Supression of Rotor Instability

The following principles are effective for increasing the stable operating speed range of a rotor: a. raise the lowest, critical speed of the system  b. increase the external system damping   The first requirement can be achieved by increasing the bearing

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SUPRESSION OF ROTOR INSTABILITY N.F. Rieger

Procedure for Suppressinf-j Unstable ·:Ihirling of Rotors The following principles are effective for increasing the stable operating speed range of a rotor: a.

raise the lowestcritical speed of the system

b.

increase the external system damping

The first requirement can be achieved by increasing the

bearin~

radial

stiffness or the bending stiffness of the rotor, or both.

The second require-

ment can be met by using a bearinrr or support type which inherently contains more non-rotating velocity damping.

It should be noted that the

increase of any damping which rotates with the shaft itself has a stabilizing effect below the bending critical speed, and a

destabilizin~

effect above the

bending critical speed. Stabilization of Rotors in Fluid-Film Bearings r.~ore-Stable

Bearing Types

Several investigations have been carried out to assess the relative stability of different bearing types. shown in figure 1.

Pinkus [1] compared the bearing types

Hydraulically loaded bearinr.s and tilting pad bearmgs

were found to be the most stable, i.e., to give rise to the highest instability threshold speed for the rotor system.

O. Mahrenholtz (ed.), Dynamics of Rotors © Springer-Verlag Wien 1984

Plain cylindrical bearings were the

142

N.F. Rieger

least stable.

The self-energizing bearing shown in figure 2 was designed

to increase bearing loading by transferring pressure from the lower half of the bearing to the upper half.

During testing, the top of the self-energizing

be a r i n g was uti Ii zed to apply an additional external load to the journal. By closing either valve 1 or valve 2, the bearing could be either selfenergi zed or externally loaded.

o o

CYlIHORICAL

PRESSURi:

Fi~ure

1

ELLIPTICAL

SYMM

AS."" THREE'LOaE

THREE' lDB£

TILTING AIIO

Bearings Tested by Pinkus to Compare Stabi lity [1]

~ PRESSURE OIL SUPPLY

PRESSUP.E OIL SUPPLY

Figure 2

Self-Energizing Bearing Pinkus [1]

Suppression of Rotor Instability

143

Tondl t 2] also undertook an experimental investigation to compare several basic types of bearinfl geometry for their ability to resist initiation of unstable self-excited whirling.

Tests were made on cylindrical and

elliptical bearings, and also on specific multi-lobe bearing designs, flexibleelement bearings, and on two loose-bushing bearing designs.

Of these, one

design had a cylindrical bushing and the other had a flexible-element loose bushing element. Tondl's tests showed that self-excited whirling was always very intensive for plain cylindrical bearings.

Elliptical bearings exhibited better

resistance to initiation of self excited whirling, i.e., the instability threshold speed was higher than for plain bearings.

Tondl found that the multi-lobe

bearings shown in figure 3 were also relatively stable.

Figure 3

In such bearings

Multi-Lobe Bearings Tested by Tondl [2]

direct contact between the journal and the bushing surface appears to be possible only in small areas.

Followinf.j the

machinin~

of the original bush-