 |
 |
|
Spiral Gears |
||
| Spiral Gears: These gears are also known as crossed helical gears. These gears are mounted on shafts that do not intersect and that are at an angle to each other. The angle is usually 90 degree. | ||
| 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. |
||
 |
 |