1. To defend the bearing surfaces as of water, dirt and so on.
2. To reduce friction and wear among sliding parts of bearing,
3. To stop rusting or corrosion of bearing surfaces,
4. To dispel the heat.
In common, oil or light grease is use for lubricating ball and roller bearings. Just pure mineral oil or a calcium-base grease must be use. If there is a chance of moisture contact, then potassium or sodium-base greases can be use. One more additional advantage of grease is that it forms a seal to keep out dirt or any other distant substance. It can be noted that too much oil or grease reason temperature of bearing to rise due to churn. Temperature must be kept less than 90ºC and in no case a bearing must function above 150ºC.