Variable preload foil bearing

An axial shaft is supported within a housing by a plurality of bearing foils which are cantilevered from the housing by a series of rotatably mounted supports. The supports are provided with gear teeth and a ring gear which surrounds the housing is engageable with the gear teeth of the supports to simultaneously rotate these supports relative to the housing and adjust the preload of the bearing foils against the shaft.

This invention relates to hydrodynamic foil bearings and specifically to a 
hydrodynamic foil bearing in which the preload of the bearing foil against 
an axial shaft may be adjusted. 
BACKGROUND OF THE INVENTION 
Typically, in a hydrodynamic bearing, a shaft rotatably contained within a 
housing is surrounded by a series of foils cantilevered from the housing 
which wrap the shaft in a series of line contacts parallel to the axis of 
the shaft under a predetermined preload or tension. This wrapping creates 
about the surface of the shaft a series of wedge shaped pockets. Relative 
high speed rotation of the shaft relative to the pockets develops or 
induces a viscous shear which wipes or draws a fluid, such as ambient air, 
between the shaft surface and the foils to create a very low friction, 
supporting gas film. The preload affects significantly the stiffness of 
the bearing and its operating characteristics, such as start-up torque. 
While such a bearing works well at high speeds, problems exist during 
startup and coast down, when the induced viscous shear support film is 
absent. At startup, the shaft rests and drags on the bottom of the 
housing, creating a large starting torque, and a large eccentricity 
between the starting and final position of the center axis of the rotating 
shaft. This eccentricity creates a problem in certain applications, such 
as turbo chargers, since such shaft misalignment decreases efficiency. 
Other unique problems are present in the turbo charger environment, 
including thermal expansion of the housing to which the foils are mounted, 
which affects the preload of the foils, and foil wear from the frequent 
starts and stops, which also affects the tension or preload. 
The prior art discloses some foil bearings in which the foil tension is 
adjustable. Gross et al U.S. Pat. No. 3,506,314 shows a foil bearing in a 
turbine application wherein a shaft is cradled within a housing by several 
foils in the form of slings, hung at one end to the inside of the housing, 
with the other end hung from a manually operable adjustable nut. This 
allows the slings to be shortened or tightened against the shaft. Marley 
U.S. Pat. No. 3,434,761 shows, in one embodiment, a foil bearing in a 
turbo expander application wherein cantilevered wrapping foils are spring 
loaded against the shaft surface. Adjusting screws threaded into the 
housing and engageable with the springs are manually adjusted to increase 
or decrease the tension of the foils against the shaft. The disadvantages 
of the systems are that a quick and simple simultaneous tension adjustment 
of the foils is not possible, each foil must be separately and manually 
adjusted. Nor may the foils be simultaneously and automatically adjusted 
in response to various bearing parameters, such as shaft angular rotation 
speed or temperature effects. 
SUMMARY OF THE INVENTION 
The subject invention solves the above outlined problems and deficiencies 
by providing a foil bearing in which the foils may be quickly, simply and 
simultaneously tension adjusted, either on a one-time basis or on a 
continuous, automatic basis responsive to bearing parameters. 
In the embodiment disclosed, an axial shaft is rotatably supported inside a 
housing by cantilevered bearing foils in a generally conventional 
configuration. Each foil wraps the shaft under tension in a 
circumferential series of line contacts. However, rather than being 
conventionally rigidly attached to the housing, the foils are anchored to 
separate foil mounts which are rotatably mounted in axially extending 
slots in the housing. The foil mounts each have toothed driven portions 
circumferentially disposed about the shaft and engageable with the teeth 
of a driving ring gear which is rotatably supported relative to the 
housing. 
As the ring gear is turned and held at a new relative position by any 
convenient drive means, such as a worm gear, the foil mounts are rotated 
to a new angular position relative to the housing, either decreasing or 
increasing the foil tension against the shaft surface. Thus, the preload 
of the foils and the bearing stiffness may be increased at startup, 
lifting the shaft away from the housing, and decreasing the eccentricity. 
Then, in response to increasing shaft angular speed, the foil tension may 
be decreased by turning the ring gear back, reducing friction as the 
hydrodynamic support film begins to form. In addition, foil tension may be 
adjusted in response to other bearing parameters, such as thermal 
expansion or sag, or to compensate for foil wear. 
It is therefore an object of the invention to provide a foil bearing in 
which foil tension and bearing stiffness may be adjusted both initially 
and during bearing operation. 
It is a further object to provide such a bearing in which the tension of 
the bearing foils may be quickly, simply and simultaneously adjusted in 
response to the shaft speed. 
