Grease is used in wheel bearings to keep the moving metal surfaces lubricated and free from contamination, usually where metal rubs against metal. Grease is used as it is made to be thick so that it will stay in place whereas oil on its own would leak out and dirt could get in causing damage to the bearings and ball joints.
Greases are made from three basic components;
- The oil which does the lubrication function
- The thickener which carries the oil
- The additives which enhance the properties of the oil to achieve superior results
The oil can be a standard mineral oil, synthetic oil, biodegradable oil, or a vegetable oil. Standard greases use mineral oils with a typical viscosity of 100 to 150 Centistoke, i.e somewhere between heavy grade hydraulic and light grade gear oil in viscosity terms.
The thickener can be a metallic soap, a clay or polyurea. The most common thickeners are Lithium and Calcium soaps. When a wider temperature range of operation is required a more advanced complex soap thickener is used. You can avoid problems by using grease with the same thickener as the existing grease.
The additives can include antiwear, anti corrosion, EP, Moly (Molybdenum Disulphide), Graphite, Teflon etc
Greases are defined by the NLGI numbering system which describes in numbers how thick or soft the grease is. At the lower end NLGI 000 is very soft semi fluid grease, NLGI 00 is frequently used in automatic greasing systems and so on getting gradually thicker up to NLGI 3 and beyond.
NLGI 2 is the most common grease thickness used in multiple applications including wheel bearings.
Once you have chosen the correct grease, using the machine manual or consulting your lubricant supplier technical help desk such as in Finol Oils, then the next most important issues are to store the grease correctly and make sure that the correct amount of grease is applied at the correct intervals. Remember that two of the main reasons for wheel bearing failure are no grease and too much grease!!
For more info on choosing the correct grease or for a full range of grease for your wheel bearings, contact Finol Oils in Dublin on 01 4555 484 or www.finol.ie.