The index is estimated with the help of kinematic viscosity measured at two temperature points therefore, unlike the empirical relationships, the index does not necessarily tell about the path that lubricants kinematic viscosity follows with respect to temperature. Mathematically, viscosity index represents a relative measure of a lubricants temperature-viscosity behavior with respect to two reference oils. an arbitrary number used to characterize the variation of the kinematic viscosity of a petroleum product with temperature. According the ASTM standard, viscosity index is: Since then, this index has been largely popularized and proven to be highly useful for a range of professionals from manufacturer to a common user of lubricants. Therefore, these two categories were initially designated as reference oils where former group was assigned a viscosity index of 100 and the latter group as 0 (as demonstrated in the illustration below).

In early days of industrial lubrication, it was realized that lubricants made from Pennsylvania crude oil is less sensitive to temperature as compared to the lubricants from Golf Coast oil while they both have same kinematic viscosity at 100. The Vogel equation is most accurate over the whole range.ĭan and Davis in 1929 introduced a term, viscosity index to compare the oils temperature-viscosity relationships in relative to each other.

The Reynolds interpolation function is accurate in the vicinity of the reference points. Here is a simple calculator that can be used to calculate dynamic viscosity of an oil at operating temperature, if the dynamic viscosities at 2 other temperatures are known (typically the viscosities at 40 ☌ and 100 ☌ are taken from the data sheet as reference points, but any two temperatures and viscosities can be used).Īs a rule of thumb, the viscosity of a machine oil fall about 25% with every 10✬ temperature increase. In simple terms, empirical relationships do not satisfy the need of a readymade metric for viscosity-temperature characteristics for a common user of lubricants. The reason behind such ambiguity lies in the need of huge experimental data in order to reliably fit each of these equations. This model proves to be more useful in engineering calculations and predictive numerical models.ĭespite of the development in temperature-viscosity characteristics in empirical terms, some of which are useful for engineering calculations nevertheless, they often appear to be less useful as a quick guidance for practitioners. to follow only an exponential curve), it may work well only over a limited temperature range for a limited range of products.Īnother model was proposed by Vogel:, where x, y, and z are empirical constants. This equation appears to be simple and can be easily implemented in mathematical models but owing to its highly conservative functionality (i.e. Where is dynamic viscosity, and are empirical constants. Reynolds have pioneered in this direction and prescribed an exponential fit to describe the declining trend of viscosity with respect to temperature. Over the years, attempts have been made to develop empirical relationships to describe the lubricants temperature-viscosity behaviors. An attention to the temperature-viscosity characteristics of lubricants is therefore of practical importance for ensuring the lubricants desirable performance in any particular system. Thereby, neither a thick nor a thin lubricant can be the suitable choice that an engineer can make during the selection of lubricant. In such cases, lubricants get thicker and add up resistance to the movement of elements. On the other hand, viscosity of lubricant can also be critical for the performance of applications at sub-zero atmospheres. Another major source of heat is the high temperature operating environment (e.g. gears, bearings, engines), temperature rise due to friction induced heating is inevitable.

Loss in lubricants viscosity may lead to severe performance issues of mechanical systems in industry and transportation applications. Oil viscosity often declines rapidly with respect to rise in temperature. Sustainability of lubricating film between contacting bodies in mechanical systems is often critical owing to the highly sensitive nature of lubricants temperature-viscosity relationship. Temperature-viscosity characteristics of lubricating oils and/or greases is one of the important determinant for lubricants performances in mechanical systems. However, it is also known that temperature impacts the viscosity. It is well-known from Hydrodynamic Lubrication Theory that viscosity plays a central role in the lubrication regime encountered in the machine element – the higher is the viscosity, the thicker is the oil film that separates the surfaces from touching each other. Viscosity is a measure of a lubricating oil’s resistant to flow.

Kinematic viscosity of engine oil