Hypoid Gear
A hypoid gear is a bevel gear in which the axes of the two bevel gears do not intersect at a point but are offset from each other. This axis offset enables higher gear ratios and greater load capacity than a spiral bevel gearbox, but creates increased sliding in the tooth contact line, which makes EP lubrication mandatory.
Design Principle and Advantages
The key difference from a spiral bevel gearbox is the hypoid offset: the pinion is positioned below (or above) the gear axis so that the axes form skew lines. This offset allows the pinion to have a larger diameter and more teeth, which increases the contact ratio and hence load capacity. At the same time, the offset enables a more compact design (the output can be mounted lower) and allows high gear ratios in a single stage.
Sliding Component and Lubrication Requirements
The axis offset causes the tooth flanks not only to roll on each other but also to slide in the longitudinal direction of the tooth (lengthwise sliding ratio). This increased sliding component reduces efficiency compared with a spiral bevel gearbox and generates higher flank pressures and temperatures. Standard gear oils are therefore unsuitable — an EP hypoid oil (extreme pressure) must be used, containing special additives that form a protective film on the flanks. Typical efficiency: 90–96% (depending on offset and ratio).
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