Description of the Worm Gear and Worm Drive in a Reducer

　　 Description of the Worm Gear Reducer Transmission Assembly

　　What is a worm gear?

　　A worm gear is a type of gear consisting of a shaft with a helical thread that meshes with and drives a gear. It is an older form of gearing and one of the six simple machines. Essentially, a worm gear comprises a screw that engages with a wheel resembling a standard spur gear, whose teeth are slightly angled and curved.

　　It converts rotational motion by 90 degrees, and the plane of motion also changes depending on the worm’s position on the worm wheel (or simply “wheel”). They typically consist of a steel worm and a brass wheel.

　　Worm gear

　　Working principle of a worm gear

　　The electric motor or engine applies rotational power via a worm gear. The worm rotates around the wheel, and its helical surface pushes against the wheel’s teeth. The wheel is then driven toward the load.

　　Applications of Worm Gears

　　There are several reasons why people choose worm gears over standard gears.

　　First is the high reduction ratio. A worm gear can effortlessly achieve an enormous reduction ratio—simply by increasing the circumference of the worm wheel. As a result, you can use it to significantly boost torque or drastically reduce speed. Typically, achieving the same level of deceleration with a conventional gear set would require multiple stages, whereas a single worm gear does so in one step—meaning fewer moving parts and fewer potential failure points for the user.

　　The second reason for using a worm gear is that it prevents reverse motion. Due to the friction between the worm and the worm wheel, the worm wheel, when subjected to an applied force, can hardly drive the worm to rotate.

　　In a standard gear set, once sufficient force is applied, the input and output shafts can rotate independently. This necessitates the addition of a locking mechanism to the standard gearbox, thereby further increasing the complexity of the gear assembly.

　　Why not use a worm gear?  

　　There is one particularly compelling reason why worm gears are often preferred over standard gears: lubrication. The motion between the worm and gear surfaces is entirely sliding; there is no rolling component in the tooth contact or interaction. This makes them relatively difficult to lubricate.

　　The lubricants required typically have very high viscosities (ISO 320 and higher), making them difficult to filter, and they are usually formulated specifically for their intended applications, necessitating site‑specific product offerings tailored to the particular type of equipment.

　　Worm gear lubrication

　　The primary challenge with worm gears lies in how they transmit power—this very feature is both a blessing and a curse. The helical motion enables substantial speed reduction within a relatively compact space, a requirement that would otherwise necessitate the use of standard helical gears.

　　This helical motion also gives rise to a particularly challenging situation: the dominant mode of power transmission, commonly referred to as sliding friction or sliding wear.

　　For a typical gear set, at least under rolling‑wear conditions, power is transmitted at the point of peak load on the tooth—known as the pitch point or pitch line. Sliding occurs on both sides of the pitch point, but at relatively low speeds.

　　For worm gears, sliding motion is the sole mode of power transmission. As the worm shaft slides over the gear teeth, it gradually wears away the lubricant film until none remains, resulting in metal-to-metal friction between the worm and the gear under boundary lubrication conditions. When the worm’s surface separates from the gear’s surface, it picks up additional lubricant and resumes this process with the next rotation.

　　Rolling friction on typical gear teeth requires virtually no lubricant film to fill the gaps and separate the two mating surfaces. Since sliding occurs on both sides of the gear tooth flanks, a lubricant with a slightly higher viscosity than that required for rolling wear is needed to overcome this load. The sliding velocity is relatively low.

　　As the worm on the worm‑gear assembly rotates, it presses against the load applied to the worm wheel. The only way to prevent direct contact between the worm and the worm wheel is to ensure that the oil film thickness is sufficient to prevent the entire tooth surface from being wiped away before that portion of the worm leaves the loaded zone.

　　This situation calls for a specialized lubricant. Not only must it be relatively high‑viscosity—higher viscosity being required as load or temperature increases—but it must also incorporate a mechanism to mitigate the prevailing sliding conditions.