Hypoid gears are similar to bevel gears, but the axes of the two connecting shafts do not intersect. They carry curved teeth, are stronger than the common types of bevel gears, and are quiet-running. These gears are mainly used in automobile rear axle drives.
A hypoid gear is a spiral bevel gear with an axis that does not intersect with the axis of the gear or pinion. Its primary application is in the differential drive of a wheeled vehicle, where the drive shaft must be at a right angle to the wheels. The helical teeth of this gear produce less vibration than a gear with spur or straight teeth. Hypoid gears are manufactured in pairs and must be replaced in pairs.
A right-hand hypoid gear is one in which the outer half of the teeth are tilted clockwise when you look at the face of the gear. Similarly, a left-handed one has the outer half of its teeth tilted counterclockwise. The gear and its pinion usually have opposite hands. This type of gear can also be classified by its spiral angle, the angle between an element of the pitch cone and the tooth track.
A hypoid gear is shaped like a rotated hyperboloid, which means that its primitive surface forms a hyperbolic surface. Its pinion is off-axis relative to the ring gear, also known as the crown gear. This allows the pinion to be larger than the hypoid gear, which causes the pinion to have more contact with the gear.
This type of gear is generally stronger and quieter than a standard spiral bevel gear. It can also handle a higher reduction ratio. The teeth experience some slippage, causing friction, which means the gear requires special oils to lubricate it at very high pressure.
A higher offset in the hypoid gear increases its torque at the expense of efficiency. Its most common use is in older rear-wheel-drive vehicles, particularly trucks, because the higher torque is more advantageous in larger vehicles. Modern car designers tend to value the greater efficiency of a standard spiral bevel gear.
A spiral bevel gear must be much larger to provide the same torque as a hypoid gear. This generally means that a spiral bevel gear has less ground clearance and produces a larger hump in the floor of cars. It is normally not practical to replace a hypoid gear with a more efficient spiral bevel gear.