机电技术专业英语徐起贺张庆良pataalessonTen课件教学.pptVIP

* 机电技术专业英语 Lesson Ten Text Each gear has and serves its own particular application. There are several kinds of gears used in modern machinery. Some of these are spur, internal spur, helical, gear racks, bevel, worm and worm wheel. Spur gear are the most widely used style of gears and are used to transmit rotary motion between parallel shafts, while maintaining uniform speed and torque. The involute tooth form, being the simplest to generate, permits high manufacturing tolerances to be attained. Internal spur gears have the teeth generated on the inside diameter of the blank. They are generally stronger and more efficient than the mating pinion gear. The pitch diameter of the internal gear must be at least as 1.5 times as the pitch diameter of the mating pinion. This condition is held to restrict interference between the tips of the teeth. Internal gears provide the designer with the ability to achieve higher contact and drive ratios than standard spur gears at shorter center distances. They also enable a velocity change without a directional change. This would require an idle gear with standard spurs. Gears Lesson Ten Helical gears are similar to spur gears with the exception that the teeth are cut at an angle to the axis of the shaft—the helix angle. The helix cut creates a wider contact area enabling higher strengths and torques to be achieved. Helical gear sets operate to be quieter and smoother than spur gears, but are slightly less efficient. Helical gears can run on parallel shafts or may be offset as much as the helix angle will permit. Axial thrust loads are developed during operation and must be considered when selecting bearings and mounting arrangements. Gear racks are best described as spur gears of infinite pitch radius. They will translate rotary motion to linear motion and viceversa. Racks will mate only with pinions of the same pitch. Bevel gears are used solely to transmit rotary motion between intersecting shafts. Thoug