Vibration damping arrangement

A damping arrangement 25 (FIG. 1) for damping axial vibrations in a shaft suspended (optionally) by a magnetic bearing has a radially extending thrust face 24 mounted on the shaft and separated by a small face gap 29 from a housing 30 and through which gap a stream of gas is forced by way of a flow restrictor 31 at the entrance to the gap. The flow restrictor acts with thrust face 26 to define a gap entrance related to the axial position/displacement of the shaft. A chamber 33, defined downstream of the restrictor and having exhaust passage 35, opens at neck 34 to face the thrust face and the gas pressure therein exerts axial thrust on the shaft. Axial vibration of the shaft modulates gas supply to the face gap and the chamber is dimensioned to create a phase lead of 90.degree. for the instantaneous pressure therein and axial thrust on the shaft, thus damping the shaft vibrations causing its displacement.

This invention relates to an axial vibration damping arrangement for a 
rotor shaft rotatable within a housing and maintained in position axially 
by a control system responsive to externally induced disturbances of the 
said axial position to apply restoring forces. Such a shaft having axial 
position control is frequently, but not exclusively, found in magnetic 
bearing arrangement in which the rotor shaft is suspended both radially 
and axially within electro magnetic fields controlled by said control 
system as a function of displacement from a desired position. 
It is characteristic of such magnetic bearings that a very small suspension 
gap separates the shaft and appendages, rotating a very high speed, from 
stationary parts. To accommodate failure of the electromagnets or control 
circuits or inability of the control circuits to restore position, back-up 
bearing arrangements are employed which permit purposely positioned shaft 
surfaces on collars or the like to run in contact with stationary bearing 
materials, both to protect the electromagnetic components and to provide a 
rotational energy dissipating, that is, braking function for the shaft. It 
is furthermore known to provide cooling gas caused to flow across the 
bearing surfaces for extracting heat therefrom, which gas may also perform 
a sealing function for process gases associated with the machine of which 
the shaft is part. 
Whereas the electromagnetic bearings and their control arrangements may be 
dimensioned and powered to suspend the shaft against continuous forces of 
any practicable magnitudes, they are less able to deal with cyclical 
variations in forces acting on the shaft, that is, vibrational 
reciprocation of the shaft, because of limited bandwidths of the control 
arrangements. 
It has been found in installations wherein the externally applied or 
internally generated forces cause reciprocation of shaft position, 
particularly in axial direction, that a magnetic axial bearing may be 
overwhelmed without there being a fault in the magnetic bearing per se and 
the back-up bearing arrangement caused to operate unnecessarily and to the 
detriment of its longevity and efficiency of the magnetic bearing as a 
whole. 
The need to provide damping for axial reciprocation of a shaft (or 
equivalent rotating body which the term `shaft` used hereinafter is 
intended to include) due to externally induced forces is not limited to 
shafts suspended in a controlled magnetic bearing, and thus preserving the 
generality of the foregoing, it is an object of this invention to provide 
an axial thrust damping arrangement, for such a shaft, of simple 
construction. 
It is also an object of the invention to provide a thrust damping 
arrangement which utilises cooling gas associated with a bearing 
arrangement of the shaft. 
It is furthermore an object of the present invention to provide an axial 
damping arrangement which may be implemented with minimal modifications to 
structures existing for bearing functions and a minimum of additional 
components. 
In accordance with this invention an axial vibration damping arrangement 
for a rotor shaft rotatable within a housing comprises at least one 
radially extending thrust face fixed with respect to the rotor shaft and 
facing along the axis of the shaft, a gas supply arranged to feed gas to 
impinge upon said thrust face and flow in a radial direction along a face 
gap between said thrust face and the housing said face gap including 
adjacent the upstream entrance thereto a gas flow restrictor extending in 
a direction between the housing and thrust face, whereby the area of the 
gap entrance is a function of the axial position of the thrust face and 
shaft with respect to the housing, and, downstream of said gas flow 
restrictor, (a) chamber means, comprising at least one chamber, opening at 
a chamber neck into said face gap adjacent the thrust face and (b) exhaust 
passage means, comprising at least one passage, having a cross-section 
greater than the maximum area of gap entrance at the gas flow restrictor, 
extending from the chamber means to downstream of the face gap, said gas 
supply being arranged to feed gas at such rate as to create a positive 
pressure within the chamber means to exert an axial thrust on the thrust 
face related to the neck area of the chamber means, said gap entrance 
being varied by axial vibrational reciprocation of the thrust face, 
induced by disturbances of the shaft, to create a variation in said 
positive pressure within the chamber means advanced in phase with respect 
to the variation of shaft position within a predetermined frequency range 
to effect a corresponding variation in axial thrust on said thrust face to 
effect damping of the shaft reciprocation.

