中英文翻译-轴承的摩擦与润滑

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南昌航空大学科技学院学士学位论文

外文文献

Friction Lubrication of Bearing

In many of the problem thus far , the student has been asked to disregard or neglect

friction . Actually , friction is present to some degree whenever two parts are in contact and

move on each other. The term friction refers to the resistance of two or more parts to

movement.

Friction is harmful or valuable depending upon where it occurs. friction is necessary

for fastening devices such as screws and rivets which depend upon friction to hold the

fastener and the parts together. Belt drivers, brakes, and tires are additional applications

where friction is necessary.

The friction of moving parts in a machine is harmful because it reduces the

mechanical advantage of the device. The heat produced by friction is lost energy because

no work takes place. Also , greater power is required to overcome the increased friction.

Heat is destructive in that it causes expansion. Expansion may cause a bearing or sliding

surface to fit tighter. If a great enough pressure builds up because made from low

temperature materials may melt.

There are three types of friction which must be overcome in moving parts: (1)starting,

(2)sliding, and(3)rolling. Starting friction is the friction between two solids that tend to

resist movement. When two parts are at a state of rest, the surface irregularities of both

parts tend to interlock and form a wedging action. To produce motion in these parts, the

wedge-shaped peaks and valleys of the stationary surfaces must be made to slide out and

over each other. The rougher the two surfaces, the greater is starting friction resulting from

their movement .

Since there is usually no fixed pattern between the peaks and valleys of two mating

parts, the irregularities do not interlock once the parts are in motion but slide over each

other. The friction of the two surfaces is known as sliding friction. As shown in

figure ,starting friction is always greater than sliding friction .

Rolling friction occurs when roller devces are subjected to tremendous stress which

cause the parts to change shape or deform. Under these conditions, the material in front of

a roller tends to pile up and forces the object to roll slightly uphill. This changing of shape ,

known as deformation, causes a movement of molecules. As a result ,heat is produced from

the added energy required to keep the parts turning and overcome friction.

The friction caused by the wedging action of surface irregularities can be overcome partly

by the precision machining of the surfaces. However, even these smooth surfaces may

require the use of a substance between them to reduce the friction still more. This

substance is usually a lubricant which provides a fine, thin oil film. The film keeps the

surfaces apart and prevents the cohesive forces of the surfaces from coming in close

contact and producing heat .

Another way to reduce friction is to use different materials for the bearing surfaces

and rotating parts. This explains why bronze bearings, soft alloys, and copper and tin iolite

bearings are used with both soft and hardened steel shaft. The iolite bearing is porous.

Thus, when the bearing is dipped in oil, capillary action carries the oil through the spaces

of the bearing. This type of bearing carries its own lubricant to the points where the

南昌航空大学科技学院学士学位论文

pressures are the greatest.

Moving parts are lubricated to reduce friction, wear, and heat. The most commonly

used lubricants are oils, greases, and graphite compounds. Each lubricant serves a different

purpose. The conditions under which two moving surfaces are to work determine the type

of lubricant to be used and the system selected for distributing the lubricant.

On slow moving parts with a minimum of pressure, an oil groove is usually sufficient

to distribute the required quantity of lubricant to the surfaces moving on each other .

A second common method of lubrication is the splash system in which parts moving in

a reservoir of lubricant pick up sufficient oil which is then distributed to all moving parts

during each cycle. This system is used in the crankcase of lawn-mower engines to lubricate

the crankshaft, connecting rod ,and parts of the piston.

A lubrication system commonly used in industrial plants is the pressure system. In this

system, a pump on a machine carries the lubricant to all of the bearing surfaces at a

constant rate and quantity.

There are numerous other systems of lubrication and a considerable number of

lubricants available for any given set of operating conditions. Modern industry pays greater

attention to the use of the proper lubricants than at previous time because of the increased

speeds, pressures, and operating demands placed on equipment and devices.

Although one of the main purposes of lubrication is reduce friction, any substance-

liquid , solid , or gaseous-capable of controlling friction and wear between sliding surfaces

can be classed as a lubricant.

