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Questions and Answers
What is the name of the company that published this booklet?
What is the name of the company that published this booklet?
Brüel & Kjær
Vibration can be caused by out-of-balance forces in rotating members.
Vibration can be caused by out-of-balance forces in rotating members.
True
What is the unit of measurement for frequency?
What is the unit of measurement for frequency?
Hertz (Hz)
Which of the following is NOT a vibration parameter discussed in the booklet?
Which of the following is NOT a vibration parameter discussed in the booklet?
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What is the name of the device typically used to measure vibration?
What is the name of the device typically used to measure vibration?
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Which of the following is a characteristic of a piezoelectric accelerometer?
Which of the following is a characteristic of a piezoelectric accelerometer?
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The law that relates force. mass. and acceleration is ______ = ______ x ______.
The law that relates force. mass. and acceleration is ______ = ______ x ______.
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What is the name of the type of accelerometer where the mass exerts a compressive force on the piezoelectric element?
What is the name of the type of accelerometer where the mass exerts a compressive force on the piezoelectric element?
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What is the name of the other type of accelerometer where the mass exerts a shear force on the piezoelectric element?
What is the name of the other type of accelerometer where the mass exerts a shear force on the piezoelectric element?
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The sensitivity of an accelerometer is always a critical factor in normal circumstances.
The sensitivity of an accelerometer is always a critical factor in normal circumstances.
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What is the general rule for the accelerometer mass compared to the dynamic mass of the vibrating part?
What is the general rule for the accelerometer mass compared to the dynamic mass of the vibrating part?
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What is the upper limit of a typical general purpose accelerometer?
What is the upper limit of a typical general purpose accelerometer?
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Which factor limits the low frequency response of an accelerometer?
Which factor limits the low frequency response of an accelerometer?
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Using a narrow bandwidth filter can lead to a faster analysis time.
Using a narrow bandwidth filter can lead to a faster analysis time.
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What are the two main types of filters used in frequency analysis?
What are the two main types of filters used in frequency analysis?
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Constant bandwidth filter gives better frequency resolution at low frequencies.
Constant bandwidth filter gives better frequency resolution at low frequencies.
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What is the purpose of a preamplifier when measuring vibrations?
What is the purpose of a preamplifier when measuring vibrations?
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A vibration meter can only measure acceleration.
A vibration meter can only measure acceleration.
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What is the purpose of frequency analysis in vibration measurement?
What is the purpose of frequency analysis in vibration measurement?
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Using vibration measurements, it’s possible to establish a basic understanding of a machine’s condition.
Using vibration measurements, it’s possible to establish a basic understanding of a machine’s condition.
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How is ‘on-condition’ maintenance beneficial compared to preset schedules for maintenance?
How is ‘on-condition’ maintenance beneficial compared to preset schedules for maintenance?
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Which of the following is NOT a potential problem associated with excessive vibrations?
Which of the following is NOT a potential problem associated with excessive vibrations?
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What is a common diagnostic tool that uses vibration measurements?
What is a common diagnostic tool that uses vibration measurements?
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What is the term for a multiple of the fundamental frequency in vibration?
What is the term for a multiple of the fundamental frequency in vibration?
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Study Notes
Introduction
- This booklet provides a basic explanation of vibration measurement for newcomers.
- It covers the reasons for measuring vibration, its source, quantification methods (acceleration, velocity, displacement), units, and environmental factors (e.g., temperature).
- The booklet details the importance of accelerometer calibration and choosing appropriate mounting positions, and the influence of environmental factors like cable noise, other environmental influences, and temperature.
Background
- Vibration isolation and reduction techniques have become integral parts of machine design.
- Accurate measurement and analysis of mechanical vibration are crucial for modern machinery.
- The use of piezoelectric accelerometers converts vibratory motion into electric signals for measurement and analysis.
Where Does It Come From?
- Vibration is often unavoidable due to manufacturing tolerances, clearances, and out-of-balance forces in rotating or reciprocating parts, and sometimes amplification of small vibrations.
- Intentional vibration can be used for purposes like component feeders, and concrete compactors, etc).
What is Vibration?
- Vibration is an oscillating motion around a reference point.
- Frequency, measured in Hertz (Hz), describes the number of cycles per second.
- Vibration signals often consist of multiple simultaneous frequencies.
