Keyed joints form a shaft, spline and wheel hub (pulley, sprocket, flywheel, etc.). Key - detail connecting the shaft and the hub. It serves to transmit torque from the shaft to the hub or vice versa.
keyed connection
Key connection: the shaft, hub, parallel key

The advantages of key connection are simple structure, low cost, ease of assembly and disassembly, and therefore are widely used in all branches of engineering. The disadvantages include the key connection loosening shaft and hub keyways. Keyway not only reduces the cross-section, but also causes a significant stress concentration. Keyed connections are not recommended for high-speed dynamically loaded shafts.