Bearing failure in a CNC spindle — when does the spectrum start talking?

There are usually two ways people find out a CNC spindle is degrading: surface quality on the workpiece drops, or the operator notices the sound has changed. In both cases the failure has already reached a certain maturity.

The same defect, however, exists as a measurable signal far earlier.

Where the defect signature forms

Micro-pitting (spalling) on a bearing’s inner race, outer race or rolling elements produces a brief impulse at every contact. Those impulses repeat at specific frequencies determined by the bearing geometry and shaft speed — ball pass frequency outer (BPFO), ball pass frequency inner (BPFI), ball spin frequency (BSF) and fundamental train frequency (FTF).

Here is the critical point: those impulses excite the structure’s natural resonances, and the energy appears as envelope modulation in a band well above the fundamental defect frequency. While the defect is early-stage, the overall vibration level (RMS) barely moves — what changes is this fine structure in the upper part of the spectrum.

Why the 1.6 kHz limit is a problem

On a spindle turning at 24,000 rpm the fundamental rotational frequency is 400 Hz. Bearing defect frequencies are multiples of it, and the resonance band that envelope analysis examines typically sits at several kilohertz.

DC – 1.6 kHz, the common band of standard industrial accelerometers, does not cover the region where the diagnostic information lives in this application. The probe works correctly, collects data, may even show a trend — but the signature you are looking for is outside the measurement band.

This is why VibroPro Sense (VIB-201) is designed with DC – 6.3 kHz flat response. Raw sampling is 26.667 kHz per axis and the analysis chain decimates that to a gap-free 4096 Hz, producing a 4096-point FFT per axis per second.

Why gap-free sampling matters

Bearing impulses are brief. If there is a gap in the sample stream, an entire group of impulses can be missed. VibroPro Sense uses ping-pong DMA for acquisition: while one buffer is being processed the other is filling, so there is no gap in the stream.

Deviation, not thresholds

On a variable-speed machine, an alarm rule watching a fixed frequency band cannot work reliably — as the speed changes, the defect frequencies move with it.

Cortex handles the problem differently. Each probe learns the reference signature of the equipment it is mounted on during commissioning, and the health score is computed as a deviation from that signature. As a result, machines with different speed and load characteristics do not need thresholds defined one by one, and a machine running in its normal regime does not produce a high score.

What changes in the field

The practical value of early warning is that a bearing replacement can be moved from an unplanned stop into a planned maintenance window. A scheduled spindle bearing change is a job measured in hours; the same bearing seizing in service can damage the shaft, the housing and sometimes the machine slide.