Eccentricity
Eccentricity in rotating machinery is the radial offset between a rotor's center of mass and its geometric center, or misalignment between rotor and stator centers in electric motors, causing a non-uniform air gap. It is a key indicator for predictive maintenance, measured via vibration analysis or laser alignment, and triggers corrective actions when exceeding 10% of the air gap length.
On the shop floor, eccentricity is measured using vibration analysis, laser alignment tools, or runout metrology, and tracked in CMMS systems like ServiceGrid as a key indicator for predictive maintenance. When eccentricity exceeds 10% of the air gap length in motors, it triggers corrective actions such as shaft balancing or rotor recentering. Maintenance teams use it to detect unbalance, bearing wear, or assembly errors, scheduling interventions to prevent unplanned downtime and extend equipment life.
How does eccentricity differ from unbalance?
Eccentricity is the physical cause (offset distance), while unbalance is the dynamic effect (centrifugal force) at a given speed; mathematically, Unbalance (U) = Mass (M) × Eccentricity (e).
What is the allowable eccentricity limit for induction motors?
Maximum allowable is 10% of the air gap length; exceeding this causes secondary failures like insulation breakdown or rotor bar fracture.
What types of eccentricity faults exist in motors?
Three types: (i) Static (unequal air gap due to non-coincident axes), (ii) Dynamic (rotor spins off its own center axis), and (iii) Mixed (combination of both).