AERODYNAMIC SUPPORTS FOR HIGH SPEED MOTORS AND TURBINES

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP80T00246A013200740001-3
• Release Decision: RIPPUB
• Original Classification: C
• Document Page Count: 10
• Document Creation Date: December 22, 2016
• Document Release Date: February 28, 2011
• Sequence Number: 1
• Publication Date: June 19, 1961
• Content Type: MISC

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50X1-HUM
Aerodynamic Supports For High Speed Motors and Turbines
Dr Techn. Science S. A. Sheinberg
Cand. of Tech. Science A. M. Kharitonov
50X1-HUM
The fundamental obstacle to the wide spread industrial application
of high speed machinery are the shortcomings of bearings now irk
use.
In rolling contact supports an increase of speed causes progressively
increasing dynamic forces which lead to extremely stringent require-
ments on the materials of the bearings and on the requirements on
the accuracy of their manufacture. Even with strict application
of these factors, endurance life of rolling bearings fall sharply
with increase of speed, which is measured by the product Dn, where ?
D - diameter of the arbor seat in MM and n - number of revolutions
per minute. Practically speaking,the useful values of Dy do not
exceed 500 thousand for rolling contact bearings of higher quality
classifications, which corresponds to a circumferential speed on
the arbor seat of 25 meters per second.
In arbor supports having hydrodynamic liquid lubricated bearings,
the situation is more favorable inasmuch as spheres subjected to
heavy centrifugal loads are eliminated. These are replaced by a
lubricant film, whose limiting factor is the viscosity of the
lubricating liquids.
As is known, the frictional moment M in the lubricant layer of the
sliding bearing is approximately
M r r
C
where 71 - absolute viscosity of the lubricant
area of the working surface of the bearing
'U' - circumferential speed of the rotating shaft
OT - average clearance, that is the difference of radii
of the bearing and the shaft (d - ra - rs1 )
From the formula it is apparent that losses due to friction can be
reduced by reducing the (sic) dimensions of the bearing since this
reduces the speed, or by lowering the viscosity of the lubricant.
Concerning the average clearance, an increase in its magnitude is
impossible due to requirements of stable rotation and prevention
of vibration.
The practicability of the first method of reducing frictional losses
is limited by-the demands of rigidity of the shaft, which falls
rapidly with reduction of shaft diameter. Therefore, for liquid
lubrication, even using the least viscous lubricants (kerosene or
water), the product b r) is limited to a value of the order of 200,000,
which corresponds to a circumferential velocity of 10 meters per
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second. Further increase of D? is successful only by introducing
special precautions, which completely complicate the design of the
machine and its use or by reducing the working surface by means of
cutting out pockets resulting in turbulence of the lubricant with
an accompanying requirement for intensive water cooling. This causes
a requirement for complicated sealing.
The second method - increasing the speed of operation by means of
using gases for lubricants of low viscosity - is far more practicable.
The application of aerodynamic supports - both radial or thrust
bearings having gas and particularly air lubrication, rad11ly solves
the problem of providing the long life desirable in the working
supports for high speed machines.
Aerodynamic supports enable practically any circumferential velocity
to be attained on the shaft surface (Dr, = 2,000,000 and more),
completely fulfilling the requirement of design rigidity of the
shaft because of its maximum possible diameter. The service life
of these supports is practically unlimited, presuming that they
are properly designed and manufactured. As a result of the fact
that viscosity of air is many times less than the viscosity of oil,
water,kerosene,and other liquids used for lubrication; the loss
of power'and heat generation in aerodynamic bearings are so small,
that there are no requirements whatever for increasing heat flow
from the support - normal ventilation is sufficient.
j tempera/ure dependence of liquids and ~
It is essential to note that the gases are directly oppos e -
the circumstance of increasing temperature causes a sharp reduction
of viscosity of liquids, gases somewhat increase in viscosity - true,
to a lesser degree.
Thus the viscosity of air at. a pressure of 1 Kg/cm2 and temperature
of -194? and 229?C are corrspondingly 0.00000056 and 000 00269
Kgm-2 see.
The comparatively insignificant effect of temperature on the viscosity
of gases enable broad possibilities for the application of gas
lubrication under conditions of low and high temperatures. There
are, for instance, reports that bearings with argon lubrication
have successfully operated at 2200?C.
