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.
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.
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[edit] Introduction
An important concept in gearing is the pitch surface. In every pair of meshing gears, each gear has a pitch surface. The pitch surfaces are the surfaces of imaginary smooth (toothless) bodies that would produce the same gearing relationship by frictional contact between their faces as the actual gears do by their tooth-to-tooth contact. They are a sort of "average" surface that one would get by evening out the peaks and valleys of the individual teeth. For an ordinary gear the pitch surface is a cylinder. For a bevel gear the pitch surface is a cone. The pitch cones of meshed bevel gears are coaxial with the gear shafts; and the apexes of the two cones are at the point of intersection of the shaft axes. The pitch angle is the angle between the face of the cone and the axis. The most familiar kinds of bevel gears, such as those in the picture at the beginning of this article, have pitch angles of less than 90 degrees. They are "pointy". This type of bevel gear is called an external bevel gear because the teeth are facing outwards. It is possible to have a pitch angle greater than ninety degrees, in which case the cone, rather than forming a point, forms a sort of conical cup. The teeth are then facing inwards, and this type of gear is called an internal bevel gear. In the border line case, a pitch angle of exactly 90 degrees, the teeth point straight forward. In this orientation, they resemble the points on a crown, and this type of gear is called a crown bevel gear or crown gear.
[edit] Teeth
There are two issues regarding tooth shape. One is the cross-sectional profile of the individual tooth. The other is the line or curve on which the tooth is set on the face of the gear: in other words the line or curve along which the cross-sectional profile is projected to form the actual three-dimensional shape of the tooth. The primary effect of both the cross-sectional profile and the tooth line or curve is on the smoothness of operation of the gears. Some result in a smoother gear action than others.
[edit] Tooth Line
The teeth on bevel gears can be straight, spiral or "zero".
- In straight bevel gears the teeth are straight and parallel to the generators of the cone. This is the simplest form of bevel gear. It resembles a spur gear, only conical rather than cylindrical. The gears in the floodgate picture are straifgt bevel gears. 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.
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.
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.
Bevel gears are used in differential drives, which can transmit power to two axles spinning at different speeds, such as those on a cornering automobile.
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.
