Need-to-Know Fluid Power Facts: The 5 W's of Hydraulic Oil Viscosity
Lubricant engineers know the value of understanding oil viscosity before choosing the right petroleum-based hydraulic oil for any application.
Selecting the right fluid viscosity, which is the measure of a fluid’s resistance to flow, is necessary to protect equipment from:
- Friction
- Abrasion
- Poor mechanical efficiency
- Adhesive hydraulic component wear
Low oil viscosity, for example, can cause a loss of lubricating film strength leading to wear between two parts. High oil viscosity can cause problems like heat generation and cavitation. Making the correct hydraulic oil choice is vital for the best performance from a hydraulic system, as well as for overall protection and longevity of the equipment.
Learn About Oil Viscosity With the 5 W’s
To understand the most important facts on any subject or situation, investigators will often seek out the “who, what, when, where, and why” of the subject.This tactic is helpful to produce a solid list of go-to information when considering oil viscosity.
Our 5 W’s on the topic of hydraulic oil viscosity are below.
Who (or in this case, “Which”): Kinematic Viscosity or Dynamic (Absolute) Viscosity
The “who” to consider in an oil viscosity quandary is really more of a “which” question that involves two types: Kinematic and Dynamic (or absolute).Kinematic Viscosity is related to a fluid’s resistance to pour and is measured in centistokes (cSt). Dynamic Viscosity is related to a fluid’s resistance to stir and is measured in centipoise (cP). The difference here is important when considering viscosity in already running machinery (kinematic) vs. getting machinery started (dynamic). Kinematic Viscosity is more useful when choosing a hydraulic oil in the interest of fluid motion and less friction propagation.
What: The Grading System
The grade of hydraulic oil viscosity is a determination of what kind of oil works best, in comparison to other kinematic viscosities at standard temperatures, as seen in the chart below.A grading system created by such organizations as ISO, American Gear Manufacturers Association (AGMA), and Society of Automotive Engineers (SAE) allows comparison of kinematic viscosities among grade levels at standard temperatures of 40° C and 100° C. Further, multi-grade lubricants allow for two different viscosity profiles depending on temperature and provide a lower-grade viscosity while starting machinery.
When: Temperature and Pressure Considerations
Number three on the list of need-to-know facts includes consideration of relationships among temperature, pressure, and kinematic viscosity — or "when" to choose an increased or decreased oil viscosity.The greater the pressure or load on the lubricant, the harder it is to flow. Therefore, the kinematic viscosity rises when there is increased pressure. Transversely, the lower the load, the easier the lubricant flows, and the kinematic viscosity lowers when pressure lowers.
Temperature also is a determinant for when adjustments need to be made with oil viscosity. The simple rules about the relationship between temperature and viscosity are:
- As temperature increases, kinematic viscosity decreases
- As temperature lowers, kinematic viscosity increases
Where: Viscosity Index
Where can grading, temperature, and viscosity type be quantified? The Society of Tribologists and Lubrication Engineers say the Viscosity Index (VI) is a good start, as it takes changes of temperature into consideration and is an indication of the rate of change of an oil’s viscosity with temperature. As oil viscosity varies with temperature, the Viscosity Index quantifies that relationship. The lower the VI, the more the viscosity is affected by changes in temperature. The higher the VI, the more stable the viscosity remains over temperature fluctuations.Viscosity Index | Classification |
| Under 35 | Low |
| 35 to 80 | Medium |
| 81-110 | High |
| Above 110 | Very high |
Why: Why is all of the above important?
This information is critical as the overall efficiency and longevity of a hydraulic system is dependent on paying attention to the:- Kinematic viscosity (who)
- Grade (what)
- Temperature and pressure (when)
- Viscosity index (where)
Ignoring the facts of oil viscosity when considering which fluid to use can make hydraulic systems prone to problems in mechanical efficiency, heat transfer, air release, volumetric efficiency, hydrodynamic lubrication, and filtration ability.
Increased heat, wear, shorter component life, and slower pump speed (and resulting cavitation) can also be the result of poor viscosity choices, so make sure to use the above information to make the right hydraulic oil choices at the outset.