Optimizing Lubrication Through Kappa and Viscosity Curve Analysis

When selecting the correct viscosity for rolling element bearings, the question is often asked: Which viscosity grade should be used? Traditionally, this is answered by referring to standard charts that indicate the required viscosity grade. However, it’s uncommon to see this data visualized as shown in the chart above.

In standard practice, we typically aim for a Kappa Ratio of 2 and choose a viscosity grade accordingly. While this approach gets you in the general ballpark, it overlooks many real-world factors—such as operating temperature range, the oil’s viscosity index, and whether the equipment operates under frequent start-stop conditions.

For those unfamiliar with the Kappa Ratio (or Rated Viscosity), it is a guideline developed by bearing manufacturers to determine the correct lubricant viscosity for rolling element bearings. The ratio should ideally fall between 1 and 4, where:

• 1 offers minimal protection,

• 4 provides maximum effective protection, and

• anything above 4 gives no additional benefit, instead causing increased bearing temperatures.

In the example above, I have plotted both the viscosity curve of the actual lubricant in use and the Kappa Ratio for the bearing at its operating speed. This visualization reveals the precise operating temperature window for the application using that specific oil. In this case, the lubricant provides optimal protection between 53°C and 92°C.

If operating temperatures were to exceed 92°C, it would be wise to either:

• move up one viscosity grade—from ISO 68 to ISO 100—which would shift the Kappa curve to a higher temperature range and maintain protection at elevated temperatures, or

• switch to a higher-performing lubricant with advanced additives that maintain effective lubrication even at ratios below 1, avoiding the need for a higher viscosity grade.

In another scenario, if equipment operates between 40°C and 100°C, the current lubricant may be inadequate. A lubricant with a higher viscosity index (VI) would be more suitable, as it broadens the Kappa operating window, enhancing both protection and efficiency across the full temperature range.

A third example involves rationalizing lubricant inventory. By plotting viscosity and Kappa curves across multiple bearings and machines, you may find that five different viscosity grades can be reduced to just two. This simplification streamlines purchasing, reduces inventory costs, and minimizes the risk of cross-contamination or misapplication during top-ups.

Finally, if your goal is to improve plant energy efficiency, you may determine that moving to a lower viscosity grade—paired with a high-VI synthetic oil—can reduce friction losses and deliver measurable energy savings.

All these insights become clear when you plot both your Viscosity Curve and Kappa Curve. With a simple visual illustration, you gain a powerful decision-making tool that brings you one step closer to precision lubrication and operational excellence.

If you’d like assistance in plotting these charts for your bearings and lubricants—or to rationalize your lubricant inventory and unlock new performance and efficiency gains—please contact us at revoserv@revolutionoil.co.za