Keyed shafts
From
Problem
Power transmission often requires rotating shafts with sprockets, brakes, pulleys, gears, wheels, and many other types of devices attached in unique configurations.
Approach
A round steel shaft with a [keyway] allows for similarly keyed interchangeable components to be positioned securely along it's entire length. Such a shaft can be manfactured from round stock with as little as a [broach] and a hammer.