Bevel gears are generally used when there is a need to transfer rotary motion at right angles. However, they can be designed in such a way that they transfer motion at other angles as required. The tooth-bearing face of a bevel gear has a conical shape.
Types of Bevel Gears
The classification of bevel gears is done on the basis of their geometry. Basically, they are of four types as explained below:
Straight Bevel Gears
The shape of the pitch surface of these gears is conical and they have straight teeth that taper towards the apex. These gears, cut from conical blanks, connect shafts are often at right angles to each other. Sometimes, a bevel gear mounted on one shaft drives two output shafts. Drive systems in machines make use of straight bevel gears to effect 90 degrees direction changes as well as in differential drives. As limitations of these gears are the same as that of spur gears, they are not useful in high speed, heavy duty applications. These low cost units often come with ratios ranging from 1:2 to 4:1.
Zerol Bevel Gears
These gears are similar to straight bevel gears, but for the teeth that are curved. In the true sense, these are spiral bevel gears with zero degree spiral angle. The advantages offered by a zerol bevel gear when compared with the straight type are smoother operation and lower vibration at higher speeds because of localised point contact between the gears instead of line contact as in the case of straight bevel gears. Straight bevel gears are often replaced with zerol bevel gears because of these benefits. These gears generally have a 20 degree pressure angle and a minimum of 14 teeth on the pinion. Zerol gears are specially designed and manufactured using special machine tools.
Spiral Bevel Gears
The teeth of these gears are curved at an angle so as to make the contact gradual as well as smooth. Spiral bevel gears are produced with the help of a spiral gear form so as to ensure a smoother drive at higher speeds and loads.
Hypoid Bevel Gears
This type of bevel gear is similar to a spiral bevel gear except for the fact that the pitch surface is hyperboloid instead of a cone. As the pinion used with these gears can be offset below or above the gear centre, they allow a larger pinion diameter, longer life as well as smoother mesh. Hypoid bevel gears are best suited for applications that call for large speed reductions and use non-intersecting shafts. They also provide a smooth as well as quiet operation. Typically, hypoid gears are used for applications in the automotive industry. The pressure angles of hypoid gears range between 19 degrees and 22 degrees. These gears are specially designed and are manufactured with the help of specialised machine tools.
Material of Construction
Bevel gears are manufactured with cast irons, non-ferrous material as well as non–metallic materials. The material is selected on the basis of the type of service, method of manufacture, peripheral speed, accuracy required, etc. Cast iron is popular because of its wearing properties, machinability and the ease with which complicated shapes can be produced by casting. Cast steel is made use of if stress on the gears is high. Plain carbon steel is used for making bevel gears for industrial applications that require toughness and strength. Bevel gears made out of alloy steels have high tooth strength as well as low tooth wear. Non–metallic materials are used if noiseless operation is required.