Types and Mechanisms of Hard-Tooth-Surface Helical Gear Reducers

Gear reducers in which the input and output shafts are offset and intersect at right angles typically employ hypoid gears or worm gears. In particular, worm gears have long been used for speed reduction and remain widely employed today. The characteristics of the gears used are shown in Table 2.3. The salient features of these reducers are as follows:

High single-stage reduction ratio
The reduction ratio ranges from 1:5 to 1:100, and a combination of two worm gear reducers can achieve a reduction ratio of 1:10,000.
Low noise and vibration
Worm gears primarily rely on sliding contact, resulting in low noise and vibration. Leveraging this characteristic, worm gear reducers were once used as drive sources. However, due to their low transmission efficiency, they have now been replaced by reducers employing hypoid gears or other types of orthogonal or parallel-axis gears, as described below.
The input shaft and the output shaft are at a right angle,
Even when integrated into the gearbox, it features a reduced number of parts, a compact footprint, and saves installation space.
Self-locking
When the reduction ratio is high, it is sometimes impossible to drive the input shaft (worm) using the output shaft (worm wheel). This phenomenon is known as self-locking. In practice, achieving self-locking can be challenging. On the other hand, this characteristic is more advantageous in cam mechanisms, which apply both positive and negative loads while rotating in a single direction.
Low transmission efficiency
Worm gear drives, due to their sliding contact, suffer significant friction losses, resulting in low transmission efficiency that varies considerably with the lead angle. (When the lead angle is between 5° and 40°, the transmission efficiency ranges from 60% to 95%.) When the lead angle approaches 15°, the transmission efficiency drops sharply, which constitutes a notable drawback.
Temperature rises rapidly
In addition to sliding along the tooth profile, worm gears also experience sliding along the tooth flank. Because this flank‑to‑flank sliding is substantial, if the contact length along the tooth flank is short, high surface pressures can develop, potentially causing the lubricant film to rupture. When the meshing point temperature is elevated and mineral oil is used, 90°C is generally regarded as the upper limit of the housing’s allowable operating temperature.