Hypoid gears have spiral teeth and a non-intersecting axis, making them ideal for differential drive in vehicles. They are stronger and quieter than regular spiral bevel gears, but require special lubrication. A larger offset increases torque but reduces efficiency. Hypoid gears cannot be replaced with spiral bevel gears due to size differences.
A hypoid gear is a spiral bevel gear with an axis that does not intersect with the axis of the gear or pinion. Its main application is in the differential drive of a wheeled vehicle, where the drive shaft must be at right angles to the wheels. The helical teeth of this gear produce less vibration than a gear with spur or spur 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 lean clockwise when looking at the gear face. Similarly, one on the left has the outer half of its teeth angled counterclockwise. The gear and its pinion usually have opposite hands. This type of gear can also be classified by its helix angle, the angle between an element of the pitch cone and the tooth track.
A hypoid gear has the shape of a rotated hyperboloid, which means that its pitch surface forms a hyperbolic surface. Its pinion is off axis with respect to the ring gear, also called ring 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 regular spiral bevel gear. It can also handle a higher reduction ratio. The teeth experience some slip, causing friction, which means 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-wheel drive vehicles, especially trucks, as higher torque is more beneficial in larger vehicles. Modern automobile designers tend to value the higher 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 produces a greater hump in the floor of passenger cars. It is not normally practical to replace a hypoid gear with a more efficient spiral bevel gear.
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