Gears are important parts of many types of machines, from clocks to elevators to cars, trucks, and tractors. Understanding what gears are and how they work can give you a better appreciation for the world of mechanical devices all around us every day.
What Is a Gear?
A gear is a rotating component featuring cut teeth, or cogs, that mesh with the teeth of another gear to transfer rotational force, or torque. Gears can alter the speed, torque, and direction of motion. Two or more gears working together are called a gear train.
Parts of a Gear
Gears consist of a hub, the central part of the gear that connects to a shaft, and teeth, the protruding parts around the edge of a gear that interlock with the teeth of another gear. The size of a gear is typically measured as if it’s a circle; the pitch circle is a circle drawn through the teeth of the gear, and the pitch diameter can then be measured based on the pitch circle. These measurements can help people to calculate which gears will fit together properly.
Parts of Gear Teeth
Drawing a pitch circle through the teeth of a gear separates those teeth into two parts: the addendum, which extends out past the circle, and the dedendum, the part of each tooth that is inside the pitch circle. At the base of each tooth is the root, the bottom of the space in between teeth. At the top of each tooth is a flat surface called the top land.
How Gears Work
Gears work by meshing their teeth with the teeth of another gear or toothed component. When one gear rotates, it transfers motion and force to the other gear, causing it to rotate as well. This interaction can change the direction, speed, and torque of the movement, depending on the sizes and types of the gears involved. For instance, a larger gear turning a smaller gear will increase speed but decrease torque, while a smaller gear turning a larger gear will increase torque but decrease speed. Whenever two gears mesh, the direction of gear motion changes; a clockwise-turning gear will cause the gears it’s connected to to turn counterclockwise and vice versa.
Materials Used in Gears
Gears can be made from many different materials, from iron to plastic, but the most common material is steel, chosen because of its strength, durability, and resistance to wear. It’s often used in high-stress applications like automotive transmissions. Cast iron gears are commonly found in industrial machinery, as they’re strong and inexpensive to manufacture. Brass is also used to make gears, especially in situations where the gear will be exposed to corrosive materials, as it’s more resistant to corrosion. Advanced plastics can also be used to make gears; they’re not as strong as metal gears, but they are lighter in weight and don’t corrode.
Types of Gears
When most people hear the word “gears,” they think of spur gears, flat circles with teeth sticking straight out around the edge that mesh together. Spur gears are used in many places, like inside of clocks or home appliances. Helical gears have teeth cut at an angle to the gear axis. This design allows for smoother and quieter operation than spur gears. These gears are used in auto transmissions and other high-speed applications. Bevel gears have teeth that are cut on an angle, allowing them to intersect at different angles, typically 90 degrees. These are used in the differential drives of vehicles to allow the wheels to turn at different speeds during cornering. Worm gears are the ones you’ll find in a screw-drive elevator; the worm is a gear shaped like a screw, and it meshes with a worm wheel to provide high torque for devices like wheelchair lifts. Construction equipment, farm machinery, and automobiles may also contain planetary gears, parts of a complex gear train including a central sun gear, planet gears, and an outer ring gear. This system allows for high torque in a compact space and is commonly used in automatic transmissions.