Definition
A series of parallel ridges (or matching grooves) machined along the length of a shaft, designed to mate with corresponding grooves (or ridges) in a hub or fitting so that the two parts rotate together as a single unit while still allowing axial sliding or removal.
Plain English
A set of straight teeth cut along a shaft that lock into matching slots in another part, so when the shaft turns, the other part turns with it — but the two pieces can still slide apart for assembly or removal.
Context Anchor
Seen in powerplant maintenance when inspecting shafts, gears, couplings, propeller connections, and accessory drives.
Derivation
From an English dialect word meaning a long thin strip or sliver of wood. The mechanical sense kept that idea — long, narrow, parallel ridges running the length of a shaft.
Why Pilots Care
Splines transmit torque between major engine components. A worn or damaged spline can slip, strip, or fail entirely, which means loss of drive to whatever the spline was turning — propeller, magneto, accessory drive. Inspection of splined connections is a routine part of overhaul.
Analogy
Think of the way a socket wrench slides onto a bolt head — the inside of the socket and the outside of the bolt have matching shapes that lock them together for turning, but pull straight apart. A spline does the same thing along a shaft.
Intuition Check
Do not think of a spline as just any tooth or gear. A spline is the matching ridge-and-groove connection between parts, usually running lengthwise along a shaft or inside a hub.
Example Sentence 1
During the propeller installation, the technician checked the spline for wear before sliding the hub onto the crankshaft.
Example Sentence 2
A damaged spline on the accessory drive can prevent proper torque transfer to the alternator.