Bearings and Fluid-Induced
Instability in Rotordynamics
Dr. Mehmet Sunar
ME 562
Fluid Induced
Instabilities
Fluid induced Instability is self-excited
vibrations induced by internal mechanism
that transfers rotational energy into the shaft
as lateral vibrations.
Properties of fluid induced
instability




Created and controlled by fluid flow around
the rotor.
Self excited.
Non synchronous.
Considered more destructive in fatigue view
point.
Categories of fluid induced
instability
Fluid Induced Instability
Lube Oil
Process Fluid
(Pumps or Compressors)
Cooling Fluids
(like air in turbines)
Areas to control Fluid Induced
Instability
Possible Areas
Bearing
Design
Rotor
Lube Oil
Preload
Flow rate
Clearance
Viscosity
Bearing type
Mathematical complexity due non-linearities in the bearing
properties. Specially, in post instability conditions.
Modeling of bearing mechanical properties. Transient rotor
response changes the fluid film bearing properties.
•Average Circumferential Velocity Ratio.
•Complex Dynamic Stiffness.
Fluid average circumferential
velocity ratio
It is the ratio of average
fluid velocity to the
average rotor velocity
Lambda ( λ )= ū/ω
Eccentricity (e)



It is the
distance from
the center of
bearing to the
center of rotor.
Ratio=e/c.
Sometimes
called radial
deflection.
e
Effect of eccentricity on
stability in fluid film bearings
Higher eccentricity
leads to lower
fluid average
velocity.
 Lower fluid
average velocity
leads to better
stability.

Bearing Stiffness
Higher eccentricity
leads to higher
stiffness.
 Higher stiffness
leads to better
stability.

Complex dynamic stiffness

Total stiffness of fluid film bearings is
considered to be complex and dynamic.





Real part is called direct stiffness
Imaginary part is called quadrature stiffness.
KT=KD+jKQ
KT=[KS-Mrω2]+j[ω(DS+D)-λDΩ]
Dynamic since it is speed dependant.
Threshold of instability

The speed at which fluid induced instability
commences. (e.g. 1900 rpm)
Two types of Fluid Induced
Instabilities




Whirl.
Forward precession.
Usually starts earlier.
Frequency holds a
constant order of rotor
speed. (dependent on
rotor speed)




Whip.
Forward precession.
Starts after whirl dies.
(it may exist without
being preceded by
whirl)
Frequency holds
constant value
(independent on rotor
speed).
Fluid Induced Instability
Whip
Whirl
Stable
Orbit= 2D shaft vibration
FII Vibration symptoms:
* Large Amplitude
* Subsynchronous
* Circular Orbit
* Forward Precession
Experimental Setup


Experiments have been carried out at
KFUPM MED Advanced Mechanics Lab.
Setup consists of




Bently Nevada rotor kit with fluid film bearing
option.
Speed controller.
Oil pump.
Vibration pickups and ADRE software.
Rotor kit
Fluid film bearing
Speed controller
Oil pump
Effect of fluid wedge support
CCW rotation
Effect of fluid wedge support
CW rotation
Typical experimental test results:
Vibration spectrum
Zoomed run-up cascade.
Instability threshold and
frequency.
Shaft average centerline,
clearance circle and average
eccentricity ratio. Gradual
concentricity.
Threshold of instability change
with oil pressure during start up
3000
Speed (rpm)
2500
2500
2250
2150
2000
2070
1620
1500
1000
500
0
0
0.6
0.8
1
1.2
Pressure (psi)
1.4
1.6
Threshold of instability change
with oil pressure during shutdown
2000
1800
Speed (rpm)
1600
1400
1200
1000
800
600
400
200
0
0.6
0.8
1
1.2
Pressure (psi)
1.4
1.6
How oil behaves at the
threshold of instability
1. Disk Location (A)
2. Shaft Length (B)
3. Disk Unbalance (Unb)
Unbalance effect (as function of rpm).
-8.5
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
-8.6
-8.7
Unb=0g
-8.8
C0930001
-8.9
Unb=2g
C0930101
C0930201
-9
-9.1
-9.2
-9.3
Conclusions from
experimental work


