灰度空间英文资料翻译(原文)Friction (1)

2021年1月19日发(作者：曲端)
Friction , Lubrication of

Bearing and How to
Extend Bearing Life
Abstract
：
This article is about the

types, harm of bearing friction and the modes
of friction reduction. Among all the modes, perhaps the lubrication is one of the most
effectively

mode, it also has many kinds of forms, functions and types.
Nature works
hard to destroy bearings, but their chances of survival can be improved by following a few
simple guidelines. Extreme neglect in a bearing leads to overheating and possibly seizure
or, at worst, an explosion. But even a failed bearing leaves clues as to what went wrong.
After a little detective work, action can be taken to avoid a repeat performance.
Keywords: bearings


friction

lubrication

failures


life

Part 1: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 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 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 of machine pars
which move relative to each other. A lubricant is any substance which, when inserted
between the moving surfaces, accomplishes these purposes. Most lubricants are
liquids(such as mineral oil, silicone fluids, and water),but they may be solid for use in
dry bearings, greases for use in rolling element bearing, or gases(such as air) for use in
gas bearings. The physical and chemical interaction between the lubricant and
lubricating surfaces must be understood in order to provide the machine elements with
satisfactory life.
The understanding of boundary lubrication is normally attributed to hardy and
doubleday , who found the extrememly thin films adhering to surfaces were often
sufficient to assist relative sliding. They concluded that under such circumstances the
chemical composition of fluid is impor
tant, and they introduced the term “boundary
lubrication”. Boundary lubrication is at the opposite end of the spectrum from
hydrodynamic lubrication.
Five distinct of forms of lubrication that may be defined :
(a)
hydrodynamic; (b)hydrostatic;(c)elastohydrodynamic (d)boundary; (e)solid film.
Hydrodynamic lubrication means that the load-carrying surfaces of the bearing are
separated by a relatively thick film of lubricant, so as to prevent metal contact, and
that the stability thus obtained can be explained by the laws of the lubricant under
pressure ,though it may be; but it does require the existence of an adequate supply at
all times. The film pressure is created by the moving surfaces itself pulling the
lubricant under pressure, though it maybe. The film pressure is created by the moving
surface to creat the pressure

necessary to separate the surfaces against the load on the
bearing . hydrodynamic lubrication is also called full film ,or fluid lubrication .
Hydrostatic lubrication is obtained by introducing the lubricant ,which

is
sometime air or water ,into the load-bearing area at a pressure high enough to separate
the surface with a relatively thick film of lubricant. So ,unlike hydrodynanmic
lubrication, motion of one surface relative to another is not required .
Elasohydrodynamic lubrication is the phenomenon that occurs when a lubricant is
introduced between surfaces which are in rolling contact, such as mating gears or
rolling bearings. The mathematical explanation requires the hertzian theory of contact
stress and fluid mechanics.
When bearing must be operated at exetreme temperatures, a solid film lubricant