Reliability Service Glossary of Terms
Acceleration: The rate of change of velocity (speed) usually expressed in ‘g’ or gravity units.
Accelerometer: Transducer or pickup that measures acceleration.
Amplitude: The extreme range of a signal, either fluctuating (AC) or static (DC).
Balance: The condition where no vibration occurs in a rotating member due to the center of mass being displaced from the center of rotation.
Bearing Defect Frequencies:
Different bearing defects generate specific frequencies:
- An outer race defect generates a Band Pass Frequency of an outer race defect (BPFO).
- An inner race defect generates a Band Pass Frequency of an inner race defect (BPFI).
- A roller/ball defect generates a Ball Spin Frequency (BSF).
Cavitation: A result of a pump not operating near its best efficiency point or having insufficient net positive suction head.
Critical Speed: The speed of a rotating member that corresponds to a resonant frequency in the member. Usually, a maximum vibration occurs at this point. A member may have several critical speeds.
Displacement: The change in position of a body measured from the point of rest. Usually expressed in mils (.001 in).
Frequency: The number of vibrations or cycles in a unit of time. Usually expressed in cycles per second (Hz) or cycles per minute (CPM).
Harmonic: One whose vibration frequency is an integral multiple of that of the fundamental.
Laser Vibrometer: State-of-the-art transducer for non-contact vibration analysis.
Modal Analysis: The analysis of structural bending characteristics.
Natural Frequency: The frequency at which a component or a complete system resonates. Every structure has a natural frequency, which, when excited, can be detrimental to the structure.
Oil Whirl: Produced when a rotating shaft within a sleeve bearing is forced into an orbital pattern by an oil pressure within the bearing that is greater than the force that the shaft can exert upon the oil.
Resonance: The natural frequency of a system. When an unbalanced system is running at its resonance or one of its resonances, the vibration reaches a peak. Any change in speed from this point will decrease the amplitude of vibration.
Resonant Frequency: That speed at which a system resonates, see Resonance.
Real Time Analyzer (RTA): Possesses superior analytical capabilities.
- Greater resolution
- Low frequency capabilities
Single Plane Balancing: Addition of balancing weight in a single plane to achieve coincidence of the mass center with the rotational center line.
Spectrum (Signature): A machine has many moving parts, such as shafts, gears, bearings, etc., each of which has its own vibration characteristics. These characteristics combine to form a vibration pattern (signature) for this machine. The vibration signature consists of multiple vibration frequencies that differ in amplitude and phase.
Static Balancing: Method of balancing without rotating the item to be balanced at full running speed.
Transducer: A device that converts a physical (mechanical) force to an electrical signal such as a velocity transducer, accelerometer or strain gauge.
Trending: Displaying the value of measurements against time is an easy and accurate method of detecting gradual changes that might otherwise be overlooked.
Two Plane Balancing: The required addition of balance weight in two planes to affect the coincidence of mass and the rotational center line of a rotor. Usually required or specified when the diameter is less than twice the length of the rotor.
Velocity: The rate of change of displacement usually expressed in inches per second (IPS or IN/SEC).