Disassembly and assembly of helical gear reducers, with open gears mounted inside the housing.

　　 Enclosed gear/Transmission

　　A closed‑type gear, also known as a gearbox or reducer, is an open gear enclosed within a housing. The housing supports the bearings and shafts, retains the lubricant, and shields the components from environmental influences. Gearboxes offer a wide range of load capacities and speed ratios.

　　The function of a gearbox is to increase or decrease speed. Consequently, the torque output is the inverse of this function: if the gearbox acts as a reducer, the torque output increases; if it increases speed, the torque output decreases. Factors to consider when selecting a gear drive include shaft orientation, gear ratio, design type, load characteristics, rated capacity, operating environment, installation location, operating temperature range, and lubrication.

　　 Basic Types of Transmissions

　　Worm gear

　　A single-stage reducer can achieve a reduction ratio of up to 100:1 in a compact package. This type of reducer, known as a right-angle gear unit, consists of a threaded cylindrical worm and a worm wheel. With a single-stage worm gear, for every 360° rotation of the worm, the worm wheel advances by only one tooth. Consequently, regardless of the worm’s size, the gear ratio is always “worm‑wheel size ratio 1.” Higher reduction ratios can be attained by employing two- or three-stage reducers.

　　 Helical gear

　　Efficient speed reduction is achieved through 1-, 2-, 3-, or 4-stage gear sets. Power is transmitted from a high-speed pinion to a low-speed gear. Helical gears typically operate with shafts that are parallel to one another. The two most common types are concentric (with the input and output shafts aligned) and parallel‑shaft (with the input and output shafts offset). Single‑stage helical gear reducers are generally used for gear ratios up to about 8:1. When lower speeds and higher ratios are required, two‑stage, three‑stage, and four‑stage gear reducers can be employed.

　　 Cycloidal gear

　　Unlike conventional gears, these mechanisms operate without the need for high-speed pinions or gear teeth, and their components engage in compression rather than shear. This is made possible by the cycloidal pinwheel design, which rolls within the ring gear housing—similar to the configuration of planetary gears. The principal components include an eccentric cam, an internally flanged output shaft, a needle wheel, and a ring gear housing.

　　 Three installation options

　　(LR) Base mounting, shaft mounting, and gear motor base mounting are the most common types of reducers; they come with mounting feet for bolted fixation.

　　The shaft-mounted reducer features a hollow output shaft that slides over the driven shaft.

　　A gear motor integrates a closed‑type gear set with a motor. An electric reducer is similar to a gear motor, except that it is driven by a separate NEMA C‑flange motor.

　　 Seals and Venting Devices

　　Seals are used between the gearbox and the input and output shafts to retain lubricant and prevent dust ingress. The most common type is the radial lip seal, which consists of a metal housing fitted into the housing bore and an elastomeric sealing lip pressed against the shaft. Labyrinth seals are employed in high-speed applications and comprise a housing with a series of rings designed to restrict leakage. A breather is a perforated plug installed in the gearbox to permit airflow and relieve internal pressure.

　　 Lubrication

　　Enclosed gears are typically lubricated with oil. The most common types of oil include rust‑preventive, antioxidant, extreme‑pressure, compounded, and synthetic oils. Other lubricants include greases and solid films. Greases can be used in worm, planetary, cycloidal, and hypoid gear reducers. Common lubrication methods are splash systems and circulating systems.