It is another object of the invention to provide such a bearing in which 
the shaft eccentricity may be controlled. 
It is still a further object of the invention to provide such a bearing in 
which the bearing stiffness may be adjusted in response to other 
parameters, such as thermal expansion and wear of the bearing foils.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawing, an axial shaft 10 is received within a housing 12 
of generally cylindrical shape and is surrounded by a series of seven 
bearing foils 14. The foils 14 surround or wrap shaft 10 in a generally 
conventional pattern, engaging the surface of shaft 10 in a 
circumferential series of parallel, axial line contacts. Foils 14 are 
cantilevered to the inside of housing 12, by means to be described below. 
The operation of shaft 10 with bearing foils 14 in a given adjusted 
position is conventional, with ambient air or other fluid being drawn in 
by the mechanism of viscous shear into the pockets to create a supporting 
gas film between foils 14 and shaft 10. It may be noted at this point that 
cylindrical housing 12 is cylindrical for convenience only, and some other 
housing or support would be feasible, provided the foils were supported in 
the same pattern. The crucial element is the wedge shaped pockets as 
described. 
Housing 12 contains seven circumferentially evenly spaced, axially 
extending slots 16 of a generally semi-circular cross section, having 
lengths generally equal to the length of the foils 14. Received within 
slots 16 are seven foil mounts consisting of rotatable rods 18. Foils 14 
are conventionally anchored into rods 18 by pins 20. In a conventional 
bearing, foils 14 would be anchored by the same pins 20 but directly to 
the inside surface of cylindrical housing 12. On the outside of each rod 
18, and accessible from the outside of cylindrical housing 12, is an 
arcuately arranged series of gear teeth 22 all of which are concentrically 
arranged relative to the center axis of the shaft 10, and which together 
constitute the driven portions of the rods 18. A ring gear 24 constitutes 
the drive mechanism and includes a series of gear teeth 26 which mesh with 
the gear teeth 22 of rods 18. Ring gear 24 is rotatably supported relative 
to housing 12 by any suitable means, not shown. Finally, a conventional 
worm gear 28 which could be driven by any suitable means, engages outside 
gear teeth 30 on ring gear 24 and releasably turns and holds ring gear 24 
in any desired position relative to housing 12. 
In operation, ring gear 24 is rotated either clockwise or counterclockwise 
by worm gear 28. The engagement of teeth 26 with teeth 22 in turn rotates 
rods 18 within slots 16 and increases or decreases the bias of foils 14 
against the outside of shaft 10. Thus, at bearing startup, ring gear 24 
may be turned counterclockwise to increase the tension or bias of foils 14 
against shaft 10. This lifts shaft 10 away from the bottom of housing 12 
to the extent desired and supports it until such time as the angular 
rotational speed of shaft 10 is enough to create a hydrodynamic fluid 
support film described above. At such time, ring gear 24 may be turned 
clockwise to decrease the rubbing force of foils 14 on shaft 10. It would 
be a relatively simple matter to calibrate worm gear 28 and ring gear 24 
to correlate various angular positions of ring gear 24 relative to housing 
12 with the resultant tension or bias of foils 14 on shaft 10. Worm gear 
28 could then be programmed through suitable electronic controls to turn 
to a desired position to create a desired tension or bias at any desired 
rotational speed of shaft 10. Thus, it may be seen that foils 14 may be 
adjusted relative to shaft 10 simultaneously, quickly, and also 
automatically. In addition, the tension or bias of the lower foils 14 
against shaft 10 could be set higher initially, prior to setting the ring 
gear 24 in place. The relative change in tension would still be equal for 
all foils 14 for the same total angular rotation of ring gear 24. In 
addition, in an environment of high temperatures, cylindrical housing 12 
might expand and decrease the tension or bias of bearing foils 14. 
Suitable electronic controls could also turn ring gear 24 to compensate 
therefor. In addition, wear of the foils 14, which would decrease the 
tension 14, could be compensated for in like manner. 
It should be understood that various modifications could be made in the 
embodiment as disclosed without changing the operation thereof. It has 
already been mentioned that the shape of housing 12 could be different. In 
addition, rods 18 could be mounted in some other rotatable fashion, the 
axial slots 16 are not strictly necessary. Gear teeth 22 could be located 
anywhere on rods 18, for example, at the end of cylindrical housing 12, 
and could, conceivably, even be on the other side of rods 18 and 
engageable by a gear with teeth on the outside rather than on the inside 
thereof. Any means which would turn and hold the rods 18 simultaneously 
could also be substituted for ring gear 24.