Gears are one of the oldest tools used to control a power source. Two gears working together comprise a simple machine, a system that has been around since the Renaissance and that provided the basis of development for the more complex machinery used today. These simple devices have been used for centuries to power watermills, mechanical clocks, manufacturing tools, and various modes of transportation. In vehicles, a gear works by connecting the driving force — like bike pedals or a car engine — to the wheels, which allows the driver to control the speed and direction of travel. There are many types of gears, and this article will provide an overview of the most common gears used in machines.

• Spur gears
  These gears are the most common type and have straight teeth that are parallel to the rotating axis. Their simple design provides an advantage for manufacturing because they are easy and inexpensive to produce. Spur gears are known as slow speed gears and are relatively quiet. They can be used to power faster machines, but they produce more noise as the speed increases.
• Helical gears
  Helical gears have angled teeth aligned along the edge of the circular gear, which produces helical shaped teeth. These high speed gears have a smoother and quieter motion than spur gears, and helical gears are also able to handle a greater load and generate more power.
• Rack and pinion gears
  Pinion gears are used in vehicles to convert the driver’s control of the steering wheel to a change in the car’s direction. A rack is a straight piece of metal that has teeth which are connected to the circular gear. Pinion gears can be thought of as a linear spur gear.
• Worm gears
  A worm gear is connected to a helical gear or spur gear and its mechanism is analogous to the motion of a screw. Worm gears have two pieces aligned parallel to each other which facilitates quiet and steady work. They are favored for industrial applications that require a smooth output.
• Face gears
  Face gears move in a circular motion at right angles. They consist of a flat metal piece with teeth around the edge, and helical or pinion gears move around the circumference. Both pieces are perpendicular to the rotational axis. These gears were used many years ago to power wagons and windmills, but they aren’t as common today because they can only handle a light load.
• Spiral bevel gears
  At first glance, a spiral bevel gear looks like a face gear. Its design is similar but the teeth are arranged at an angle, which means they operate more quietly and can handle a heavier load.
• Straight bevel gears
  These gears have straight teeth that are parallel to a cone in the center of the gear. They can be thought of as a conical variation of the spur gear. This type is the most basic form of a bevel gear.
• Herringbone gears
  A herringbone gear is unique because two helical gears are arranged side by side. The teeth of each gear are aligned during motion, which produces quiet and smooth movement. They are more stable and balanced than many other types of gears. The herringbone gear was an important advance that led to developments in powering steam engines and other modern machinery because they can handle a greater load with smooth movement. The primary drawback of these gears is their complex design which makes manufacturing difficult. Check out this site for more.