Increase of pressure similarly insignificantly affects the viscosity
of the gas lubricant. Thus at q temperature of 20?C and a change
of pressure from 1 to 100 Kg/cm, the viscosity of air correspondingly
is 0.00000180 and 0.00000199 Kg m -2 sec.
As the pressure in the lubricant film of aerodynamic supports, in
general, does not exceed 10 Kg/CM2 then it is practical to consider
that viscosity is independent of pressure.
Work on high speed electromotors on aerodynamic supports was begun
in ENIMS back in 1950.
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The purpose of this work was embodied in the development of electric
spindles for grinding small diameter openings, at speeds of rotation
of the order of 30 - 50,000 RPM.
Testing of the initial examples of electrospindles underscored the
basic practicability of solving the above discussed problem.
Simultaneously it was demonstrated that reworked equipment was
unsatisfactory. The primary difficulties were the following:
a) The increase of rotor vibration of the rotor with increase of
rotor speed which led to scoring of the working surfaces of
the aerodynamic supports. which were made of quenched steel
in the first examples.
b) Insufficient accuracy of coaxiality of the bearings,
c) A high friction moment on the shaft of the electric spindle
from the surface of the stationary bearing at start up.
d) Poor accuracy of dynamic balance of the rotor.
As a result of extensive work by the authors, in 1957 there was
established a construction of a high speed electric motor for
application in electric spindle drives (announced in authors
communication No. 579255/25) in which the earlier limitations were
overcome and the solution of a series of more important problems
associated with aerodynamic supports were given.
The electro spindle of the type 3 W -19 (fig. 1) was designated
for grinding bores to 25 mm. It is a 3 phase asynchronous short
circuited electric motor with following technical specifications:
power 1 kvt,,. 220 volt a, amperage -3,& ;
0.85, k.p.d. -80%, speed of rotation -
48000 synchronous rpm; diameter of shaft 32 mm.
At synchronous speed, the circumferential velocity at the rotor
surface is 124 meters per second and on the bearing seats - 80
meters per second. The D product is equal to one and one half
million, which is three t1mes the analogous parameter for ball
bearings.
The shaft 1, with the pressed on rotor 2, rotates in two cylindrical
bearings 3, located axially in supporting conical surfaces of the
front and rear end bell of the motor 4. The bearings are pulled
up by means of threaded collars 5 and 6. The end bells are assembled
into the shell 7 on accurate locating surfaces and are fastened in
place by screws B.On the end surface of the extension of each and
bell is installed the thrust washer 9. The accurate correction of
coaxiality of the two bearings is achieved by means of the screws 10.
Into the lubricant clearances of both bearings air or some other
gas under pressure is introduced through the inlet 11 and dis-
tributive tubing 12.
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The single direction of gas feed into the bearing applies a load
on the shaft which damps its vibration on the air supporting pad
which otherwise occurs at high speeds. These vibrations, called
half speed whirl, are the result of rotation of the shaft in the
bearings with small relative eccentricities, under which the
location of the shaft axis becomes instable. The fixed direction
of radial load forces the axis of the shaft into the zone of
maximum eccentricity, corresponding to its stable position. This
eliminates half speed whirl. The compressed gas impinges on the
loaded upper portion of the bearing where the pressure is somewhat
less than atmospheric. Increase of pressure in that region
significantly increases the carrying capacity of the bearing. At
a pressure of 3 atmos. and minimum clearance h - 4 microns,the
carrying capacity of the bearing (diameter 32 millimeters, length
55 millimeters) is equal to 85 kilograms, when under a pressure of
one atmosphere; the same bearing can carry a load of 30 kilograms.
The compressed gas escaping from the bearings in the axial direction
establish a load bearing air film which supports an axial load.
This occurs in the following manners. The gas passing through the
lubricating clearance, collects under the threaded ring 6 and= loads
against the end surface of the shaft, pressing the rotor against
the thrust bearing at the left. Simultaneously these surfaces
continue to be divided by the gas lubricant pad, formed as the
result of efflux of gas, entering the left bearing. Such
construction enables acdurate axial location of the rotor, i.e.,
a means for reducing the requirement of accurate correspondence
of the distance between thrust bearings and rotor length.
Increasing the pressure in the lubricant film of the bearing prevents
the possibility. of entry of abrasive dust or particles, etc. This
reduces the requirement of protecting the bearings by some kind
of sealing, simplifies the construction and enables use of the
described motor in atmosphere conditions which are contaminated
by corrosive gases, dust and abrasives which, in particular, is
very important for grinding heads.
