09.09.2025 by Viktor Siebert
Overhaul of a heavily contaminated Mitsubishi Spindle Motor SJ-18.5BW4
The motor arrived at our workshop with a clear symptom: loud bearing noise. The customer reported that the machine was still operational, but vibrations had increased significantly. On initial inspection, the motor was visibly dirty covered in metal shavings, oil residues, and machining contaminants.
After disassembly, our suspicion was confirmed: the bearings were completely worn out. It was almost surprising that the motor was still capable of operating under these conditions. Had it been run for several more months, a catastrophic failure of the spindle and encoder could have occurred leading to not only high repair costs but also extensive machine downtime.
The condition of the encoder was particularly critical. Electrically, it was already operating out of tolerance. The pickup area was heavily contaminated, increasing the risk of complete encoder failure if operation had continued. Such a failure in live production could easily cause an axis crash or part loss. The customer was fortunate to schedule the overhaul before reaching that point.
Our repair measures included:
- Complete disassembly of the motor
- Thorough cleaning of all components with emulsion and oven-drying
- Replacement of all bearings with high-precision spindle bearings
- Renewal of seals and connectors
- Encoder reconditioning, re-adjustment, and fine-tuning
- Surface treatment and repainting of the housing to protect against corrosion
Test procedures:
After repair, the motor underwent extensive testing on our dedicated test bench:
- Insulation resistance measurement
- Test run across full speed range
- Load simulation under different torque conditions
- Encoder signal quality verification
- Final documentation via detailed checklists
The result: The motor now runs smoothly, free of vibrations, and with perfect encoder signal quality.
Causes of Encoder or Motor Failures
- Temperature fluctuations → condensation leading to corrosion
- Moisture & oil mist → short circuits, insulation degradation
- Vibration → premature bearing and encoder wear
- Aging → electronic components drifting out of tolerance
- Cable/connectors → intermittent faults, insulation cracks
Failures typically develop gradually: at first sporadic errors appear, then more frequently, until final breakdown occurs.
Preventive Tips for Customers
- Keep the operating environment clean and dry
- Regularly inspect cables and connectors
- Monitor vibrations (e.g., through vibration analysis)
- Avoid extreme temperature fluctuations
- Plan preventive overhauls every 5–7 years, depending on load and environment
To mentioned Mitsubishi Motor: Mitsubishi AC Spindle Motor SJ-18.5BW4
More details about our Mitsubishi repair services can be found here:
Mitsubishi motor Repair by Industrypart
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Device Data
| Parameter | Value |
|---|
| Motor type | Mitsubishi AC Spindle Motor SJ-18.5BW4 (SJ-18.5-BW4) |
| Rated power (S1 continuous) | 18.5 kW |
| Speed range | 150 – 4600 rpm (continuous) / up to 6000 rpm |
| Torque | ~118 Nm at rated load (calculated) |
| Voltage | 200–230 V AC, 3-phase, 50/60 Hz |
| Current | Approx. 60 A (depending on load profile) |
| Power factor | 0.84 |
| Insulation | Class F |
| Thermal classification | 180 °C |
| Protection class | IP54 (typical, depending on installation) |
| Cooling | Forced-air cooling with external fan |
| Weight | approx. 150 kg |
| Frame size | 132L |
| Encoder | Incremental/Resolver feedback (depending on variant) |
| Manufacturer | Mitsubishi Electric Corporation, Japan |
Application Environment & Compatible Devices
The Mitsubishi AC Spindle Motor SJ-18.5BW4 is typically used in CNC machine tools such as turning centers, milling machines, and machining centers. It is compatible with spindle drives from the Mitsubishi MDS-B, MDS-C, and MDS-D series.
Application areas:
- Metalworking machines (turning, milling, drilling)
- Machines requiring a wide speed range up to 6000 rpm
- Operating environment with controlled temperature (10–40 °C) and low dust/moisture
Functional Description
The SJ-18.5BW4 spindle motor is designed for continuous high-performance operation with excellent dynamics. Its primary task is to drive the spindle of a CNC machine reliably and with precise torque output.
Key features:
- High power density in a compact size
- Stable torque delivery across a wide speed range
- Accurate feedback via integrated encoder
- Optimized for continuous duty cycles under heavy loads
Alarm Messages & Troubleshooting (MDS-B-SP based)
| Alarm code | Description | Cause | Solution |
|---|
| 03 | Overcurrent | Short circuit in motor, defective power module | Check motor insulation, replace power module |
| 05 | Overvoltage | Line voltage too high, brake circuit fault | Check mains supply, inspect brake resistor/transistor |
| 07 | Undervoltage | Line voltage too low or momentary drop | Check power supply, stabilize mains |
| 08 | Overheating | Cooling blocked, fan failure, ambient too hot | Replace fan, clean cooling fins, improve environment |
| 11 | Overload (continuous operation) | Mechanical overload, worn bearings | Check load, inspect bearings, schedule overhaul |
| 17 | Encoder error | Out-of-tolerance signal, cable break, contamination | Clean/adjust encoder, check cabling, replace encoder if necessary |
| 50 | Ground fault | Damaged winding insulation, oil/moisture ingress | Perform insulation test, dry/repair motor |
| 51 | Phase loss | Cable break, loose connector, faulty power stage | Check wiring, replace connectors, replace power board if needed |
| 58 | Brake overload | Brake resistor overloaded or defective | Inspect resistor, replace brake components |
| 59 | Brake open circuit | Open brake circuit, cable break | Check wiring, replace resistor if open |
| 70 | Communication error | Fault between spindle drive and NC | Inspect cabling, re-establish NC connection |
| 79 | Thermistor error | Faulty or contaminated thermal sensor | Test thermistor, check motor windings |
| 9B | Vibration/positioning error | Bearing wear, encoder drift, mechanical imbalance | Replace bearings, realign encoder, balance rotor |
Components
| Component | Description |
|---|
| Rotor | Permanent magnet rotor for spindle drive |
| Stator | Winding assembly, insulation class F |
| Encoder | Incremental/resolver feedback for speed and position control |
| Bearings | Precision bearings for high-speed spindle operation |
| Housing | Cast-iron, black coated, protection class IP54 |
| Cooling | External fan and ventilation channels |
| Connectors | Power and encoder terminals, vibration-resistant |
| Seals | Protection against oil mist and coolant ingress |