One of the most important pieces of any good lubrication strategy is selecting the right grease for a given application. That may seem like an obvious statement, but we often get customers in the field asking us how they should choose a grease for their equipment.
So, this week’s Productivity Tip takes a look at the grease selection process to help simplify the decision making process.
Selecting the correct grease
Selecting the correct grease for an application requires evaluating the application, environment and grease composition,
Let’s first consider the grease application.
A grease is composed of three things: base oil, thickener, and additives. Of these components, base oil viscosity is the most important.
The best and easiest way to determine the viscosity requirement for an application is to perform a DN calculation. This calculation takes into consideration the speed and temperature of the application, and the DN Factor for an application is calculated as follows:
DN = RPM x (Bearing OD (mm) + Bearing ID (mm))/2
As a general rule - the operating temperature is typically calculated as the bearing housing temperature plus 10 degrees Celsius.
The DN calculation provides the DN factor, and when paired with the operating temperature, it may be used to determine the grease base oil viscosity requirement. You can refer to the following graph:
Now you may wonder, where am I going to get the bearing information (OD and ID)?
That is actually easier than you think. If you can get the bearing number from the customer, you can find the OD and ID on the internet. As for RPM, you can get that from the motor data plate. Remember to take into consideration any step down provided by a gear drive between the motor and the application.
The next factor to consider is the thickener type. You should choose a thickener type that’s compatible with the other greases being used throughout the operation. If thickener choice is not driven by equipment compatibility, then you may consider choosing a grease based on operating temperature and DN speed factor. Refer to the following table to help with your decision:
The second consideration when selecting a grease is the operating environment. For example, do you need to think about operating temperature? Is water washout a factor? Does the application require additional wear protection?
Finally, it’s important to consider the grease composition. For example, a bearing operating at high temperature may require some anti-oxidation additives or synthetic base oil. A grease operating under a shock loading conditions would require anti-wear or extreme pressure additives.
The chart below is a quick reference guide for additive technology based on application type:
In short, when selecting a grease for a given application, you need to consider:
These topics of application, environment and composition – plus DN – can help drive your choice of the right grease.
Hope this was helpful, and if you have any more questions, don’t hesitate to leave a comment in the section below!
thanks for sharing, great information!
Felipe Brepohl Great article!
The temperature at which the grease will fall away from the application due to heat.
Hi Rick, just curious, where did you get that viscosity selection chart? I use the 2 part chart that most brg manufacturers use but yours is easier to use.
Great information Rick
My question is what is the dropping point of a grease?
If you are applying the grease directly at the bearing, an NLGI 2 would be appropriate. If you are applying the grease remotely, and the grease must travel through tubing to get to the bearing, then an NLGI 1 would be better.
For a wide temperature of 160º - 180º C. In an asphalt smelting machine, would it have to be a synthetic grease with NLGI 2?