Saturday, January 16, 2016

Vibration Measurement Basics


 Measured Characteristics:

Amplitude
Frequency
Phase Angle
Absolute Phase Angle

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
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.

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









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










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).



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.


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?

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 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.