A keyway transmits torque through a single fitted key; a spline uses many integral teeth.
A keyway is cheap and standardised, and it deliberately weakens the shaft at one point so the key shears before the shaft yields. That sacrificial behaviour is a feature — it is why keys are sized from the shaft diameter rather than from the torque.
A spline distributes the same torque over many teeth, so the shaft is not weakened by a single concentrated keyseat and the joint handles reversing loads better. It costs more to produce and is used where the drive is highly loaded, reversible, or must slide.
Sizing a key from the torque rather than from the shaft diameter. It removes the sacrificial protection and risks a broken shaft instead of a sheared key.
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