Blade or Vane pass frequency

The number of fan blades or pump vanes times the rotational speed equals the specific frequency.

Base-line Spectrum

A vibration spectrum taken when a machine is in good operating condition; used as a reference for monitoring and analysis.

Bandwidth

The spacing between frequencies at which a bandpass filter attenuates the signal by 3 dB.

Band-Pass Filter

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

Ball Pass Frequency

The frequency generated when a rolling element passes over a flaw in the inner race, BPFI, or over the outer race, BPFO.

broaching

The cutting of holes of various shapes or cutting of an outside surface, with a ‘broach’consisting of a tapered bar with cutting edges. The broach moves in a reciprocating axial manner.

buckling

Sudden large-scale deformation of a strut, thin cylinder, etc., due to instability when loaded, e.g. an axial load on a strut.

bulk modulus

The ratio of pressure (three-dimensional stress) to volumetric strain of a material.

buoyancy

The apparent loss of weight experienced by a submerged or floating body due to the upthrust caused by fluid pressure.

boundary layer

A thin layer of fluid adjacent to a surface over which the fluid flows, which exerts a viscous drag on the surface due to the large velocity gradient.