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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to bearings and, more particularly
in order of increasing specificity, to hydrodynamic bearings, compliant
bearings, foil bearings, and foil thrust bearings.
2. Related Art
The load capacity of a foil thrust beating depends on the compliance of the
bearing with pressure exerted by a fluid film developed between the
bearing and the associated runner, or rotatable member of the device
incorporating the bearing. The pressure profile for a thrust beating
varies, and in order to accommodate the optimum pressure profile and
attendant fluid film thickness associated with maximum lead capacity, the
thrust bearing should be designed to provide stiffness which varies in a
manner similar to the pressure profile. Prior foil thrust bearings have
been known to exhibit limited lead capacity resulting from the
incorporation of springs designed with limited accommodation for the
variance in fluid pressure profile and the resultant effect on lead
capacity of the bearing. A typical spring utilized in such bearings is
illustrated in FIGS. 2-6 of U.S. Pat. No. 4,668,106 issued to Gu. While
such spring designs provide varying stiffness in radial directions, they
provide limited lead capacity because of excess pad deflection over the
spring support points. The excessive pad deflection loads to a divergent
fluid film at the trailing edge of the pad and prevents the bearing from
developing an optimum pressure profile.
More recent foil thrust bearings, such as those illustrated in U.S. Pat.
No. 5,318,366 issued to Nadjafi and U.S. Pat. No. 5,248,205 issued to Gu,
et al. incorporate springs which are configured to provide a variation in
bearing stiffness in both the radial and circumferential directions. While
these thrust bearings solve the aforementioned problems with earlier
bearings, they are not suited for use in bi-directional devices, i.e.
those in which the runner disposed adjacent the bearing may rotate in
either direction. This occurs since, in one direction of rotation, there
is a mismatch between the boating stiffness profile and the fluid film
pressure profile, causing a deterioration in the conformity of the bearing
shape to the rotatable member, or runner.
Accordingly, prior to the present invention, engineer continue to search
for improved foil thrust boating designs.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a bi-directional foil
thrust bearing for use in an apparatus having a first member rotatable
about a centerline axis and a second, stationary member, with the thrust
boating disposed between the rotatable and stationary members. According
to a preferred embodiment, the bi-directional foil thrust bearing
comprises an annular spring cluster disk disposed coaxially about the
centerline axis and a plurality of circumferentially spaced, generally
trapezoidal spring sots attached to the spring cluster disk. Each of the
spring sets includes a radially extending central portion attached to the
spring cluster disk and a plurality of radially spaced spring members
attached to the central portion. Each of the spring members includes a
first portion attached to and extending generally circumferentially away
from a first side of the central portion and a second portion attached to
and extending generally circumferentially away from an opposite side of
the central portion.
BRIEF DESCRIPTION OF THE DRAWINGS
The structural features and functions of the present invention, as well as
the advantages derived therefrom, will become apparent from the subsequent
detailed description of the preferred embodiments when taken in
conjunction with the accompanying drawings in which:
FIG. 1 is an exploded perspective view partially frustrating an apparatus
incorporating a bi-directional foil thrust bearing according to the
present invention;
FIG. 2 is a fragmentary plan view illustrating the bearing stiffener disk
and associated foils or pads, included in the thrust bearing of the
present invention, according to an alternative embodiment;
FIG. 3 is a fragmentary plan view frustrating the bearing stiffener disk
and associated foils or pads, according to another alternative embodiment;
FIG. 4 is a fragmentary plan view further illustrating the bearing
stiffener disk and spring cluster disk shown in FIG. 1;
FIG. 5 is a fragmentary plan view further illustrating the spring cluster
disk and associated spring sets shown in FIG. 1;
FIG. 6 is a cross-sectional view taken along line 6-6 in FIG. 5;
FIG. 7 is a plan view further frustrating one of the spring sets shown in
FIG. 5;
FIG. 8 is a cross-sectional view frustrating the foil thrust bearing of the
present invention incorporating a follower spring according to an
alternative embodiment;
FIG. 9 is a graphic representation frustrating the relationship between
circumferential distance along a bearing foil and both pressure of the
fluid film over the boating foil and stiffness of the thrust boating;
FIG. 10 is a graphic representation illustrating the relationship between
radial distance along a boating foil and both the fluid film pressure over
the foil and the stiffness of the thrust bearing.
