Straight Bevel Gears

Bevel Gears are gears where the axes of the two member shafts intersect and the shape of the tooth-bearing surfaces are conical. Bevel gear sets are usually mounted 90° apart however; other angles are used as well. The pitch surface of a bevel gear is cone-shaped.

The two most important characteristics of a gear are pitch surface and pitch angle. The pitch surface of a gear is the imaginary (toothless) surface that would be created if you average the top lands (peaks) and bottom lands (valleys) of the gear in cross-section. The pitch surface of an ordinary gear – like the spur gear found on a bicycle – is cylindrical. The pitch angle of a gear is the angle between the face of the pitch surface and the axis of the gear. The pitch angle of an ordinary spur gear is 0°, whereas the pitch angle of a bevel gear is usually between 0° and 90°.

The most familiar kinds of bevel gears have pitch angles that are cone-shaped and 90° or less. This kind of bevel gear is called external because the teeth point outward. The pitch surfaces of meshed external bevel gears are coaxial with the gear shafts; the apexes of the two surfaces are always at the point of intersection of the shaft axes. Bevel gears that have pitch angles of greater than 90° are called internal bevel gears; their teeth point inward. Bevel gears that have pitch angles of exactly 90° have teeth that point parallel with the axis. This type of bevel gear is called a “crown” gear because the gear configuration resembles the points on a crown. Miter gears are mating bevel gears with equal numbers of teeth and axes at right angles. Skew bevel gears use a crown gear where the teeth are straight and oblique.

Spiral Bevel Gears

A spiral bevel gear is a bevel gear with helical teeth formed along spiral lines on the gear face. The main advantage of spiral gears over straight gears is the smoothness of operation as the teeth engage more gradually. The contact between the teeth starts at one of the tooth and spreads across the entire tooth. The result is a smoother transfer of force when each pair of teeth comes into contact. Spiral bevel gears should be replaced in pairs or run against reference master spiral gears. Spiral bevel gears are most often used in vehicle differentials – automotive, marine and aerospace – where the helical design produces less vibration and noise then straight bevel gears.

Zerol® Bevel Gears

Zerol® bevel gears are an intermediate type of gear between straight and bevel gears. Their teeth are curved like a spiral gear, but they are not angled. They have the same orientation on the gear face as straight bevel gears.

Hypoid Gears

Hypoid gears are spiral bevel gears where the axes of the two gears do not intersect. The shape of a hypoid gear is a hyperboloid of revolution, i.e. the pitch surface of a hypoid gear is a hyperbolic surface, as opposed to a spiral bevel gear which is normally conical. The hypoid gear configuration places the pinion gear off-axis relative to the ring gear, which allows the pinion gear to be larger in diameter with more contact area. In hypoid gear design, the pinion and ring gears are always of opposite hands, and the spiral angle of the pinion gear is usually larger than that of the ring gear. The hypoid pinion gear can be designed with fewer teeth then a corresponding spiral bevel gear, resulting in gear rations of 60:1 and higher. Hypoid gears can be considered halfway between a spiral bevel gear and a worm gear.

Teeth

Gear teeth have two main features: 1) The cross-sectional profile of the individual tooth and; 2) The line or curve of the tooth on the face of the gear, i.e. the line or curve on the gear face along which the cross-sectional tooth profile is projected to form the three-dimensional shape of the tooth. The primary effect of these two features is the operating smoothness of the gears, with some configurations resulting in a smoother gear action than others. The teeth on bevel gears can be straight, spiral or Zerol®.

Tooth Lines

Straight bevel gears have teeth that are straight and parallel to the generators of the cone. This is the simplest form of a bevel gear. It resembles a spur gear, only conical rather than cylindrical. In straight bevel gears, each tooth impacts upon engaging its corresponding tooth, this can be solved by curving the gear teeth.

Spiral bevel gears have teeth formed along spiral lines on the gear face. The main advantage of the spiral tooth over the straight tooth is that they engage more gradually and thus more smoothly. The contact between the teeth starts at one end of the tooth and spreads across the entire tooth. The result is a less abrupt transfer of force when a new pair of teeth comes into contact.

Zerol® bevel gears are an intermediate type of gear between straight and bevel gears. Their teeth are curved like a spiral gear, but they are not angled.


Straight Bevel Gears, Spiral Bevel Gears, Zerol® Bevel Gears, Hypoid Gears, and Face Clutches

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