Билет 4 riveted joints

A rivet is a round bar consisting of an upset end called the head, and a long part called the shank. The rivet blank is heated to a red glow, inserted into one of the holes, and, while the head is held firmly against the plate by a heavy sledge, the projecting end is formed into a second head, called the point, by means of a hand hammer and a set or by a press.

Button heads are used for small rivets up to 5/16 in. in diameter, which are driven cold; pan heads are used chiefly in ship work; countersunk heads are used only in special cases, chiefly in structural work and below the water line in ships; the countersunk points weaken the plate so much that they should be used only when unavoidable; the others, including button heads, are used in boiler and structural work.

Rivets are made of tough and ductile low-carbon or nickel steel. The rivet materials must have a tensile strength between 45,000 and 55,000 psi.

Brass rivets are used only cold and in small sizes.

Rivet holes are either punched or drilled. Punching injures the metal around the holes, for which reason holes should be punched at least 1/8 in. smaller than the required size, and then reamed. Simultaneous reaming of the hole through the plates to be connected by a rivet straightens the hole and eliminates offsets, which weaken the rivet.

a second head, called the point – вторая головка, называемая замыкающей головкой

for which reason – по причине чего

Билет 5 bearings with sliding contact

A bearing is a machine part which supports a moving part and confines its motion. That part of a shaft which rotates in a bearing is called a journal. Bearings in which one rubbing surface slides over another are called plain bearings and may be divided into two classes: those with a continuous rotary motion and those with an intermittent motion. To the first class belong journal bearings, which carry a load acting at right angles to the shaft axis, and thrust bearings, which take a load acting in the direction of the shaft axis. To the second class belong bearings of parts having a rocking motion, as wrist pins, or a linear reciprocating motion, as cross-heads. The latter can be considered as rocking parts with an infinitely large radius of the bearing surface.

Bearings with a continuous rotary motion form the great majority of all bearings. They are also the only ones in which an oil film pressure adequate to support the journal can be created by the journal itself. Bearings with an intermittent motion must depend for proper operation either upon an outside source for obtaining the necessary oil pressure or upon an abundant oil supply and a low specific bearing pressure.

Билет 6

Bearing Failures. The failure or need of replacement of a bearing with sliding contact may be due to excessive wear of the bearing surfaces, overheating or cracking of the bearing metal.

Wear is caused by metal-to-metal contact. It cannot be entirely eliminated but can be appreciably reduced by providing a sufficient bearing area and by adequate lubrication.

Overheating, if not stopped in time, may result either in seizure of the journal if it runs in a hard-metal bearing, such as a copper-alloy bearing, or in melting the bearing surface, if the bearing is lined with a metal having a low melting temperature. Overheating is primarily caused by metal-to-metal contact. Lubrication decreases the danger of overheating. However, it may start even with proper lubrication, if the heat dissipation of the bearing is not adequate.

Cracking of the bearing metal is due to heavy shock loads, such as are taking place in the running gear of internal combustion engines when the compressive stresses in the bearing metal exceed its endurance limit. The remedies are to lower the specific bearing pressure or to use a bearing metal with a proportionately higher endurance limit. However, even in this case the presence of an oil film is useful as a shock-absorber.