Bevel Gear Types: Straight, Spiral, Zerol, Mitre and Hypoid Gears
Bevel gears are gears where the axes of two shafts intersect, and the tooth-bearing faces of the gears themselves are conically shaped. Bevel gears are typically mounted on shafts that are 90 degrees apart but can be designed to function at other angles as well. One of the most common use of bevel gears is in the differential of a vehicle, where it transmits power generated by the power engine to the vehicle’s wheels.
The two most important concepts in gearing are pitch surface and pitch angle. The pitch surface of a gear is the imaginary toothless surface that one would have by averaging out the peaks and valleys of the individual teeth. The pitch surface of an ordinary gear is in the shape of a cylinder. The pitch angle of a gear is the angle between the face of the pitch surface and the axis. Most kinds of bevel gears have pitch angles of less than 90 degrees and are therefore cone shaped. These are called external bevel gears, because their gear teeth point outwards. Internal bevel gears, meanwhile, have pitch angles of greater than ninety degrees, and have teeth that point inward. Bevel gears with pitch angles of exactly 90 degrees are known as crown gears.
Bevel gears can be classified into several different types, based on their geometry. Straight bevel gears have a conical pitch surface and teeth that are straight and tapering towards the apex. Straight bevel gears have a conical pitch surface and teeth that are straight and taper towards the apex. Spiral bevel gears have teeth that curve at an angle, which allows contact to be gradual and smooth. Zerol bevel gears are technically spiral bevel gears but have a spiral angle of zero at the middle of the face width, effectively combining the strengths of both straight and spiral bevel gears. Lastly, hypoid bevel gears are similar to spiral level, but the pitch surfaces are hyperbolic and not conical. A hypoid pinion is also larger in diameter than an equivalent bevel pinion. Lastly, mitre gears are a type of bevel gear with equal numbers of teeth and are primarily used to transmit rotational motion at a 1:1 ratio.