Greases, mainly used for roller bearings, may be considered as semi-solid lubricants. The primary functions of greases are:
1.) Reduce Friction
2.) Minimize Wear
3.) Seal against contaminants
4.) Prevent Rust and Corrosion
Its pasty consistency (its non-flowing nature) allows for ease of application compared to oils. One simply has to pack a bearing with grease, or dispense via a grease gun through a nipple. Sumps, tubings, filters, coolers, and nozzles are typically not necessary, unlike with oils
Greases provide other advantages vs oils:
1.) There is no lubricant drop-off at shut-down, preventing wear during start-up,
2.) Greases provide noise and dampening owing to its consistency,
3.) It acts as a strong seal or barrier against contaminants, and,
4.) Leakages and throw-offs are minimized
On the other hand, greases cannot be used as a circulating oil, cannot generate force (hydraulic systems), and has a limited ability to absorb heat.
Greases may also be used with ball-screws, linear guides, open gears, journal bearings, gear couplings, and wire-ropes.