求英文高手帮我翻译一个文章（汉译英,机械类）

求英文高手帮我翻译一个文章（汉译英,机械类）Mechanics ComponentsCharles W. BeardsleyGearsSpur and helical gears. A gear having tooth elements that are straight and parallelto its 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 gearcan be mated with a gear of a different type.To prevent jamming as a result of thermal expansion, to aid lubrication, and tocompensate 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, backlash can be eliminated by using a gear split downits middle, one half being rot table relative to the other A spring forces the split gear teeth to occupy the full width of the pinion space.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 numbersand cut with the same cutter. Because of the overlapping action of the teeth, they aresmoother in action and can operate at higher pitch-line velocities than spur gears. Thepitch-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 beabsorbed in the shaft bearings. The thrust problem can be overcome by cutting two setsof 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 gearwith a space between the two halves to permit the cutting tool to run out. Double-helicalgears are well suited for the efficient transmission of power at high speeds.Helical gears can also be used to connect nonparallel, non-intersecting shafts at anyangle to one another. Ninety degrees is the commonest angle at which such gears areused.Worm and bevel gears. In order to achieve line contact and improve theload-carrying capacity of the crossed axis helical gears, the gear can be made to curvepartially around the pinion, in somewhat the same way that a nut envelops a screw. Theresult would be a cylindrical worm and gear.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 55 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.