Vibration Reference Guide Basic Vibration Theory Time vs. Frequency Analysis
Rule 1 for Frequency Analysis
Rule 2 for Frequency Analysis
Individual frequency components are separate in spectrum
A sine wave only has one frequency component
Periodic signals in machinery produce Harmonics
Rule 4 for Frequency Analysis
Rule 5 for Frequency Analysis
Amplitude Modulation produces Sidebands (e.g. gear fault)
Beats look like amplitude modulation in the waveform
Rule 3 for Frequency Analysis An impulse or random noise produces a continuous spectrum
Sidebands equally spaced apart
2 discrete peaks from 2 machines running close by
Presentation of Data
Vibration Analysis – 3 Step Process
Each curve below contains the same information! Select the units for the flattest curve – provides the most visual information. information. Displacement (distance from a reference point) is used for low speeds. Velocity (rate of change of displacement) is the most commonly used for machine diagnostic work. Acceleration (rate of change of velocity) is is used for high-speeds.
1. Identify Peaks: Peaks: Relationship to machine component 2. Trend Amplitude of Peak: Peak: Severity of machine fault 3. Repair Priority: Priority: Based on fault severity
Vibration Transducers Low range
Proximity Probe Displacement
Bearing Fault grows
Mid-range
Velocity Probe – Velocity
High range
Accelerometer Acceleration
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Vibration Reference Guide Simplified Diagnostic Chart Vibration Source Imbalance, Supported Imbalance, Overhung Bent or bowed shaft
Exciting Freq. 1X 1X 1X
Dominant Axes R&T A, R, T A, R, T
Comments Most common Axial deflection Mimic imbalance
Parallel misalignment Angular misalignment Combination Coupling Wear
2X 1X 1X, 2X 3X
R&T A A, R, T any, all
Both sides of coupling Both sides of coupling Most common Both sides of coupling
Rolling Bearing
Non Integer
R, T, A
With harmonics, 1X sidebands, noise floor
1X and Harmonics 1X MB 120 Hz PV or FB Noise
R, T, A
High 4X – 15X
T R, T, A R, T, A R, T, A R, T, A
T > R by 6 dB 120 Hz sidebands (twice line freq.) Twice line freq Vane or blade pass freq. and harmonics Hump of random high freq. noise 20X – 50X
Shaft Looseness And Journal Bearing Foundation flexibility Motor Lamination Unbalanced Phase Fan or Pump wear Cavitation
1X
3 Axes of Data Horizontal Shaft: Radial = Vertical Tangental = Horiz.
1X
Imbalance
2X
Misalignment
3X
Roller Bearing 3.1X 6.2X
2 Frequency Ranges Low Range = 0-10 Orders of shaft rotation High Range = 0-100 Orders o f shaft rotation
3 Axis and 2 Frequency Ranges Low Range Data
High Range Data
Imbalance, Misalignment, Bearings, Fan blades, Pump Impeller, Looseness, Foundation Flexibility
First bearing fault indication, Motor Bars, Gear Mesh, Turbine vanes, harmonics
Angular Misalignment
Motor Bars
Axial (Axial)
Motor Bars Parallel Misalignment
Parallel Misalignment
Vertical (Radial)
Horizontal (Tangental)
Motor Bars
Data Validity Bad Data comes from: - Improper test conditions - Wrong machine tested - Sensor amplifier problem - Incorrect orientation Loose Sensor (magnet mount)
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Overheated sensor (ski slope)
