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A hypoid gear is a spiral bevel gear with an axis that does not intersect with the axis of the meshing 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 in this gear produce less vibration than a gear with spur-cut or straight-cut teeth. Hypoid gears are manufactured in pairs and should be replaced in pairs.
A right-hand hypoid gear is one in which the outer half of the teeth are inclined in the clockwise direction as one looks at the face of the gear. Similarly, a left-hand one has the outer half of its teeth inclined in the counterclockwise direction. The gear and its pinion usually have opposite handedness. This type of gear also can be classified according to its spiral angle — the angle between an element of the pitch cone and the tooth trace.
A hypoid gear has the shape of a revolved hyperboloid, meaning that its pitch surface forms a hyperbolic surface. Its pinion is off-axis with respect to the ring gear, also known as the crown wheel. This allows the pinion to be larger than the hypoid gear, which causes the pinion to have greater contact with the gear.
This type of gear is generally stronger and quieter than an ordinary spiral bevel gear. It can also handle a higher reduction ratio. The teeth experience some sliding, causing friction, which means that the gear requires special oils to lubricate it under very high pressure.
A larger offset in the hypoid gear increases its torque at the expense of efficiency. Its most common use is in older rear-drive vehicles, especially trucks, since the greater torque is most beneficial in larger vehicles. Designers of modern automobiles have tended to value the greater efficiency of an ordinary 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 it produces a greater hump in the floor of passenger cars. It's normally impractical to replace a hypoid gear with a more efficient spiral bevel gear.
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