Lesson 17

I. Read and memorize the following words and word-combinations:

II. Read and translate the following text:

The Four-stroke Cycle Engine

The starting motor is a small electric motor. When it is actuated, a driving pinion is shifted and meshes with the toothed rim of the flywheel. Thus the crank-drive as­sembly begins cranking (working).

The highest position of the piston in the cylinder is called "top dead centre," the lowest "bottom dead centre". The path covered between these two positions is the "pi­ston stroke," the place cleared in this travel is called "piston displacement." The motion of the piston from dead centre to dead centre is called "stroke." The "combustion space" is the space between the piston in top dead centre and the cylinder head. In the combustion space, the fuel-air mixture is compressed by the upward movement of the piston and ignited by the electric spark.

Let us analyze the working method of a four-stroke cycle carburettor engine, as an example.

With all the carburettor engines, the fuel-air mixture is sucked into the cylinder by the downward movement of the piston. The fuel-air mixture is broken up and par­tially vaporized in the carburettor; then, it is ignited by an incorporated ignition-supplying device, the spark plug.

In case of the four-stroke cycle engine, the path co­vered between top dead centre and bottom dead centre and vice versa is a stroke. Each stroke is accompanied by one-half of a crankshaft revolution. The complete cycle requires the following four strokes:

Suction stroke: the piston moving downward takes in fuel-air mixtu­re through the open in­take valve, while the exhaust valve is closed. Immediately after the piston has reached the bottom dead centre, the intake valve is clo­sed. The first stroke has been finished. The crankshaft has turned one-half of a revolution.

Piston поршень 1 — piston head головка поршня; 2 - piston rings поршневі кільця; 3 — piston skirt юбка поршня; 4 — wrist pin boss бо­бишка поршневого пальця Four-stroke engine cycle робочий цикл четирьохтактного двигуна 1 — intake stroke всмоктування; 2 — compression stroke стискування; З — power stroke робочий хід; 4 — exhaust stroke вихлоп; 5 — spark plug свічка запалювання; 6 — cylinder циліндр; 7 — poppet-valve тарільчатий клапан; о — exhaust port вихлопний отвір; 9 — piston поршень; 10 — con-netting rod шатун; 11 — flywheel маховик; 12 — crankshaft колінчатий вал; 13 — Inlet port всмоктуючий отвір

Lesson 18

I. Read and memorize the following words and word-combinations:

II. Read and translate the following text: Screw fastenings

Machine parts are held together by parts: working in tension, working in shear, creating friction, and using both shear and friction forces.

Types of Fastenings.—All fastenings can be divided into two classes — disconnectable fastenings and permanent joints.

Disconnectable fastenings, in turn, are effected by: bolts and screws, wedges, dowel pins, keys.

Permanent joints are obtained by means of: (a) press fits, (b) shrink fits, (c) rivets, (d) welding, brazing, and soldering and (e) casting.

Forms of Threads.— Screw fastenings are used for holding two or more machine parts together or for adjusting one part with relation to another. In screw fastenings the threads are made in several forms but are always of triangular-type single thread.

Screw threads are made right-hand and left-hand.

Pins

Geometrically pins can be divided into cylindrical pins, called straight pins, and conical or taper pins. Dyna­mically pins can be classified as those used only to locate the relative position of two parts when there is little or no force acting upon the pin, and those that fasten two or more parts together and are subjected to considerable stresses, which are mostly in shear but sometimes in bending. Locating pins are called dowel pins or simply dowels. A connecting pin, like a dowel, may be used either as a permanent connection or as a fulcrum for a movable joint.

Keys

Types of Keys.— The main function of a key is to transmit torque between a shaft and a machine part assembled on it. In most cases keys prevent relative motion, both rotary and axial. In some constructions keys allow an axial motion bet­ween the shaft and the hub, such keys are called feather or spline keys. In spite of the tendency to standardize there are many key types in use by various manufacturers.

According to various characteristics keys can be distinguish­ed as straight and tapered; rectangular, dovetailed, chamfered round, and disk-shaped; radial and tangential; and (according to their use) for light duty and for heavy duty.