Thanks for the post!

The Output Shaft Key is necessary, yes.

Here is a basic definition:

The channel, or keyway, provides a place for the key to lock a pulley or sprocket onto the round bar (in this case the engine shaft). The square key fits in both the keyway and a square slot machined into the sprocket or gear. By fitting securely between the round bar and the gear, the key creates a sturdy connection in the assembly. Many components utilize a keyway to produce a slip-proof method of attaching rotating parts. Axles, crank shafts and drive shafts all use a keyway to hold other components in place as they spin and provide power to drive other mechanisms.

Hope this helps.

- MTR