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Task 4. Read and translate the text.






Task 1. What is the role of gears in modern world? What types of gear can you name?

Task 2. Label the gear components on the diagram below. Use the words in the box.

  idler, spur gear, output shaft, input shaft, free-wheel disk

 

Task 3. Watch the video and try to explain the main principle of a system of gears. Discuss the process with your groupmates.

Task 4. Read and translate the text.

Gear and Gearing (part 1)

External spur gears are cylindrical gears with straight teeth cut parallel to the axes. Gears transmit drive between parallel shafts. Tooth loads produce no axial thrust. Excellent at moderate speeds but tend to be noisy at high speeds. Shafts rotate in opposite directions.

Internal spur gears provide compact drive arrangements for transmitting motion between parallel shafts rotating in the same direction.

Helical gears are cylindrical gears with teeth cut at an angle to the axes. Provides drive between shafts rotating in opposite directions, with superior load carrying capacity and quietness than spur gears. Tooth loads produce axial thrust. Crossed helical gears are helical gears that mesh together on non-parallel axes. Straight bevel gears have teeth that are radial toward the apex and are of conical form. Designed to operate on intersecting axes, bevel gears are used to connect two shafts on intersecting axes. The angle between the shafts equals the angle between the two axes of the meshing teeth. End thrust developed under load tends to separate the gears.

Spiral bevel gears have curved oblique teeth that contact each other smoothly and gradu­ally from one end of a tooth to the other. Meshing is similar to that of straight bevel gears but is smoother and quieter in use. Left hand spiral teeth incline away from the axis in an anti-clockwise direction looking on small end of pinion or face of the gear; right-hand teeth incline away from axis in clockwise direction. The hand of spiral of the pinion is always opposite to that of the gear and is used to identify the hand of the gear pair and to connect two shafts on intersecting axes as with straight bevel gears. The spiral angle does not affect the smoothness and quietness of operation or the efficiency but does affect the direction of the thrust loads created. A left-hand spiral pinion driving clockwise when viewed from the large end of the pinion creates an axial thrust that tends to move the pinion out of mesh.

Zero bevel gears have curved teeth lying in the same general direction as straight bevel teeth but should be considered to be spiral bevel gears with zero spiral angles.

Hypoid bevel gears are a cross between spiral bevel gears and worm gears. The axes of hypoid bevel gears are non-intersecting and non-parallel. The distance between the axes is called the offset. The offset permits higher ratios of reduction than is practicable with other bevel gears. Hypoid bevel gears have curved oblique teeth on which contact begins gradu­ally and continues smoothly from one end of the tooth to the other.

Worm gears are used to transmit motion between shafts at right angles, that do not lie in a common plane and sometimes to connect shafts at other angles. Worm gears have line tooth contact and are used for power transmission, but the higher the ratio the lower the efficiency.

If a gear tooth of the involute curvature acts against the involute tooth of a mating gear while rotating at a uniform rate, the angular motion of the driven gear will also be uniform, even though the center-to-center distance is varied.

The relative rate of motion between driving and driven gears having involute tooth curves is established by the diameters of their base circles.

Contact between intermeshing involute teeth on a driving and driven gear is along a straight line that is tangent to the two base circles of these gears. This is the line of action.

The point where the line of action intersects the common center-line of the mating involute gears, establishes the radii of the pitch circles of these gears; hence true pitch cir­cle diameters are affected by a change in the center distance. (Pitch diameters obtained by dividing the number of teeth by the diametric pitch apply when the center distance equals the total number of teeth on both gears divided by twice the diametric pitch.)

The pitch diameters of mating involute gears are directly proportional to the diameters of their respective base circles; thus, if the base circles of one mating gear is three times as large as the other, the pitch circle diameters will be in the same ratio.

The angle between the line of action and a line perpendicular to the common center-line of the mating gears, is the pressure angle; hence the pressure angle is affected by any change in the center distance.

When an involute curve acts against a straight line (as in the case of an involute pinion acting against straight-sided rack teeth), the straight line is tangent to the involute and per­pendicular to its line of action.

The pressure angle, in the case of the involute pinion acting against straight-sided rack teeth, is the angle between the line of action and the line of the rack's motion. If the involute pinion rotates at a uniform rate, movement of the rack will also be uniform.

 

Task 5. The definitions of the terms are given. Guess these terms.

1. The surface of the gear between the flanks of adjacent teeth. a)
2. The distance on the base circle in the plane of rotation between invo­lutes of the same pitch. b)
3. The angle at the base cylinder of an involute gear that the tooth makes with the gear axis. c)
4. The circle from which the involute tooth curve is generated or developed. d)
5. The shortest distance between the non-driving surfaces of adjacent teeth when the working flanks are in contact. e)
6. The distance parallel to the axis between two pitch line elements of the same tooth. f)
7. The dimension of the tooth face width that makes contact with a mating gear. g)





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