Precision of Helical Gear Reducers and the Function of Gears

　　 Spur gear

　　Gears with cylindrical tooth surfaces are called cylindrical gears. Spur gears belong to the family of parallel-axis gears; they are cylindrical gears whose teeth have straight lines and are parallel to the axis. Spur gears are the most widely used type of gear, offering high precision and relatively simple manufacturing processes. A key characteristic is that they do not experience axial loads (thrust loads). In a meshing pair, the larger gear is referred to as the gear, while the smaller one is called the pinion.

　　 Helical gear

　　Helical gears, used in conjunction with parallel shafts like spur gears, are cylindrical gears featuring helical tooth profiles. They offer superior tooth engagement compared to spur gears, exhibit excellent quietness, and can transmit higher loads, making them well suited for high-speed applications. When operating, helical gears generate axial thrust forces, necessitating the use of thrust bearings. Helical gears come in right-hand and left-hand variants, requiring mating gears with opposite hand directions to ensure proper meshing.

　　 Rack

　　A rack is a straight bar or flat surface with equally spaced teeth of identical size and shape. It is essentially a cylindrical gear whose pitch cylinder has an infinite radius. By meshing with a cylindrical pinion, it converts rotary motion into linear motion. Racks are broadly classified into spur racks and helical racks, though both feature straight tooth profiles. By machining the ends of a rack, its ends can be joined to form a closed loop.

　　 Bevel gear

　　Bevel gears have a conical shape and are used to transmit power between two shafts that intersect at a single point (intersecting axes). The tooth surfaces of bevel gears are conical, with the teeth cut along these conical surfaces. Types of bevel gears include straight bevel gears, spiral bevel gears, hypoid bevel gears, helical bevel gears, angular bevel gears, crown gears, zero‑degree bevel gears, and quasi‑hypoid gears.

　　 Spiral bevel gear

　　Spiral bevel gears are bevel gears with a curved tooth profile. Thanks to their high tooth contact ratio, they outperform straight‑bevel gears in terms of efficiency, strength, vibration, and noise. However, they are also more challenging to manufacture. Moreover, because the teeth follow a curve, an axial thrust force is generated. Among spiral bevel gears, those with a zero helix angle are referred to as zero‑degree bevel gears.

　　 Helical gear

　　Helical gears consist of a pair of helical gears with a helix angle of 45°, mounted on non‑parallel, non‑intersecting shafts. Because the tooth surfaces make point contact, their load-carrying capacity is relatively low, making them unsuitable for high‑power transmission. Moreover, since power is transmitted via sliding contact between the tooth surfaces, proper lubrication is essential when using helical gears. There are no restrictions on the combination of the number of teeth.

　　 Helical gear

　　Equal‑diameter bevel gears are bevel gears with a gear ratio of 1. They are used to change the direction of power transmission without altering the speed. There are straight‑tooth equal‑diameter bevel gears and helical‑tooth equal‑diameter bevel gears. When using helical‑tooth equal‑diameter bevel gears, it is necessary to account for thrust bearings, as they generate axial thrust. In addition to the conventional 90°‑angle equal‑diameter bevel gears, any equal‑diameter bevel gear with a non‑right‑angle shaft intersection is referred to as an angular equal‑diameter bevel gear.

　　 Worm gear  

　　A helical screw cut onto a shaft is called a worm, and the gear that meshes with it is called a worm wheel. When the two are mounted on non-intersecting axes, the assembly is referred to as a worm gear set. Worms and worm wheels are not limited to cylindrical forms; conical (hourglass‑shaped) variants also exist, which can increase the contact ratio but are more challenging to manufacture. Because the gear surfaces are in sliding contact, friction must be minimized. To this end, worms are typically made from hard materials, while worm wheels are made from softer materials. Although sliding contact results in lower efficiency, it yields smooth, quiet operation. When the lead angle of the worm is small, self‑locking characteristics emerge.