Exercises. I. Use the following wards and phrases in sentences of your own:

I. Use the following wards and phrases in sentences of your own:

round thread, internal thread, triple thread, profiles of thread, triangular thread, square thread, buttress thread, pitch of thread, right-hand thread, screw thread, single thread, left-hand thread

II. Retell the text giving answers to the following questions:

1. What are threads used for? 2. What types of threads do you know? 3.What are the main elements of a thread? 4. What types of screw threads do you know? 5. What are the most widely used systems of triangular threads in machine-building?

III. Find in the text synonyms of the following words and word combinations:

square thread, trapezoidal thread, major diameter, minor diameter, root diameter

IV. Analyse and translate the following sentences:

1. The metal cannot be compressed, but is cold forged into a different form, and what takes place when the thread shaped dies are forced into the metal to form a thread is displacement of the surface which is forced into grooves, that is, the material from the depressions forms the elevations on either side. 2. The V-shaped thread must not be made sharp because the sharp crest of the V-thread is very easily broken, and the taps and dies will not stand long due to the sharp thread. 3. The finished thread is of greater diameter than the original diameter of the wire, which in turn is larger than the diameter of the finished thread at the root. 4. A screw or bolt of a given size has a greater minor diameter and a greater strength if the pitch is fine rather than coarse. 5. The finished product is therefore larger in diameter than the blank by an amount approximately equal to the depth of the thread. 6. This process of rolling threads, grooves and screw blanks is used in the manufacture of many articles in preference to cutting the threads or grooves with ordinary slow-cutting tools or threading dies.

V. Make up as many questions as possible concerning the following sentences:

1. During the operation the metal is cold forged into a different form. 2. The finished thread is of greater diameter than the original diameter of the wire. 3. A crest of a thread is the top surface joining the two sides of the thread. 4. A base of a thread is the bottom surface joining the two adjacent threads. 5. In screw fastenings threads are made of triangular type.

VI. (a) Read and translate the following text using a dictionary:

The process of rolling screw threads has been greatly developed and its usefulness extended by the rather recent introduction into common use of a material suitable for roll threading. Iron of ordinary quality does not lend itself well to this process on account of its fibrous structure, which makes this material liable to split or fracture under pressure, but in the modern low-carbon steel we find a material in

every way suited to the rapid and economical forming of screw threads. By this process the blank is rolled between two flat dies with their working surfaces grooved to the shape of the thread required, these grooves being cut across the face of the dies at the proper angle to suit the pitch and diameter of the screw.

(b) Retell the text.

VII. On the basis of Fig. 38 describe what profiles of thread are obtained depending on the shape of the threading tool.

VIII. Giving answers to the following questions describe the main elements of a thread as shown in Fig. 34:

l. What are the main elements of a thread? 2. What is the angle included between the sides of the thread called? 3. What is the distance between the two extreme outside points of the thread in the direction square to the axis called? 4. What is the largest diameter of a thread called? 5. What is the minor diameter of a thread? 6. What is called the "pitch diameter" of a thread? 7. What is the distance from a point of thread to the corresponding point on the next thread measured to, the axis called? 8. What is the depth of a thread?

IX. Describe the shapes of threads in screw fastenings on the basis of Fig, 35.

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".

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