Hypoid gears are a type of spiral bevel gear, with the difference being that hypoid gears have non-parallel axes and do not intersect. In other words, the hypoid gear axes are offset from each other. The basic geometry of the hypoid gear is hyperbolic, rather than having the conical geometry of a spiral bevel gear.
In a hypoid gearbox, the pinion spiral angle is greater than the gear spiral angle, so the pinion diameter can be larger than that of a bevel pinion. This provides a larger contact area and better tooth strength, which allows more torque to be transmitted and high gear ratios (up to 200:1) to be used. Because the axes of hypoid gears do not intersect, bearings can be used on both sides of the gear to provide greater rigidity.
The difference in spiral angles between the pinion and the crown (larger gear) causes slippage along the teeth, but the slippage is uniform, both in the direction of the tooth profile and longitudinally. This gives hypoid gearboxes very smooth running properties and silent operation. But it also requires special EP (extreme pressure) gear oil to maintain effective lubrication, due to the pressure between the teeth.
Hypoid gearboxes are generally used where speeds exceed 1,000 rpm (although above 8,000 rpm, earth gears are recommended). However, they are also useful for low-speed applications that require extreme smoothness of motion or quiet operation. In multistage gearboxes, hypoid gears are often used for the output stage where lower speeds and high torques are required.
The most common application of hypoid gearboxes is in the automotive industry, where they are used on rear axles, mainly in large trucks. With a left spiral angle on the pinion and a right spiral angle on the ring gear, these applications have what is known as an “off-center” offset, which allows the driveshaft to be positioned lower in the vehicle. This lowers the vehicle's center of gravity and, in some cases, reduces interference with the vehicle's interior space.
