Choosing the right type of grease
Grease versus oil
There are many advantages to using grease rather than oil in an application. Because of its consistency, grease remains in place better than oil. Grease also offers excellent sealing capacity, good corrosion protection and can withstand heavy loads. A typical application at the moment is in forestry, where lubricating the cutting bar and chain with oil is being replaced by central lubrication with grease.
However, there are certain performance limitations which readily explain why grease is not used in certain applications. Greases cannot lead heat away as well as oil where cooling is required, such as in an engine. Greases cannot be filtered to increase its purity, and it is hard to separate water from the grease.
How to choose the right type of grease
Each application places specific demands on the grease and its performance. Water, dirt, chemicals, temperature, operating speed, and load are all examples of parameters that need to be considered when choosing a product.
Base oil viscosity is an important factor just as it is when using lubricating oils, such as gearbox oils. You do not use an ISO VG 46 where you need an ISO VG 320. One NLGI grade 2 grease is not the same as another NLGI grade 2 grease if the viscosity of the base oils is different. When a machine or bearing manufacturer stipulates an ISO VG 46 grease at 40°C, an ISO VG 150 or ISO VG 220 will not work. The wrong grease could lead to overheating and shorter bearing life.
The base oil in a grease usually has a viscosity of between 20 and 500 mm2/s at 40°C. The application where the grease is to be used governs what base oil viscosity is required.
Low viscosity oils generally work better at low temperatures, while greases with a higher base oil viscosity are used more for heavier loads and higher working temperatures. It is also important to consider the speed/revs per minute of the application. Low speed calls for a high viscosity base oil, while low viscosity oils are to be preferred in fast-moving applications.
Choosing base oil viscosity
There are however exceptions when it comes to guidelines for base oil viscosity. Greases based on the NOVA technology does not follow the common rules and experiences of conventional greases. Read more about this here.
Generally speaking, there are some guidelines based on the NLGI rating:
NLGI 00/0
- Lubrication of shafts/axles where good fluidity is required
- Where excellent pumpability is key
NLGI 1
- In low surrounding temperatures
- For oscillating applications
- When good pumpability is required
NLGI 2
- For normal bearing applications
More about grease
Handbook
The Law of Least Resistance - The FUCHS Lubricants book regarding greases.