appropriate hydrocarbon chain length for its viscosity grade, AMSOIL Synthetic Motor Oils naturally have high viscosity indices.

Shear stability describes a lubricant’s resistance to viscosity loss due to shearing forces in the engine. In journal bearings, the lubricant moves around the rotating shaft, slower as it enters the large end of the wedge and faster as it exits the small end. However, because the lubricant clings to the shaft and bearing, it moves very slowly against those surfaces, which means it moves very fast in the center of the lubricant film to maintain a constant volume of lubricant in the wedge. The difference in velocity between the center and outside of the lubricant film can shear lubricant molecules, causing them to lose viscosity. Viscosity index improver molecules, used to insure appropriate viscosity in high temperature operations, are particularly susceptible to shear. Due to the naturally high viscosity indices of synthetic base stocks, AMSOIL Synthetic Motor Oils require the addition of a much smaller quantity of Viscosity Index Improver than conventional oils do – and AMSOIL uses shear stable chemistry only. Result: AMSOIL Synthetic Motor Oils maintain viscosity even after exposure to highly stressful engine conditions.

Anti-wear additives are physically attracted to the metal surfaces in the engine and cling to them even after the engine is shut down and the oil has settled to the sump. The unique blend of synthetic base stocks in AMSOIL Synthetic Motor Oils actually helps larger quantities of AMSOIL’s anti-wear additive system cling longer for superior boundary lubrication protection at startup.

Picture showing Abrasive and Adhesive Wear

*All trademarked names and images are the property of their respective owners and may be registered marks in some countries. No affiliation or endorsement claim, express or implied, is made by their use. All products advertised here are developed by AMSOIL for the use in the application shown.

Reproduced With The Permission Of AMSOIL INC. All Rights Reserved.

For More Information On How AMSOIL Lubricants Perform Now, Check Out The AMSOIL Performance Tests.

Comments are closed.