If the action of spur and helical gears can be related to that of rolling cylinders, then the action of bevel gears can be compared to a friction cone drive. Bevel gears are used to connect shafts which lie in the same plane and whose axes intersect. The size of the tooth decreases as it passes from the back edge towards the apex of the pitch cone, hence the cross-section varies along the whole length of the tooth. When viewed on the curved surface which forms part of the back cone, the teeth normally have the same profiles as spur gears. The addendum and dedendum have the same proportions as a spur gear, being measured radially from the pitch circle, parallel to the pitch-cone generator.
Data relating to bevel gear teeth is shown in Fig. 24.24. Note that the crown wheel is a bevel gear where the pitch angle is 90°. Mitre gears are bevel gears where the pitch-cone angle is 45°.
The teeth on a bevel gear may be produced in several different ways, e.g. straight, spiral, helical, or spiraloid. The advantages of spiral bevels over straight bevels lies in quieter running at high speed and greater load-carrying capacity.
The angle between the shafts is generally a right angle, but may be greater or less than 90°, as shown in Fig. 24.25.
Bevel gearing is used extensively in the automotive industry for the differential gearing connecting the drive shaft to the back axle of motor vehicles.
Fig. 24.22 Double helical gears. (a) On same wheel (b) On separate wheels
For shafts lying at 90° to each other, both gears will have the same hand of helix, Fig. 24.23.
Helical gears can be used for shafts which lie at an
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