Compliant hydrodynamic bearing with improved support element

A compliant, hydrodynamic fluid film bearing for supporting a rotating rotor on a stationary mount, includes a bearing sheet supported by a compliant resilient support member formed of two sheets, each having raised resilient elevations in the form of corrugations. One of these sheets is inverted with respect to the other and the corrugations on the two sheets are vertically aligned and face in opposite directions, each corrugation forming one-half of an elongated tubular spring. A metal sheet lies between the two support sheets and all three sheets, and the bearing sheet, are fastened at one end to a pair of spacer blocks. The yield strength of the support member is thus raised above the yield strength of a single support sheet while maintaining low stiffness.

BACKGROUND OF THE INVENTION 
This invention relates to compliant hydrodynamic fluid film bearings, and 
more particularly to an improved compliant support element for a bearing 
of this type. 
A compliant hydrodynamic fluid film bearing is a thin, lightweight, 
inexpensive structure that supports extremely high speed rotors on a fluid 
film with small frictional losses, good damping characteristics, excellent 
durability and reliability. It includes a thin, flexible bearing sheet 
supported on a compliant support element which, in turn, is supported on a 
stationary mounting member such as a bearing sleeve or a thrust plate. 
The hydrodynamic fluid film which supports the rotor on, and separates it 
from, the bearing sheet is created by the viscous or shear forces acting 
in the fluid parallel to the direction of relative movement between the 
rotating rotor bearing surface and the bearing surface of the bearing 
sheet. A rotating thrust runner, for example, drags the boundary layer of 
fluid with it as it rotates over the bearing sheet. The boundary layer, in 
turn, drags in the layer of fluid immediately adjacent and in this way a 
velocity gradient is established in the fluid in the gap between the 
thrust runner and the bearing sheet. This gap is wedge-shaped, tapering in 
the direction of movement of the rotating rotor. The wedge-shaped gap is 
inherent in the journal bearing and is created in the case of the thrust 
bearing by various techniques. The pressure of the fluid drawn into the 
wedge-shaped gap tends to increase toward the narrow end of the gap, thus 
creating the pressurized cushion or fluid film which dynamically supports 
the rotating rotor. 
The compliant support element for the flexible bearing sheet enables it to 
conform to the bearing surface of the rotating rotor despite thermal 
distortion and centrigual growth, and despite rotor run-out due to 
eccentric loads or rotor unbalance. In prior art rigid hydrodynamic fluid 
bearings, these effects can interfere with the conformance of the 
stationary bearing surface with the bearing surface of the rotating rotor 
and thereby adversely affect the hydrodynamic action by which the 
supporting fluid film is generated. The compliant support element in 
compliant hydrodynamic bearings can deflect and expand to support the 
bearing sheet in correct hydrodynamic relationship to the rotating rotor 
despite these deviations of the rotor bearing surface from its normal 
plane of operation. The pressurized fluid cushion or film on which the 
rotor is supported obviates the need for rolling element bearings, and the 
self-pressurizing nature of this bearing frees it from dependence on 
external pressurizing equipment needed in hydrostatic bearings. Thus, this 
bearing offers the potential advantages of a virtually limitless speed 
ceiling, a miniscule wear rate, and long reliable operation. 
Because of these advantages, the compliant hydrodynamic fluid bearing has 
attracted a great deal of attention in the recent past for extremely high 
rotational speed applications. In addition, when the lubricating fluid 
used in these bearings is a gas such as air or helium, the temperature 
limitations imposed upon conventional bearings by reason of the coking 
temperature of oil or other liquid lubricants does not apply and the 
temperature limitation then becomes that of the metal elements in the 
bearing. These and other properties make these bearings extremely 
attractive for applications such as turbomachinery, high speed industrial 
applications, and certain consumer products. 
