Read this lovely article laying out the different gear drive transmissions and a basic description of each. Thought it’d be a nice visual resource to have here so credit goes to Chris Young.
1. Spur gears
Spur gears are one of the most common types of gear out there. They have cylindrical pitch surfaces and belong to the parallel shaft gear group. They have a tooth line that is straight and parallel to the shaft.
Spur gears are commonly used because they can be relatively easily produced while achieving a high performance accuracy. The larger of the meshing pair is called the gear (pictured above) and the smaller one is called the pinion. They are typically used to increase or decrease torque in machines such as washing machines, clothes dryers, screwdrivers, windup alarm clocks, and blenders.
2. Helical gears
Similarly to spur gears, helical gears are used with parallel shafts. They are also cyclindrical gears with winding tooth lines. However, they are a definite improvement in terms of design over the spur gear. Unlike spur gears the leading edges of the teeth on helical gears are not parallel to the axis of rotation — instead, they are set at an angle.
This smooth teeth meshing means that the gears can transmit higher loads and are quieter than spur gears, which can be noisy at higher speeds. As these gears create thrust force in the axial direction, they require the use of thrust bearings. They are used in machines such as elevators and in factory automation.
3. Bevel gears
Bevel gears have the appearance of a cone that has had its top cut off. Within the bevel gear family, there are several different subsets, including helical bevel gears, straight bevel gears, spiral bevel gears, and miter bevel gears.
Bevel gears are used to transmit force between two shafts which intersect at a specific point. They are used in differential drives, such as those on a cornering automobile, as they can transmit power to two axles spinning at different speeds.
4. Spiral bevel gears
Spiral bevel gears are bevel gears that have curved tooth lines. Similarly to helical gears when compared to spur gears, the curvature allows for a higher tooth contact ratio, meaning that spiral bevel gears are more efficient than the standard bevel gear; they are stronger, less noisy, and vibrate less.
Why have spiral bevel gears not completely displaced the use of standard bevel gears you ask? Spiral bevel gears are harder to produce and, in some uses, they can also create unwanted thrust in axial directions, due to the curved teeth.
5. Worm gears
The “worm” of the worm gear refers to a screw shape cut into a shaft that the mating gear, or worm wheel, is attached to. Due to sliding contact of the gear surfaces a hard material is generally used for the worm, so as to reduce friction. Though the sliding contact means that worm gears aren’t incredibly efficient, their rotation is very smooth and quiet. As such, they are often used for industrial applications, heavy equipment, and sometimes consumer goods.
Worm gears provide very high reduction ratios and are often self-locking, as they cannot function in the reverse direction. This inherent feature makes them a safe option to use in certain types of machinery. A common example of a self-locking worm gear is the machine tuning head found on many string instruments, including the guitar.
6. Crown gears
Crown gears, also known as contrate gears, are a type of bevel gear whose teeth project at right angles to the plane of the wheel. This makes the teeth resemble the points of a crown, giving the gear its name. Unlike conical bevel gears, crown gears are cylindrical. They can either be paired with other bevel gears or spur gears, depending on the tooth design.
Crown gears are typically used for applications where low noise-emitting gears are required. A crown gear used with a rack’s interlocking clogs allows the gear to roll along with the rack even if it has to go uphill or sideways. They are used for trains on uphill tracks, rollercoasters, locking doors on tracks, and car steering wheels.
7. Sun and planet gears
We probably don’t have to tell you why the sun and planet gear is called the way it is. The motion of its orbit-imitating flywheel allows the sun and planet gear to convert reciprocating motion into rotary motion. That’s why James Watt used it in his early steam engines.
In the illustration above, the sun is yellow, the planet is red, while the reciprocating arm is blue, the flywheel is green and the driveshaft gray. Sun and planet gears are an example of epicyclic gearing, or planet gearing, in which the center of one gear revolves around the center of another. They are used on anything from pencil sharpeners to locomotive engines.
Planetary gearboxes generally have single or two-gear stages for reduction ratios ranging from 3:1 to 100:1
8. Cycloidal gearboxes
Cycloidal gearboxes or reducers consist of four basic components: a high-speed input shaft, a single or compound cycloidal cam, cam followers or rollers, and a slow-speed output shaft. The input shaft attaches to an eccentric drive member that induces eccentric rotation of the cycloidal cam. In compound reducers, the first track of the cycloidal cam lobes engages cam followers in the housing. Cylindrical cam followers act as teeth on the internal gear, and the number of cam followers exceeds the number of cam lobes. The second track of compound cam lobes engages with cam followers on the output shaft and transforms the cam’s eccentric rotation into concentric rotation of the output shaft, thus increasing torque and reducing speed.
Compound cycloidal gearboxes offer ratios ranging from as low as 10:1 to 300:1 without stacking stages, as in standard planetary gearboxes.
