Spiral bevel gears have spiral angles, which gives performance
improvements. They are designed for an angle change of 90 degrees, where
the two axes are concurrent and in the same plane. These gears have a
double function of being helical and beveled at the same time. The
contact between the teeth starts at one end of the gear and then spreads
across the whole tooth. In this type of gears the shaft must be
perpendicular to each other and must be in the same plane. They are the most complex forms of bevel gears. The teeth are curved by cutting them obliquely, resulting in higher overall contact ratios. Because of higher contact ratio, these have better load carrying capacity an this allows them to be smaller in size for a given load capacity than an equivalent gear. Thus they can transmit more power with smaller gears.
Methods of Manufacturing
Face milled and face hobbed are two methods of manufacturing for spiral bevel gears. In the method of face milled, the grinding of the contacting surface is the last step. Whereas, in the face hobbed method, hard cutting is the final step. The same machine is used to rough cut and finish cut the gears. The desirable design of the gear is to minimize the weight of the gear and it is done by reducing the material in the gear's rim.
Applications
Spiral bevel gears are important components on all current rotorcraft drive systems. These components are required to operate at high speeds, high loads, and for an extremely large number of load cycles. In this application, spiral bevel gears are used to redirect the shaft from the horizontal gas turbine engine to the vertical rotor. They are also used in power windows and power seats. They are used where speed and strength are desirable along with the change in angle of power flow.