Referring to FIG. 1 a rotor shaft 11 is suspended within a housing 12 by 
electromagnetic bearing means, part of which is shown at 13. To 
accommodate failure of the electromagnetic bearings to support the shaft a 
back-up bearing arrangement is provided shown generally at 15. 
The back-up bearing arrangement includes a cylindrical block 16 of dry 
running bearing material extending about the shaft to provide radial 
back-up for a portion 17 of the shaft itself and a flat annular block 18 
of similar dry running bearing material to provide axial back-up for a 
face 19 of a collar 20 secured to the shaft. 
An annular clearance 21 exists between the shaft and housing and source 22 
provides a supply of cooling gas by way of the clearance 21 to flow across 
the contact-making faces of the collar 20 and bearing material 18 and 
between the shaft portion 17 and bearing material 16. In this construction 
the axial back-up bearing 18, 19 provides support for the shaft against 
excessive displacement in a single axial direction and the opposite 
radially extending face 24 of the collar 20 is unused. A similar back-up 
bearing arrangement (not shown) spaced axially along the shaft provides 
support for the shaft against excessive axial displacement in the opposite 
direction. 
In accordance with the invention an axial thrust damping arrangement is 
provided as shown generally at 25. 
It comprises a radially extending thrust face provided by the face 24 of 
collar 20 that is facing along the shaft and a gas supply, provided by the 
cooling gas passing along clearance 21 and passages 27 in the housing 
which causes some of the gas to by-pass the back-up bearing arrangement 
and pass, like the used cooling gas but at increased pressure and flow 
rate, across the periphery 28 of collar 20 and in a radial direction along 
a face gap 29 defined between the thrust face 24 and the housing at 30 
which overlies it radially. 
The face gap 29 includes, adjacent the upstream entrance thereto, a gas 
flow restrictor 31 which extends in a direction between the housing and 
the thrust face 24. The flow restrictor 31, which also extends about the 
periphery of the shaft, is conveniently mounted on, or formed integrally 
with, the housing but may be carried by, or formed of the collar thrust 
face 24. It will be appreciated that the flow restrictor 31 defines with 
the thrust face 24 a gap entrance 32 to the face gap 29 whose area is a 
function of the axial position of the thrust face and shaft with respect 
to the housing. 
Downstream of the flow restrictor 31 there is provided chamber means 33 
comprising at least one chamber, such as the annular chamber shown which 
extends completely about the shaft. The chamber means, that is, each 
chamber thereof, opens at a chamber neck 34 into the face gap 29 adjacent 
the collar thrust face. The chamber is of substantially uniform 
cross-section in an axial direction and the chamber neck of substantially 
identical cross-sectional area with the remainder of the chamber. 
Exhaust passage means 35 extends from the chamber through the housing from 
a wall 36 of the bearing disposed opposite to the chamber neck, to a point 
37 downstream of the chamber gap, the exhaust passage having a 
cross-sectional area greater than the maximum area of gap entrance 32 at 
the gas flow restrictor. 
The dimensions of the chamber means, gap entrance and exhaust passage and 
rate and pressure of gas supply are chosen having regard to achieving a 
functional result to be described. 
The gas is supplied upstream of the flow restrictor and flows therethrough 
and through the face gap to exit by way of clearance between the radial 
back-up bearing 16, 17 where it may provide a cooling function. Some of 
the gas flows into, and through, the chamber means 33, exiting by way of 
exhaust passage 35. 
The gas is arranged to feed into the face gap at such a rate as to create a 
positive pressure in the chamber means 33, notwithstanding the flow of gas 
therefrom by way of exhaust passage 35. The positive pressure within the 
chamber means exerts and axial thrust on the collar thrust face 24 which 
is related not only to instantaneous chamber pressure but also the chamber 
neck area. 
