Bearing grease is combustible, not flammable.
In other words, grease will burn when introduced to ambient temperatures, but it will not instantly burst into flame. In order to be considered flammable, a substance must have a flash point below 100 degrees Fahrenheit, and very rarely would a lubricating grease fall into this category. This means bearing grease is typically only flammable in the presence of an open flame or sparks.
With that said, both combustible and flammable materials should be stored properly in flammable-safe cabinets or containers when not in use. In addition, you should always review the Safety Data Sheet when purchasing a new grease. The information provided in the SDS will not only tell you the grease’s flash point, but other useful information regarding toxicology, storage and handling, and its physical and chemical properties.