The two axes of hypoid gear are straight lines with different surfaces, that is, there is an offset distance between the axes of the two gears, as shown in Figure. Due to the existence of offset distance, the spatial layout of hypoid bevel gear pair has greater flexibility. If the pinion is offset, the height of the chassis from the ground can be increased and the trafficability of the chassis can be improved; The lower offset can reduce the center of gravity of the chassis, so as to make the whole machine run more smoothly. The relative offset of the pinion enables it to realize cross mounting support in the installation process, so as to increase the strength of the gear pair and improve the load bearing capacity. Due to the existence of offset distance, the small bevel gear can have a large helix angle, which improves the coincidence degree and makes the gear pair meshing more stable. At the same time, the increase of the helix angle of the small gear makes the end face modulus of the small gear increase accordingly, which increases the shaft diameter of the small gear, which greatly enhances the strength and prolongs the service life of the small gear. Due to the relative sliding along the tooth length direction and tooth height direction in the meshing process, the hypoid gear is easy to run in in the machining process and easy to grind after heat treatment, so as to improve the contact area, improve the tooth surface finish and reduce the noise. Hypoid gear has strong bearing capacity and smooth movement. Its transmission stability is almost close to that of worm gear pair, and has the same or better bearing capacity compared with worm gear and worm transmission. It does not need to use wear-resistant materials, and its manufacturing is far simpler than that of worm gear and worm.
Compared with other gear pair transmission, hypoid gear also has some disadvantages. The offset distance makes the hypoid gear slide relatively along the tooth length direction and tooth height direction in the meshing process, which increases the tooth surface wear and even leads to gluing or fatigue pitting corrosion on the tooth surface, which greatly reduces the service life of the gear pair. Therefore, the gear pair requires higher lubrication conditions and special high-grade anti-wear lubricating oil. Compared with other gears, the calculation and design of hypoid gear pair are more cumbersome and complex. According to the Gleason system method, taking geometric calculation as an example, there are as many as 150 basic formulas, including three iterative calculations (usually three iterations, sometimes more). Compared with other spiral bevel gears, hypoid gears have low transmission efficiency, and the calculation of tooth cutting adjustment is relatively complex. They are sensitive to installation errors, so contact matching is also difficult.
With the development of science and technology, hypoid gear transmission has put forward higher requirements. High speed, heavy load, low noise and high reliability have become the development direction in the future. The geometric design theory of hypoid gear directly affects the meshing performance and service life of gear pair. Therefore, seeking an efficient design method which is easy to operate and easy to analyze and study is the development direction of gear pair at present. In view of many advantages of hypoid gear, the hypoid gear pair with vertical axis is emphatically studied.