More bearings are lost to lubrication problems than to fatigue. The three questions that matter are which lubricant, how much, and how often.
Grease Or Oil
Grease is used in the large majority of positions. It stays where it is put, helps seal the bearing and needs no circulation system. Oil is used where speed or temperature is high, where heat must be carried away, or where the bearing shares a system with gears.
Choosing A Grease
Base oil viscosity is the first decision and is set by speed, load and temperature: slow and heavily loaded wants a high viscosity, fast and lightly loaded wants a low one. Thickener type then sets temperature range, water resistance and pumpability. Lithium complex covers most general industrial duty; polyurea suits sealed electric motors; calcium sulphonate suits wet and heavily loaded positions.
How Much
For most applications, fill the bearing cavity fully and the surrounding housing space about one third to one half. Packing a housing solid traps heat, churns the grease and raises operating temperature sharply. On high speed positions the fill should be lighter still.
Relubrication
Intervals shorten with speed, temperature and contamination. As a working rule, every 15 degrees Celsius of sustained temperature rise above roughly 70 degrees halves grease life. Relubricate in small quantities frequently rather than large quantities occasionally, and provide a path for old grease to escape.
Contamination
Solid contamination is the most common single cause of shortened life. A grease gun nipple wiped with a dirty rag introduces more particles than most seals will ever admit. Clean the fitting, use clean equipment and keep containers closed.
