Definition
A description of the lobes on an engine camshaft. Each lobe is shaped so that its outer surface curves around a point that is offset from the centerline of the camshaft itself. As the shaft turns, this offset causes the lobe to push the valve lifter outward and then allow it to return, opening and closing the engine valve at the correct time.
Plain English
The bumps on a camshaft are not centered on the shaft. They sit off to one side, so when the shaft spins, those bumps swing out and push the engine's valves open, then let them close again.
Context Anchor
Seen in aircraft engine and accessory discussions, especially where rotating engine parts operate valves, pumps, switches, or other moving parts.
Derivation
Cam comes from an old word meaning a projecting part or tooth on a wheel. A cam is essentially a shaped lobe used to convert rotating motion into back-and-forth motion. Knowing this helps the term make sense: the shaft rotates evenly, but the off-center lobe creates the rise-and-fall action that operates the valves.
Why Pilots Care
Correct offset action maintains precise valve opening and closing, which directly affects engine power, fuel efficiency, and in-flight reliability.
Analogy
Think of a round wheel with its hole drilled off-center. As it turns, the outside edge moves closer and farther away, so it can push on something once each turn.
Intuition Check
Do not read this as an ordinary straight shaft that simply spins in place. The important idea is that the turning part is shaped or positioned so its rotation creates a controlled movement in another part.
Example Sentence 1
The camshaft lobes rotate about a center different from the shaft's center, which is what allows them to open the valves as the engine turns.
Example Sentence 2
The eccentric design of the cam shaft that rotate about a center different from the shaft's center allows the fuel pump to operate without additional linkages.