There is one limitation, however, which has in the past restricted the use 
of these bearings to fewer fields than their potential application. The 
relatively low yield strength of the thin and flexible materials used in 
these bearings to achieve the desired compliance have imposed a limited 
load carrying capacity. The performance characteristics of these bearings, 
well known to experts in the field of high speed rotating machinery, have 
inspired continuous research to discover ways to increase the load 
carrying capacity of these bearings in order to apply them to higher load 
applications. These approaches have generally involved increasing the 
gauge of the bearing materials or adding other stiffening or strengthening 
members. These expedients have produced an increase in load carrying 
capacity but, have usually been accompanied by other undesirable effects 
such as a decrease in compliance and therefore a decrease in the tolerance 
of the bearing to misalignment and transient rotor excursions referred to 
above. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide a compliant 
hydrodynamic fluid bearing having improved load bearing capacity without 
imparing the ability of the bearing to conform to the bearing surface of a 
misaligned rotor, and which produces only small reaction forces when an 
imperfectly balanced rotor passes through critical speeds. 
These and other objects of the invention are achieved by the disclosed 
preferred embodiments of the invention wherein the compliant support 
element for the bearing is formed of two support sheets having raised 
resilient projections. The projections of the two sheets are vertically 
aligned with each other or separated by a flexible separator sheet, and 
one support sheet is inverted with respect to the other so that the 
projections extend in opposite directions. In this configuration, the two 
support sheets act as two springs in series to offer a higher degree of 
compliance than either individual sheet acting alone could offer. To 
achieve this degree of compliance with a single sheet would require the 
use of thinner gauge materials which might not be able to support the 
required load without becoming overstressed. The series stack enables the 
use of heavier gauge bearing materials which can withstand the stress of 
the higher load exerted on the bearing without producing a stiffness which 
would give undesirable large reaction forces and prevent the degree of 
conformance with the rotor bearing surface necessary to maintain the 
hydrodynamic effect without loss of load carrying capacity in misalignment 
situations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings wherein like reference characters designate 
identical or corresponding parts, and more particularly to FIG. 1 thereof, 
a combined thrust and journal bearing is shown having a thrust plate 10 
mounted on one end of a journal sleeve 11. An axial bore 12 extends 
through the thrust plate 10 and journal sleeve 11 and receives the rotor 
shaft 13 of the rotor assembly 14 shown removed from the bearing housing. 
Six identical pad modules or assemblies 15 are mounted on, and spaced 
equally around the thrust plate 10. Although six pad assemblies 15 are 
shown, other numbers may be used, depending on the size of the thrust 
plate 10, with more pads being used on a larger diameter thrust plate and 
fewer pads being used on a smaller diameter thrust plate. A narrow space 
16 is maintained between the pad assemblies 15 for ease of fabrication and 
repair, and also as an inlet ramp for the hydrodynamic fluid wedge and as 
a channel for cooling fluid. 
Referring now to FIG. 2, each pad 15 includes a bearing sheet 17 having an 
upwardly facing bearing surface 18. The bearing surface 18 faces an 
opposing bearing surface 20 of a thrust runner 22 which is rotating 
relative to the thrust plate 10 and its bearing pad assemblies 15 in the 
direction indicated by the arrow 24. 
The bearing sheet 17 in each pad assembly 15 is supported by a resilient, 
compliant support element 26. The support element 26 supports the bearing 
sheet 17 in bearing relationship to the opposing surface 20 of the thrust 
runner 22 while permitting the bearing sheet 17 to deflect and conform to 
the opposing bearing surface 20 when the thrust runner 22 experiences 
transient excursions out of its normal operating plane parallel to the 
thrust plate 10. 
The support element 26 includes a first support member 28, a second support 
member 30, and an intervening sheet 32. Each of the support elements 28 
and 30 is formed of thin, flexible spring metal and includes a regular 
series of elevations 34 separated by flat land portions 36. The elevations 
34 on the two sheets 28 and 30 are vertically aligned and inverted with 
respect to each other on the two members so that the elevations 34 project 
in opposite directions and the flat land portions 36 on the two members 
are adjacent. In this way, the elevations 34 form a parallel array of 
series spring pairs, and the forces exerted by the sheet 17 upon the top 
member 30 are transmitted through the intervening sheet 32 directly to the 
land portions 36 on the first support member 28 and thence to the thrust 
plate 10. This arrangement ensures that the stress in both support members 
will be equal and they will deflect equally to maintain the aligned 
relationship. 
