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