What Do We Know About Bearings?
Page 32
Page 35
If you've noticed an error in this article please click here to report it so we can fix it.
The Development of Superfinish for Bearing Surfaces May, Says our Contributor, Have Revolutionary Effects on Lubrication Practice as We Now Know It
IF we be in an inquisitive state of mind, we cannot talk about bearings without finding it necessary to con, sider other matters at the same time, that is, if we be anxious to satisfy our curiosity and hope to •learn something that will make further progress a result of our investigations.
. Other subjects quite tightly tied up ,with bearings are lubrication, the making of bearing surfaces, the condition of the surfaces when made, friction and the chemical and other forces that are in operationwhen a shaft runs in its bearing. • 1 •
We suffer, to-day, -under the disadvantage of having inherited bearing, friction and lubrication theories which we have since found, were based on presumed conditions and, in the light of modern knowledge, that were incorrect,
Our presumptions were that shafts and •bearings had smooth surfaces. Of recent years we have satisfied ourselves that such conditions never existed and, although we nbw have that knowledge, it is still doubtful if we have even reasonably perfect surfaces.
The discovery of the deficiency of surface conditions was made as recently as 10 years or so ago, and in America. It was arrived at during an investigation by some automobile manufacturers into the persistent failure of axle bearings. Much progress was made in that country, but it attracted little attention over here until the war came, -when it was impossible to undertake-further research work
in this direction, American research led to the introduction of other methods of surfape finishing, instruments to measure such surfaces, and machine tools developed to produce the kind of finish that the investigations suggested was desirable and necessary, It was named Superfmish.
Considerable developmental work has taken place recently in this country and the original methods of obtaining Superfinish have been revised and added to, whilst some conventional methods of finishing have been developed to produce Superfinish.
Shall We See a New Technique in Lubrication Practice? '
With the recognition of what improved surfaces can now be, compared with what was possible in the days which produced the theories to which engineers have been working since the days of Towers and Reynolds (among others), it is logical to consider whether those theories, based upon false premises, should not he rejected, and a completely new technique, based upon conditions now known to exist, be put in their piace.
The Institution of Mechanical Engineers has taken a leading part in the progress that has taken place. The investigations undertaken by Towers in 1879 were sponsored
by the Institution.. The discussions on lubiication, in 1937, were also the result of its efforts, and most valuable exchanges of -views then took place; including some outspoken doubts and critiCisms of the old theories. Unfortunately, the War came too close on these meetings to permit of further developments taking place.
Again, recently, the Institution held a conference at which the chief subject of, discussion was that of Super. finish. It was noticeable, at that Meeting, that much progress had been made, but it was also clear that the progress had not been co-ordinated; research had taken different directions, and results, therefore, were not comparable. For instance, there seemed to be more than one standard of reference. Generally; however, engineers recognized the existence of the Problem, and through the ventilation of ideas at the meeting, it was felt that collaboration would be forthcoming to smooth out the creases and difficulties and to institute a single "language " which all could speak and understand. The third meeting, in 1945, nearly 70 years after the first one, may possibly lead to as important results as did • the first; it •may be the means for so advancing surface technique to the extent that, owingsto the great differences in quality of surfaces that are now beginning to show themselves, lubrication may be affected in possibly unexpected ways.
Of the three classes of surface considered in relation to lubrication, i.e., completely lubricated, or hydrodynamic film lubrication; the dry surface, which had no practical meaning so far .a..sv lubrication was concerned; and the inbetween, or boundary lubrication condition, in which the surface was sometimes lubricated, and ofttimes not, the only practical condition that existed was that of the boundary lubricated bearing.
It is recognized that a shaft, at rest, has no oil film separating it from the metallic surface of its bearing. Before the shaft can be lubricated, it must revolve to bring up the oil. During its initial movement metal-to-metal contact exists, bringing surface destructive forces to work. There is no protective oil film; the .condition of lubrication is boundary.
It would appear that boundary lubilcation is but a description of faulty lubricating conditions, and that, from the conditions of shafts at rest, there iS no such thing as a lubricated bearing.
Some Pertinent Questions Which Still Need Answering At the 1937 conference in London, questions asked were various and instructive, as indicating the trend of thought and the desire for more definite information than the textbooks of that period, which were well over, half-a-century old, could provide. Here are some of them :—
What are the 'mechanics of oil lubrication ? What are the conditions of surfaces ? How can they be compared, in the (then) absence of a standard of measurement, or of instruments suitable for the purpose ? Is the Beilby layer a fact or \a.. myth ? Is oil the only lubricant ? Why does the beautifully shiny shaft, straight from the grinder, show up so differently when it has been run-in for a time ? What are the marks which became visible after the run-in ? Where do they come from, and why ?
