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The main purpose of lubrication is to minimize friction and wear between two machine parts which move relatively to each other. This is achieved by building a load-carrying lubricant film between both machine parts.

Friction conditions combined with a lubricant:

• dry friction
• mixed friction
• fluid friction

Dry friction or dry lubrication:

The load is mainly carried by the roughness peaks of both solid objects (see Fig. 11).

Mixed friction or mixed lubrication:

The load is carried by the roughness peaks and the lubricant (see Fig. 12).

Fluid friction or full fluid film lubrication:

The load is fully carried by the lubricant (see Fig. 13).