DETAILED DESCRIPTION
Referring now to the drawings, wherein like reference numerals have been
used for similar elements throughout, FIG. 1 is an exploded perspective
view illustrating a bi-directional foil thrust bearing 10 according to the
present invention. Bearing 10 may be used in a variety of turbomachinery
devices including, but not limited to, turbocompressors, turbopumps and
turboexpanders. One such turbomachine device, or apparatus 12, is
partially frustrated in FIG. 1. Apparatus 12 includes a rotatable member,
or runner 14 and a shaft 16 attached to runner 14. Both the shaft 16 and
runner 14 are rotatable about a longitudinal centerline axis 18, in either
a clockwise direction as indicated by arrow 20 in FIG. 1 or in a
counterclockwise direction as indicated by arrow 22 in FIG. 1. Apparatus
12 further includes a stationary member, or thrust plate 24, which is
longitudinally spaced from runner 14 as shown in FIG. 1. Thrust plate 24
may be a separate unit as illustrated in FIG. 1, but may also form a
portion of a housing for bearing 10, or another structural member which is
rotationally stationary relative to runner 14. As illustrated in FIG. 1,
bearing 10 is disposed, or positioned, longitudinally between runner 14
and thrust plate 24. Due to the subsequently described unique structural
features and functions of bearing 10, bearing 10 may be utilized in
apparatus 12 when runner 14 is rotated in either clockwise direction 20 or
counterclockwise direction 22, unlike prior foil thrust bearings which are
only suitable for use in uni-directional devices.
Thrust boating 10 includes an annular bearing stiffener disk 26, a spring
cluster disk 28, and a follower spring disk 30. Each of the disks 26, 28
and 30 are coaxially disposed about longitudinal centerline axis 18 and
are constructed of a metallic material, preferably from sheet stock.
Preferably, disks 26, 28 and 30 are made of stainless steel, a
nickel-based alloy such as Inconel X750, 718 or others, or a copper-based
alloy such as beryllium copper. However, the particular selection of
materials for disks 26, 2g and 30 is dependent upon the particular
application and other suitable metals such as iron-based, or
aluminum-based alloys may be utilized for particular applications. Thrust
plate 24 includes a substantially flat surface 32 which faces toward the
spring follower disk 30 so as to provide a proper seating surface for disk
30 of bearing 10. A plurality of alignment pins 34 are attached to, and
may be integrally formed with, thrust plate 24. Bearing stiffener disk 26
includes a plurality of circumferentially extending guide slots 36 (only
one shown). A minimum of two slots 36 are required, but the particular
number of slots may vary with application. Spring cluster disk 28 and
follower spring disk 30 each include a plurality of circumferentially
spaced notches 38 and 40, respectively, disposed about the periphery of
disks 28 and 30, respectively. Guide slots 36 and notches 38 and 40
cooperate with pins 34 to provide the proper clocking, or circumferential
alignment of disks 26, 28 and 30 relative to one another as subsequently
discussed in greater detail.
