Bearing support system with controllable spring rate

A control system for bypassing critical speeds in a high speed rotating turbine shaft supported by multiple journal bearings which includes pairs of apertures disposed in each bearing seat. The pairs of apertures are linked by fluid vent lines having valves disposed therein. The valves are responsive to vibration in the shaft and serve to vent lubricant between opposing apertures in order to vary the stiffness and hence the spring rate of the bearing as the shaft approaches critical speed.

BACKGROUND OF THE INVENTION 
The field of the present invention is bearing support systems for high 
speed rotating machinery. 
Conventional journal bearings that are lubricated with viscous lubricants 
and used to support a rotating shaft have the characteristic of allowing 
the shaft to displace itself under bearing load in proportion to the force 
of the load. Thus, the bearing has a so-called spring rate. The rotating 
assembly also has a mass, so the combination of this mass and the spring 
rate of the bearing comprises the elements of an oscillating system the 
characteristics of which may be derived mathematically. The system 
characteristic of concern here is a so-called critical speed of the shaft 
and bearing system. This critical speed is the rotational frequency at 
which the shaft will resonate in the bearing. Frequently, it is very 
difficult to get sufficiently rigid bearings to place this resonance 
frequency above the shaft running speed, and thus it often occurs within 
the operating range. The result can be severe vibration and possible 
bearing damage. 
One solution to this problem is suggested by Sternlicht, U.S. Pat. No. 
3,124,395. Sternlicht discloses a method and apparatus for bypassing 
critical speeds of rotors by varying either the lubricant film thickness, 
lubricant viscosity or the lubricant temperature in hydro-dynamic bearings 
at predetermined shaft speeds. Sternlicht's method, however, is somewhat 
complicated, would appear to suffer from rather slow response time, and 
being responsive only to predetermined speeds, would not be readily 
adaptable to varying shaft loading conditions that would alter the 
critical speeds. Thus, it is desirable to have a journal bearing support 
system with a controllable spring rate which is highly responsive to the 
onset of a critical speed condition and which is operable under variable 
loading conditions. 
SUMMARY OF THE INVENTION 
The present invention is directed to a control system for bypassing 
critical speeds in high speed rotating shafts supported by bearings. 
Associated with such a bearing is a pair of fluid apertures and a 
connecting fluid vent line having a fluid control therein. The fluid 
control is to be responsive to shaft vibration in the vicinity of an 
associated bearing. Should the vibration in a given bearing approach a 
critical point, fluid may be appropriately vented around the bearing, 
altering the stiffness thereof. The system may be tuned to provide an 
advantageous phase shift in the flow of lubricant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a high speed rotating shaft 10 is supported by 
hydro-dynamic journal bearings 12 and 14 having bearing seats 13. The 
bearings 12 and 14 are supplied with pressurized lubricant through a main 
lubricant feed line 16 and individual lubricant inlet lines 18. Fluid 
emerging from the lubricant inlet lines 18 circulates circumferencally 
around the bearing through channel 18a as shown in FIG. 3. The lubricant 
is then forced through roughly axially extending channels 18b to then be 
swept across the bearing surfaces. Principal lubricant flow is then axial 
along the shaft and discharged outwardly of the bearings. 
The bearings 12 and 14 include fluid vent lines 20 and 22, respectively. 
Disposed in the vent lines 20 and 22 are control valves 24 and 26. The 
control valves 24 and 26 include, in turn, valve controllers 28 and 30. 
The controllers are responsive to a vibration control signal generated by 
vibration sensors 32 and 33 positioned adjacent the bearings. Should shaft 
vibration in the vicinity of either bearing approach a predetermined 
critical level, the respective control sensor 32 or 33 will generate a 
vibration signal causing the associated valve 24 or 26 to modulate fluid 
flow through fluid vent lines 20 or 22, as the case may be. 
Fluid vent line 20 is shown providing fluid communication between the 
apertures 34 and 36 located in the bearing seat 13. A second fluid vent 
line 20a provides fluid communication between the apertures 38 and 40 also 
located in the bearing seat 13. Associated with the vent line 20a is a 
valve 24a which functions in the same manner as the valve 24. Additional 
vent lines and aperture pairs could also be employed to provide even 
greater control. 
In FIG. 2, the shaft 10 is shown in an eccentric position such as would 
result from a radial load. The oil film is shown thin adjacent the 
aperture 34 and thick on the opposite side of the shaft 10 adjacent the 
aperture 36. If the valve 24 were open, it would allow lubricant to flow 
from its pressurized zone at the aperture 34 to the underpressurized zone 
adjacent the aperture 36, thus resulting in a lower bearing spring rate. 
About one-half a revolution from this position oil would flow backwards 
through valve 24 and have a similar effect if the radial loading rotates 
with the shaft 10. It is generally such loading due to imbalance or shaft 
geometry which must be addressed in overcoming phenomena associated with 
the natural frequency of a system. 
This bleeding of oil and some throttling or friction not only reduces the 
spring rate but also absorbs power from the oscillating shaft system and 
thus has a damping effect. By incorporating two pairs of connected 
apertures, i.e. 34 and 36 and 38 and 40, still greater control and damping 
is possible. For convenience, valves 24 and 24a could be combined into a 
single mechanism if desired and be built into the bearing body or quite 
near it. 
In addition, the vent lines 20 and/or 20a may be "tuned," that is, the 
length and the cross-sectional area along the length of the vent line may 
be so selected that the inertia of the oil moving first in one direction 
and then the other will cause the oil velocity to increase after its 
driving pressure differential passes its maximum and begins to decrease. 
This may be achieved by empirical testing. This flow will continue to some 
extent after the driving force becomes negative; that is, the phase of the 
oscillating oil flow will be shifted. Further, if the tuning of the lines 
20 and 20a is required to be varied, this can be accomplished by one or 
more bypass lines such as 20-1, each having its own control valve such as 
24-1. The phase shift of this oil flow out of phase with the shaft 
oscillation provides the further benefit of increasing the protection from 
possible metal-to-metal contact by delaying the maximum oil withdrawal 
from the oil film until after the instant of minimum clearance. 
The present system is especially useful when the journal bearing has 
Rayleigh type oil grooving. Such grooving includes a dam just ahead of the 
axial oil grooves. The apertures 34-40 may be located ahead of the dams in 
zones of larger clearance where more oil is or can be pumped by the shaft 
rotation. Thus the lateral shaft displacement has less effect on this zone 
than in a closer clearance zone of a simple journal bearing, and the tuned 
vents and vent line can be more effective. 
Thus, a control system for bypassing critical speeds in a high speed 
rotating turbine shaft supported by multiple journal bearings is disclosed 
wherein lubricant stiffness in each bearing may be adjusted by venting 
lubricant between opposing sides of the shaft in response to a vibration 
control signal indicating a predetermined vibration level in the vicinity 
of the bearing. 
While embodiments and applications of this invention have been shown and 
described, it would be apparent to those of ordinary skill in the art that 
many more modifications would be possible without departing from the 
inventive concept herein. Thus, the invention is not to be limited except 
in the spirit of the appended claims.