SHEARING

You probably have heard of the word “oil breakdown”. Many people use this term to represent what they think is a motor oil that has “broken down” and is in need ofchanging, when in reality the actual process of “break down “ is not properly understood. The correct word for this is oil breakdown due to shear forces. An internal combustion engine imparts high shear forces on a motor oil, which is sandwiched between two rotating or sliding forces under load and heat. The molecular structure is essentially torn apart by these mechanical shear forces. The component of the oil that is affected most by these shear forces is the viscosity improvers. These viscosity improvers allow the manufacturer of the oil to create multi-grade oils suitable for a wider temperature range of operation and are covered in detail in another section of this book. The end result of these shear forces is a decrease in the viscosity of the oil as well as a decrease in the viscosity index. Once a motor oil has sheared beyond a specific point it will not revert back to it’s base structure when it cools down and the shear forces have ceased. Keep in mind also that I am referring to petroleum oils only. Synthetic motor oils are extremely resistant to the detrimental effects of shear forces.