Hypoid

A hypoid is the name given to a type of helical (spiral) gear. The main application of this is in a motor vehicle differential, where the direction of the drive carried by the propeller shaft (drive shaft) 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 (ring gear) 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 (see zinc dialkyldithiophosphate).