Bevel gears are used to connect shafts which intersect usually but not necessarily at 90 degrees. The teeth on a bevel gear are subjected to much the same action as spur gear teeth. Bevel gears are not interchangeable and in consequence are designed in pairs (except in the case of mitre bevel gears). Definitions
Shaft angle.- The angle between the intersecting shaft axes.
Pitch angle.- The angle between the gear axis and the pitch cone generator.
Pitch diameter.- The diameter of the circle which forms the base of the pitch cone.
Cone distance.- The distance between the apex of the cone and the pitch circle.
Back cone.- The complementary cone generated by a line at right angles to the pitch cone generator at the pitch diameter and intersecting the axis.
Outside diameter.- The diameter across the tips of the teeth.
Addendum.- The height from the pitch circle to the tip of the tooth measured along the back cone generator.
Dedendum.-The depth of the tooth space below the pitch circle measured along the back cone generator.
Face width.- Length of the teeth measured along the pitch cone generator; the British Standards Institution recommend that the face width should not normally exceed either one third of the cone distance or three times the pitch.
1:1 / 1:1.5 Ratio Bevel Gear Data
These gears are to straight bevel gears as helical gears are to straight tooth spur gears; they have the same advantages as helical gears - smoother tooth engagement, quietness of operation, greater strength and higher permissible speeds. Spiral bevel gears are more expensive than straight bevel gears and are not normally used where straight bevel gears are adequate.
In appearance hypoid gears are similar to spiral bevel gears, but their pitch surfaces are not the same. Instead of a pitch cone the pitch surface is a hyperboloid. When two hyperboloidal pitch surfaces are in contact along a common element, their axes do not intersect, which is the way in which hypoid gears differ from spiral bevel gears. Since the pinion shaft is offset in relation to the gear shaft, hypoid gears are often used in the automobile industry in the rear axle drive of a car, the advantage being that their use makes it possible to lower the centre of gravity of the car. Because of the difference between spiral angles, hypoid gears are larger than spiral bevel gears of the same capacity, which makes it possible to use a larger shaft on the hypoid pinion than on a spiral bevel pinion. For this reason it is sometimes an advantage to use hypoid gears even though it is not necessary to offset the shafts for any other reason.
These are a pair of bevel gears of the same size mounted on shafts which intersect at right angles. The gear ratio is of course 1:1.
Mechanical Engineering Design Topics Page - Gearing Selection Page
Pages created by David Price - 9/08/95
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