Vibration Flashcards
(32 cards)
Define vibration
Back and forth motion in a machine that deviates from normal operation
What are the four motion characteristics of vibration?
1) Displacement
2) Velocity
3) Acceleration
4) Frequency
Why does a vibration curve for a piece of equipment not normally look like a smooth vibration curve?
Each source of vibration has its own displacement time domain waveform characterized by a frequency, shape, and amplitude
Why is it useful to determine the frequency or frequencies of vibration?
To determine the cause of the vibration
List 5 causes of vibration
1) Unbalance
2) Misalignment
3) Mechanical looseness
4) Fan blade problems
5) Component wear
What is displacement?
Distance a mass moves relative to a reference point
What is velocity?
Rate of change of displacement
What is acceleration?
Rate of change of velocity
If you measure one vibration parameter, how can you calculate another?
Vibration parameters are related by factors that depend on vibration frequency
What is the difference between vibration monitoring and vibration analysis?
Vibration analysis: periodic measurement and vibration analysis for preventative maintenance
Vibration monitoring: permanently installed sensors that continuously monitor equipment vibration using indicators or alarms
What are the benefits of predictive maintenance?
1) Extends machinery lifespan
2) Minimizes downtime
3) Minimizes the risk of failure
4) Improves safety
5) Reduces energy cost
What is natural frequency? How does the natural frequency relate to resonance and critical
speed?
Natural frequency is a frequency at which an object tends to vibrate. Exaggerated vibration caused by a force acting on the natural frequency is resonance. Resonant frequencies that cause severe vibration are called critical speeds.
True/false: You should operate equipment at its critical speed.
False
What are the three types of vibration transducers?
1) Proximity/ displacement
2) Velocity
3) Accelerometer
When are displacement/proximity sensors most commonly used?
Used on equipment where the vibrations are not well transmitted through the machine case.
What is the measurement principle for a displacement transducer?
- Oscillator signal is sent through a coil creating a magnetic field
- Shaft movement creates eddy currents that disrupts the magnetic field
- The closer the shaft, the smaller the output of the signal
Can a displacement probe be used with a non-magnetic shaft?
Yes, but the shaft needs to be conductive
Should a displacement probe be used with a shaft that has a scratch in it?
No, becaused discontinuities affect magnetic spots on the shaft
What are you determining/updating when you calibrate the probe for a different material?
Scale Factor, a change in output voltage over a change in distance
What device is used to calibrate, validate and test a displacement transducer?
Wobulator
Displacement/proximity transducers are best for monitoring _________________
vibrations.
Low frequency
What is the measurement principle for a velocity transducer?
A transducer is attached to a moving object. Inside the transducer a permanent magnet moves back and forth in a coil inducing a voltage. The voltage is proportional to the velocity.
What are the disadvantages of velocity transducers?
1) Large in size making installations difficult
2) Affected by extreme temperatures
3) Expensive
4) Not accurate at low frequencies
What is the frequency range for velocity transducers?
8-1500 Hz
