Hypoid gears and Comparison of spiral bevel gears to hypoid gears

A hypoid gear is a spiral bevel gear whose axes do not intersect the axes of the meshing gears.The shape of a hypoid gear is a hypoid of revolution (that is, the pitch surface of a hypoid gear is a hyperboloid), while the shape of a spiral bevel gear is usually conical.Hypoid gears place the pinion off-axis from the crown wheel (ring gear), which allows the pinion to have a larger diameter and larger contact area.In a hypoid gear design, the pinion and gear are practically always counter-rotated, and the helix angle of the pinion is usually greater than that of the gear.A hypoid pinion has a larger diameter than an equivalent bevel gear.A hypoid gear contains some slip and can be thought of as an intermediate gear between a spur gear and a worm gear.Hypoid gears require special gear oils because the sliding action requires effective lubrication under extreme pressure between the teeth.Hypoid gears are used in power transmission products that are more efficient than traditional worm gear drives.They are fairly strong because any load is transmitted through multiple teeth simultaneously.In contrast, bevel gears are loaded through one tooth at a time.The multiple contact points of the hypoid gear, when properly lubricated, can be virtually silent,.

Spiral angle 

• The helix angle in a spiral bevel gear is the angle between the tooth line and one element of the pitch cone, corresponding to the helix angle in a helical tooth.Unless stated otherwise, the term helix angle is understood to mean the average helix angle.Mean helix angle is the specific name for the helix angle at the mean pitch in bevel gears.

• External helix angle is the helix angle of the bevel gear at the outer pitch.

• The internal helix angle is the helix angle of the bevel gear at the internal pitch.

Comparison of spiral bevel gears to hypoid gears

Hypoid gears are stronger, operate more quietly and can be used for higher reduction ratios, however they also have some sliding action along the teeth, which reduces mechanical efficiency, the energy losses being in the form of heat produced in the gear surfaces and the lubricating fluid.Hypoid gears are commonly used in rear wheel drive car drivetrains.A higher hypoid offset allows the gear to transmit higher torque.However, increasing the hypoid offset results in a reduction in mechanical efficiency, which in turn results in lower fuel economy.For practical purposes, it is generally not possible to replace a less efficient hypoid gear with a more efficient spiral bevel gear in automotive use because the spiral bevel gear would require a larger diameter to transmit the same torque. Increasing the size of transaxle gears will require increasing the size of the gearbox and reducing ground clearance, interior space and increasing weight.The hypoid gear is also commonly used in some railcar transmissions with diesel power units where the engine and gearbox are similar to those used in traditional trucks and busses (not diesel/electric hybrid type drive).The transmission, to allow the input shaft to always rotate in one specific direction (either clockwise or anti-clockwise) while allowing the output shafts to change their rotational direction; thus allowing a vehicle to drive either direction.

Another advantage of hypoid gear is that the ring gear of the differential and the input pinion gear are both hypoid.In most passenger cars this allows the pinion to be offset to the bottom of the crown wheel.This provides for longer tooth contact and allows the shaft that drives the pinion to be lowered, reducing the "hump" intrusion in the passenger compartment floor. However, the greater the displacement of the input shaft axis from the crown wheel axis, the lower the mechanical efficiency.