Тема 22. Гвинтові скріплення

Мета: удосконалювати навички студентів правильно використовувати нову професійну лексику; розвивати логічне мислення; виховувати культуру мовлення. Навчитися будувати монологічні та діалогічні повідомлення по темі. Формувати граматичні навички по теміSequence of Tenses

Обладнання: малюнки до теми, роздруківки текстів для читання та завдань, словники.

Час проведення: 2 год.

План заняття

І Організаційний момент. Привітання

ІІ. Перевірка домашнього завдання.

ІІІ. Основна частина.

BEARINGS

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 crossheads. Bearings with a continuous rotary motion form the great majority of all bearings.

Bearing Failures.— The failure or need of replacement of a bearing with sliding contact may be due to:

(a) excessive wear of the bearing surfaces;

(b) overheating; or

(c) cracking of the bearing metal.

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

Overheating, if not stopped in time, may cause either seizure of the journal if it runs in a hard-metal bearing or melting of 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.

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.

PRESS, SHRINK, AND FRICTION JOINTS

Definitions.— A press joint, also called a force joint, is obtained by forcing a shaft into a hole that is slightly smaller than the shaft. This is possible because of the elasticity of the materials which produces the grip that holds the hub and shaft together.

A shrink joint differs from a press joint chiefly by the method of assembling it. The hub is heated to expand its bore and to slip it on the shaft. When the hub cools down to the temperature of the shaft, the grip is produced in the same way as in the force joint. The shrink joint is also used to connect machine parts by means of special rings, anchors, and tie rods.

In a friction joint the holding grip is produced by the conical shape of the shaft end and the hub bore and by the pull of a nut, or by a slotted hub whose bore is smaller than the shaft and which is spread by a wedge when the joint is being assembled.

Comparison of Joints.— The assembling of a shaft and a hub by means of a press joint is simpler than that with a shrink joint especially if a hydraulic press of a sufficient capacity is available. Shrink joints are used mainly in places where it is difficult or impossible to assemble a press joint, as in the case of rings or anchors. On the other hand, shrink joints assemblies with the same interference as press joints give more than three times the holding power against both torsion and axial pull. This superior effectiveness is due to the absence of abrasion between the surfaces of the shaft and the hub during assembly.

All three joints are used when machine parts must be connected more securely than can be accomplished with a key or screw joint, especially when they are subjected to shock or vibration.

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