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"What are lubricant churning and bearing torque, and how do they affect gears, bearings and the lubricant?"
Bearings and gears require a certain amount of force to begin turning and to continue turning once they are set in motion. The measure of this force is commonly referred to as torque. Running torque is usually less than starting torque. These forces are variable depending on several parameters.
Lubricant churning is one of the more common factors that affects both bearings and gears. It occurs when the bearing or gear must churn through the lubricant as it performs its regular task. The majority of these machines are splash-lubricated, which means they must operate at the proper lubricant level to be able to lift and splash lubricant to all surfaces inside the machine. This is where lubricant churning can become an issue.
If the lubricant level is too high, either because too much oil has been added or the bearings have been overgreased, the machine must work harder to push through the added lubricant. This is the basis of that churning condition.
It can be compared to walking along the beach. If you reach ankle-deep water, it is still fairly easy to move (equivalent to running torque). However, if the tide comes in or you venture out into deeper water, it becomes much more difficult to walk. This requires you to work harder and causes you to tire out quicker.
The same thing happens to a machine when the lubricant level is too high. It works much harder to push through the added lubricant, which results in higher operating temperatures, decreased efficiency and a reduction in the life of both the lubricant and the machine.
Besides the lubricant level, another variable is the lubricant being used. Since the most important physical property of a lubricant is viscosity, the proper viscosity must be selected according to the speed, load, temperature and general running conditions of the machine. If the viscosity is too thin, then excess friction due to metal-on-metal friction is generated, causing machine wear and premature failure. If the viscosity is too thick, this leads to viscous drag, which causes very similar issues as having a lubricant level that is too high. Neither of these situations is desirable for the equipment.
By choosing the right lubricant as well as ensuring the appropriate lubricant level, you can give your equipment the best chance for error-free operation.