Unbalance
Unbalance is a rotor condition where unequal weight distribution causes the principal axis of inertia to deviate from the rotation axis, generating centrifugal force proportional to the square of rotational speed (F = U · ω²). This results in vibration and accelerated wear, commonly detected via vibration analysis.
In shop floor maintenance, unbalance is detected through vibration analysis in CMMS systems like ServiceGrid, where high 1x radial amplitude and phase differences of 90° ± 30° between vertical and horizontal measurements indicate unbalance. Mitigation involves adding or removing weights to realign the center of gravity with the rotation axis. Poor practices, such as skipping post-overhaul balancing or failing to re-balance after component replacement (e.g., impeller wear), lead to accelerated bearing wear and shaft fatigue. Condition-based maintenance (CBM) tracks daily vibration to identify deterioration curves, enabling proactive scheduling.
- Static unbalance: Center of gravity offset in a single plane, causing vertical vibration in overhung masses.
- Dynamic unbalance: Inertia and rotation axes intersect but are not coincident, causing vibration in both axial and radial planes.
- Poor balancing practices: Skipping post-overhaul balancing or failing to re-balance after component replacement, leading to accelerated bearing wear and shaft fatigue.
How is unbalance quantified in ISO standards?
Unbalance U is defined as U = m · R (mass × radial distance), and ISO standards specify allowable residual unbalance based on rotor type and speed.
What distinguishes static from dynamic unbalance in vibration signatures?
Static unbalance shows high 1x radial vibration with axial phase in-phase; dynamic unbalance shows high 1x in both radial/axial with axial phase out-of-phase.
How does CBM use unbalance monitoring for prognostics?
CBM tracks daily vibration at bearings to identify unbalance deterioration curves, extrapolating future values to schedule maintenance before failure.