Maintenance gearboxes are normally filled with which fluid?

• A. Water
• B. Air
• C. Grease
• D. Oil

Answer: (D)

Lubrication in gearboxes relies on a fluid that can continually flow over and between moving parts to form a protective film, carry away heat, and reduce wear. Oil provides that because it can circulate through the gears and bearings, cooling them and creating a consistent lubricating film.

Water would cause corrosion and wash away lubrication, and air offers no lubricating film. Grease stays in place but is too thick and not easily circulated to all moving parts, which leads to poor heat removal and insufficient lubrication in a rotating gearbox. That’s why the normal fill is oil.