12. Discuss the following problems using the words below.

1. Compare the advantages and disadvantages of different fastenings applied in joints. 2. Describe the application of fastenings in mechanisms.

1) depend on, distinguish, tighten, choose, restrict, select, differ, take, protect, damage, presence, purpose, form, rotation, load, bolt, nut, head, joint, design.

13. Word the mathematical symbols.

1) y →y tends to infinity or y is tending to infinity. 2) h(z) = 4 the value of the function h in the_:point z is equal to four. 3) sin y = h the sine of y is equal to h. 4) 1/k one over k or one divided by k. 5) b(√2) =

2 b of the square root of two equals (is equal to) two, or the value of the square root of two equals (is equal to) two. 6) z(√2)>z(l½) the value, of the function z of the square root of two is greater than the value of this function of one and one half or z of the square root of two is greater than its value in the point one and one half.

Text 2

1. Read the text to describe the key notions.

Threads

Threads are applied for interconnection of machine parts and for transmitting motion from one part to another. When a thread is cut on the outside of 3 part it is known as an external or "male" thre­ad. A thread is called an internal or "female" thread when iti s cut inside of apart. Depending on the shape of the threading tool different profiles of thread are obtai­ned, such as triangular, square or trape­zoidal.

a b

Fig. 13. a. triangular thread, b. trapezoidal thread.

In practice triangular threads are most widely used. The main

elements of a thread are: the angle of the thread, tie major, minor

and pitch diameters, depth and the pitch.

The angle of the thread is the angle included between the

sides of the thread and measured in an axial plane. The ma­jor or outside diameter of a thread (some­times referred to as "full diameter") is the distance between the two extreme outside points of the thread in the direc­tion square to the axis.The major diameter is the largest dia­meter of the thread of a screw or a nut. The minor diameter, being the smal­lest diameter of the thread, is the distance between the two extreme inside points of the thread measured at the right angle to the axis. The minor diameter is also called the "core diameter" or "root diameter".

The pitch diameter is the distance between the two opposite parallel sides of he thread profile perpendicular to the thread axis. The depth of the thread is he distance between the crest and the base of the thread measured normal to the axis.

Classification of Threads. Classification into cylindrical and ta­per threads depends on the basic surface on which the thread is formed.

Threads can be cut into the surface or hole of a cylindrical bar or cone, and on the surface, the former being an interal thread and the latter  an external thread.

When the threading cylinder is turned clockwise to engage the second

member, the thread is said to be right-hand and when counterclockwise—left-hand. The former of the contour that deter­mines the thread profile in the plane passing through its axis gives us vee, trapezoidal, buttress, square and round threads. The number of starts classify threads as single and multiple (double, triple, etc.)

By the purpose, the threads are subdi­vided into fastening and lead. For faste­ning use is made of vee threads. They are distinguished by a higher strength of turns and can ensure a more stable

tightening than other types of threads due to a higher coefficient of friction. The principal parameters of a cylindrical thread are: major diameter, minor diameter, base thread depth, thread pitch, effective thread angle, helix angle.