The bi-directional foil thrust bearing 10 further includes a plurality of
circumferentially spaced, generally trapezoidal foils, or pads 42 which
are fixedly attached to the bearing stiffener disk 26 by conventional
means, such as weldments 44. Alternatively, pads 42 may be fixedly
attached to disk 26 by other conventional means. Each foil 42 is bounded
by a radially inner arcuate edge 46, a radially outer arcuate edge 48, and
opposing radially extending edges 50 and 52. Preferably, the radially
extending edges 50 and 52 extend along radial lines. Weldments 44 may be
positioned proximate edge 50, as shown in FIG. 1, or alternatively, may be
positioned proximate edge 52. When runner 14 rotates in clockwise
direction 20, edge 50 comprises the leading edge of foil 42 with edge 52
comprising the trailing edge. However, when runner 14 rotates in
counterclockwise direction 22 edges 52 and 50 comprise the leading and
trailing edges, respectively, of foil 42. Each foil 42 further includes a
slightly convex surface 56 which faces runner 14 and an opposite surface
(not shown) which is slightly concave and faces the stiffener disk 26. The
desired curvature of surface 56 and the opposite surface varies with
application, and depends upon the available starting torque applied to
shaft 16 in a given application. The bearing stiffener disk 26 illustrated
in FIG. 1 comprises a substantially solid, i.e. devoid of apertures,
annular disk. However, thrust bearing 10 may alternatively incorporate a
boating stiffener disk 126 as illustrated in FIG. 2, or a bearing
stiffener disk 26 as frustrated in FIG. 3, in lieu of the bearing
stiffener disk 26. Disks 126 and 226 each incorporate a plurality of sets
(two shown for each disk) of apertures, indicated at 58 and 60,
respectively, which are utilized to increase the compliance, or to soften,
disks 126 and 226, respectively, in selected areas or regions. Each sot 58
of apertures included in disk 126 is disposed within a region 62 of disk
126 which is aligned with one of the foils 42. The number of sets 58 of
apertures and therefore the number of regions 62 is equal to twice the
number of foils 42, with each set 58 of apertures being biased toward one
of the radially extending edges 50 and 52 of one of the foils 42. Each
region 62 may be sized such that it is approximately equal to one quarter
of the size of foils 42, or somewhat smaller. Each set 58 of apertures
comprises a plurality of generally circular holes 64. Each sot 60 of
apertures formed through disk 226 is disposed within a region 66 of disk
226 which is aligned with a corresponding one of foils, or pads 42.
Similar to the configuration of disk 126, the number of sets 60 of
apertures, and therefore the number of regions 66 of disk 226 is twice the
number of foils 42, with each set 60 of apertures biased toward one of the
radially extending edges 50 and 52 of one of foils 42 as described
previously with respect to disk 126. Each region 66 is sized such that it
is approximately equal to quarter of the size of foils 42, or may be sized
to be somewhat smaller. Each set 60 of apertures comprises a plurality of
circumferentially extending, and radially spaced slots 68. As with disk
26, foils 42 are attached to disks 126 and 226 via conventional means,
such as weldments 44 which are radially spaced and disposed proximate edge
50 or 52 of each pad 42. Sets 58 and sets 60 are biased toward edges 50
and 52 of foils 42 since, as subsequently discussed in greater detail, the
film pressure existing over the leading portion of foils 42 is relatively
low, thereby requiring a relatively low stiffness of bearing 10 in these
areas, and since either edge 50 or edge 52 of each foil 42 may comprise a
loading edge of the corresponding foil 42 due to the bi-directional
capability of shaft 16 and runner 14.