- Frequency analysis breaks down complex signals into individual frequency components, often depicted in a spectrogram (vibration level plotted against frequency).
- This analysis is useful in identifying the source of disruptive vibrations.
Quantifying the Vibration Level
- Peak-to-peak: Maximum excursion in a wave. Useful for shock impacts, but only for maximum excursion.
- Peak: Maximum value of the wave. Useful for shock impacts, but again only for the maximum value.
- Average: Takes the wave's time history into account but is not a practical measure for vibration measurements.
- RMS: A relevant amplitude measure accounting for wave time history that is related to energy content (destructive abilities from vibration).
The Vibration Parameters, Acceleration, Velocity, and Displacement - Measuring Units
- Amplitude, velocity, and acceleration are related mathematically.
- ISO requirements specify metric units for vibration parameter measurements.
- Frequency of the mechanical phenomenon has a direct impact on the magnitude of each variable.
- Different instruments measure these parameters.
Considerations in Choosing Acceleration, Velocity, or Displacement Parameters
- Acceleration measurements are useful for identifying high-frequency components.
- Velocity measurements provide a good overall assessment of vibration severity across a range of frequencies.
- Displacement measurements are typically useful for lower frequency analysis and imbalances in rotating parts.
- Selecting the most appropriate parameter depends on the frequency analysis purpose and frequency range.
The Piezoelectric Accelerometer
- The piezoelectric accelerometer is the most common vibration transducer.
- It's self-generating, doesn't need a power supply, and provides an acceleration-proportional output.
- Piezoelectric effect: Converts mechanical stress into an electrical charge.
- This characteristic allows the transducer to measure acceleration and, with integration, velocity or displacement.
Practical Accelerometer Designs
- Piezoelectric element is used to measure acceleration and is proportional to the force, while the mounting method is either compressive or shear-type.
- The compression type uses compressive force; the shear type uses shear force.
Accelerometer Types
- Most manufacturers offer a range of accelerometers suited for different applications.
- General-purpose types are for the most common uses.
- Some are compact.
- Others are optimized for special applications or working environments (e.g., high temperatures, vibration, shocks) and with different sensitivity and characteristics
Accelerometer Characteristics
- Sensitivity: Ideally a high output level, but compromise due to piezoelectric size and weight.
- Mass: Important when measuring on light objects. Should be significantly less than the object under examination.
- Dynamic Range: Important when measuring high or low levels, the device has an upper and lower limit or range.
Accelerometer Frequency Range Considerations
- The frequency range should be sufficient to fully cover the range of interest.
- There's an upper frequency limit defined by the accelerometer's resonant frequency
- Lower limits are generally impacted by the amplifier's cut-off frequency.
Avoiding Errors Due to Accelerometer Resonance
- Accelerometer resonance causes higher sensitivity at the high-frequency end leading to an inaccurate interpretation of the vibration level at those frequencies.
- Low-pass filtering within vibration measuring instruments can resolve the issues of inaccurate readings.
- Mechanical filters can be used to reduce the effects of high-frequency vibration or increase reliability of the measurements.
Choosing a Mounting Position for the Accelerometer
- The accelerometer's mounting position should match its sensitivity axis for accurate results.
- This is important in different situations.
- Avoiding obstructions to vibrations signal from the object being measured.
Mounting the Accelerometer
- Mounting methods vary and depend on the frequency and type of measurements desired.
- Using proper techniques for mounting helps to avoid undesirable vibration resonances, which could distort the readings.
The Influence of Environment - General
- Humidity, temperature fluctuations, nuclear radiation, base strains, acoustic noise, corrosive substances, electromagnetic interference, and transverse vibrations can affect measurement accuracy.
Environmental Influences - Temperature
- Temperature affects the sensitivity of piezoelectric materials.
- Accelerometers with appropriate ceramic materials can tolerate high temperatures
- Heat sinks and other measures can be used to maintain a stable temperature for measurements when the object under examination is at high temperature.
Environmental Influences − Cable Noise
- Ground loops, triboelectric noise, and electromagnetic fields can induce noise issues with the data readings.
- Using double-shielded cable, ground isolation techniques or a balanced accelerometer and differential preamplifier to alleviate these issues.
