There are two types of gear reducer and they both serve very different functions. Both forms can be found in almost every automotive application on the planet, though if you don't know the difference, finding information can be somewhat bewildering. One form of gear reducer allows very large gears to turn very small ones, while the other form is used to slow down an input shaft's velocity.
Big to Small: When most people picture a gear reducer, what they picture is a large gear to small gear solution. This is very common in automotive applications, where pulleys and belts can't be used. Often, a large gear will be the termination of a moving shaft. This gear must be used to turn many other gears, sometimes very small ones. While it is possible to create a small gear that can be used with the larger one, it is more common to simply use a gear reducer. This gear does nothing more than act as a go between for the two differently sized gears.
The other type of gear reducer is used to slow down the velocity of an input shaft. These are commonly found within automatic transmissions and look very similar to an external gear pump. This type of gear reducer slows down the rotation of the gears, while providing more torque for the overall application, resulting in better performance at lower RPMs. Gear reducers are used by almost all major automotive manufacturers within their transmissions, allowing for greater power to be delivered, without increasing the cost of power and still decreasing wear.
You'll find many different styles of gear reducers in use throughout many different industries. Worm gear reducers, inline gear reducers, helical gear reduces; these all reflect the style and type of the gear. They job they perform is the same. Each gear type has different characteristics, which lends them to different applications. Some types of gear reducer are much better for low horsepower or lower speed applications, while others are not well suited to anything other than high output applications. You'll find that the different types of gear reducer have many different industries that they are suited to, from manufacturing to the automotive industry to food processing. Gear reducers are one of those applications that have immense appeal to the broadest range of industries possible.
Each type of gear used in the application has certain characteristics, as has been mentioned. These characteristics go far beyond simply the size of the gears involved. Some of these gears can be mounted at 90-degree angles to the shaft, greatly reducing the size of the gear reducer, enabling just as much performance from a much smaller package. These are ideal in today's automatic transmissions, with the ever-growing number of items that must be fit into the transmission housing. Other characteristics involve different tooth styles, such as curves and herringbone shaped teeth. These allow greater power to be applied, without the force of impact hitting the entire tooth at one time. They also enable much more quiet operation and facilitate a greater range of applications than simple straight-toothed gears can provide.