Many people might not be aware but there is a whole range of different types of gears on the market today. Every mechanical engineer must have a basic understanding of the main types of gears that exist.
In layman’s terms gears are rotating mechanical elements that are generally used for the purpose of transferring torque from one element to another. In order to achieve this, their outer rims directly contact each other with common features in the shape of teeth/cogs, so that the rotational motion of the one gear results in the forced movement of the other. This kind of transmission of torque is very efficient and effective because very little energy is lost in the process. In addition to this, gears can be manufactured in a way that allows them to withstand very high rotational speeds and forces while still maintaining a high level of effectiveness. This effectiveness as well as their versatility has made gears popular since the ancient times, and nowadays they are to be found almost everywhere, from aircraft to clockworks.
There are many different types of gears aimed at a different kind of utilization scope, and all types are characterized by the position of their connected axis or shaft which can be parallel, intersecting, or non-canonical, and the shape of their teeth. Here is a list of the most common types and what they are typically used for.
| Spur Gears: A spur gear is a cylindrical parallel axis and straight-teeth gear which is one of the most commonly used types of gears thanks to their simplicity, wide utilization spectrum, and cost-effectiveness. Spur gears carry and transfer radial loads, and are typically used in low rotational speed applications as they tend to get noisy at higher speeds. | |
 | Helical Gears / Dry Fixed Gears: Helical gears are also cylindrical but feature angled teeth forming a shape of a helix so that they can transfer both torque and thrust, although the latter isn’t always useful in their applications and must be accommodated. The advantage brought by the helical shape of the teeth is the low noise and vibration level of operation even at high rotational speeds, as well as the ability to transfer even larger loads than the spur gears. |
 | Double Helical Gears / Herringbone Gears: The Herringbone gear is basically two helical gears glued together side by side with their teeth having opposite orientation. The reason behind this arrangement is to eliminate the thrust that is generated in the single-helical gear by making the loads cancel each other out. The major disadvantage is that they are hard to manufacture with the required precision, so they are quite expensive to acquire. |
 | Bevel Gears / Miter Gears: Bevel gears are of the intersecting axes/shafts type, so their connection is done on vertical inclination. Their shape is conical and their teeth can be either straight, or spiralled for quieter operation. Although the 90 degrees of angle between the interconnected axis isn’t mandatory, they are typically found in this arrangement and have the exact same number of teeth so as the rotation speed between the two is the same. This type of bevel gears is called a miter gear, and it is used in cases where speed modifications are not required. Any other angle and difference in the number of the teeth is simply called a bevel gear arrangement. |
 | Hypoid Gears: Hypoid gears are very similar to spiral bevel gears with the major difference being that their axes of rotation do not coincide. As a consequence of this, the design of the teeth that are otherwise placed on hyperbolic conical parts must be very carefully manufactured in regards to their angle. They are typically used in applications where speed reduction/increase is required, so they are often found in vehicle drive trains that allow for more compact implementations since the pinion doesn’t have to be above the hypoid gear but on its side. |
 | Worm Gears: The worm gear is a type of gear arrangement consisting of a cylindrical gear that is similar to a helical gear. However, it also includes a shaft gear that features a screw thread placed on a parallel plane but at an axial rotation of 90 degrees in relation to the first element. Thanks to the tight connection of the two elements, the operation of a worm gear arrangement is quiet and free of vibrations. The most common type of applications for a worm gear have the characteristics of decisive speed reduction, locking, and precision. A typical example of these types of gears are the tuning keys in stringed musical instruments. |
 | Rack and Pinion Gears: Similar to the worm arrangement, the rack and pinion features a kind of a spur gear connected to a straight gear rack with one of the two being stably fixed in position and the other one moving left and right. A typical application of the rack and pinion is the steering wheel where the spur gear position is locked and the rack is moving left and right, subsequently moving the tie rods and steering arm to turn the wheels. Since the rotation speed of the rack and pinion is very low, the teeth don’t need to be inclined or spiralled and do not produce any sound or vibrations whatsoever. |
 | Sprockets: Sprockets are types of gears that feature sharper teeth because they are not meant to be connected to other gears but to belts or chains. A typical example is the bicycle sprocket that has its teeth enter the gaps in the metallic chain, moving the rear wheel forwards. In industry, sprockets are usually found in conveyor systems, although other types of gears which are safer and less prone to wear and tear are generally preferred. |
| Internal Gears: Internal gears are cylinders that have their teeth on the inside of their radius. The advantage of having such a gear is the saving on space and the fact that they do not inverse the rotational direction, so they are used in applications that require those characteristics. Most typically, they are to be found in planetary gear systems. | |
 | Epicyclic Gears / Planetary Gears: The typical arrangement for epicyclic gears – or planetary gears – involves a central spur gear and an external spur gear that moves around its perimeter. The main point of having such an arrangement is to achieve the efficient conversion of the reciprocating motion to rotation. A typical example of such a system is the early age steam locomotives showing that different types of gears have played important roles in history. |
| Harmonic Gears: Harmonic Drive / Harmonic Gears or Strain Wave Gears are rather a rather complex types of gears, and are actually sets of planetary gears connected inside a shell and rotating co-axially. These systems are very accurate, feature a complete absence of backlash, and can achieve high gear ratios while remaining very compact. Their main application sector is differential gearing where accuracy and compactness are critical, so they are found in aerospace, robotic, and industrial motion control applications. More details on Harmonic Drive can be found here. |
