Introduction to Vibration and Position Measurements
CopyrightBently Nevada CorporationAll Rights Reserved, 1998
The following marks are Bently Nevada Corporation’s Registered Trade Names:
MicroPROX ADRE REBAMKeyphasor Proximitor SeismoprobeKey Velomitor TorXimitorTrendmaster Smart Monitor Data ManagerDynamic Data Manager Transient Data Manager Bently Nevada
What is Vibration?
Mechanical vibration is the dynamic motion of machine components.
Vibration measurement is the measurement of this mechanical vibration relative to a known reference.
How does Vibration Start?Energy must be put into the system through an
applied force, either internal or external. The force may be instantaneous, an impulse, or
continuous. In machines this energy is diverted from the process. This reduces the machine’s efficiency and may also result in damage to the machine’s components.
What Characteristics are Measured?
AmplitudeFrequencyPhase Angle
Absolute Phase Angle Relative Phase Angle
What Affects These Characteristics?
MassSpring StiffnessDamping
AMPLITUDE
pk
0
pk
pk
rms
Peak-to-peak refers to the total amount of vibration.Zero-to-peak refers to the total amount of vibration from themaximum height of either the positive or negative peak to thezero voltage axis.Root mean square (RMS) is a function of the signal conditioningperformed in the monitor or diagnostic instrument and not theoutput of the transducer.
Frequency
Frequency is defined as the repetition rate of a periodic vibration within a unit of time.
The frequency of vibration (cycles per minute) is most often expressed in multiples of rotative speed of the machine.
Basic frequency measurements can be performed with an oscilloscope and Keyphasor signal.
FREQUENCY
TIME/CYCLE
Phase
Phase is defined as the timing relationship, in degrees between two or more signals.
PHASE
A
B
TIME(DEGREES)
PHASE
TIME(DEGREES)
AMPLITUDE
PHASE(BETWEEN VIBRATION SIGNALS)
PHASE ANGLE
0° 360°
PHASELAG
VIBRATIONSIGNAL
KEYPHASORSIGNAL
TIME
DEGREESOF
ROTATION
The phase angle is defined as the number ofdegrees from the Keyphasor pulse to the firstpositive peak of vibration.
Form
Vibration form is the raw waveform displayed on an oscilloscope and can be separated into two categories: Timebase presentation Orbit presentation
VIBRATION FORM
TIME
TIMEBASE- an oscilloscope displays the vibration in units of thetransducer (amplitude) versus time horizontally across the display.
AM
PL
ITU
DE
TIME
VIBRATION FORM
ORBIT- is the output of two transducers at 90 angles to one another (XYplane) in the X-Y mode of the oscilloscope. The orbit is the representation of the shaft centerline movement.
ORBIT
Shape
Measurement References
The three basic frames of reference for vibration measurements on rotating machines are: Rotor motion relative to the bearing (Shaft
relative Vibration).
MEASUREMENT REFERENCES
Shaft Relative
Measurement References
The three basic frames of reference for vibration measurements on rotating machines are: Rotor motion relative to the bearing (Shaft
relative Vibration). Casing motion relative to a fixed reference
(Casing Absolute Vibration). Rotor motion relative to a fixed reference
(Shaft Absolute Vibration).
MEASUREMENT REFERENCES
Casing Absolute
MEASUREMENT REFERENCES
Shaft Absolute
PositionMeasurements
Axial PositionThrust PositionRotor Position
Radial PositionDifferential ExpansionCase ExpansionEccentricity
AXIAL POSITION
THRUSTBEARINGASSEMBLY
THRUSTPADS
THRUSTCOLLAR
Measurement of the position of the thrust collar in the thrust bearing assembly and how much bearing material may be used up.
The probes are mounted on the thrust bearing observing the movement of the rotor position within the thrust bearing.
Thrust Position
THRUST POSITION
20
15
10
5
0
20 40 60 80 100MILS
0.5 1.0 1.5 2.0 2.5mm0
HOT FLOAT ZONECOLD FLOAT ZONE
COUNTERDIRECTION
NORMALDIRECTION
Thrust PositionNORMAL/COUNTER
Normal - direction the rotor would normally be forced to move due to the designed operation of the machine. Steam turbine typically Away from high
pressure (HP) inlet towards the LP inlet.
Counter - direction of thrust opposite the direction the rotor is normally designed to move. indicates the machine is experiencing an
abnormal operation
THRUST POSITION
NORMALDIRECTION
NORMALDIRECTION
HP
LP LP
HP
STEAM TURBINE COMPRESSOR
RADIAL POSITION
Proximity Probes are used inthe X-Y configuration to measureradial vibration, the dc signal fromthe transducer can be used toindicate the radial positionof the rotor within the bearing
DIFFERENTIAL EXPANSION
CLEARANCESTHRUSTBEARING
EXPANSION
CASE EXPANSION
FIXED FOOTSLIDING FOOT
FIXED FOOT
FIXED REFERENCEFOUNDATION
EXPANSION
FRONT STANDARD
LVDTsEXPANSION
LVDT
CASING EXPANSION
DUALCASEEXPANSION
SINGLECASEEXPANSION
ECCENTRICITY
SHAFT BOW
ECCENTRICITY
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