AIR LUBRICATION OF BEARINGS IN CENTRIFUGES AND OTHER HIGH-SPEED MACHINERY IN USSR

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP80-00809A000600350369-4
• Release Decision: RIPPUB
• Original Classification: C
• Document Page Count: 6
• Document Creation Date: December 22, 2016
• Document Release Date: August 22, 2011
• Sequence Number: 369
• Publication Date: October 16, 1950
• Content Type: REPORT

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CLASSIFICATION CONFIDENTIAL COH~~~11t1a_
CENTRAL INTELLIGENCE AGENCY REPORT
INFORMATION FROM
FOREIGN DOCUMENTS OR RADIO BROADCASTS CD NO.
HOW
PUBLISHED Monthly periodical
WHERE
PUBLISHED Moscow
DATE
PUBLISHED,. Aug 1950
LANGUAGE Russian
THIS DOCUMENT CONTAINS INFORMATION AFFECTING THE NATIONAL DEFENSE
OF THE UNITED STATES WITHIN THE MEANING OF SSPIONAGE ACT EO
U. U C.. SI AND 30. AS AMENDED ITS TRANSMISSION OR THE REVELATION
OF_ITS CONTENTS IN ANY MANNER TO AN UNAUTHORIZED PERSON IS PRO.
N IEITE0 SY LAW. REPRODUCTION OF THIS FORM IS PROHIBITED.
?
SOURCE
Vestnik mashinostroyeniya, No 8, 1950.
DATE OF
INFORMATION 1950
DATE DIST. l(, Oct 1950
SUPPLEMENT TO
REPORT NO.
AIR LUBRICATION OF BEARINGS IN CENTRIFUGES
AND OTHER HIGH-SPEED MACHINERY IN USSR
development of machines whose separate elements operate at high speeds. In
recent years, the operating speed of the working parts has risen considerably.
This is true of machine tools for turning, milling, and grinding. It is also
true in the field of light industry. The operating speed of spinning spindles
is already reaching 20,000-30,000 revolutions per minute. In many cases high
speeds are a necessary condition to the normal course of the technological
process, as in electric spindles for grinding small holes, turbine-driven com-
pressed-gas engines, and supercentrifugal and ultracentrifugal apparatus. In
the'last 10-15 years, the operating speed of hydroscopic instruments has in-
creased from 8,000-10,000 to 20,000-25,000 revolutions per minute. The in-
creased speed also increases the accuracy and dependability of the instrument.
It is very difficult to achieve high operating speeds with the prevail-
ing types of bearings. In the case of rolling friction bearings, an increase
in'speed produces a progressive increase in dynamic intensities, which makes
great demands on the materials used and the accuracy of manufacture. The
practical speed limit at which it is possible to use standard rolling friction
bearings is 10,000 revolutions per minute. The production of beariiiks for
speeds in the 20,000-revolution-per-minute range already appears to be expen-
sive and complicated) and their period of service does not exceed 200-300 hours.
In the case of sliding friction bearings with liquid lubrication, the sit-
uation is basically more favorable. However, losses due to friction increase
proportionally, at the rate of approximately the square of the increase in speed.
Therefore, with oils or liquids of great viscosity, work at high speeds is pos-
sible only with the use of complex apparatus for forcible lubrication supply and
heat elimination.
Modern Requirents
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Ti
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The low viscosity of gases, and in particular of air, opens up new pos-
sibilities for the use of sliding friction bearings. Spindles on bearings
lubricated with air can rotate at practically unlimited speed, with negligible
heat generation and small losses due to friction.
Work has been going on in this direction for some time. A number of ma-
chines and instruments with air-lubricated bearings have been designed. A
survey of the work done abroad can be found in Fuller's article "Hydrostatic
Lubrication" in Machine Design, No 6,7,8,9, 1947. In the bearings described
by Fuller, a buoyant supporting cushion is formed by blowing in compressed
air from the outside. One of the best known designi. of this type is the ul-
tracentrifuge, which rotates at 80,000 revolutions per minute. Compressed
air at a pressure of 0.5 atmosphere is used for lubricating the step bearing.
t
The most significant failing of such aerostatic bearings is the fact
that they require a steady and dependable source of compressed air. If the
blowing is momentarily interrupted, irreparable damage is inevitable.
