Definitions of Vibration Terms

Acceleration: The rate of change in velocity with respect to time. For the purpose of this document, and most vibration work, acceleration is measured in the unit g's (Peak).

Alignment: The objective of alignment is to have all rotating shafts in a perfect parallel, linear relationship under normal operating conditions. Misalignment may be parallel, angular, or a combination of both. Misalignment can also be introduced by not considering machine thermal growth of the shaft if there is a considerable difference in temperature between operating and start-up conditions. It is very common to find misalignment in belt driven machinery, since belt systems allow more "forgiveness".

Amplitude: The value or magnitude of the measured vibration quantity (displacement, velocity, acceleration) expressed in peak-to-peak (P-P), peak (P) or root-mean-squared (RMS).

Critical Speed: The speed of a rotating system that corresponds to a resonant frequency of the system. The "1st Critical" is the lowest natural frequency at which a system will resonate.

Displacement: The distance traveled by a vibrating object. For purpose of this document, displacement represents the maximum distance traveled by a vibrating part or surface from the maximum position of travel in one direction to the maximum position of travel in the opposite direction (Peak to Peak) and is measured in mils (1 mil = 0.001 inch).

Forced Vibration: The response of a system is caused and determined by the nature of the vibration excitation. If the excitation is periodic and continuing, such as imbalance, the vibration is steady-state.

Frequency: The number of cycles completed in a specific time period. For the purpose of this document, frequency is measured in the unit of cycles per minute (CPM). Frequency may also be expressed in Hertz (cycles per second), but does not provide the precision necessary for mechanical diagnostics. For example, a 60 Hz vibration may be anywhere between 3570 CPM and 3629 CPM. That may not seem significant, but when multiplied as a harmonic the error is compounded.

NOTE: Vibration frequencies are expressed in the unit CPM. Rotational speeds (Running Speed) is expressed in the units RPM.

Fundamental Vibration Frequency: The lowest natural frequency at which a system will vibrate.

g: The acceleration produced by the force due to gravity (weight), variable with latitude and elevation. By international agreement, the value of 980 cm/sec² = 386.087 inch/sec² = 32.1739 ft/sec² has been designated as the standard acceleration due to gravity.

Harmonic: A frequency that is related to and is an integral multiple (2X, 3X, 4X, etc.) of the source or excitation frequency.

Mil: One (1) mil equal one-thousandth, 1/1000 (0.001), of an inch and is a unit of displacement.

Non-Rigid (Flexible) Structure/Mounting/Rotor: A structure for which the shaft rotating speed is near the structure's first natural frequency.

Period: The amount of time, usually expressed in seconds or minutes, required to complete one cycle of motion of a vibrating machine or machine part. The reciprocal of the period is the frequency of vibration.

Phase (Phase Angle): Compares the motion of a vibrating part with a fixed reference, or compares the motion between two vibrating parts. The Phase Angle (usually in degrees) is the angle between the instantaneous position of a vibrating part and the reference position. It represent the portion of the vibration cycle through which the part has moved relative to the reference position. Phase is the measure of timing between two events. In a rotating system, the time delay between two events can be represented by how many degrees the shaft rotates during the delay period. Phase is most often used in balancing. In this instance, phase is a measurement of how far the shaft rotates between the time a reference mark passes a fixed reference point and when the most positive part of the motion passes the same fixed point. Parts that are in phase move together, parts the are 180 degrees out of phase move opposite each other.

Resonance: Resonance of a system occurs when a frequency of a forced vibration excitation (forcing function) is the same as a natural frequency of the vibrating system. At resonance a significant increase in vibration amplitude will occur limited only by the amount of damping in the system.

Rigid Structure/Mounting/Rotor: A structure which has a fundamental natural frequency higher than the shaft maximum rotating speed for the installation.

Rotational Speed (Running Speed): The speed at which a rotating body, such as a fan, is operated. For the purposes of this document, rotational speed/running speed is measured in the units Revolutions Per Minute (RPM).

Spectrum (Signature): A representation of the amplitude (magnitude) and frequency of the different components that constitute the total vibration quantity. Usually presented as a plot of vibration amplitude (displacement, velocity, or acceleration) versus time or versus frequency. Typically a time plot is referred to as a signature and a frequency plot is a spectra or FFT. The FFT is derived from the time signature.

Transducer (Pickup): A device that converts shock or vibratory motion into a "signal" that is proportional to a parameter of the motion. Transducer selection is dependent on the frequencies of vibration to be measured.

o Acceleration Transducer: Generates an output signal which varies in proportion to the acceleration of the vibrating surface to which it is attached. Accelerometer sensitivity is characteristically very low by comparison with a velocity transducer; however an accelerometer has a wide range of frequency response, typically 600 to 600,000 CPM, with special designs extending this range both higher and lower. An accelerometer designed for high frequency response may not have sufficient sensitivity for use in a low frequency situation; conversely, an accelerometer designed for low frequency response may not be useable in a high frequency situation due to its resonance. Accelerometers should be used for detection of high speed gear defects and early warning on ball and roller bearing deterioration.

o Velocity Transducer: Generates an output signal which varies in proportion to the velocity of the vibrating surface to which it is attached. Usually limited to frequencies between 10 and 2000 Hz (600 to 120,000 CPM).

o Displacement Transducer: Generates an output signal which varies in proportion to the distance between the probe tip and an adjacent conducting material. Can sense shaft motion relative to a bearing if mounted to the bearing housing. Usually limited to frequencies below 1000-1500 Hz (60,000-90,000 CPM).

Velocity: The rate of change of displacement with respect to a referenced direction (horizontal, vertical, or axial). For purpose of this document, velocity is measured in the unit in/s (Peak).

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Last updated: November 24, 2002