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Vibration Measurement Basics

 Measured Characteristics:

l Amplitude

l Frequency

l Phase Angle

w Absolute Phase Angle

w Relative Phase Angle

AMPLITUDE

Peak-to-peak refers to the total amount of vibration.

Zero-to-peak refers to the total amount of vibration from the

maximum height of either the positive or negative peak to the

zero voltage axis.

Root mean square (RMS) is a function of the signal conditioning performed in the                                                                                      monitor or diagnostic instrument and not                                                                              the output of the transducer.

Frequency

l Frequency is defined as the repetition rate of a periodic vibration within a unit of time.

l The frequency of vibration (cycles per minute) is most often expressed in multiples of rotative speed of the machine.

l Basic frequency measurements can be performed with an oscilloscope and Keyphasor signal.

PHASE

l Phase is defined as the timing relationship, in degrees between two or more signals.

PHASE ANGLE

          .The phase angle is defined as     the number of degrees from the   Keyphasor pulse to the first positive peak of vibration.

Measurement References

l The three basic frames of reference for vibration measurements on rotating machines are:

w Rotor motion relative to the bearing (Shaft relative Vibration).

w Casing motion relative to a fixed reference (Casing Absolute Vibration).

w Rotor motion relative to a fixed reference (Shaft Absolute Vibration).

Position  of  Measurements

lAxial Position

wThrust Position

wRotor Position

lRadial Position

lDifferential Expansion

lCase Expansion

lEccentricity

Axial Position

Thrust Position

lMeasurement of the position of the thrust collar in the thrust bearing assembly and how much bearing material may be used up.

lThe probes are mounted on the thrust bearing observing the movement of the rotor position within the thrust bearing.

Rotor Position 

Radial Position: 

    Proximity Probes are used in The X-Y configuration to measure

radial vibration, the dc signal from  the transducer can be used to

indicate the radial position   of the rotor within the bearing

Differential Expansion

Case Expansion

                                  Eccentricity

How does Vibration Start?

l Energy must be put into the system through an applied force, either internal or external. 

l 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 is Vibration?

l Mechanical vibration is the dynamic motion of machine components.

l Vibration measurement is the measurement of this mechanical vibration relative to a known reference.