I. Key Points at a Glance
• Single-bearing: The rotor is supported by a bearing at only one end of the generator, with the other end supported by the coupling/engine bearing or structure. It is smaller in size, lower in cost, and easier to assemble. It is suitable for low-power or applications with good fixed base and low vibration.
• Double-bearing: The rotor is supported by bearings at both ends. The rotor position is more constrained, with better radial and radial/axial rigidity during rotation. It can maintain a stable air gap and has strong resistance to lateral deflection. It is suitable for higher power, marine/moving/vibrating conditions, or applications with high requirements for air gap stability.
• Selection Principles (Simplified Version): Determine single-bearing or double-bearing based on power/rotor length (cantilever ratio), marine/moving environment, base quality, allowable air gap variation, coupling method (direct coupling/flexible coupling), vibration and torsional vibration analysis results, maintenance accessibility, and cost.
II. Structural Differences
2.1 Mechanical Support Method
Single Bearing: Only one end near the generator housing has a rolling/sliding bearing, while the other end is constrained by a coupling to the engine end or the base. The advantages are compactness, low weight, and reduced material usage; the disadvantages are that the rotor is a "cantilever", and the air gap is prone to unevenness under radial loads or deflection, affecting electrical performance and vibration.
2.2 Rigidness and Air Gap Stability
Double Bearings: Double-end support significantly increases rotor rigidity, maintains a more uniform air gap between the stator and rotor, reduces electromagnetic asymmetry, local thermal load, and noise caused by deflection. Designs that are sensitive to misalignment and deflection typically use double bearings.
2.3 Tolerance to Vibration/Shock
On high-vibration or low-mass bases (such as ships or mobile platforms), double bearings can better absorb/distribute lateral loads and reduce concentrated fatigue on a single bearing.
2.4 Maintenance and Cost
Single bearings have fewer parts, are easier to assemble and disassemble, and have lower maintenance costs; however, when bearing or coupling problems occur, the fault impact may be more complex (for example, the engine end may also be affected). Double bearings have higher initial costs and more maintenance points, but offer higher operational stability and are suitable for heavy-load long-term operation.
III. Selection
3.1 Determine Basic Parameters: Rated power, speed, total rotor length (especially the length protruding from the stator or cantilever), and rotor mass distribution.
3.2 Evaluate Operating Environment: Offshore/onshore, base rigidity, expected vibration spectrum, and whether there are impacts or frequent start-stop cycles. Double bearings are preferred for offshore or mobile equipment.
3.3 Coupling/Installation Method: Directly fixed coupling (rigid coupling) or elastic coupling; if the rotor is supported by the engine end bearing, the single bearing solution must ensure that the engine end bearing and base can handle the additional load.
3.4 Electrical/Mechanical Coupling Analysis: Conduct air gap sensitivity assessment and torsional vibration analysis to see if the single bearing will cause air gap eccentricity or excite electromagnetic imbalance due to deflection. If the analysis shows that the air gap or dynamic balance is easily affected, double bearings should be used.
3.5 Reliability and Maintenance Strategy: For long-term inaccessible or highly reliable requirements (such as ocean-going vessels, critical backup power sources), double bearings are recommended; conversely, if volume/cost priority is required and regular maintenance is possible, single bearings can be selected.
3.6 Manufacturer and Specification Requirements: Some generator manufacturers may directly recommend or mandate the use of double bearings for specific applications (above a certain power, specific ship types), and the manufacturer's manual and maritime regulations should be referred to.
Ⅳ. Common Failure Modes
4.1 Rolling Contact Fatigue
Cause: Cyclic stress, load concentration, exhaustion of material fatigue life. Commonly occurs in areas with high cycle loads or local overload.
Diagnosis: Increased vibration, spalling/powder on the bearing raceway, increased metal particles in the oil.
4.2 Wear
Causes: Insufficient lubrication, contamination particles, metal contact due to boundary/mixed lubrication.
Diagnosis: Oil particle analysis, increased bearing temperature, visible surface scratches.
4.3 Lubrication Failure (Insufficient Oil, Deteriorated Oil Quality)
Causes: Insufficient oil quantity, blocked oil passage, incorrect grease/oil, thermal degradation or emulsification (seawater intrusion).
Diagnosis: Sudden increase in temperature, oil analysis (viscosity/pollution/water content changes), lubrication system alarm.
4.4 Electrical Discharge Machining, EDM
Causes: Grounding/leakage of the rotor or unit, or free current generated by the excitation/rectifier device flowing through the bearing to the ground, causing pitting/fluting on the bearing raceway.
Diagnosis: Needle-like or groove-like pitting on the bearing raceway, early damage that is difficult to explain by mechanical reasons. Common in generators.
4.5 Corrosion (including seawater corrosion)
Cause: Sealing failure allowing seawater/moisture ingress, or chemical contamination causing damage to white metal/bearings.
Diagnosis: Surface rust, white oxidation products, early fatigue fractures accompanied by corrosion spots.
4.6 Misalignment / Mounting Errors
Causes: Incorrect centering, improper tightening, and incorrect axial positioning. Single-bearing designs are particularly sensitive (cantilever deflection).
Diagnosis: Specific frequency vibration spectra (2X, 3X), local heating, and early uneven wear.
4.7 Brinelling / False brinelling / Fretting
Cause: Vibration during transportation or storage, axial fretting or intermittent loads cause dents or fretting wear on the contact surface.
Diagnosis: Dents/wear streaks on the raceway, vibration/noise after startup.
4.8 Cage fracture/wear
Causes: Poor lubrication, foreign matter intrusion, manufacturing defects or excessive transient impact.
Diagnosis: Abnormal noise, misalignment of bearing rollers, debris in the oil.