Bevel gear
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Bevel gears are gears where the axes of the two shafts intersect and the tooth-bearing faces of the gears themselves are conically shaped.
The bevel gear planer was invented by William Gleason at Gleason Works in 1874.
These gears permit minor adjustment during assembly and allow for some displacement due to deflection under operating loads without concentrating the load on the end of the tooth. For reliable performance, Gears must be pinned to shaft with a dowel or taper pin.
Bevel gears are most often mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well. The pitch surface of bevel gears is a cone. They are useful when the direction of a shaft's rotation needs to be changed. Using gears of differing numbers of teeth can change the speed of rotation.
Four bevel gears in a square make a differential gear, which can transmit power to two axles spinning at different speeds, such as those on a cornering automobile.
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[edit] Tooth Types
The teeth on bevel gears can be straight, spiral or "zero".
- Straight bevel gears resemble simple spur gears, except that pitch surface of a spur gear is cylindrical whereas the pitch surface of a bevel gear is conical. The teeth of a straight bevel gear have no helix angle. In straight, when each tooth engages it impacts the corresponding tooth and simply curving the gear teeth can solve the problem.
- Spiral bevel gears have their teeth formed along spiral lines. They are somewhat analogous to helical gears, a cylindrical type, in that the teeth are angled; however with spiral gears the teeth are also curved. The advantage of the spiral tooth over the straight tooth is that they engage more gradually. The contact between the teeth starts at one end of the gear and then spreads across the whole tooth. This results in a less abrupt transfer of force when a new pair of teeth come in to play. With straight bevel gears, the abrupt tooth engagement causes noise, especially at high speeds, and impact stress on the teeth which makes them unable to take heavy loads at high speeds without breaking. For these reasons straight bevel gears are generally limited to use at linear speeds less than 1000 feet/min; or, for small gears, under 1000 r.p.m.1
- Zero bevel gears are an intermediate type between straight and spiral bevel gears. Their teeth are curved, but not angled.
Hypoid bevel gears can engage with the axes in different planes. This is used in many car differentials. The ring gear of the differential and the input pinion gear are both hypoid. This allows input pinion to be mounted lower than the axis of the ring gear. Hypoid gears are stronger, operate more quietly and can be used for higher reduction ratios. They also have sliding action along the teeth, potentially reducing efficiency.
[edit] Applications
A good example of bevel gears is seen as the main mechanism for a hand drill. As the handle of the drill is turned in a vertical direction, the bevel gears change the rotation of the chuck to a horizontal rotation. The bevel gears in a hand drill have the added advantage of increasing the speed of rotation of the chuck and this makes it possible to drill a range of materials.
The bevel gear find its application in locomotives, marine applications, automobiles, printing presses, cooling towers, power plants, steel plants, defence and also in railway track inspection machine.
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
[edit] Advantages
- This gear allows to change the operating angle
[edit] Disadvantages
- One wheel of such gear is designed to work with its complementary wheel and no other.
- Must be rather precisely mounted.
- The axes must be capable to support significant forces.
[edit] See also
- Another type of gear that also allows to change the operating angle is a crown gear.
[edit] Notes
- Doughty and Vallance, Design of Machine Members.
