Basic Machines Types
Simple machines are designed to make work easier using few parts. A doorknob is a simple machine that only has two main parts. Six basic types of simple machines exist: the lever, inclined plane, wedge, pulley, screw and the wheel and axle. Of these, the doorknob most closely resembles the wheel and axle.
Wheel and Axle
A wheel and axle is made by putting a shaft through the center of a larger wheel. Twisting the axle by itself is difficult, but attaching a wheel makes the job easier. In the case of a doorknob, the knob is the wheel and the central shaft through the door is the axle. The knob requires application of less force to turn the knob than what's needed to turn the shaft by itself.
When the knob on one side of the door is turned, the shaft retracts the spring-loaded latch that holds the door closed. Without the knob in place, more force would be needed to turn the shaft and retract the latch.