Definition
A series of parallel ridges machined along the length of a shaft that mate with matching grooves cut into the bore of a hub, gear, or coupling. Splines lock the two parts together so they rotate as one, while still allowing the hub to slide along the shaft if needed.
Plain English
Long straight teeth cut into a shaft (and matching slots in the part that fits over it) so the two pieces turn together and don't slip.
Context Anchor
Seen in aircraft maintenance discussions involving shafts, propellers, engine accessories, and other parts that must rotate together without slipping.
Derivation
From an old English/dialect word 'spline' meaning a long, thin strip of wood or metal. The mechanical sense kept the idea of a long, narrow ridge running lengthwise along a shaft.
Why Pilots Care
Ensures secure power transfer from the engine to the propeller or accessories; worn or damaged splines can cause slippage, vibration, or sudden loss of thrust.
Analogy
Splines work a little like the ridges on a key fitting into a lock: the matching shapes keep the parts lined up and allow force to be transferred.
Intuition Check
Do not think of splines as separate bolts or pins. They are shaped ridges and grooves built into the parts themselves.
Example Sentence 1
The mechanic inspected the splines on the propeller shaft for wear before installing the new propeller.
Example Sentence 2
Wear on the propeller shaft splines was discovered during the annual inspection.