Hypoid gears are beveled gears similar to spiral ones, except they transmit motion between axes that do not intersect. The small gear shaft (hypoid pinion side) is offset from the giant gear shaft (hypoid gear side). Given the proper offset amount, the pinion shaft and large gear shaft can pass without interference, enabling the shafts to be supported securely on both ends.
Generally, compared with bevel gears, hypoid gears can obtain higher speed reduction. Their extensive contact ratio allows heavier load transmission than similar bevel gears. Also, the smooth meshing makes it possible to suppress noise and vibration further. However, the meshing is very complicated, and the production is more difficult. As for applications, they are mainly used in automotive drive systems such as differential gears.
It is possible to obtain a very high-speed reduction with one pair of hypoid gears. Compared to worm gear drives of similar high-speed reduction, hypoid gears generally have the following advantages.
- Both pinion and gear can be heat treated, resulting in high rigidity, which leads to a smaller unit.
- Less sliding and high efficiency result in smaller capacity motors.
- Compared to the worm, the offset between pinion and gear is slight with resultant space savings.
Comparison of hypoid gears to spiral bevel gears
Hypoid gears are more robust, operate more quietly, and can be used for higher reduction ratios. However, they also have some sliding action along with the teeth, which reduces mechanical efficiency. The energy losses are in the form of heat produced in the gear surfaces and the lubricating fluid.
Hypoid gears are typically used in rear-drive automobile drivetrains.
A higher hypoid offset allows the gear to transmit higher torque. However, increasing the hypoid compensation reduces mechanical efficiency and a consequent reduction in fuel economy. It is often impossible to replace low-efficiency hypoid gears with more efficient spiral bevel gears in automotive use because the spiral bevel gear would need a much larger diameter to transmit the same torque. Increasing the size of the drive axle gear would require an increase in the size of the gear housing and a reduction in the ground clearance, interior space, and an increase in weight.
The hypoid gear is also commonly used in some railcar transmissions with diesel power units – where the engine and gearbox are similar to those used in traditional trucks and busses (not diesel/electric hybrid type drive). The transmission allows the input shaft to rotate in one specific direction (either clockwise or anti-clockwise) while allowing the output shafts to change their rotational movement, thus allowing a vehicle to drive in either direction continuously.
Another advantage of hypoid gear is that the ring gear of the differential and the input pinion gear are both hypoid. In most passenger cars, this allows the pinion to be offset to the bottom of the crown wheel. This allows for more extended tooth contact and lowers the shaft that drives the pinion, reducing the “hump” intrusion in the passenger compartment floor. However, the greater the displacement of the input shaft axis from the crown wheel axis, the lower the mechanical efficiency.
How hypoid gears work in reducers
Single-stage hypoid reducers can achieve reductions with ratios of 3:1 to 10:1. In comparison to straight or spiral bevel reducers, which need an additional planetary step to accomplish the removal, the single location hypoid is well suited for compact applications that fall in this range of reduction ratios.
Hypoid gears can be combined with planetary gears in multiple stage gearboxes to reach
higher reduction ratios, typically up to 100:1 with a single additional planetary stage. In that case, hypoid gears should be chosen over bevel gears for the 90° angle transmission if the system’s configuration requires non-intersecting shafts or if higher torques need to be transmitted with low noise levels.
When compared to worm gear reducers, hypoid reducers are a better option in terms of efficiency and heat generation. They require less maintenance and fit into tighter places while delivering the same torque. For long-term cost savings, hypoid reducers are an alternative to worm gear reducers that should be considered.
Hypoid gear examples used
- ATC for machine tools
(Automatic tool changer)
- Four-wheel differential, etc.