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Hypoid Gears: A hypoid is the name given to a type of
helical (spiral) gear. The main application of this is in the final drive
of an automobile, where the
direction of the drive carried by the has to be turned through 90
degrees in order to power the driven wheels. Conventional straight-cut or
spur-cut gears, with perpendicular teeth, are considered to be too noisy in use,
and a normal spiral bevel does not always give sufficient contact area. The
hypoid gear places the pinion off-axis
to the crown wheel which
allows the pinion to be larger in diameter. In a normal passenger car the pinion
is always offset to the bottom of the crown wheel. This provides for longer
tooth contact and serendipitously allows the propeller shaft that drives the
pinion to be lowered, reducing the "hump" intrusion in the passenger compartment
floor.
A hypoid gear incorporates some sliding and can be considered halfway between
a straight-cut gear and a worm
gear. Special gear oils are
required for hypoid gears because the sliding action creates extreme pressure
between the teeth.
These gears are similar to spiral
bevel gears with the difference that the axes of the shafts do not intersect. The
distance between a hypoid pinion axis and the axis of the hypoid gear is called
the “ offset”. If the offset is zero, the hypoid gears become spiral bevel
gears. Shaft angle is usually 90 degrees but other angles are also possible.
Because the shafts do not intersect, bearings can be provided on both sides
of the gear which will result in extra rigidity. High reduction hypoid gears permit
ratios between 10:1 and 20:1 and even high as 360:1, in fine pitches.
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