When dealing with lubricants, hydraulic fluids, or industrial machinery, one of the most important parameters engineers and maintenance teams must understand is kinematic viscosity. It is not just a theoretical concept from fluid mechanics—it has direct implications for the performance, safety, and longevity of equipment across a wide range of industries.
At Bijur Exim, a trusted name in the supply of high-quality industrial lubrication systems and components, we recognize the critical role that fluid properties play in efficient operations. In this comprehensive blog, we delve deep into what kinematic viscosity is, how it differs from other types of viscosity, why it matters in industrial applications, and how to measure and choose the right fluid based on this property.
What is Kinematic Viscosity?
Kinematic viscosity is defined as the ratio of dynamic viscosity to fluid density. It describes how easily a fluid flows under the influence of gravity, without external pressure.
Difference Between Dynamic and Kinematic Viscosity
- Dynamic viscosity (also called absolute viscosity) measures the internal resistance of a fluid to flow when an external force is applied.
- Kinematic viscosity, on the other hand, takes gravity into account and expresses how fast a fluid can move.
This makes kinematic viscosity a better indicator of fluid flow in gravity-fed systems, such as lubrication lines, hydraulic reservoirs, and gravity drain lines—a common feature in the systems designed and supplied by Bijur Exim.
Units of Measurement
Kinematic viscosity is typically expressed in:
- Centistokes (cSt) – Commonly used in industrial lubrication
- Square millimeters per second (mm²/s) – SI unit (1 cSt = 1 mm²/s)
For example:
- Water at 20°C has a kinematic viscosity of ~1 cSt.
- Motor oil at 40°C may have a kinematic viscosity of 100 cSt or more.
Why Kinematic Viscosity Matters in Industry
1. Lubrication Performance
A fluid with incorrect viscosity can lead to inadequate film formation, causing metal-to-metal contact and premature wear. If the viscosity is too high, the fluid may not flow properly, increasing energy consumption and causing system overheating.
2. Pump and Valve Efficiency
For hydraulic systems, kinematic viscosity affects the efficiency of pumps and valves. Fluids that are too viscous may cause sluggish operation, while low-viscosity fluids can cause internal leakage.
3. Temperature Sensitivity
Kinematic viscosity varies significantly with temperature. As temperature increases, viscosity decreases. That’s why choosing a lubricant or fluid with the right viscosity index is essential for systems that operate across varying temperature conditions.
Common Applications in Industrial Sectors
At Bijur Exim, we cater to multiple industries where kinematic viscosity plays a vital role:
A. Cement Industry
Lubricants used in ball mills, gearboxes, and rotary kilns must maintain stable viscosity under high temperatures and dust exposure.
B. Food & Beverage Industry
Food-grade lubricants with specific viscosity are essential to ensure safe, contamination-free processing, especially in conveyors, mixers, and filling systems.
C. Automotive & Heavy Equipment
Engine oils, gear oils, and transmission fluids are all specified based on their kinematic viscosity to ensure peak performance and component longevity.
D. Mining & Construction
Heavy-duty machinery in this sector operates under extreme loads and dusty conditions. Using lubricants with the correct viscosity ensures reduced downtime and equipment failure.
How Kinematic Viscosity is Measured
Kinematic viscosity is measured using capillary viscometers, the most common of which is the Ubbelohde viscometer. Here’s a simple breakdown of the process:
- The fluid is poured into a glass capillary tube.
- The time it takes for the fluid to flow between two marks under gravity is recorded.
- This time, combined with a calibration constant, is used to calculate viscosity in centistokes.
These measurements are typically done at 40°C and 100°C for industrial oils, and the results are reported as ISO VG grades (e.g., ISO VG 32, ISO VG 68).
ISO Viscosity Grades (VG) and Kinematic Viscosity
The ISO VG system classifies lubricants based on their kinematic viscosity at 40°C. Each grade represents a range:
| ISO VG Grade | Kinematic Viscosity at 40°C (cSt) |
|---|---|
| VG 22 | 19.8 – 24.2 |
| VG 32 | 28.8 – 35.2 |
| VG 46 | 41.4 – 50.6 |
| VG 68 | 61.2 – 74.8 |
| VG 100 | 90 – 110 |
| VG 150 | 135 – 165 |