Referring now to FIGS. 4-7, the foil thrust bearing 10 further comprises a
plurality of circumferentially spaced, generally trapezoidal spring sets
indicated generally at 70. Each of the spring sets 70 is attached to the
spring cluster disk 28, and the total number of spring sets 70 is equal to
the number of foils 42 attached to the stiffener disk 26. Each spring sets
70 includes a radially extending central portion 72 which is attached to
spring cluster disk 28 via conventional means such as radially spaced
weldments 74. Each spring set 70 further includes a plurality of radially
spaced spring members 76. Each spring member 76 includes a first portion
78 attached to and extending generally circumferentially away from a first
side 80 of central portion 72, and a second portion 82 which is attached
to and extends generally circumferentially away from a second side 84 of
central portion 72. Portions 78 and 82, for each spring member 76, are
mirror images of one another. Each spring member 76 includes a radial
width 86 and spring members 76 are sized such that the radial widths 86
progressively increase in magnitude from a radially innermost one of
spring members 76, indicated as 76A in FIG. 5, to a radially outermost one
of spring members 76, indicated as 76B in FIG. 5. Accordingly, spring
members 76 provide a variation in stiffness of bearing 10 in a radial
direction. A total of five spring members 76 is illustrated in each spring
set 70. However, other numbers of spring members 76 may be used. It is
preferable to provide as many spring members 76 in each spring set 70 as
is possible in view of dimensional constraints and manufacturing costs to
provide a relatively smooth stiffness gradient of beating 10. It should be
noted that, unlike some prior spring designs, the thickness 86 for any
given spring member 76, is substantially constant in a circumferentially
extending direction. Each spring set 70 may optionally include a first set
of radially aligned and radially extending slots 86, and a second set of
radially aligned and radially extending slots 88. Each of the slots 86 is
formed in the central portion 72 of the corresponding spring set 70 and is
disposed proximate the first portion 78 of the corresponding spring member
76. Each of the slots 88 is formed in the central portion 72 of the
corresponding spring set 70 and is disposed proximate the second portion
82 of the corresponding spring member 76. Slots 86 and 88 are not included
in spring set 70 for purposes of stiffness variation, but rather are
included to ensure that the first and second portions 78 and 82,
respectively, of each spring members 76 are free to move independently of
the central portion 72 of spring set 70. Each of the spring members 76
comprises a corrugated spring as shown in FIG. 6. The only portion of
spring set 70 which is fixedly attached to the spring cluster disk 28 is
central portion 72, thereby permitting spring members 76 to deflect
relative to the spring cluster disk 28. Spring sets 70 are preferably
manufactured separately and apart from disk 28 and subsequently attached
to disk 28 as discussed previously. Alternatively, spring sets 70 may be
integrally formed from a relatively thicker spring cluster disk. In this
instance, the two dimensional shape of each spring cluster disk, best seen
in FIGS. 5 and 7, may be created by chemical etching of the spring cluster
disk 28. Each spring member 76 may then be stamped in a forming die to
form the wavy cross-section or corrugations of spring member 76. The
corrugations produce a plurality of crests, or peaks 90 for each spring
member 76, as best seen in FIG. 6. The crests 90 of adjacent spring
members 76 may be aligned in a direction parallel to a centerline axis 92
of spring set 70, or alternatively, may be aligned in a direction parallel
to a radial line passing through a distal end 94 of the first portions 78
of spring members 76 or a radial line passing through a distal end 96 of
the second portions 82 of spring members 76. If thrust bearing 10 is
configured to include either of the alternative bearing stiffener disks
126 or 226 in lieu of disk 26, thrust bearing 10 may optionally include a
relatively thin, flat, solid annular disk (not shown) coaxially disposed
about axis 18 and positioned longitudinally between disk 126 or disk 226
and disk 28. The purpose of this optional thin, flat, solid disk is to
prevent the crests 90 of spring members 76 from becoming engaged with or
caught in either holes 64 of disk 126 or slots 68 of disk 226. The
necessity for including the optional thin, flat, solid disk depends on the
size of holes 64 or slots 68, which is dependent upon the particular
application of bearing 10.
FIG. 4 illustrates the orientation, or alignment between the beating
stiffener disk 26 and the spring cluster disk 28, which exists in a
circumferential direction. Each foil 42 may be subdivided into a first
region or portion 98 extending from the radially extending edge 50 to a
median line 100, and a second region or portion 102 extending from the
median line 100 to the radially extending edge 52. Portions 98 and 102
comprise either a leading or trailing portion of foil 42, depending upon
the direction of rotation of runner 14. When runner 14 rotates in
clockwise direction 20, portion 98 comprises the leading portion of foil
42, while portion 102 comprises the trailing portion of foil 42. When
runner 14 rotates in counterclockwise direction 22, regions 102 and 98
comprise the loading and trailing portions, respectively, of foil 42.
