Grease bleeding or oil separation is an expression used to refer to grease that has released oil during static (storage) or normal operating conditions. In static conditions, oil bleeding is identified by the presence of small pools of oil, particularly when the grease surface is not flat or even. In dynamic conditions, it is distinguished by oil leaking from a lubricated component.
Oil separation is a natural behavior of primarily soap-thickened greases. The property is required for the grease to lubricate properly when in the load zone, such as with a rolling-element bearing. The load “squeezes” the grease, which releases oil to lubricate the component. Additives may help to form a better lubricant film. In certain cases, the thickener can contribute to lubricate as well.
Oil separation will vary based on the storage time and temperature. The higher the storage temperature, the more likely oil will be released. Similarly, the lower the base oil viscosity, the more oil separation may occur. Some studies have suggested that when grease is stored in static conditions, it is normal to have oil separation of up to 5 percent.
While bleeding is a natural grease property, it should be minimized during storage to ensure the lubricant is in the proper condition when needed. Of course, bleeding will not be eliminated completely, as you may still see a little free oil.
If you observe grease bleeding during storage conditions, you may be able to mix the oil to reincorporate it into the grease prior to use. Blend the oil into the top 2 inches of the grease using a clean spatula and in a clean environment so as not to introduce contaminants that could damage the lubricated components.
New grease cartridges or tubes should be stored upright (vertically) with the plastic cap up at all times. This will help to prevent oil from leaking out of the tube.
If the cartridge is left in a grease gun, the gun should be depressurized and stored in a horizontal position inside a clean, cool and dry area. This stops oil from bleeding to one end of the grease gun by keeping the oil level and consistent throughout the length of the tube.
When the grease is in use, if some oil leaks out of the equipment, the remaining grease in the cavity will be hardened. In this situation, it is important to regrease the component more frequently, purge any excess grease and do not overlubricate. Finally, you must always verify that the correct grease is being used for the application.