Why can grease help keep contaminants out of moving machine parts?

Grease functions as an effective barrier against contaminants because of its physical consistency. Unlike liquid lubricants, grease is a thick, semi-solid substance composed of a thickening agent—such as soap or an inorganic gel—dispersed in a liquid lubricant like petroleum or synthetic oil.1

This high viscosity and structural stability allow grease to act as a sealant, effectively filling gaps and creating a protective seal around moving machine parts. By remaining in place rather than flowing away, this seal prevents external debris, dust, and moisture from entering critical areas, thereby protecting components from wear and corrosion.21