How plotting your Kappa curve against your Viscosity curve can help rationalize lubricants and take you one step closer to precision lubrication.
When considering viscosity selection for rolling element bearings the questions is always asked as to what viscosity grade should be used, where numerous charts are used to determine which grade is necessary, but it is very seldom that these are plotted on a chart such as the one above.
Standard practice is to target a Kappa Ratio of 2 and select a viscosity grade from there, although this method will put you in the general ballpark it does not take into account the multitude of operational and environmental circumstances which may arise, such as operating temperature windows, the viscosity index of the oil in use and whether the rotating equipment is operated under frequent stop/start conditions.
For those who are not familiar with the Kappa Ratio / Rated Viscosity, it is a tool developed by bearing manufacturers to establish the correct viscosity grade to use for rolling element bearing applications and which should be maintained between a ratio of 1 and 4, with a ratio of 1 offering the least amount of protection and a ratio of 4 giving the highest. Anything above 4 offers no further bearing protection and will only cause increased bearing temperatures.
In the image above I have plotted a viscosity curve for the exact lubricant in use as well as the Kappa Ratio for the bearing at its operating speed. By plotting the information in this way, I can see the exact operating window for this specific application using the exact oil that is in use. From this I can determine that the oil in use to lubricate this particular bearing offers optimum protection throughout temperature ranges of 53°C – 92°C.
Going back to the example in the image you can determine that if the equipment were to exceed the maximum temperature of 92°C it may be prudent to move one notch up in VG from an ISO 68 (which it is currently) to an ISO 100, which will have the effect of shifting the Kappa curve to a higher temperature range offering better protection during high temperatures. Alternatively, one could switch to a higher performing lubricant with select additives that will allow the lubricant to perform at ratios that fall below 1 without the need for switching to a high viscosity grade.
In another example, you could conclude that the equipment being operated has an operating window of 40°C – 100°C, which means that the current lubricant being used is inadequate and a lubricant with a higher viscosity index would be better suited for this application as it would broaden the Kappa operating window, giving you better efficiency and protection across the full temperature spectrum.
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In a 3rd example, if you are wanting to rationalize onsite lubricant inventory so that you do not have a multitude of viscosity grades within the plant, you could plot these curves across multiple bearings in a variety of different equipment and arrive to the conclusion that you can reduce your inventory from 5 different grades down to 2 which would streamline purchasing and minimize the likelihood of mistakes when topping up equipment around the plant.
In a 4th and final example, you may be looking to improve the energy efficiency of your plant and can safely arrive to the conclusion that by moving down in viscosity grade and switching to a high viscosity index oil you could potentially save a significant amount on energy.
All these decision-making tools are available by plotting both your Viscosity curve as well as your Kappa curve, and through a simple visual illustration get one step closer to lubrication excellence.
If you require assistance with plotting these charts for your bearings and lubricants and are wanting to rationalize or achieve precision lubrication and unlock performance and efficiency, please contact us on [email protected]