Our own technologists have developed aerodynamic bearings with compres-
sors which are not inadequate. Air is drawn into the lubrication opening
directly from the atmosphere as the shaft rotates. These bearings are sim-
ple in their working principle and, as experience has shown, dependable in
operation. These features open up wide possibilities for their use.
The Machine-Building Institu-:e of the Academy of Science USSR has
worked with industrial enterprises for a number of years on the problems of
gas lubrication. The successful solution of a number of design problems and
the invention of an aerodynamic step bearing made it possible to use aerody-
namic bearings in 1948 in the production of a continuous-motion rotor super-
centrifuge.
Rotor centrifuges greatly surpass previous types in productivity. Their
working principle consists of passing a purifying liquid through a simple steel
cylinder which rotates on a vertical axis. The normal acceleration of the liq-
uid can reach a magnitude of tens and even hundreds of thousands of g (the ac-
celeration of gravity, equal to 9.81 meters per second.) This requires a ro-
tating speed of 15,000 or more revolutions per minute. Thus, supercentrifuges,
and, to a greater extent, ultracentrifuges working at speeds of up to 150,000
revolutions per minute, are adaptable to gas lubrication.
Observation of the wcrk of supercentrifuges with a speed of 21,000 revolu-
tions per minute on aerodynamic bearings has shown the bearings to be highly ef-
ficient. One of the first machines has already been in operation on a produc-
tion basis for more than 10,000 working hours, with no breakdown of the bear-
ing unit. This fact gives reason to suppose that aerodynamic bearings will
soon find wide application in technical fields. Therefore, a knowledge of
their working principle, methods of computation, and special features is of
interest to those who are concerned with the subject of high speeds.
Working Principle of Aerodynamic Bearings
The principle can be explained on the basis of a gas-lubricated cylindri-
cal_ bearing, which differs from a liquid-lubricated bearing only in the fact
that its working surface is executed with greater accuracy and fineness of
finish. The same is true of the surface of the journal. The difference be-
tween the diameters of the journal and the bearing is ordinarily from 10 to
30 microns.
CONEIDENTlAC
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7
When it is not rotating, the journal lies on the bottom of the bearing.
The eccentricity, which is equal to the mean clearance, coincides with the
alignment of the load on the journal. The space between the journal and the
bearing, filled with air at atmospheric pressure, is of unequal breadth, vary-
ing from zero at the bottom to twice the mean clearance at the top. After the
journal begins to rotate, air, trapped by its working surface, is forced into
the wedge-shaped clearance and compressed. The increased pressure in this lub-
rication layer separates the journal from the bearing. The magnitude of the
eccentricity depends on the size of the bearing, the magnitude of the mean
clearnace, the load of the journal, and the speed of rotation. The eccentric-
ity of operation increases with an increased load. Thus, to insure a buoyant
air cushion, the breadth of the clearance must change in the direction of the
sliding.
In cylindrical bearings, this condition is automatically fufilled. In
step bearings, which serve for the pickup of axial loads, the working sur-
face must impart the required contour. Research has 'shown that -a gab-
lubricated step bearing with an oscillating working surface has good sup-
porting capacity.
The step bearing consists of a round block with three or more radial
ribs, to which the supporting plate is fastened with screws, one in each of
the interradial spaces. The supporting surfaces of the ribs are polished
and ground. The plate is a tempered steel, polished and ground, plane-
parallel disk. When the screws are tightened, the plate undergoes elastic
deformations, assuming an oscillating form very much like a sinusoid. The
plate should be of such thickness as to insure the depth of curvature nec-
cessary for maximum supporting capacity when the screws are firmly tightened.
When the toe rotates, the air forced into the narrowing space is com-
pressed, forming a buoyant air cushion which separates the toe from the step
bearing. The plate has radial grooves which intersect the screw axes. These
grooves facilitate the drawing of air from the a+.aosphere into the lubrica-
tion opening. In this way they increase the supporting capacity of the step
bearing, at the expense of increasing the pressure on the whole of its sur-
face. If there were no grooves, the pressure at the point of maximum clear-
ance would be less than atmospheric pressure.
Structure of the Supercentrifuge and Its Basic Elements
The rotor centrifuge, composed of nine units mounted on one base, is
designed for the continuous centrifugal purification of liquids. The rotor
is suspended by a flexible shaft to a spindle which rotates in the aerody-
namic bearing unit. Transfer of the rotation from the pulley of the motor
is accomplished by a knitted belt. The motor is secured to a revolving mech-
anism for maintaing belt tension.
