Glossary
Key terms used across the knowledge base, in plain language.
- Balance quality grade (G)
- A single number (e.g. G6.3) from ISO 21940-11 setting the permissible residual unbalance of a rigid rotor. Numerically it equals the orbital velocity of the rotor's mass centre at service speed (eper × Ω) in mm/s. A lower G means stricter balancing.
- Residual unbalance (U)
- The unbalance remaining after balancing, in g·mm. The target value is Uper = eper × rotor mass.
- Specific unbalance (e)
- Unbalance per unit rotor mass, in µm (= g·mm/kg). eper = 9549 × G / n, where n is the speed in rpm.
- Vibration velocity (mm/s RMS)
- The standard measure of machine-vibration severity, read on the bearing housings and judged against the ISO 20816-3 zones A–D. This is distinct from the G grade.
- FFT (Fast Fourier Transform)
- The algorithm that converts a time-domain vibration signal into a spectrum of frequencies and amplitudes — the basis of vibration diagnostics.
- Running-speed orders (1×, 2×, …)
- Frequencies expressed as multiples of the shaft's rotational frequency. Imbalance appears at 1×; misalignment adds a strong 2×; mechanical looseness adds many orders.
- BPFO — Ball Pass Frequency, Outer race
- The rate at which rolling elements pass a defect on the stationary outer race — a non-synchronous bearing-fault frequency.
- BPFI — Ball Pass Frequency, Inner race
- The rate at which rolling elements pass a defect on the rotating inner race; usually shows sidebands spaced at 1×.
- BSF — Ball Spin Frequency
- The spin frequency of a rolling element about its own axis; a defect on a ball/roller excites it (and 2×BSF).
- FTF — Fundamental Train Frequency
- The rotational frequency of the cage, typically 0.35–0.45× shaft speed — sub-synchronous.
- Critical speed
- A rotational speed at which the shaft frequency coincides with a natural frequency of the rotor–support system, producing a sharp vibration peak.
- Rigid vs. flexible rotor
- A rotor running well below its first critical speed behaves rigidly (balance in one or two fixed planes). One running near or above a critical speed bends and needs flexible-rotor methods.
- Static / couple / dynamic imbalance
- Static = mass-centre offset (one plane); couple = equal masses 180° apart in two planes (a rocking moment); dynamic = the general combination, corrected in two planes.
- ISO 21940-11
- Mechanical vibration — Rotor balancing: procedures and tolerances for rotors with rigid behaviour. Defines the G grades. Superseded ISO 1940-1.
- ISO 20816-3
- Mechanical vibration — Measurement and evaluation of machine vibration. Sets the A/B/C/D vibration-severity zones. Superseded the withdrawn ISO 10816-3.