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.
Two important concepts in gearing are pitch surface and pitch angle. The pitch surface of a gear is the imaginary toothless surface that you would have by averaging out the peaks and valleys of the individual teeth. The pitch surface of an ordinary gear is the shape of a cylinder. The pitch angle of a gear is the angle between the face of the pitch surface and the axis.
The most familiar kinds of bevel gears have pitch angles of less than 90 degrees and therefore are cone-shaped. This type of bevel gear is called external because the gear teeth point outward. The pitch surfaces of meshed external bevel gears are coaxial with the gear shafts; the apexes of the two surfaces are at the point of intersection of the shaft axes.
Bevel gears that have pitch angles of greater than ninety degrees have teeth that point inward and are called internal bevel gears.
Bevel gears that have pitch angles of exactly 90 degrees have teeth that point outward parallel with the axis and resemble the points on a crown. That's why this type of bevel gear is called a crown gear.
Miter gears are mating bevel gears with equal numbers of teeth and with axes at right angles.
Skew bevel gears are those for which the corresponding crown gear has teeth that are straight and oblique.
Bevel gears are classified in different types according to geometry:
* Np - No. of teeth on Pinion.
* Ng - No. of teeth on given Gear.
* Dg - Pitch Dia. of given Gear.
* Dp - Pitch Dia. of given Pinion.
* F - Face Width (Length of single tooth).
* γ - Pinion Pitch Angle (Radians).
* Γ - Gear Pitch Angle (Radians).
* Ao - Cone Distance (Distance from pitch circle to intersection of shaft axes).
* rb - Back-Cone Radius.
* P - Diametrical Pitch. Teeth per inch of Pitch Diameter (N/D).
* p - Circular Pitch. Inches of circumference per tooth (Π/P).
Tooth shape for bevel gears is determined by scaling spur gear tooth shapes along the face width. The further from the intersection of the gear and pinion axes, the bigger the tooth cross sections are. If the tooth face were to extend all the way to the axes intersection, the teeth would approach infinitesimal size there. The tooth cross-section at the largest part of the tooth is identical to the tooth cross-section of a tooth from a spur gear with Pitch Diameter of 2* rb, or twice the Back-Cone Radius, and with an imaginary number of teeth (N’) equal to 2*Π times the Back-Cone Radius (rb) divided by the Circular Pitch of the bevel gear (p). This method of obtaining the dimensions and shape of the largest tooth profile is known at the “Tredgold” tooth-shape approximation. Refer to the profiles shown near the Back-cone radius dimension in the drawing above. Mean radius- Hp=Tx n/63000 → T = Hp x 63000/n T = Rm x Wt → Wt = Hp x 63000/ n x Rm
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.
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 straight 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 cylindrical type helical gears 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.
Zerol bevel gears are an intermediate type between straight and spiral bevel gears. Their teeth are curved, but not angled. Zerol bevel gears are designed with the intent of duplicating the characteristics of a straight bevel gear but they are produced using a spiral bevel cutting process.
The various materials used for gears include a wide variety of cast irons, non ferrous material &non – material materials the selection of the gear material depends upon: i) Type of service ii) Peripheral speed iii) Degree of accuracy required iv) Method of manufacture v) Required dimensions & weight of the drive vi) Allowable stress vii) Shock resistance viii) Wear resistance.
1) Cast iron is popular due to its good wearing properties, excellent machinability & ease of producing complicated shapes by the casting method. It is suitable where large gears of complicated shapes are needed.
2) Steel is sufficiently strong & highly resistant to wear by abrasion.
3) Cast steel is used where stress on gear is high & it is difficult to fabricate the gears.
4) Plain carbon steels find application for industrial gears where high toughness combined with high strength.
5) Alloy steels are used where high tooth strength & low tooth wear are required.
6) Aluminum is used where low inertia of rotating mass is desired.
7) Gears made of non–metallic materials give noiseless operation at high peripheral speeds.
Two bevel gears in mesh is known as bevel gearing. In bevel gearing, the pitch cone angles of the pinion and gear are to be determined from the shaft angle, i.e., the angle between the intersecting shafts. Figure shows two views of a bevel gearing.
The bevel gear has many diverse applications such as locomotives, marine applications, automobiles, printing presses, cooling towers, power plants, steel plants, railway track inspection machines, etc.
For examples, see the following articles on:
Working of Bevel Gear-
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