Published: 8 November, 2006

Doorknob employs 2 simple machines: Wheel-and-axle, lever

How are simple machines used in doorknobs?

When you use a doorknob, the rotary motion of the handle is transformed into a sliding motion of the bolt that unlocks the door. The horizontal bolt or pin moves out of the door jam plate and allows you to open the door. A device that changes motion from one kind to another is called a mechanism and the science that engineers use to invent and design mechanisms is called kinematics. Mechanisms can be found in bicycles, CD players or robots. In fact most complex machines are composed of a set of mechanisms to change motion from one form to another.

Simple machines include the lever, pulley, wheel and axel, wedge and screw. Simple machines allow the user to produce a large force by applying a small force. To help answer this question, I took apart a doorknob mechanism in my house to look for simple machines. I discovered two parts that are similar to simple machines. First the circular knob was attached to a shaft like a wheel and axel. The square shaped shaft was connected to a plate that had two pins. These pins act against the bolt to push it back and forth. These pins acted like a lever. The bolt also had a helical spring to push it back when I let go of the handle. The arm of the lever was from the center of the doorknob shaft to the pin. When I applied a torque on the doorknob, it placed a force on the pin that moved the bolt against the spring. Some doorknobs are not round or circular but are straight and when you push down, it acts like another lever. Engineers today design mechanisms using geometric principles of kinematics as well as the mechanics of simple machines. You can learn more about kinematics and the geometry of motion at the Cornell website KMODDL. There are pictures, movies and descriptions of over 300 mechanisms.