Basic Concepts inVibration

Basic ConceptsSHAFT.

Basic ConceptsSHAFT.

pk0pkpkrmsAmplitude of Vibration:Displacement, mils or microns, peak-to-peakVelocity, in/sec or mm/sec, peakAcceleration, gs , peakBasic Concepts

Vibration TransducersWhat is a Transducer?

Vibration TransducersWhat is a Transducer?. . . is a devise that converts a physical measurement to a proportional electrical signal

Vibration TransducersDisplacement Transducers(eddy-current or proximity xducers)Velocity Transducers(Seismic transducers)AccelerometersThree Types of Transducers:

Proximity Transducer System

Proximity Transducer System

Velocity TransducerCasingDamping fluidMagnetSpringCut away view of a Velocity ProbeCoil

ACCELEROMETERCut away view of AccelerometerMICAINSULATORCONDUCTIVEPLATEELECTRICALINSULATORELECTRONICSCOMPONENTPRELOAD BOLTMASSPIEZOELECTRICCRYSTALBASEASSEMBLY

VELOMITOR

Comparative Frequency ResponseBasic Concepts

Vibration Signal CharacteristicsAmplitudeFrequencyPhaseShape & FormPositionAll of them together are used to reach an accurate assessment of the mechanical condition of a machineThere are five basic signal characteristics that need to be observed:

AMPLITUDEPeak-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.

Radial Vibration Shaft Centerline Position Shaft Attitude Angle Eccentricity Shaft Trend Plots Alignment Along ShaftFluid Film Bearing Monitored byX-Y Proximity Probe Transducers

X-Y Proximity Probes Mounted To Each Bearing

PhasePhase is defined as the timing relationship, in degrees between two or more signals.relativeabsolute

ONE REVOLUTIONONE REVOLUTIONPhase & Frequency fromKeyphasor Reference Signal

Absolute PHASE LAGThe phase angle is defined as the number ofdegrees from the Keyphasor pulse to the firstpositive peak of vibration.

Relative PHASEABTIME(DEGREES)PHASETIME(DEGREES)AMPLITUDEPHASE(BETWEEN VIBRATION SIGNALS)

Deflection ShapeAB

FrequencyFrequency 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, i.e. 1X, 2X, 3X nX.Basic frequency measurements can be performed with an oscilloscope and Keyphasor signal.

FREQUENCYTIME/CYCLE

Shape & FormVibration form is the raw waveform displayed on an oscilloscope and can be separated into two categories:Timebase presentationOrbital presentation

VIBRATION FORMTIMETIMEBASE- an oscilloscope displays the vibration in units of thetransducer (amplitude) versus time horizontally across the display.AMPLITUDETIME

Orbital Presentation

VIBRATION FORMORBIT- is the output of two transducers at 90 angles to one another (XY plane) in the X-Y mode of the oscilloscope. The orbit is the representation of the shaft centerline movement.ORBIT

Shape

PositionRadial shaft position is a measurement of the shaft centerline radial position within the radial bearing.Derived from the dc information provided by the proximity system.Used to determine bearing wear, misalignment, external preloads and other malfunctions.

Average Shaft CenterlineAmplitude 0.20 mils / divX to Y (CCW) Rotation(Not Orbit or Polar Plot)Top

Thank you!!!

The End.The student will be able to relate the relative phase measurements from the inboard and outboard ends of a two plane rotor kit to the rotors mode shape. If the vertical inboard and vertical outboard 1X compensated vibration signals are in phase, the rotor kit is operating in a cylindrical or translational mode shape. Q. What is the phase relationship between probes 2 and 3 for example A ? Q. What is the phase relationship between probes 2 and 3 for example B ? Review Questions: Q. What happens to the signal of a proximity transducer when the shaft moves closer to the probe tip? A. The signal goes more positive.

Q. What does a positive peak on a vibration waveform represent? A. The shafts closest approach to the probe tip. DEMO Rotor Kit Demonstration: Run a two plane rotor kit at a speed above its first critical, but slow enough that it is still in its first mode. Display the Inboard and Outboard vertical signals, filtered at 1X and compensated, using a timebase wave form. Be sure to explain the setup to the class. Ask the students for the relative phase between the Inboard and Outboard ends of the rotor. It should be obvious to the class that the signals are in phase.

Ask the students to describe the apparent mode shape of the rotor. It should also be obvious that the apparent mode shape is cylindrical or translational. Q. Could you tell the mode shape of the rotor if you were only looking at vibration amplitude (Frequency, or Position)? A. Obviously Not. If the vertical inboard and vertical outboard 1X compensated vibration signals are 180 degrees out of phase, the rotor kit is operating in a pivotal or conical mode shape. DEMO Rotor Kit Demonstration: Run the rotor kit to a speed above its second balance resonance. Using the same set up as before, ask the class for the relative phase of the inboard and outboard vibration signals. It should be obvious that the signals are approximately 180 degrees out of phase. Ask the class what the apparent mode shape of the rotor is at this speed. It should be obvious that the rotor is now apparently operating in a pivotal or conical mode shape. Review Question: Again ask the class if they would be able to tell the mode shape of the rotor if they were only looking at amplitude, or frequency, or position. It should be obvious that it would not be possible.The student will be able to read or calculate the measurements available from the average shaft centerline plot.

The transient average shaft centerline plot is a plot of the relative X-Y DC displacement components of the vibration signals plotted against machine speed.

Because the average shaft centerline plot is derived from relative displacement transducers, a starting point must be assumed.

- In the example provided, the starting point was assumed to be at the bottom of the bearing. It is difficult to determine whether the assumption is valid, until after the machine is run and the data analyzed.

- The shaft centerline plot correlated with the multiple orbits plot can provide indications of misalignment or other heavy preloads.