The bearing sheet 17, the two support elements 28 and 30, and the 
intervening sheet 32 are all fastened together along one radially 
extending edge, which is the leading edge 38 with respect to the rotation 
of the thrust runner 22. To provide a secure fastening of the bearing 
elements to the thrust plate 10, and to provide a leading edge which is 
free of distortion and is the optimum height for the initiation of a 
hydrodynamic fluid wedge, a pair of spacer blocks 40 and 42 are used to 
space the leading edge portions of the bearing elements above the surface 
of the thrust plate 10 and to hold them in correct relationship to each 
other. The first spacer block 40 is welded to the thrust plate 10 by spot 
welding or resistence seam welding, and the aligned and juxtaposed edges 
of the leading edge of the two support members 28 and 30 and the 
intervening sheet 32 are welded to the spacer block 40 with the leading 
edge of the support members 28 and 30 on either side of the leading edge 
of the intervening sheet 32. A second spacer block 42 is welded to the 
welded edge of the two support members and the spacer block 40 and the 
leading edge of the bearing sheet 17 is welded to the top of the spacer 
block 42. 
The function of the intervening sheet 32 is twofold. Its first function is 
to insure that the land portions of the two support members 28 and 30 do 
not slip past one another in the event of a slight misalignment or 
momentary non-uniform deflection of the two support elements. If the land 
portions of the two support members slipped past one another, the bearing 
sheet 16 would lie slightly lower than its leading edge portion which 
would be the wrong slope for the bearing sheet. In addition, if the 
"springs" of the corrugations or elevations 34 were nested instead of in 
series, the stiffness of the resulting structure would be excessive. 
The other function of the intervening sheet 32 is to facilitate coulomb 
damping which occurs when the support members 28 and 30 flex and expand, 
thereby scrubbing their land portions 36 against the opposite sides of the 
intervening sheet 32. The frictional losses inherent in the scrubbing 
action tend to absorb the energy of undesirable bearing effects such as 
self-speed whirl, thereby increasing the bearing stability. 
The spacer block 40 raises the level of the leading edge of the intervening 
sheet 32 to the plane defined by the top surface of the land portions 36 
of the first support member 28. The second spacer block 42 raises the 
level of the bearing sheet 17 to the plane defined by the crests of the 
elevations 34 on the second support member 30. In this way, it is 
unnecessary to bend the intervening sheet 32 or the bearing sheet 17 and 
each member of the pad assembly 15 lies flat and level on its allotted 
position without distortion and with such ease of assembly. 
As shown in FIG. 2, it may be desirable to arrange the elevations 34 of the 
two support members 28 and 30 in gradually increasing height. In this way, 
the support element 26 supports the bearing sheet 17 so that it slopes 
upwardly away from the leading edge 38. The upward slope facilitates the 
generation of a hydrodynamic supporting fluid film cushion or film between 
the bearing sheet 17 and the opposing bearing surface 20 of the rotating 
thrust runner 22. 
The two support members 28 and 30 are mirror images in design and 
dimensions and identical in physical characteristics such as stiffness, 
spring constant, coefficient of thermal expansion, etc. Thus, when the 
support members flex under load, they each flex in the same way so that 
the elevations and land portions remain vertically aligned. In practice, 
this is achieved by making the two support members 28 and 30 mirror 
images, that is, made from the same material blanks oriented 180.degree. 
apart in the same or identical dies. Likewise, the intervening sheet 32 
can be identical to the bearing sheet 17, and the two spacer blocks 40 and 
42 can be identical. This arrangement facilitates the fabrication and 
assembly process and presents a very simple inventory situation. 
When the bearing is used in high temperature applications and uses a gas as 
a lubricant, the surface 18 of the bearing sheet 17 is preferably coated 
with a dry, high temperature coating such as "HL-800", a proprietary 
coating of Mechanical Technology Incorporated of Latham, New York and 
disclosed in a copending application of Bharat Bhushan Serial No. (File 
No. 2-D-443) the disclosure of which is hereby incorporated by reference. 
