To build a hydraulic system that works well, designers must choose the right fluid properties—viscosity, compressibility, ability to separate from water (demulsibility), fire resistance, etc. Among these, viscosity is often the most critical because it directly influences wear, precision, and overall efficiency.
What Is Viscosity?
Viscosity describes a fluid’s resistance to flow or deformation. In everyday terms, it’s often thought of as how “thick” or “thin” the fluid is. For example, honey is much more viscous than water — moving a spoon through honey takes considerably more effort. More precisely, viscosity quantifies the internal friction between adjacent layers of fluid sliding past each other.
There are two related measures:
- Dynamic viscosity (also called absolute viscosity), often denoted μ, with units like Pa·s (pascal-seconds).
- Kinematic viscosity (often denoted ν), which is the dynamic viscosity divided by density. It has units like m²/s (or more commonly mm²/s or centistokes, cSt).
What Influences Viscosity?
Temperature is the biggest factor in a hydraulic system: as temperature goes up, viscosity tends to drop (fluid becomes “thinner”). As temperature falls, viscosity rises (fluid thickens).
Molecular structure also matters. Fluids with large, complex molecules or strong intermolecular forces resist flow more.
Note – In hydraulic systems, since the fluid’s chemical composition is fixed, designers mostly worry about how viscosity changes with temperature.
Viscosity Index (VI): Measuring Stability Over Temperature
The viscosity index (VI) is a dimensionless number that describes how much a fluid’s viscosity changes with temperature. A high VI means the fluid’s viscosity changes less as temperature changes (which is desirable). A low VI means viscosity is more sensitive to temperature.
- The VI is typically calculated (for lubricants) using standards (e.g. ASTM D 2279) based on viscosity measurements at 40 °C and 100 °C.
- Because of viscosity changes, fluids are often classified using ISO viscosity grades (ISO VG). For example, ISO VG 32 means the fluid’s kinematic viscosity is roughly 32 cSt at 40 °C (within tolerance).
- ISO also maintains a classification standard, ISO 3448 (formerly ISO 3348 in older texts), for liquid lubricant viscosity grades.
The Significance of Viscosity Index
While viscosity is one of the most if not the most important aspects of hydraulic fluids, viscosity index (VI) is very important too. Viscosity index is a numerical measurement of how fluid increases in viscosity in cold weather and decreases in viscosity in hot weather. The ISO grade of a AW 32 is 32 cSt at 40C. VI will indicate how the product performs at temperatures less than 40C or greater than 40C.
Normal VI measurements will be 95-105 for paraffinic mineral oils. Synthetic hydraulic oils will have a VI of 140-160. Natural esters (Canola) will have VI measurements of 200 and higher. Some synthetic specialty oils (such as for aviation) may have VI’s over 500. One other note is that there are additives that increase the VI that can be included in the product formulas.
How Viscosity Affects a Hydraulic System
1. Mechanical Efficiency / Friction
The fluid should help reduce friction between moving parts. If viscosity is too high (fluid too thick), it becomes harder to move parts, increasing friction losses and reducing mechanical efficiency. If viscosity is too low (too thin), lubrication fails and surfaces may contact each other, causing wear.
2. Volumetric Efficiency / Internal Leakage
Volumetric efficiency deals with how much fluid “leaks” internally (slips past seals, pistons, clearances). If viscosity is too low, more fluid leaks, reducing pressure and responsiveness. If viscosity is too high, flow is restricted and movement slows.
3. Hydrodynamic (Fluid Film) Lubrication
Proper lubrication often relies on a thin continuous oil film separating surfaces. This is called hydrodynamic lubrication. If the fluid is too thin, that film breaks and surfaces contact. If it is too thick, drag and energy losses increase. Finding a “sweet spot” is essential.
4. Cavitation / Foaming / Air Entrapment
Cavitation is the formation and collapse of vapor bubbles in fluid under local low pressure; it can cause pitting and erosion of metal surfaces. If viscosity is high, the fluid is “slower” to respond, which can exacerbate cavitation.
If viscosity is too low, the fluid may not carry away dissolved gas or bubbles, increasing the risk of foaming. High-viscosity fluids may trap air more readily, slowing air release.
5. Heat Dissipation
Hydraulic systems generate heat (through friction, compression, etc.). The fluid must carry away that heat to avoid overheating components. However, as temperature increases, viscosity falls, which can reduce the fluid’s effectiveness. If the fluid’s VI is too low, its viscosity may drop excessively at high temperature, degrading lubrication.
6. Filtering / Flow Through Filters
When fluid must pass through filters, strainers, narrow passages, it must overcome resistance (pressure drop). The thicker (higher viscosity) a fluid is, the more pressure is needed to drive it through filters. This means that high-viscosity fluids are harder to filter and may cause higher pressure losses across filters.
7. Air Release / Degassing
Trapped air can reduce system efficiency, cause noise, and damage components. The fluid’s ability to release entrained air (air release) is influenced by viscosity: more viscous fluids tend to hold air bubbles longer. Designers may use larger reservoirs or slower flow zones when higher viscosity fluids are used.
How to Choose the Right Viscosity
- Always follow the manufacturer’s recommended viscosity grade (ISO VG or equivalent) for your equipment. Deviating too far can degrade performance or damage components.
- Consider your machine’s operating temperature range. The fluid should maintain sufficient viscosity at the highest expected temperature, and still flow reasonably at cold startup.
- In systems with very high pressures or precise controls, a slightly higher viscosity may help maintain film thickness; but this must be balanced against increased drag and energy loss.
- Check that fluid additives (e.g. VI improvers, antiwear agents) do not produce negative side effects (e.g. foam, instability) when viscosity changes.
Summary
Understanding hydraulic fluid viscosity is essential for selecting the right fluid to maximize equipment efficiency, protect critical components, and enhance system performance. At BioBlend, we offer a wide range of lubrifiants respectueux de l'environnement designed to maintain optimal viscosity under various operating conditions. Our premium hydraulic oils provide exceptional performance while reducing environmental impact. Explore our full product lineup, learn how we support various sectors on our industries page, or visit our homepage for more information. Need expert advice? Contact us today for assistance.
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