The measurement of an oil’s resistance to flow is known as its viscosity. Usually, with a temperature increase, we can expect viscosity to diminish, and reduce with a temperature decrease.
Temperature and viscosity are considered to be inversely proportional. In oil analysis, viscosity is typically reported in centistokes (cSt) and calculated by utilizing kinematic viscometers.
Viscosity can also be measured employing absolute (dynamic) viscosity methods and recorded in centipoise. Absolute methods usually employ rotational viscometers, but kinematic methods tend to use flow viscometers which are dependent on gravity. The two methods are differentiated by fluid density.
When choosing the right viscosity oil for your equipment there are some important factors to consider: shear stress conditions, Viscosity Index (VI), and component temperature are all crucial. Viscosity Index is a dimensionless value which quantifies relative changes in viscosity with changes in temperature.
Under extreme temperature changes, oils with a higher VI usually possess less variation in viscosity. A common way to enhance the oil’s VI for mineral base oils are Viscosity Index Improvers. Oils with higher VI can perform at a larger scope of temperatures and reduce wear rates effectively. A number of synthetic base oils have VI values which are naturally high, but not all of them.
Whilst they are effective in decreasing viscosity changes which are temperature dependent, VI Improvers can be susceptible to mechanical shearing. Excessive shearing can result in a decrease in viscosity values at higher temperatures, and lead to the oil being ineffective at producing the necessary fluid film at operating conditions.
Excessive shearing can lead to boundary lubrication conditions, which occurs when two surfaces are no longer achieving full-fluid film (hydrodynamic or elastohydrodynamic). Boundary lubrication is sometimes unavoidable and in these cases we can use anti-wear and/or EP additives to protect the machine surface.
Extreme temperatures, continuous heavy loading, shock loading, and degraded or mixed lubricants can also contribute to boundary lubrication conditions and result in inadequate lubricant conditions. It is vital to be aware of these conditions and ensure the correct oil (and additives) are chosen to manage these issues.
Choosing the Right Viscosity
The load, size, speed, and temperature of the lubricated component must be considered when selecting the right viscosity. This may mean selecting a grease rather than an oil in some cases. There are a number of viscosity calculators and tools available that can assist in selecting the right viscosity for the component.
Typically, higher angular velocity (size and speed), higher temperature applications will usually require oil, yet lower angular velocity applications use grease. To understand which lubricant is right for the equipment, ensure that the OEM is consulted.
Causes of Viscosity Change
Generally, viscosity is considered a lagging indicator test, meaning that something happened to prompt a change in the oil’s viscosity. Most commonly, using an incorrect grade of oil is the reason for a sudden and significant viscosity change, but other root causes include loss/shear of VI Improvers or contamination of water, fuel or other solvents.
Excessive heat, moisture, exposure to air and elevated metal concentrations (acting as metal catalysts) can result in oxidation of the oil, which will also lead to an alteration in the viscosity. It is helpful to utilize instruments like the Spectroil or FluidScan to trend changes in oil chemistry and elemental values, in order to establish the root cause of viscosity changes.
By baselining the brand new oil, setting alarm limits for viscosity can be performed. Baselining the oil is a vital first step because during the blending process, ISO Grades usually possess a +/- 5% cSt tolerance. It is crucial to know the starting point so condemning limits can be set accordingly.
Typically, +/- 5% is cautionary and +/- 10% is alarm. These limits can be altered accordingly. It is possible to go as high as +/- 20% as an alarm, depending on the criticality and history of the part.
There are a number of ways to monitor viscosity, including Ametek Spectro Scientific’s MiniVisc 3000 per ASTM D8092, Kinematic Viscometers (u-tubes) per ASTM D445, Visgages, Rheometers and Rotational Viscometers. In general, ASTM D445 methods are run in a laboratory setting because of the glass capillary tubes and large constant temperature baths which are challenging to maintain in the field.
Spectro’s MiniVisc 3000 can perform kinematic viscosity quickly with just a few drops of oil and the results are reported in centistokes per ASTM D8092. The robust design and small footprint permits easy transportation of the unit.
This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.
For more information on this source, please visit AMETEK Spectro Scientific.