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Baffle

A device, usually filled with foam or fiber, which is used to block noise from entering or exiting an confined space.

Balancing (mechanical)

A procedure for adjusting the radial mass distribution of a rotor so that the mass centerline approaches the rotor geometric centerline, thereby reducing the vibratory forces generated by rotation.

Balancing Resonance Speed(s)

A rotative speed that corresponds to a natural resonance frequency.

Balanced Condition

For rotating machinery, a condition where the shaft geometric centerline coincides with the mass centerline.

Band-Pass Filter

A filter with a single transmission band extending from lower to upper cutoff frequencies. The width of the band-pass filter (band width) is determined by the separation of frequencies at which amplitude is attenuated by 3 dB (0.707).

Bandwidth (Measurement)

The frequency range (usually stated in hertz or Hz) within which a measuring system can accurately measure an amplitude quantity.

Bandwidth (Digital Filters)

The spacing between frequencies at which a band-pass filter attenuates the signal by 3 dB. In a digital signal analyzer, the measurement bandwidth is equal to [(frequency span)/(number of filters) x (window factor)]. Window factors are: 1 for uniform , 1.5 for Hanning, and 3.63 for flat top (Uniform, Hanning and Flat Top are digital filter shapes).

Baseline spectrum

A vibration spectrum taken when a machine is in good working condition (new or just overhauled), which is then used as reference for future monitoring or analysis.

Bearing (rolling Element)

A Rolling element bearing has four parts: an inner race, an outer race, balls or rollers, and a cage to maintain the proper separation of the rolling elements.

Bearing (sleeve)

A sleeve bearing is a cylinder of alloy metal surrounding the rotating shaft. Contact between the shaft and sleeve is prevented by a lubrication film.

Beat Frequency

Where two cyclic components are close together in frequency they combine in such a way that their sum will vary in amplitude at a rate equal to the difference in frequency between the two components. This phenomenon is known as beating, and its frequency is known as the beat frequency.

Bias

Refers to a more or less persistent tendency for the measurements, as a group, to be too large or too small.

Biased Hall Effect Sensor

Sensor that operates with either magnets or ferrous metal targets, Generates a square wave output.

Bins (lines)

In an FFT spectrum, the individual frequencies at which the amplitudes are calculated.

BIT

Short for binary digit. A number expressed in binary notation utilizes the digits 1 and 0, and these are called bits. Any number can be expressed with combinations of them.

Blade Passing Frequency

A potential vibration frequency on any bladed machine (turbine, axial compressor, fan, etc.). It is represented by the number of blades times shaft-rotating frequency.

Block Size

The number of samples used in a Digital Signal Aanalyser (DSA) to compute the Fast Fourier Transform (FFT). Also the number of samples in a DSA time display. Most DSAs use a block size of 1024. Smaller block size reduces frequency resolution.

Bode

Rectangular coordinate plot of 1x component amplitude and phase (relative to a keyphasor) vs. running speed / Magnitude and phase of vibration in a machine plotted against speed.

Broadband

Vibration (or other) signals which are unfiltered. Signals at all frequencies contribute to the measured value.

Bode plot

The magnitude of vibration at 1x shaft speed, also its phase relative to the key phaser, both plotted against running speed.

Bow

In Rotating Machinery. A shaft condition in which the geometric shaft centerline is not straight.Can be caused due to inadequate baring during cool down.

BPFO, BPFI.

Common abbreviations for ball pass frequency of defects on outer and inner bearing races, respectively.

Brinneling

Impressions made by bearing rolling elements on the bearing race; typically caused by external vibration when the shaft is stationary.

Broadband

Vibration (or other) signals which are unfiltered. Signals at all frequencies contribute to the measured value.

Buffer

A memory location in a computer or digital instrument that is set aside for temporarily storing digital information while it is waiting to be processed.

Buzz

A sound exemplified by loose power transformer laminations (dominated by 120 Hz where the power frequency is 60 Hz).