Turning now to FIG. 3, the journal bearing of the bearing shown in FIG. 1 
is shown having bearing insert 54 lining the bore 2 for dynamically 
supporting the rotating shaft 13. The bearing insert 54 includes a bearing 
sheet 56 compliantly supported by a support element 58 so that the bearing 
sheet 56 can conform to the shape of the rotating shaft 13 and assume the 
optimum shape for the generation of a high pressure, large area 
hydrodynamic supporting fluid film. 
The theory and mode of operation of this bearing is based on the action of 
the rotating shaft carrying the lubricant into, and pressurizing it in, 
the wedge shaped cup which lies between the shaft 55 and the bearing sheet 
56. This wedge shaped gap is shown, great exaggerated for purposes of 
illustration, in FIG. 3. The conformance of the bearing sheet 56 to the 
shape of the rotating shaft 13 makes the wedge shaped space between the 
shaft and the bearing sheet 56 much wider and more gradual than a rigid 
bore of the same diameter would be, and therefore increases the area of 
the interface between the shaft and the bearing sheet in which a 
hydrodynamic supporting fluid film pressure can be generated and exist. In 
addition, because the effective area supporting the shaft load is greater 
in a compliant bearing, the pressure of the load is less and therefore the 
total load capacity can be greater. Finally, the compliance of the support 
element enables the bearing sheet to deflect in the event of transient 
excursions of the shaft 13 from its normal axis of rotation which can 
occur during high speed operation of an unbalanced shaft, under operation 
with eccentric applied loads, or with a misaligned shaft or bearing. Under 
these effects, the compliant support element 58 will deflect and enable 
the bearing sheet 56 to conform to the bearing surface of the shaft 55 
despite its deviations from its intended axis of rotation. 
The supporting element 58, best shown in FIG. 4, includes a first support 
member 60, a second support member 62, and an intervening sheet 64 
positioned between the two support members. Both support members 60 and 62 
and the intervening sheet 64 are fastened to a spacer block 66 at one end 
of the assembly, and a second spacer block 68 is fastened on top of the 
end of the second support member 62. The first spacer block 66 is welded 
to the bore 52 of the bearing sleeve 50 by spot welding or resistence seam 
welding and the other members of the bearing insert are welded, in turn, 
atop each other as shown in FIG. 4. Finally, the bearing sheet 56 is 
similarly welded to the top of the second spacer block 68. The two support 
members each include a regular array of raised resilient elevations in the 
form of corrugations 69 separated by flat land portions 67. The elevations 
69 and the flat land portions 67 are radially aligned so that forces 
exerted on the supporting element 58 from the bearing sheet 56 during 
operation are transmitted directly through the flat land portions 67 from 
one member to the other, and the elevations act as springs in series. 
Because the radius of the second support member 62 is less than the radius 
of the first support member 60, its circumference is less and the pitch of 
the elevations must be less than the pitch of the elevations on the first 
support member 60 so that the elevations and the land portions are 
radially aligned when the bearing is in place in the bore 52. 
A modification of the support elements 28, 30, 60, and 62 is shown in FIGS. 
6 and 7. The support elements 70 and 72 are formed of thin sheet metal of 
the same material used in the embodiments of FIGS. 1 and 2 and FIGS. 3-5. 
Instead of the bump form or corrugation elevations used in the support 
elements of the first two embodiments however, the embodiment of FIGS. 6 
and 7 employs leaf-like projections which are formed by cutting or 
shearing the material in a U-shaped cut and bending the material within 
the U-shaped cut out of the plane of the material. Because of the 
relatively wide area of engagement of the two sheets 70 and 72 and because 
the leaf projections 74 do not cause movement of the sheet when the leaf 
projections deflect, there is no need for an intervening sheet such as 
sheet 32 and sheet 64 in the first two embodiments. 
Obviously, numerous other modifications and variations are possible in 
light of the above disclosure.