During rotation of runner 14, the pressure profile of the fluid film
passing over foils 42 increases in a circumferential direction.
Accordingly, it is desirable to align the spring sets 70 with the trailing
portion of the foils 42. This circumferential alignment is illustrated in
FIG. 4 which corresponds to the alignment achieved during the clockwise
direction of rotation 20 of runner 14, and is achieve, d by the unique
features of thrust bearing 10. As shown in FIG. 4, each spring set 70 is
aligned with region or portion 102 of the corresponding one of foils 42,
with portion 102 comprising the trailing portion of foil 42 during
rotation of runner 14 in clockwise direction 20. As shown in FIG. 4, each
foil 42 subtends an angle 104 and, as shown in FIG. 5, each spring set 70
subtends an angle 106. Angle 106 is less than angle 104 and preferably the
magnitude of angle 106 is one half of the magnitude of angle 104.
Consequently, each spring set 70 is sized to correspond to one half of the
size of foils 42 and is therefore equivalent in size to either portion 98
or 102. The desired alignment between disks 26 and 28 which is illustrated
in FIG. 4, is achieved by the cooperation among pins 34, notches 38 in
disk 28 and slots 36 in disk 26. Each of the pins 34, which protrude from
the thrust plate 24, is received in one of the notches 40 formed in the
spring follower disk 30, one of the notches 38 formed in spring cluster
disk 28, and one of the slots 36 formed in the bearing stiffener disk 26,
thereby providing alignment among disks 26, 28 and 30. When runner 14
rotates in clockwise direction 20, disks 26 and 28 rotate relative to each
other, by a predetermined amount defined by the circumferential extent of
slot 36, until pins 34 bottom out, or contact a circumferential end of
slot 36 as illustrated in FIG. 4. It should be understood that when runner
14 rotates in the opposite, counterclockwise direction 22, the pins 34
bottom out in the opposite circumferential end of slots 36. The relative
rotation between disks 26 and 28 during the initial, or startup rotation
of shaft 16 is caused by the frictional force between the runner 14 and
foils 42. After pins 34 contact one of the ends of slots 36, disks 26 and
28 do not rotate relative to one another until the direction of rotation
of runner 14 is reversed.
In order to provide an acceptable service life, the bearing stiffener disks
26, 126, or 226, or at least portions of oath disk surrounding guide slots
36, must be hardened by conventional means such as carborizing, nitriding,
or the application of a hard coating. Additionally, the interface between
beating disk 26, 126, or 226 and tho spring cluster disk 28 may be coated
with a durable and low friction coating to allow relatively smooth
relative rotation of either disk 26, 126, or 226 relative to disk 28
during reversal of tho direction of rotation of shaft 16. Surfaces 56 of
foils 42 may be coated with a solid lubricant such as Teflon.RTM. (Teflon
is a registered trademark of the E. I. Dupont De Nemours & Company),
molydisulfide or polyimide to further enhance the service life of beating
10.
Referring now to FIG. 1, the spring follower disk 30 includes a plurality
of generally U-shaped tabs 108 which are formed by cutting a corresponding
number of generally U-shaped slots 109 in disk 30 and bending the
resulting tabs 108 away from disk 30 and toward the thrust plate 24 to
form a plurality of spaced apart preload springs. Alternatively, tabs 108
and slots 109 may be included in the spring cluster disk 28, at locations
spaced from spring sets 70, thereby eliminating the need for disk 30. As
yet another alternative, thrust bearing 10 may incorporate a spring
follower disk 130, which is partially illustrated in the cross-sectional
view shown in FIG. 8, with disk 130 including a plurality of
circumferentially spaced and generally circumferentially exuding
corrugated preload springs 113 (only one shown in FIG. 8). FIG. 8
frustrates bearing 10 in a loaded condition which occurs during the
operation of apparatus 12. Although the corrugations of the spring 113 are
shown in FIG. 8 for purposes of illustration, the corrugations of each
spring 113 are substantially flattened during the operation of apparatus
12. Each corrugation of spring 113 converges in a radially inward
direction toward tho center of disk 130 and may be formed by stamping disk
130 in a forming die. Each preload spring 113 may comprise a single
corrugated spring member or alternatively may comprise a plurality of
radially spaced corrugated spring members. Regardless of whether beating
10 includes tabs 108, incorporated in either disk 30 or disk 28, or
corrugated springs 113 included in disk 130, the total preload spring
force exerted by either tabs 108 or springs 113 is substantially less than
the spring force exerted by spring members 76 of spring sets 70.
