Vibration of a physical quantity is said to occur when its time profile shows regular or irregular reversal points.
Mechanical vibrations are almost unavoidable with rotating or oscillating machinery, and with flowing fluids. As with all turbomachinery, vibrations are also evident in centrifugal pumps and machinery influenced by the fluid handled (see Smooth running).
Measurable acceleration, displacement and velocities of a mechanical vibration are governed by national and international codes such as DIN ISO 10816; DIN ISO 7919; ISO 2372; ISO 5799; DIN 45662; VDI 3839; VDMA 24297 and API 610. These contain the general values for overall assessment of vibrations in machinery.
There are special codes relating to the effects of mechanical vibrations on humans, for example DIN 45 675, VDI 2057 and ISO 2631.
Vibrations are important indicators when monitoring the operating behaviour of machines. When selecting and designing machinery and systems, it is important to eliminate foreseeable vibration-induced problems such as resonances (see Critical speed or Unbalance).