In operation, when the shaft is subjected to consistent externally induced 
axial force, the chamber provides a substantially constant axial thrust on 
the collar. When the shaft is subjected to a axial vibration that results 
in reciprocation of its desired axial position, the variations in entrance 
gap at the flow restrictor modulates the flow of gas into the face gap and 
chamber means and thus the pressure in the latter, the consequence being a 
modulation in axial thrust on the thrust face out of phase with the forced 
reciprocation of the shaft and collar. 
The location of the chamber means causes the modulation in axial thrust, 
due to instantaneous pressure in the chamber means, to be advanced in 
phase with respect to the shaft reciprocation and thus acts towards 
damping such reciprocation. 
By dimensioning the chamber in accordance with standard equations known 
(for example, from the book entitled "Compressible Fluid Flow" by Shapiro, 
Ascher H, published by the Ronald Press Co, 1950/1955) and widely use in 
compressible fluid theory, it is possible to tune the response to a 
particular vibration frequency band to effect a phase advance of pressure 
variations in the chamber which lead shaft displacements due to the 
vibrational forces by 90.degree. and exerts an optimum damping effect on 
the shaft reciprocation. 
It will be appreciated that the 90.degree. phase advance is optimum and 
achieved for a particular chamber size for a limited range of 
reciprocation frequencies. Some adjustment of frequency can be 
accommodated by changes in gas supply rate but significant changes are 
determined by the dimensions of the chamber means. 
However, a useful degree of damping is provided for chamber pressure phase 
advances between 45.degree. and 135.degree. so that a practicable 
bandwidth for damping is readily achievable in practice. 
It will be appreciated that many items described above may be varied 
without substantially altering the function. 
Referring now to FIG. 2, a similar arrangement of shaft and housing is 
shown in which identical parts are given corresponding reference numbers 
but the axial thrust damping arrangement shown generally at 50 has the 
chamber means 51 provided with an exhaust passage 52 defined as a recess 
in the housing open to, and extending radially of, the face gap 29. 
A further variation that is shown in FIG. 2 is the supply of gas by way of 
a separate supply duct 53 through the housing which may be employed 
instead of axially flowing cooling gas or when the latter is omitted. 
It will be appreciated that the gas damping arrangements 25 and 50 may be 
employed as shown and described separately from a back-up bearing 
arrangement 18 and the absence of such back-up bearing arrangement 
increases the possibility for structural variations. 
It will be appreciated that the gas may be supplied so that it flows across 
the thrust face in a radially outward direction, in which case the 
upstream flow restrictor would be located adjacent the shaft. 
Furthermore, absent a back-up bearing arrangement, the face gap may be 
formed by either or both faces of collar 19. 
It will be appreciated that if desired the chamber means may be formed as a 
series of discrete chambers, each with their own exhaust passage arrayed 
about the periphery of the shaft. 
Furthermore, a chamber or chambers of the chamber means may be formed 
entirely or partly by recesses in the thrust faces of the collar 20 as 
illustrated schematically at 60 in FIG. 3. 
It will be appreciated that where a single thrust face is employed at any 
one shaft location an axial thrust damping arrangement per se may have a 
thrust face provided, for example as illustrated in FIG. 4, by a simple 
shoulder 70 intermediate two shaft portions of different diameters. 
Other variations are possible to vary the centre frequency of the band in 
which a useful phase advance is achieved; for instance, as illustrated in 
FIG. 5, the dimensions of the chamber means 80 may be altered by providing 
that at least one wall 81 of one or more chambers of the means is 
adjustable in position by means of adjusting screw or push rod 82. 
Alternatively, any other wall or walls of the chamber means may be 
adjustable in position or the chamber volume altered by means of a tuning 
screw or stub corresponding to the adjusting screw or push rod 82. 
It will be appreciated that the above described axial damping arrangement 
may be employed with a shaft supported by conventional contact-making 
bearings if such a shaft is capable of making limited axial displacements 
and control of axial vibration is necessary, and irrespective of whether 
or not it is subject to active axial positional control. 
Other changes may be made without departing from the scope of the 
invention.