The most important condition for normal operation of the rotor on
aerodynamic bearings is the coaxiality of both rotor bearings. A
non coaxiality over the length of each bearing should not exceed
2-3 microns. In view of this, to provide such a high accuracy
under rigid assembly of bearings has presented, up till now,
tremendous difficulties in known constructions of aerodynamic
supports used in self ac-ting bearings assembled into pivots or
in flexible membranes. Similar constructions work well only at
comparatively low speeds (to 20,000 RPM). With an increase of
speed such a non rigid support of the bearings enablesoccurrence
of vibration. They are also inappropriate for transmitting
significant radial loads and cannot transmit axial loads. The
thrust bearing in that case must be installed into an independent
Cardan support.
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In the above described motor,the bearings are rigidly installed
in end bells, but the extensions in which the bearings are installed
is associated with the thin rim of the and bell by a thin ring which
can deform as a result of tightening the screws 10. This enables
the possibility of rotating the axis of each bearing relative to
the axis.of the motor housing and, therefore, accurate correction
of the co-axialitt of both bearings, completely compensating for
errors in manufacturing the motor parts.
The rigidity of the installation of bearings enables application
of the simplest thrust bearings, using the gas consumed by radial
bearing operation. Simultaneously achieving accurate location of
the rotor in an axial direction by means of single direction
application of gas pressure on the open shaft end.
The verification of coaxiality of the bearings is accomplished by
an accurate cylindrical insert which closely fits the bearings.
As a result of the cumulative errors.o'f manufacture of parts of
the motor motor frame, and bells, inserts) the clearance between
the end surfaces of the installed surfaces vary around the circum-
ferential'direction, as is shown schematically on fig. 2. Thanks
to its abort length, the finished cylindrical band does not hinder
such axi'a,;l divergence. Let o A - maximum clearance around the circle
Moving the scale and accompanying fastened.end bells relative to
the motor. frame., we can establish th#:a, :clearance equal for instance
0.1 mm, and then with the aid of the , e;ar:ew G make equal ( by
means of a compensating element) clearances G 6' and o A. With
the aid of three or more regulating .screws located around the
circumference of the'end bell it is'ea,sy to obtain an adjustment
of constant clearance around the circle, to an accuracy of 0.005 mm.
For a relation o//i ' = 2.5 this corresponds to a coaxiality error
over the 'lehgth of the bearing of no' more than 0.002 nun.
By the same method the second bearing is adjusted.
In the final adjustment, the insert is removed, the motor is
assembled and the toroidal locating surfaces of the end bells and
the body (frame) are tied together by bolts' as shown on figure 1. c
The shortcomings of' nown constructions of 'aerodynamic bearings
are the impossibility of regulating the bore of the bearing, which
hinders attainment of required clearance and requires bearing change
even after insignificant damage to its working surface.
In the electrospindle 3 W-19,the exterior surface of the bearing is
made in conical form. Thanks to the fact that the bearing is made
of material having significant elasticity, its exterior conical
surface deforms under tightening axially, compensating for-errors
of manufacture of contacting conical surfaces. The final turning
of the cylindrical working surface occurs after assembly with the
and bell. Later the diameter of the bearing can be reduced by
proper tightening of the threaded ring, during which the geometric
accuracy is not lost, since the motion along the axis is not great
and the contacting cones are always in contact.
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At present the Moscow Electrode Works manufactures for aerodynamic
supports graphitic materials a and # , which are improved
versions of materials Ayr-3 and Ayr 4 and having improved mechanical
and antifriction properties according to Inst. of Machine Develop-
ment AN USSR. (2).
Impregnbtingrthese materials by metals (lead, babbet, cadmium and
others) prepared by Moscow Electrode. Works, almost two-fold increased
their strength, somewhat reduced brittleness and eliminated porosity,
which was one of the primary impediments to establishing the air 1
film between the shaft and insert.
In the electrospindle 3W-19, supports are of graphitic materials,
designation E, having a higher heat conduction than f3 material,
better antifriction properties and lower wear.
Coefficient of dry friction of this material depends on the quality
of shaft surface.finish and insert and for speeds to 10 meters per
second is in the range 0.0L-0.05.
