Bearings are among the first components to communicate distress in a submersible pump — but the signals often go unrecognised until the damage has progressed beyond simple bearing replacement. Understanding what triggers bearing failure and how to read the early indicators is the foundation of effective maintenance for any submersible pump installation.
Why Bearings Are Critical in Submersible Pumps
In a submersible pump, the bearing assembly carries the full axial and radial loads generated by the rotating impeller, while operating in an environment of elevated temperature, vibration, and — in the event of seal degradation — potential moisture contamination. Unlike surface pump bearings which can be monitored and lubricated in service, submersible bearings are inaccessible during operation. Failure is therefore always discovered after the fact — either during a planned inspection or following a breakdown.
Flygt submersible pumps use bearings that are pre-greased, dimensionally matched to Flygt-specific tolerances, and selected by approved suppliers for each pump model. These tolerances are tighter than standard commercial bearing specifications in many cases — which is why using generic replacement bearings frequently results in shorter service life.
The Four Primary Causes of Bearing Failure
1. Moisture Ingress Following Seal Failure
The most common cause of submersible pump bearing failure is moisture ingress through a degraded or failed mechanical seal. Water in the bearing cavity causes immediate grease contamination and accelerated corrosion of the bearing races and rolling elements. A bearing operating in contaminated grease will fail in a fraction of its designed service life. The bearing failure in this scenario is a consequence — the root cause is the seal that allowed ingress.
2. Overloading — Axial and Radial
Bearings are rated for specific axial and radial load combinations. When a pump operates significantly off its Best Efficiency Point — either through incorrect system design or progressive wear ring deterioration — the internal hydraulic forces change and may generate loads that exceed the bearing design envelope. Pumps running in recirculation or at very low flow are particularly susceptible to elevated axial thrust.
3. Incorrect Bearing Specification
Non-genuine replacement bearings may have the correct bore and outer diameter but differ in internal clearance, cage material, or grease specification. Flygt-specific bearings often carry tighter internal clearances than the equivalent standard commercial bearing — a difference that matters significantly at operating speed and temperature. An incorrect clearance results in either insufficient preload (allowing ball slip and skidding) or excessive preload (generating heat and accelerating raceway fatigue).
4. Thermal Cycling and Inadequate Cooling
Submersible pump motors are cooled by the surrounding fluid. When a pump operates dry — even briefly — the motor temperature rises rapidly and the bearing grease may reach temperatures beyond its rated range. Thermal degradation of the grease accelerates bearing wear and shortens the regreasing interval significantly. Repeated thermal cycling also fatigues the bearing steel through differential thermal expansion.
Early Warning Signs
Early bearing distress in a submersible pump can be detected through several indicators, even without physical inspection:
- Elevated vibration signature — bearing defect frequencies (BPFO, BPFI, BSF, FTF) appear in vibration spectra before any audible noise develops
- Increased current draw — higher friction in degraded bearings increases motor load, detectable through current monitoring
- Reduced flow at constant head — shaft misalignment from bearing wear can reduce hydraulic efficiency and shift the operating point
- Motor winding temperature alarm — if motor protection includes thermistors, bearing friction heating may trigger temperature alerts before catastrophic failure
What Inspection Reveals
When a submersible pump is pulled for inspection, the bearing condition tells a clear story:
- Blue or brown discolouration of races indicates thermal overload
- Rust and pitting on rolling elements and races indicates moisture contamination
- Flaking or spalling of the raceway surface indicates advanced fatigue — typically following an extended period of overload
- Contaminated or washed-out grease is a direct indicator of seal failure allowing water ingress
Key Takeaways
- Most submersible pump bearing failures are consequences of seal failure — always inspect the seal when replacing a bearing
- Flygt-specific bearings carry tighter tolerances than standard commercial equivalents — generic replacements frequently underperform
- Operating significantly off BEP generates abnormal hydraulic loads that reduce bearing life
- Vibration monitoring can detect bearing defect frequencies before audible noise develops
- Pre-greased, genuine replacement bearings ensure correct lubrication from first start-up
Maintenance Checklist
- When replacing a bearing, always inspect and replace the mechanical seal — the two failures are frequently linked
- Use only genuine Flygt bearings with unique part numbers for your specific pump model
- Check operating point — ensure the pump is running within 70–110% of BEP flow
- Implement motor current monitoring as an early indicator of bearing degradation between planned service intervals
- Record bearing condition at each inspection — patterns across multiple units reveal systemic causes