Exercises. A gear is a toothed wheel used to transmit rotary motion from one shaft to another

GEARS

A gear is a toothed wheel used to transmit rotary motion from one shaft to another. If power is transmitted between the two shafts, the angular velocity ratio of these two shafts is constant and the driving shaft and the driven shaft rotate at a uniform rate. Shafts may be parallel, intersecting, and non-coplanar. Types of gears may be diverse depending upon the above positions of the shafts. Gears may be classified according to their shape and according to the position which the teeth occupy respectively to the axis of rotation. The teeth cut on the face of л gear may be curved, straight or helical.

The main types of gears are: bevel gears, eccentric gears, helical or spiral gears, herringbone gears, screw gears, spur gears and worm gears.

Fig. 36. Bevel Gearing

Bevel gearing (Fig. 36) is used to transmit power between two shafts, which liein a common plane and whose axes intersect each other. The axes may be inclined

to each other at any angle, although 90' is the most common one. The teeth of bevel gears may be either straight or spiral. In the straight bevel gears the elements of teeth converge to a common point called the "apex".

Eccentric gears operating on parallel shafts are used to transmit a varying angular velocity either continuously or for a portion of revolution.

Fig. 37. Gearing: a - spiral bevel gear, b - hypoid gear

Helical or spiral gears (Fig.37) operate on parallel shafts at high speeds, providing maximum strength of gear teeth for a given width of face. Such gears are heat-treated and then ground to accurate shape and size, necessary for smooth and quiet running at high speeds. The teeth of helical gears, having been cut on a conical surface, curve continually toward or away from the apex of the cone upon which they are cut. These gears closely resemble • bevel gears and are frequently called spiral bevel gears.

Similarly to helical gears, herringbone gears also operate on parallel shafts. Herringbone gears have helical teeth radiating from the center of the face towards the sides of the gear body. They are used where high speeds and high gear ratios are necessary.

Screw gearing is used for converting some rotary motion into a forward motion, and for connecting shafts which are not intersecting. Spur gears are gears having straight or helical teeth cut on a cylindrical surface at an angle to the shaft axis.

Fig. 40. Spur Gearing: a - parallel shafts

Fig. 39. Spur Gear Wheels with External Gearing

Spur gearing (Fig. 38) is used to transmit power between two shafts, the axes of which are parallel. Spur gearing may be divided into three types such as: external gearing (Fig. 39), internal gearing (Fig. 40) and rack-and-pinion gearing (Fig. 41). Rack-and-pinion gearing serves for converting rotary motion into forward motion and is widely used in lathes. It consists of a rack-and-pinion.

Fig. 40. Spur Gear Wheels with Internal Gearing

Fig. 41. Rack-and-Pinion Gearing

Fig. 42. Worm Gearing

A worm gear (Fig. 42) is a gear having the teeth cut at an angle to the axis of rotation of the gear body and radially in the gear face. A worm gear is driven by a worm which resembles a large screw. Worm gearing is applied for transmitting power between non-intersecting shafts which are at right angles to each other.

In practice friction gearing and toothed gearing are most widely used for transmitting power from one shaft to another and for connecting the shafts. Friction gears are used for light and medium powers in machinery which is frequently started and stopped. Their advantages are flexibility and noiselessness. The disadvantages of friction gears are the thrust on the bearings and slippage. Toothed gears are used when a constant speed is desirable and the distance between the shafts is rather small. Transmission of rotary motion is performed by means of shafts and gears or gear trains mounted on them with the help of inserted keys.

Shafts may be of different length and diameters. When rotating, the shafts transmit both the rotation and the torque. Gears replace belt-and-pulley drives where positive motion is required. Gear teeth for all types mentioned above are made in mass production by the generating process on specially designed machines. In this process, the cutter used for cutting teeth has the form of a tooth of the mating gear. One of the most important gear-cutting processes is that of hobbing. In this process, the cutter used for hobbing gear teeth is made like a worm with gashes parallel to the axis to provide cutting edges on the worm. Such a cutter is called the "hob".

I. Use the following words and word combinations in sentences of your own:

a toothed gear, gearing, to hob, to run, to generate, rotary motion, intersecting shafts, spur gearing, bevel gearing, worm gearing, driven shaft, angular velocity, positive motion, inserted key

II. Answer the following questions:

1. What is a gear and what is it used for? 2. Where do gears replace belt-and-pulley drives? 3. What types of shafts do you know? 4. V/hat do types of gears depend on? 5. According to what features may gears be classified? 6. What kinds of teeth cut on the gear face do you know? 7. What main types of gears can you enumerate? 8. By what processes are gear teeth produced?

III. Choose synonymical groups out of the following list:

velocity, to transmit, to intersect, smooth, to converge, to transfer, speed, to cross, quiet, to approach

IV. Underline the suffixes and prefixes and translate into Russian the following groups of words:

controllable, controller, incontrollable; caller, calling, recall; section, sectional; intersect, intersection, non-intersecting

V. Translate the following sentences observing different meanings of the words in italics:

1. When the rain has stopped I noticed a worm on the lawn of our garden. 2. Worm gearing is applied to transmit power between shafts placed at right angles to each other. 3. The Great October Revolution broke out in 1917 and liberated the working class in our country from the yoke of capitalism. 4.Eccentric gears are applied for transmitting a varying angular velocity for a portion of revolution. 5. I locked the door of my flat and left the key on the shelf. 6. Gears are mounted on shafts by means of keys.

VI. Find in the text English equivalents of:

Передача с внешним зацеплением; передача с внутренним зацеплением; винтовая передача; закладная шпонка; прямой зуб; кривой зуб, винтовой зуб, косой зуб, реечная передача, фрикционная передача, цилиндрическое зубчатое колесо, коническое зубчатое колесо, шестерня с косыми или спиральными зубьями, шестерня с шевронными зубьями, вращательное движение, давление, зубчатая передача, червячная передача

VII. Make up questions to which the italicized words are the answers:

I. Modem requirements for gears demand that they run quietly at high speeds. 2. Gear-cutting machines have been developed for the rapid mass production of similar parts. 3. The gear blank is pressed on an arbor and held between centers of the dividing head and the index tailstock. 4. Bevel gearing is used to transmit power between two shafts whose axes intersect each other. 5. Spur gearing is used to transmit power between parallel shafts.

VIII. Using the following words and word combinations describe the bevel gears shown in Figs 36 and 37 and explain the difference between their teeth:

bevel gears, to be used, to transmit power, two shafts, axes, to intersect each other, the teeth, to be straight or spiral, Fig. 36, one can see, the straight bevel gears, to converge, a common point, to be called the apex, Fig. 37, a spiral bevel gear, to have helical or spiral teeth, to have been cut, a conical surface, to curve toward or away, the apex, helical or spiral gears, to be applied, operation, parallel shafts, high speeds

IX. Giving answers to the following questions describe the difference between gearings shown in Figs 39 and 40:

1. What types of gearings do you see in Figs 39 and 40? 2. What type of gearing do external and internal gearings belong to? 3. Are the teeth in Figs 39 and 40 straight or helical? 4. How must two shafts be placed when external and internal gearings are used for transmitting power between them? 5. What engagement of two gears does an external gearing provide? 6. What engagement of two gears does an internal gearing provide?

X. Look at Figs 38, 41 and 42 and describe in detail the difference between the given gearings.

Дата добавления: 2014-12-27