Higher flow rate of lubricating oil should raise the
threshold of instability.
The unbalance has effect on instability.
THERORITICAL WORK
System response at P=0.6 psi
at 1800 rpm
Displacement Transient Response
Pressure is 0.6 psi at 1800 rpm
Stn (2): Fluid Film Bearing
15
Displacement, mils
10
5
D(2)x
0
D(2)y
0
0.5
1
-5
-10
-15
Tim e, sec
1.5
2
System response at P=0.6 psi
at 1800 rpm (Cont’d)
Displacement Transient Response
Pressure is 0.6 psi at 1800 rpm
Stn (2): Fluid Film Bearing
15
D(2)y, mils
10
5
D(2)x
0
-15
-5
-5
5
15
-10
-15
D(2)x, m ils
25
35
System response at P=0.6 psi
at 1800 rpm (Cont’d)
Displacement Transient Response
Pressure is 0.6 psi at 1800 rpm
Stn (2): Fluid Film Bearing
Displacement, mils
7
6
F=11.72 Hz=703 cpm
5
D(2)x
4
D(2)y
3
F=30 Hz=1800 cpm
2
1
0
0
50
100
Frequency, Hz
150
200
System response at P=0.6 psi
at 1700 rpm
Displacement Transient Response
Pressure is 0.6 psi at 1700 rpm
Stn (2): Fluid Film Bearing
4
Displacement, mils
3
2
1
D(2)x
0
D(2)y
-1 0
0.5
1
-2
-3
-4
Tim e, sec
1.5
2
System response at P=0.6 psi
at 1700 rpm (Cont’d)
Displacement Transient Response
Pressure is 0.6 psi at 1700 rpm
Stn (2): Fluid Film Bearing
4
D(2)y, mils
3
2
1
D(2)x
0
-4
-2
-1 0
2
4
-2
-3
-4
D(2)x, m ils
6
8
10
System response at P=1.2 psi
at 2300 rpm
Displacement Transient Response
Pressure is 1.2 psi at 2300 rpm
Stn (2): Fluid Film Bearing
Displacement, mils
10
8
6
4
2
D(2)x
0
D(2)y
-2 0
-4
0.5
1
1.5
2
-6
-8
-10
Tim e, sec
2.5
3
3.5
System response at P=1.2 psi
at 2300 rpm (Cont’d)
Displacement Transient Response
Pressure is 1.2 psi at 2300 rpm
Stn (2): Fluid Film Bearing
D(2)y, mils
10
5
D(2)x
0
-8
-3
2
7
12
-5
-10
D(2)x, m ils
17
22
27
System response at P=1.2 psi
at 2300 rpm (Cont’d)
Displacement Transient Response
Pressure is 1.2 psi at 2300 rpm
Stn (2): Fluid Film Bearing
Displacement, mils
4.5
4
3.5
F=13.67 Hz=820 cpm
3
2.5
D(2)x
D(2)y
2
1.5
F=38.4 Hz=2300 cpm
1
0.5
0
0
50
100
Frequency, Hz
150
200
System response at P=1.2 psi
at 2200 rpm
Displacement Transient Response
Pressure is 1.2 psi at 2200
Stn (2): Fluid Film Bearing
4
Displacement, mils
3
2
1
D(2)x
0
D(2)y
-1 0
0.5
1
1.5
2
-2
-3
-4
Tim e, sec
2.5
3
3.5
System response at P=1.2 psi
at 2200 rpm (Cont’d)
Displacement Transient Response
Pressure is 1.2 psi at 2200
Stn (2): Fluid Film Bearing
4
D(2)y, mils
3
2
1
D(2)x
0
-4
-2
-1 0
2
4
-2
-3
-4
D(2)x, m ils
6
8
10
Conclusions from theoretical
work


Effect of Viscosity on the stability is expected.
Higher bearing pressure leads to higher
threshold of instability.