A spiral bevel gear is a bevel gear with helical teeth. The main application of this is where the direction of drive from the drive shaft must be turned 90 degrees to drive the wheels. They are somewhat analogous to cylindrical type helical gears in that the teeth are angled; however, with spiral gears, the teeth are also curved. Spiral bevel gears have a strength, oscillation, and noise level advantage over straight bevel gears. 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.