Varieties of lubrication

Unlubricated sliding. Metals that have been carefully treated to

remove all foreign materials seize and weld to one another when slid

together. In the absence of such a high degree of cleanliness, adsorbed

gases, water vapor ,oxides, and contaminants reduce frictio9n and the

tendency to seize but usually result in severe wear; this is called

“unlubricated ”or dry sliding.

Fluid-film lubrication. Interposing a fluid film that completely

separates the sliding surfaces results in fluid-film lubrication. The

fluid may be introduced intentionally as the oil in the main bearing of

an automobile, or unintentionally, as in the case of water between a

smooth tuber tire and a wet pavement. Although the fluid is usually a

liquid such as oil, water, and a wide range of other materials, it may

also be a gas. The gas most commonly employed is air.

Boundary lubrication. A condition that lies between unlubricated

sliding and fluid-film lubrication is referred to as boundary

lubrication, also defined as that condition of lubrication in which the

friction between surfaces is determined by the properties of the surfaces

and properties of the lubricant other than viscosity. Boundary

lubrication encompasses a significant portion of lubrication phenomena

and commonly occurs during the starting and stopping off machines.

Solid lubrication. Solid such as graphite and molybdenum disulfide

南昌航空大学科技学院学士学位论文

are widely used when normal lubricants do not possess sufficient

resistance to load or temperature extremes. But lubricants need not take

only such familiar forms as fats, powders, and gases; even some metals

commonly serve as sliding surfaces in some sophisticated machines.

Function of lubricants

Although a lubricant primarily controls friction and ordinarily does

perform numerous other functions, which vary with the application and

usually are interrelated .

Friction control. The amount and character of the lubricant made

available to sliding surfaces have a profound effect upon the friction

that is encountered. For example, disregarding such related factors as

heat and wear but considering friction alone between the same surfaces

with on lubricant. Under fluid-film conditions, friction is encountered.

In a great range of viscosities and thus can satisfy a broad spectrum of

functional requirements. Under boundary lubrication conditions , the

effect of viscosity on friction becomes less significant than the

chemical nature of the lubricant.

Wear control. wear occurs on lubricated surfaces by abrasion,

corrosion ,and solid-to-solid contact wear by providing a film that

increases the distance between the sliding surfaces ,thereby lessening

the damage by abrasive contaminants and surface asperities.

Temperature control. Lubricants assist in controlling corrosion of

the surfaces themselves is twofold. When machinery is idle, the lubricant

acts as a preservative. When machinery is in use, the lubricant controls

corrosion by coating lubricated parts with a protective film that may

contain additives to neutralize corrosive materials. The ability of a

lubricant to control corrosion is directly relatly to the thickness of

the lubricant film remaining on the metal surfaces and the chermical

composition of the lubricant.

Other functions

Lubrication are frequently used for purposes other than the reduction

of friction. Some of these applications are described below.

Power transmission. Lubricants are widely employed as hydraulic

fluids in fluid transmission devices.

Insulation. In specialized applications such as transformers and

switchgear , lubricants with high dielectric constants acts as electrical

insulators. For maximum insulating properties, a lubricant must be kept

free of contaminants and water.

Shock dampening. Lubricants act as shock-dampening fluids in energy

transferring devices such as shock absorbers and around machine parts

such as gears that are subjected to high intermittent loads.

Sealing. Lubricating grease frequently performs the special function

of forming a seal to retain lubricants or to exclude contaminants.

The object of lubrication is to reduce friction ,wear , and heating

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南昌航空大学科技学院学士学位论文外文文献FrictionLubricationofBearingInmanyoftheproblemthusfar,thestudenthasbeenaskedtodisregardorneglectfriction.Actually,frictionispresenttosomedegreewhenevertwopartsareincontactandmoveoneachother.Thetermfrictionreferstotheresistanceoftwoormorepartstomovement.Frictionisharmfulorvaluable...

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