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Gears

editor：gdmachinepopularity：3765pubtime：2017-10-12 14:34:55

Spur and helical gears. A gear having tooth elements that are straight and parallel to this axis is known as a spur gear. A spur pair can be used to connect parallel shafts only. Parallel shafts, however, can also be connected by gears of another type, and a spur gear can be mated with a gear of a different type (Fig. 1). www.mapeng.net 马棚网机械资讯
To prevent jamming as a result of thermal expansion, to aid lubrication, and to compensate for unavoidable inaccuracies in manufacture, all power-transmitting gears must have backlash. This means that on the pitch circles of a mating pair, the space width on the pinion must be slightly greater than the tooth thickness on the gear ,and vice versa. On instrument gears, backiash can be eliminated by using a gear split down its middle, one half being rotatable relative to the other. A spring forces the split gear teeth to occupy the full width of the pinion space. www.mapeng.net 马棚网机械资讯
Helical gears have certain advantages; for example, when connecting parallel shafts they have a higher load carrying capacity than spur gears with the same tooth numbers and cut with the same cutter. Because of the overlapping action of the teeth, they are smoother in action and can operate at higher pitch-line velocities than spur gears. The pitch-line velocity is the velocity of the pitch circle. Since the teeth are inclined to the axis of rotation, helical gears create an axial thrust. If used singly, this thrust must be absorbed in the shaft bearings. The thrust problem can be overcome by cutting two sets of opposed helical teeth on the same blank. Depending on the method of manufacture, the gear may be of the continuous tooth herringbone variety or a double-helical gear with a space between the two halves to permit the cutting tool to run out. Double-helical gears are well suited for the efficient transmission of power at high speeds. www.mapeng.net 马棚网机械资讯
Helical gears can also be used to connect nonparallel, non-intersecting shafts t any angle to one another. Ninety degrees is the commonest angle at which such gears are used. www.mapeng.net 马棚网机械资讯

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Fig I Spur gears www.mapeng.net 马棚网机械资讯
Worm and bevel gears. In order to achieve line contact and improve the load- carrying capacity of the crossed axis helical gears, the gear can be made to curve partially around the pinion, in somewhat the same way that a nut envelops a screw. The result would be a cylindrical worm and gear. www.mapeng.net 马棚网机械资讯
Worm gears provide the simplest means of obtaining large ratios in a single pair. They are usually less efficient than parallel-shaft gears, however, because of an additional sliding movement along the teeth. Because of their similarity, the efficiency of a worm and gear depends on the same factors as the efficiency of a screw. Single-thread worms of large diameter have small lead angles and low efficiencies. Multiple-thread worms have larger lead angles and higher efficiencies (Fig. 2). www.mapeng.net 马棚网机械资讯
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. www.mapeng.net 马棚网机械资讯
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. www.mapeng.net 马棚网机械资讯
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. www.mapeng.net 马棚网机械资讯

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Fig. 2 Worm gear set www.mapeng.net 马棚网机械资讯
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. www.mapeng.net 马棚网机械资讯
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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