Other Environmental Influences
- Base strains: Affect measurements when the surface being measured is under strain
- Nuclear Radiation: Some accelerometers can be used in high radiation environments.
- Magnetic fields: A minimal effect on measurements.
- Humidity: Sealed accelerometers are suitable
- Corrosive substances: Many accelerometers are designed to be resistant to corrosion.
- Acoustic noise: Usually not a concern.
- Transverse vibrations: Accurately measure low vibrations.
Accelerometer Calibration
- Accelerometers calibration may be affected by shocks, temperatures, etc.
- Periodic checks are recommended for optimal accuracy.
A Simple Calibrator
- A small, battery-operated device that vibrates at a precise frequency (e.g., 10 m/s²).
- Used to calibrate accelerometers by measuring output ratios with a reference accelerometer.
- Can check the complete measuring or analyzing setup.
Force and Impedance Measurements
- Force transducers are employed along with accelerometers to determine dynamic forces and resulting vibrations.
- This analysis helps determine the mechanical impedance of the structure being examined.
- Force can be determined from the piezoelectric effect, which transforms force to an electrical signal.
Logarithmic Scales and Decibels
- Logarithmic scales are often used for frequency and amplitude to plot vibration signals in diagrams to avoid errors in plotting.
- Decibels (dB) are used to compare vibration levels to a reference level.
Why use an Accelerometer Preamplifier?
- Direct loading of piezoelectric accelerometers can reduce the frequency response and sensitivity.
- Preamplifiers offer impedance matching, adjustable gain, integration capabilities, and filtering.
- Essential for ensuring the proper measurement range.
The Vibration Meter
- Vibration meters typically combine accelerometer, amplifier, filters, and displays.
- A range of frequencies and level types is available and can be chosen depending on measuring needs.
- They have both AC and DC amplification features.
- External filtering may be used
What is Frequency Analysis?
- Vibration signals often contain various frequencies.
- Frequency analysis isolates individual frequency components.
- Filters are used in this analysis to isolate individual frequencies within a wide frequency range.
Constant Bandwidth or Constant Percentage Bandwidth Frequency Analysis
- Two types of filters used in frequency analysis for vibration signals.
- Constant bandwidth filters offer a constant absolute bandwidth.
- Constant percentage filters offer a bandwidth that's a constant percentage of the center frequency.
Filter Bandwidth Considerations
- Filter bandwidth affects frequency resolution. A narrow bandwidth provides more details.
- A wide bandwidth is important for a wider frequency range coverage of the device or structure being examined.
Measuring Instrumentation
- Portable and laboratory-based vibration measuring instruments are used.
- Many include a variety of features such as filtering, integration, and recording capabilities.
- Frequency analyzing systems are also used.
Recording Results
- Method of recording varies and depends on the number of measurements and frequency analyses performed.
- A level recorder gives a visual record of vibration frequencies over time.
- Automatic recording is useful for data analysis with an ease.
- Time-based recording is also useful for specific occurrences
Using Vibration Measurements
- Single-wide frequency band vibration measurements can quickly evaluate overall machine condition or vibration isolation techniques.
- Frequency analysis is required to identify frequency components related to operational conditions, etc., thus allowing better diagnosis.
Vibration as a Machine Condition Indicator
- Machines often show signs of impending failure in the form of increasing vibrations.
- Vibrations are useful as indicators of a machine's condition and can demonstrate needs for maintenance or repair.
- Vibration level plotted against time. This helps in maintenance schedules or identifying warning signs.
Vibration Trouble Shooting Chart (A) & (B)
- Charts (A) and (B) provide troubleshooting guidance for identifying vibration causes in machinery.
- The charts link vibration frequencies with potential machine faults.
- For each possible fault, the dominant vibration frequency and direction are listed.
Vibration and the Human Body
- Direct vibration on the human body can cause harmful effects (e.g., blurred vision, loss of balance).
- Certain frequencies and levels can permanently damage body organs.
- International standards define acceptable levels of worker exposure to vibration in different directions (transverse and longitudinal).
Vibration and the Human Body - Recommendations
- International standard ISO 2631 (1978) sets exposure limit curves for vibration effects on the human body between 1Hz to 80Hz.
- Exposure times and severity criteria are defined.
- Certain frequency bands resonate with the human body therefore need to be avoided as they can cause issues.
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