SPIRAL BEVEL GEAR
A spiral bevel gear is a bevel gear with helical teeth. The main application of this is in a vehicle differential, where the direction of drive from the drive shaft must be turned 90 degrees to drive the wheels. The helical design produces less vibration and noise than conventional straight-cut or spur-cut gear with straight teeth.
A spiral bevel gear set should always be replaced in pairs i.e. both the left hand and right hand gears should be replaced together since the gears are manufactured and lapped in pairs.
To understand the bevel gear tooth geometry, one might first observe the case of straight bevel gears. If the generating rack used to derive the cylindrical gear involute is bent in a horizontal plane into a circular shape, it results in a crown gear, which is used to derive the flank form of bevel pinion and gear. In the case of straight bevel gears, the crown gear or generating gear can be placed between the pinion and gear assembly.
Reference : Wikipedia