Which statement is true about planetary axles in trucks?

• A. Inboard planetary axles are used on most trucks
• B. Outboard planetary axles need a separate oil supply
• C. Both statements true
• D. Neither statement true

Answer: (B)

Planetary axles have lubrication that depends on where the planet gears sit. When the planet gears are inside the axle housing (inboard), they can be fully immersed in the axle oil, so they’re lubricated by the same lubricant that bathes the rest of the axle. If the planet gears are mounted outside the housing (outboard), they aren’t in that oil bath, so they need their own separate oil supply to keep the gears and bearings properly lubricated. That makes outboard planetary axles needing a separate oil supply the true statement. The claim about inboard axles being used on most trucks isn’t universally true, since designs vary across trucks and configurations.