For aerodynamic supports in addition to a low coefficient of
friction, the non-scoring or welding of graphite with metals during
contact at high sliding speeds is extremely important. During
application of inserts of steel, bronze, iron-graphite and other
materials, dry friction occurring as a result of inaccurate
manufacture and selection (for example distortion of inserts)
resulted in serious damage of working surfaces of shaft and inserts
even to welding. Application of graphite removed such occurrences.
In the worst case the problem is limited to damage to the insert,
which is easy to change-The shaft remains satisfactory for further
use. Steel P-9 quenched to 58-60 Rc has been used for shafts.
To reduce dimensional change during use the quenched parts are cold
treated in liquid nitrogen.
Dynamic balance is extremely important. in achieving stable non
vibrating operation at high speed of rotors. This is equally true
of rotors operating in aerodynamic bearings.
The minimum air space h, between shaft and supports at limiting
load can be reduced to 3 microns. Considering that the relative
motion of the axis of the rotor under the effect of imbalance should
not exceed 0.15 ho , we obtain a maximum allowable motion of
approximately 0.5 microns.
As is known
Ma=C,e =rg
where M a - moment of remaining imbalance'
G - weight of rotor
divergence of physical and geometric axes'of
rotor under effect of imbalance
r - radius at which material is removed
9 - weight of removed metal
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In our instance (for G c 1 kg, e = 0.5 microns, r = 1.5 cm)
the balancing machine must respond to a load 9 = 0.32 gram.
i
For achieving these requirements a special rig was designed and
constructed (fig. 3), in which a mechanical indicator is used.
The rig is quite simple and can be used in any machine construction
plant. The mechanical indicator, exhibiting low sensitiivity, was
replaced with a piezoelectric pickup from a tonearm. The balance
rotor i is mounted on sliding bearings similar to those in the
electrospindle ( in view of the low speed of rotation they are
lubricated by kerosene) disposed in the frame 2 such that one of
its end surfaces is located in the plane of fastening the frame
on knife blades, the second moves with the framework.
The vibration of the framework is transmitted as an angle to the
quartz'pickup 4 by means of the crank arm and by means of the
amplifier 5 is applied to the oscillograph 6.The pickup is pressed
against the crank arm 3 by a weak spring.
The magnitude and location of imbalance is determined by means of
selection and relocation of balancing, weights.
For reducing extraneous effects, the machine is supported on four
springs 7 in assembly 8.
The speed of rotor rotation during balancing is 3 - 4.000 RPM.
In the head end of the balancing machine there 'is a special electric
motor 9 . With the purpose of maximum reduction of rotor un-
balance the eletromotor rotor is solid. The support bearings of
the electric motor are of the sliding type of antifriction graphite b
with adjustable bore diameter. The sheave of the electric motor
is finish machined after installation on the shaft as a result
the eccentricity of the sheave is held to 0.01 mm.
These measures lead to a minimum of vibration of the driving motor
and its effect on the rig.
The transmission from the drive to the balance rotor is achieved by
a belt of chlorovinyl tube which is heat welded.
The accuracy of balance achieved on the rig is less than 0.02 grams
centimeters which meets requirements discussed above.
In co~clusivn it can be said that the, high speed motor on aerodynamic
support bearings has been successfully tested under laboratory
conditions as aninternal grinding head. This head enabled high
quality grinding - during finish grinding to 13th class of surface
finish - and higher productivity. This is explained initially on
the basis of the greater bearing shaft diameter and therefore high
shaft stiffness. The grinding head is simple and desirable for
production.
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In the process of manufacturing aerodynamic supports of the motor
the question is of construction and support of the stationary
bearing parts in the end bells, accurate verification of coaxiality
of stationary bearing parts, entering into their lbad carrying
capacity, damping shaft vibration on the air bearing support, axial
location of the rotor, protection of bearing against dirt contamin-
ation and selection of materials for shaft and stationary bearing
part.
Founded on this basis,aerodynamic bearings and technology of their
manufacture can be successfully applied in all kinds of high speed
motors and machinery.
LITERATURE
1. Sheinberg,'S. A. as Lubrication of Sliding Bearing
(Theory and Calculation) Trenie. Iznos
Mashinakh Sb. VIII, 1Zb A.N. USSR, 1953
2. Plutalova, L. A. Graphitic Sealing of Rotating Shafts
Veatnik Mashinostroeniya No. 2, 1957
C W Cr V Mo
P-9 0.85-0.95 8.5-10 3.8-4.4 2-2.6 0.3
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