The revelations made by the development of Superfinish have provided answers to a number of these questions, but it does not yet appear to have led to any serious study of the lubrication problem, with a view to the possibility of the adoption of new s'stems.
Another phase of the matter is the mating of materials' suitable to run together as shaft and bearing. The conventional practice is a hard (comparatively) and a soft metal together, which is almost universal to-day, although two hardened surfaces are sometimes run together, but the cost of this particular combination prevents its development to any great extent, and its efficacy is not universally conceded.
The exigencies of the war have forced higher and higher duties from all kinds of 'machinery, more particularly internal-combustion engines of all types.
Lubrication has suffered from this requirement, inasmuch as temperatures of operation are much higher than they used to be, and a certain amount of dissociation of the oil takes place. Sludge in the sum.p, carbon under the rings, and varnish on the piston skirts have all been much increased. Bearings are-increasingly attacked, as acids are released into the oil due to the dissociation.
It is found that hydrocarbon oils alone no longer _give satisfactory service; it is necessary to add to the oil other compound.s, chiefly of sulphur and phosphorous, as well as those of a metal such as potassium. There are various proposals for improving the lubricating properties of hydro carbons, patents for which are being constantly taken out chiefly by the American oil companies. Another cause of the demand for imprcived, lubrication is that of the hypoid gear, used in some American vehicles, and in which the pressure on the teeth is several times higher than in the conventional spiral-bevel sear. It comes about by the fact that the centre of the bevel pinion is below the centre line of the axle.. 'Special lubricants, with greatly improved surface tension, have been developed for this specialized purpose. The reason for the adoption of the hypoid gear' is that it permits a lower floor line of the vehicle in which it is fitted.
For fast-running machinery, oil is almost exclusively the lubricant used; there does not appear to be any thought of
any other possibility.. Some attention ,has been given to graphite, but it is used almost exclusively with oil, or, in some cases, with water.
There are some quite conclusive figures of the improvement that graphite makes when used with oil; they have been published from time to firm) in the technical Press. For instance, according to an N.P.L. report, the friction of graphited oil as compared with plain oil is lower, seizing
temperature higher and ail consumption lower-the lastnamed to the extent. of over three titnes-0.017 to 0.005 fluid ounces per minute. For 100 years, efforts have been made to introduce different-and varying methods of lubrication from those which
have been used for so long, 'apart from the employment of graphite, which has attained to a fair amount of popularity among those who have studied its possibilities.
Breaking Entirely New Ground In these experiments, mediums concerned include air under pressure, oil under extreme pressure, oil and tiny steel balls mixed, and the forces of magnetism, among others.
Air under pressgre was first experimented with over 100 years ago, and later, some 50 or so years ago in America, by Kingsbury, who referred in his 'report to these earlier attempts. Recently, a patent was taken out on the operation of a shaft in a bearing with compressed-air lubrication. It describes the conditions as those of a 4-in.-diameter shaft which was run at a 'speed of 20,000 r.p.rn in compressed air at a pressure of approximately 36 lb. per sq. in.
A German experiment employed tiny steel balls a tenth of a millinetre in diameter, introduced into the oil. Although little information was given, it was stated that the usual high-starting friction of a plain blaring was reduced to about that of a ball or roller bearing. The balls were evolved by Melting steel wire in a gas flame which, it is stated, produced a " fog " of steel balls which were used as blown.
Another proposal, using a variation of conventional oil conditions, is one in which, the oil pressure is greater than the load carried bythe bearing, the idea being to float the shaft away from metallic' contactwith its bearing; if the pressure can be maintained during inaction, the destructive metallic running contact of starting up from a stationary position would be advantageously. affected.
The possibilities of magnetism are coming in for considerable attention as a potential " lubricant " or. more correctly, lubricant eliminator.
The general idea of magnetism is that it is a plaything; that the fish we useil to float on the top of a bowl of water by a wire magnet, is still the limit of the power of that force.
Exploiting the liorce of Magnetism
Nothing is firther from the truth; a, indication may be given, without entering into the complicated and somewhat obscure standards of measurement of the powers of magnetiC pieces of material, 'by saying that, to-day, the power of .a magnet, as compared with that of, say, 25 years ago, is as 50 is to 3. .
During the past few years, several of the large electrical concerns, among others, have, taken out patents for the application of magnetism n to shafts and bearings.
In the initial stages of this development the shaft was specified as being a vertical one; its end was to be magnetized, as was the bearing, but to the opposite polarity.. These magnetic applications were also specified as applicable to instruments, 'that is, for very light magnetic duty. The 'latest patent taken out refers definitely 'to a horizontal shaft that is magnetically supported in its bearing. This claims that the patentees have supported by bearing magnets weighing lb., a shaft and its magnets of the same weight. This is an attempt to neutralize the force of gravity in running shafts; its successful application can have untold results in many kinds of machinery. " AZOTE."