The liquid, supplied by a feed pipe, is injected into the intake nozzle
of the rotor. Inside the rotor the liquid is distributed in an even layer
along the rotor?s walls. Under the action of centrifugal force the residue
is precipitated from the liquid and deposited on the walls. The "centrifugat"
(the purified liquid) passes into the mouth of the rotor and is ejected through
holes in the latter into an overflow trough. From the trough it drains into a
receiving container. The machine is stopped periodically, and the rotor dis-
mantled so that the residue which has accumulated may be cleaned off its walls.
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CONFIDENTIAL
The speed of the rotor is 21,000 revolutions per minute. The centrifugal
force generated is 12,000 times as great as t.? force of gravity. Productiv-
ity is about 10 liters per hour, depending on the liquid processed and the ex-
tent of cleaning required.
When the design of the centrifuge was being developed, it was necessary
to limit the rotating speed to a comparatively low rate. Although the aerody-
namic bearings caused no difficulty at higher speeds, the drive wao a limiting
factor in any given case. During experimentation, the number of revolutions
reached 30,000, but the durability of the belt at this speed was completely
inadequate. Experiments have shown that by far the best belts are those com-
posed of rings of seamless knitted hose which have been saturated with a 5-
percent solution of shoemaker's wax in petroleum ether.
The best solution would be to use a built-in electric drive operating on
a stepped-up frequency. However, in this case, it would be necessary for every
consumer to have a stepped-up frequency generator. This would scarcely be feasi-
ble under the conditions which exist in medical, nutrition, and similar labora-
tories which are the basic consumers of the centrifuges described above.
The aerodynamic bearing unit consists of a hollow spindle pressed into a
pulley and rotating on three supports, two bearings and one step bearing. The
toe is screwed onto the pulley. The bearings are attached by spring fixators
on the bushing and the cap. Since the balls of the fixators are adjusted by
springs, and the edges are rounded off at the fitting sockets of the bushing
and cap, the bearings are self-adjusting along the axis of the spindle. The
step bearing is mounted on the spherical edge of the bushing, so that it also
is self-adjusting.
When the spindle rotates, air is drawn from the atmosphere into the spaces
between the working surfaces and the supporting surfaces of the bearings and
step bearing, forming air cushions which separate the spindle from the bearings.
At the moment of starting, the air cushions are lacking. The moment of friction
between the toe and the step bearing, which are squeezed together by the weight
of the rotor and the spindle (about 2.5 kilograms), is so great that the belt
cannot make the spindle start to rotate. A centrifugal clutch coupling placed
inside the spindle and attached to the rotor serves to decrease the starting
torque and insures fast warming up of the spindle. It thus lessens wear and
tear on the working surfaces in starting, when dry friction occurs between the
spindle and the bearings.
When the machine runs down, the air cushion between the toe and the step
bearing is retained down to speeds of 400-500 revolutions per minute, after
which the spindle is stopped almost instantaneously as a result of the dry
friction which has set in. The action of the clutch coupling permits the
rotor to revolve freely until the machine stops completely. Thus wear and
tear on the step bearing when the machine runs down is also avoided.
Another centrifugal clutch coupling, by which the motor pulley is con-
nected with the shaft of the electric motor, serves to decrease wear and tear
on the belt at the sbarting of the machine.
During normal operation of the machine, the rrindle is completely insu-
lated from the bearings by the air cushions. This is the basis for automatic
control of the bearings, which is achieved by an electric signaling device.
It consists of a galvanometer switched into the electric circuit of a dry cell
with a voltage of 1.5 volts. The circuit runs through the spindle, the bear-
ings, and the step bearing, but carries no current unless normal operation of
the unit is interrupted.
IONFIDE :0 iAL
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Method of Computing Bearing_CaPacities
The bearing in the aerodynamic bearing unit of the centrifuge may be
used as an example for illustratirg the method of computing the technical
characteristics of gas lubrication. The technical data is as follows: dia-
meter of the spindle, 17 millimeters; mean clearance between the spindle and
the bearing, 8 microns; breadth of the bearing, 50 millimeters; speed of ro-
tation of the spindle, 21,000 revolutions per minute; atmospheric pressure,
one kilogram per square centimeter; working temperature of the bearing, 30
degrees.
