The tooth profile of bevel gears can be classified as straight and spiral; both types are suitable for intersecting axes; in straight bevel gears, the tooth profile is straight as in spur gears, cut along the gear axis, and kept inclined. As the teeth of two mating gears come into sudden contact, the teeth are subjected to shock loads, which cause vibration and noise. Face and helical gears also have straight teeth.
The advantages of spiral bevel gears over straight bevel gears are similar to those of helical gears over spur gears. The teeth of spiral bevel gears have a helical profile and thus come into contact gradually during mating. Therefore, it provides smooth, vibration-free, and quiet operation. However, it places high thrust loads on the bearings. Hypoid gears also have helical teeth.
Spiral Bevel Gear
Spiral bevel gears are beveled gears with helical teeth formed on the gear face along a winding line. The main advantage of helical gears over spur gears is smooth operation because the teeth mesh gradually. When each pair of gears is in contact, force transmission is smoother. Spiral bevel gears should be replaced in pairs and run together concerning the main helical gear. Spiral bevel gears are more commonly used in vehicle differentials, automotive, and aerospace. The spiral design produces less vibration and noise than straight bevel gears.
Straight Bevel Gear
A straight bevel gear is where the axes of the two-member shafts intersect, and the tooth flanks are conical in shape. However, straight bevel gear sets are usually mounted at 90°; other angles are also used. The pitch faces of bevel gears are conical. Two essential properties of a gear are tooth flank and pitch angle.
Spur gears have a zero pitch angle, while bevel gears typically have a pitch angle between 0° and 90°. The more common bevel gears have a conical shape and a pitch angle of 90° or less. This type of bevel gear is called an external bevel gear because the teeth face outward. The pitch faces of the meshing external bevel gears are coaxial with the gear shaft. The vertices of the two surfaces are always at the intersection of the axes. A bevel gear with a pitch angle greater than 90° is called an internal bevel gear; the tooth top of the gear faces inward. A bevel gear with a pitch angle of precisely 90° has teeth parallel to the axis.
Comparing Spiral Bevel Gears and Straight Bevel Gears
1. The teeth of the straight bevel gear are straight and cut along the axis of the cone, while the teeth of the spiral bevel gear are helical and cut in the form of a spiral curve on the pitch cone.
2. The meshing of the tooth surfaces of straight bevel gears is a sudden contact, so it will generate noise, while the spiral bevel gears are gradually meshing during operation, so the noise is relatively small.
3. Straight-toothed bevel gears have the teeth of two mating gears in sudden contact. Contact is a line of length equal to the width of the tooth flanks, whereas the teeth of the two mating gears of a spiral bevel gear gradually come into contact.
4. The impact load will cause vibration in the straight bevel gear, so its operation is not smooth. Still, the spiral bevel gear gradually increases the load, and the vibration generated is slight, so it runs smoothly.
5. In straight bevel gears, the teeth are also subject to impact or impact loads due to sudden contact, while in spiral bevel gears, the teeth are subject to gradual loads.
6. The straight bevel gear exerts a small thrust on the bearing supporting the shaft, while the spiral bevel gear exerts a significant thrust on the bearing.
7. Straight bevel gears are easier to design and manufacture, so the cost is lower. In contrast, spiral bevel gears are more complicated to design and manufacture, resulting in higher prices for such gears.
8. The service life of the straight bevel gear is shorter because it is subjected to shock loads and vibrations, while the life of the spiral bevel gear is longer.
9. In most processing methods, the two are different in the gear-cutting process. Straight bevel gears are mainly cut by gear shaping, while spiral bevel gears are processed primarily by milling.
10. Straight bevel gears are generally used in equipment with low speed and low load. Spiral bevel gears are more widely used and suitable for high-speed and high-load equipment.
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