Comparing types of gearboxes

Gearboxes use a variety of power-transmission methods including, but not limited to, planetary gears, worm gears, spur gears, helical gears, and shaft-mount gear units. In servo applications, planetary gearboxes are often the most suitable.

Worm gears use a worm or screw gear to turn a larger transverse gear. They can provide high gear reductions in small packages but aren’t that efficient, on the order of 70%. Worm gears aren’t reversible either and thus can’t be back driven, so they cannot be used to increase speed.

Spur gears use straight-cut teeth on parallel shafts for power transmission. They are available in a wide range of ratios and are cost effective, but they can be noisy and prone to wear.

Helical inline gearboxes also use gears on parallel shafts, but the teeth are cut in helical shapes to allow gradually increasing contact between mating teeth.

Helical inline gearboxes and spur gears typically have more backlash than planetary gearboxes, and there is an undesirable thrust acting along the gear’s axis. Helically cut gear teeth are quieter and are used in other types of gearboxes because of this.

Shaft-mounted gear units are popular for parallel shaft gears, such as spur and helical gears. They are well suited to conveyors and other material-handling applications, and are easy to mount. However, they suffer from the same disadvantages as their constituent parts.

Planetary gearboxes are named for their resemblance to a simple solar system. They consist of a ring gear, several planet gears, and a sun gear. The ring gear is typically fixed and is often part of the gearbox’s outer casing, and the input shaft drives the sun gear.

Rotation of the sun gear drives the planet gears to spin about their own axes and revolve about the sun gear. A carrier attached to the planet-gear shafts harnesses output. This arrangement creates a balanced and compact design that is concentric about the shaft. If multiple stages are needed, it is relatively simple to connect the output of one set of planet gears to the sun gear of a secondary stage.

High efficiency, low backlash, and high power density make planetary gearboxes the best of these alternatives in high-precision servosystem applications.

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