That's an excellent question about predictive maintenance for servo motors! I can see you're looking to distinguish between normal operation and early warning signs of bearing failure. Here's what I found from my research:

For impending bearing failure, the most critical frequency ranges to monitor are:

1. **Ultrasonic range (20-60 kHz)**: This is where the earliest indications appear. When signals show up in the 30-60 kHz range, you're looking at early Stage 2 defects. Ultrasound testing in this range can detect issues long before traditional vibration analysis.

2. **Characteristic Defect Frequencies (CDFs)**: These are specific to your bearing type and include:

• **BPFI (Ball Pass Frequency Inner race)**: 8-10 harmonics with RPM sidebands

• **BPFO (Ball Pass Frequency Outer race)**

• **BSF (Ball Spin Frequency)** for roller defects

• **FTF (Fundamental Train Frequency)** for cage defects

Under normal operation, bearings shouldn't produce these CDFs in the vibration spectrum. When they appear, that's your red flag.

For normal operational harmonics, you'll typically see vibration at multiples of the rotating frequency (RPM harmonics). The key difference is that normal harmonics are predictable and stable, while bearing failure frequencies appear as new peaks that weren't there in your baseline measurements.

The smart approach is to establish a baseline when the motor is healthy, then monitor for the appearance of these specific defect frequencies and ultrasonic signals. That way, you can catch bearing issues months before they cause catastrophic failure!