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For transmitting rotary motion and torque around corners, bevel gears are commonly used. The connected shafts, whose axes would intersect if extended, are usually but not necessarily at right angles to one another.
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When adapted for shafts that do not intersect, spiral bevel gears are called hypoid gears. The pitch surfaces of these gears are not rolling cones, and the ratio of their mean diameters is not equal to the speed ratio. Consequently, the pinion may have few teeth and be made as large as necessary to carry the load.
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The profiles of the teeth on bevel gears are not involutes, they are of such a shape that the tools .for cutting the teeth are easier to make and maintain than involute cutting tools. Since bevel gears come in pairs, as long as they are conjugate to one another they need not be conjugate to other gears with different tooth numbers.
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Fig. 2 Worm gear set
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Gear trains The maximum gear ratio obtainable with a single of gears varies with the type of gear and the application. The following are approximate maximum for the various types for average load conditions: spur 8, parallel-shaft helical, 10; straight bevel, 6; spiral bevel, 8; hypoid, 12; and worm, 80. For lightly loaded, instrument, and positioning gears, these ratios can be exceeded. Ratios as high as 400 or higher can be obtained with gears that resemble tapered worms meshing with hypoid gears. For heavily loaded gears, the given ratios may be so high that a reasonable gear size precludes a satisfactory pinion.
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Since ratio in a single pair of gears is the quotient of the tooth numbers, and since there usually are limitations on both the minimum and maximum numbers of teeth on the available gears) it follows that the number of ratios obtainable in a single pair is limited. To enlarge the coverage it is necessary to use multiple pairs, or trains .The overall Speed ratio in a train is the product of the ratios in each pair. In certain cases an exact ratio cannot be obtained with gears, but by using two or more pairs, the desired ratio can be approximated to any degree of precision.
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