The position of the journal in the bearing shifts when the load on the
journal is changed. With an increased load, the eccentricity is increased
and the minimum clearance is decreased. The mean clearance remains constant,
since it does not depend on the eccentricity or the minimum clearance.
It is impossible, however, to decrease the minimum clearance beyond a
certain limit. If the clearance is too small, dry friction will occur in cer-
tain places as a result of unavoidable errors in the macrogeometry of the work-
ing surfaces of the journal and the bearing, and the machine will break down.
Experimental research has shown that 3 microns can be considered the minimum
for bearings with gas lubrication. This is the extent of error in ovalness
and conicity with which the journals and bearings are made at present. When
greater accuracy in their manufacture is achieved, the minimum clearance can
be decreased.
If the minimum clearance is assumed to be 3 microns, the load capacity of
the bearing is 11 kilograms. Since the tensile strength of the belt, distrib-
uted over the two bearings, does not exceed 4-5 kilograms, there is a fourfold
margin of supporting power.
In designing the bearing, it is important to determine the proper mean
clearance, in order that the minimum clearance will be such as to afford the
greatest supporting capacity to the bearing.
It must be kept in mind that when the mean clearance is increased, there
is a greater shift of the spindle axis from its central position under the
weight of the load. In certain cases (grinding spindles, for example), the
extent to which the eccentricity may shift is limited by the nature of the
work. Therefore, after the proper mean clearance has been determined, it is
necessary to make sure that the eccentricity produced by the load will not
exceed the prescribed limit.
The moment of friction on the journal is greater than on the bearing.
However, in gas lubrication, and especially at high speeds of rotation, the
difference. is negligible. The greatest moment of friction is equal to 12.7
gram-centimeters. The total power expended in friction, including both bear-
ings, is less than .01 horsepower. The increase in temperature of the bear-
ings does not exceed 3 degrees.
Computation of the step bearing is done by the same methods. It may be
noted only that the supporting capacity. of the step bearing is 12 kilograms,
whereas its load does not exceed 3 kilograms. The diameter of the step bear-
ing is 70 millimeters; the moment of friction under the working load is 57
gram-centimeters.
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CON, WERIRRI
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QQQ~IQ~~~i~l`:,
Some Special Features of Gas Lubrication
The supporting capacity of a gas lubrication layer is limited, whereas
the supporting capacity of a liquid layer increases with speed (where a laminar
flow exists). This is the fundamental difference between gas and liquid lubri-
cation.
At high rotating speeds, the load on the bearings is usually small, and
the supporting capacity of the gas layer is adequate, as in the case of the
centrifuge. Where great supporting capacity is required, it can be achieved
by an increase in atmospheric pressure. At high rotating speeds, the support-
ing capacity is almost directly proportionate to the atmospheric pressure.
It is sometimes convenient to feed compressed air directly into the bear-
ing. It is particularly advantageous to blow it into the section where the
pressure is lowest. It is important to note that losses due to friction are
not increased by the blowing, since the viscosity of gases is not dependent
to any great extent on pressure. Temperature also has small effect on the
viscosity of gases as compared with oils.
This feature opens greater possibilities for using aerodynamic bearings
under conditions of extreme temperatures. Their efficiency is attested by
the steady work of the centrifuges which have been installed in refrigeration
compartments.
Conclusions
The proper use of aerodynamic bearings is at high rotating speeds. How-
ever, it is impossible to state the minimum speed at which they can be used,
since, with small loads, the gas cushions can exist ev~>n at low rotating speeds.
Generally speaking, it is not efficient to use aerodynamic bearings at low
speeds, because under these conditions, oil-lubricated bearings and rolling
friction bearings have a much greater supporting capacity.
When the dependability and durability of other types of bearings are
inadequate, the lower speed limit for the use of aerodynamic bearings may
be fixed at 8,000-10,000 revolutions per minute. The upper limit depends
only on the sturdiness of the material in the rotor, since losses due to
friction are always small because of the low viscosity of gases.
Among the merits of aerodynamic bearings are their simplicity -- since
the surrounding atmosphere is a lubrication reservoir -- and their insensi-
tivity to high and low temperatures. In view of their operational attain-
ments, it may be assumed that in the near future they will find varied ap-
plication in machines and instruments which require high rotating speeds
under moderate loads.
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