In operation, the thrust bearing 10 is initially biased longitudinally into
contact with the rotatable member, or runner 14 due to the engagement of
either disk 30 or 130 with thrust plate 24 and the resultant spring force
exerted by tabs 108 or corrugated springs 113. During operation of
apparatus 12, the runner 14 may rotate in either the clockwise direction
20 or the counterclockwise direction 22. Regardless of the direction of
rotation, a hydrodynamic film develops between foils 42 and the runner 14,
causing the beating stiffener disk 26 and the spring cluster disk 28 to
rotate slightly relative to one another so as to align the spring sets 70
with the trailing portion of the corresponding foils 42. The fluid
pressure associated with the development of the film overcomes the
opposing preload spring force associated with either tabs 108 or
corrugated springs 113, and the bearing 10 moves toward the thrust plate
24 until the tabs 108 or springs 113 are flattened relative to the
remainder of disks 30 or 130, respectively.
FIG. 9 generally frustrates the pressure profile of the film existing over
each of the foils 42 versus the circumferential distance from the leading
edge to the trailing edge of the foil 42, and a corresponding stiffness
profile of beating 10 which is required to achieve an optimum load
capacity of beating 10. FIG. 10 generally illustrates the pressure
gradient and required stiffness of bearing 10 from the radially inner edge
to the radially outer edge of each foil 42 at any given circumferential
location on the trailing portion of foil 42. The film pressure existing
over the leading portion of foils 42 is relatively low, and accordingly
the required stiffness gradient of beating 10 in a radial direction in the
leading portion of foils 42 is negligible. As shown in FIG. 9, the film
pressure profile includes a first range 115 which gradually increases
throughout the leading portion of each foil 42 and a second range of 117
which is substantially constant throughout the trailing portion of each
foil 42. The required bearing stiffness illustrated in FIG. 9 is
substantially achieved by spring sets 70 which are aligned with the
trailing portions of the corresponding foils 42 and since the leading
portion of foils 42 are substantially unsupported. The bearing stiffness
required in the leading portion may be further reduced by the use of
bearing stiffener disk 126 incorporating the plurality of generally
circular holes 64 or the use of bearing stiffener disk 226 incorporating
the plurality of slots 68, in lieu of stiffener disk 26 which is devoid of
any apertures. The required bearing stiffness variation in a radial
direction, illustrated in FIG. 10, is approximated by the use of spring
sets 70 which comprise a plurality of spring members 76 having widths
which progressively increase from an innermost one 76A of the spring
members 76 to an outermost one 76B of the spring members 76. Although the
use of spring members 76 approximates the required bearing stiffness in a
step-wise fashion, the provision of a smoother stiffness gradient in a
radial direction, which more closely approximates the desired bearing
stiffness illustrated in FIG. 10, is considered within the scope of the
present invention, and is limited only by the cost one is willing to incur
for analysis and manufacture.
While the foregoing-description has set forth the preferred embodiments of
the invention in particular detail, it must be understood that numerous
modifications, substitutions and changes can be undertaken without
departing from the true spirit and scope of the present invention as
defined by the ensuing claims. The invention is therefore not limited to
specific preferred embodiments as described, but is only limited as
defined by the following claims.
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