The fundamentals of vibration measurement are essential for monitoring and maintaining the health of industrial machinery. With accurate vibration measurement, you can identify problems before they worsen, preventing failures and unexpected downtime. To achieve this, sensors and specific techniques capture vibration signals from equipment such as pumps, motors, and turbines.
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Use of vibration throughout history
Machines and other mechanical systems vibrate due to the varying forces and stiffnesses acting on them. This vibration provides valuable insights into how the machine operates and where potential problems arise. While complex and precise systems exist today, the historical uses of vibration monitoring evolved until more modern technologies were developed.
For example, the fishtail method uses a piece of wood with a V-shaped notch, which rests against the rotating shaft. A technician carefully applies a stick to the shaft to measure the vibration level. Another old method is the screw method, in which the technician uses the screw's conductive path to locate the vibration in the machine. Although rudimentary and requiring experience, these methods had their value at the time. Fortunately, vibration measurement methods have advanced significantly since then.
Today, monitoring systems collect data on how a machine vibrates and use this information to identify patterns or irregularities in vibration output. This allows operators to detect and repair problems before they occur, without waiting for visible or audible signals. By the time an operator without equipment notices these changes, the problem is often already aggravated, potentially causing further damage. Therefore, since companies have recognized the value of monitoring, sensors have become part of comprehensive systems that ensure process continuity and the safety of people and facilities.
What is vibration?
To understand the fundamentals of vibration measurement, it's essential to understand what vibration is. Vibration is the mechanical movement or oscillation around the equilibrium position of a component or machine. It can be random, like the movement of a tire on a gravel road, or periodic, like the movement of a pendulum. Technically, vibration results from the relationship between forces and stiffness. In rotating, reciprocating, or structural machines, vibration levels change as forces or stiffness vary.
There are two main forms of vibration:
- Free vibration: It occurs when an object is impacted or displaced and oscillates naturally. For example, a tuning fork vibrates at its natural frequency after being struck, and the sound diminishes due to damping. Musical instruments such as the piano, guitar, and drums also exhibit free vibration.
- Forced vibration: It occurs when external forces act on the system, such as imbalance in a washing machine. In rotating machines, residual imbalance is common and acceptable within limits, allowing continuous operation without degradation.
How is vibration measured?
Vibration is measured in terms of velocity, acceleration, and/or displacement. Units vary depending on the transducer and the type of measurement.
- Displacement: distance traveled by the object, usually measured in mils (1/1000 inch) or microns (1/1000 mm) peak to peak. Proximity probes are used in machines with fluid film bearings.
- Velocity: rate of change of displacement, measured in inches per second (ips) or mm/s peak or RMS. Applies to rotating or reciprocating machines, especially those with rolling element bearings.
- Acceleration: rate of change of velocity, measured in peak g's or peak m/s². It can also be expressed in RMS values for greater accuracy.
Typically, proximity sensors measure displacement, while seismic sensors measure velocity and acceleration. The choice depends on the bearing type and machine speed to ensure the best signal-to-noise ratio.
Tools for detecting machine vibrations
Vibration detection equipment has evolved and now provides early warnings of changes in machine condition. Key tools include:
- Vibration Monitoring Sensors and Systems: They provide the overall amplitude of the vibration and monitor variations from normal, indicating potential problems. Sophisticated systems also monitor amplitude, phase angle, and position.
- Vibration Control Systems and Transmitters: convert complex signals into 4-20mA outputs, facilitating integration with control systems.
- Vibration Meters: measure vibration at specific points on the machine, using a sensor, cable and reader.
- Vibration Analyzers: Monitor vibration simultaneously at multiple points along the machine train, providing a detailed view of the equipment's condition. They analyze amplitude, frequency, phase angle, and position.
To learn more about the basics of vibration measurement and to understand the full line of Metrix solutions, contact usWe are ready to offer technical support and equipment that enhance the monitoring and security of your industrial plant.
Content adapted from an article published in metrixvibration.com
