Hypoid VS Spiral Bevel Gear Systems

http://en.hbaxle.com/productz1/typeid/11.html

From a physical standpoint, there is just one main difference between the two different types of gears: In a spiral bevel gearset, the pinion center line intersects the ring gear center line. Also, the shape of a spiral bevel gear is normally conical.

The hypoid gear system has an offset or difference between the pin-ion center line and ring gear center line. This offset is given the variable E.

Most automotive applications utilize a hypoid gear system, meaning that the gears have an offset. Typical rear axles have a left hand spiral angle on the pinion gear and a right-hand spiral angle on the ring gear to accommodate a below center offset. A below center offset allows the propshaft to be located lower in the vehicle relative to the axle shafts. This allows the tunnel in the vehicle to be shallower and protrude less into the passenger compartment.

This pinion and ring gear spiral hand and below center offset arrangement are such that the pinion is thrust for-ward in the vehicle, or toward the head bearing, during forward-drive conditions. This is the main reason that the head bearing is typically larger than the tail bearing.

The spiral bevel gear is not commonly found in automotive applications, but this arrangement lines up the gears at a right angle to one another, as with the hypoid. The pinion centerline actually lines up with the ring gear center line.

This offset increases the contact ratio of the gear system and provides more torque-carrying capacity. Contact ratio is a term used to describe the average number of tooth pairs that are in contact between the mating gears. Typical values for total contact ratio are in the 2.2 to 2.9:1 range (2.2 means that two pairs of teeth are incontact at all times, and a third pair is in contact 20 percent of the time).

Also, hypoid gear systems are quieter due to this higher contact ratio.

This offset, however, has the negative side-effect of sliding friction at the tooth interface. This sliding generates heat and, without the correct lubrication, extreme wear known as scoring. As a result, extreme pressure (EP) additives are blended into the gear oil. This sliding also allows for lapping during the manufacturing process.