On a machine running at constant speed, a standard frequency spectrum works fine because every running-speed harmonic sits at a steady number of hertz. The moment the speed drifts, as it constantly does on variable-speed drives, those harmonics smear across the spectrum and turn sharp diagnostic peaks into blurred humps. Order tracking fixes this by sampling the vibration in step with shaft rotation rather than in step with the clock. The result is a spectrum measured in orders of running speed, where the running-speed harmonics stay put no matter how the speed varies.
Order tracking in one line: Order tracking is a vibration analysis technique that samples or resamples the vibration signal against shaft angle using a tachometer, rather than against fixed time, so that measurements are made a set number of times per revolution. This keeps running-speed harmonics locked to fixed order lines even when the machine's speed changes, which prevents the spectral smearing that a normal fixed-frequency FFT suffers on variable-speed equipment.
A conventional FFT spectrum samples the vibration signal at a steady rate set by a clock, so many samples per second, and reports amplitude against frequency in hertz. This is built on the assumption that the machine's speed holds constant over the whole capture window. When the assumption holds, the running-speed peak and its harmonics land on clean, narrow bins and the spectrum is easy to read. Everything about the standard technique depends on that steady speed during the measurement.
On a variable-speed machine that assumption fails. A pump on a variable-frequency drive, a fan trimming to a process setpoint, or any machine ramping between operating points changes speed during the capture. As the speed drifts, the running-speed frequency and every harmonic of it drift with it, so instead of sitting in one bin a harmonic wanders across several bins during the measurement. The FFT spreads that energy out, and the sharp peak collapses into a low, wide smear that is hard to distinguish from background and easy to underestimate.
The consequence is diagnostic blindness at exactly the frequencies analysts care most about. Imbalance lives at one times running speed, misalignment at two times, and many defect frequencies at specific multiples of speed, and all of these smear together when speed varies. A fixed-Hz spectrum on a swinging-speed machine can hide a serious fault simply because its energy is spread too thin to stand out, which is why the technique has to be adapted before it is trusted on drive-controlled equipment.
Order tracking removes the speed dependence by anchoring the analysis to shaft angle instead of time. A tachometer or an encoder provides a pulse each revolution, or many pulses per revolution, giving a precise record of where the shaft is at every instant. Using that angle reference, the technique samples, or more commonly resamples an already-captured time signal, so that a fixed number of samples is taken per revolution rather than per second. In effect the data is stretched and compressed to undo the speed changes, so one revolution always contains the same number of samples regardless of how fast that revolution actually turned.
Once the signal is expressed against shaft angle, the horizontal axis of the resulting spectrum is orders rather than hertz, where an order is a multiple of running speed. Running speed is exactly one order by definition, its second harmonic is two orders, and so on, and these order lines stay fixed no matter what the machine's speed does, because the axis is now referenced to rotation itself. The smearing disappears because a harmonic that used to wander in hertz now sits on a stationary order line, sharp and easy to read.
This makes order tracking the natural tool for run-ups, coastdowns, and any machine whose speed is never truly constant. It is also what makes a clean cascade plot possible on variable-speed equipment, since each captured spectrum is already referenced to orders and can be stacked against speed without the individual peaks blurring. The one firm requirement is a reliable speed signal; without a tachometer or encoder pulse to define shaft angle, there is no reference to resample against and the technique cannot be applied.
Variable-frequency drives are now standard on pumps, fans, and compressors because they save energy and let the process control flow directly, but they also mean almost nothing runs at a fixed speed anymore. That shift is precisely what makes order tracking relevant to routine condition monitoring rather than a specialist niche. A monitoring channel that only produces fixed-Hz spectra will steadily lose diagnostic value as more of the plant moves onto drives, because more of its readings will be taken while speed is drifting.
For this to work in an automated system, the speed reference has to travel alongside the vibration data. Many drives already publish shaft or motor speed over their control network, and a tachometer or encoder can supply a hardware pulse where higher precision is needed. A monitoring platform that pulls both the vibration signal and the live speed into the same record can resample against angle and store order spectra, so the diagnostic peaks stay sharp even though the machine never sat still. Tying the vibration measurement to the speed the drive was commanding at that instant is what keeps the analysis honest.
Merobix brings the vibration and the drive speed into one browser-accessible history, which lets order-referenced analysis happen against machines that are genuinely running at varying speed in the field. Because the speed context is stored with every reading, an analyst can revisit a past measurement, confirm what speed the machine was at, and interpret order lines with confidence rather than guessing whether a smeared peak was a real fault or just a speed excursion. For a fleet of drive-controlled equipment spread across remote sites, keeping speed and vibration together in the record is what turns raw signals into trustworthy order spectra.
Hertz measures frequency against the clock, so a peak at 30 Hz is 30 cycles per second regardless of the machine. Orders measure frequency against shaft rotation, so one order is exactly running speed, two orders is twice running speed, and so on. On a constant-speed machine the two are simply related by the running speed, but on a variable-speed machine only the order axis keeps the running-speed harmonics in fixed positions.
You need some reliable measure of shaft speed or angle, which usually means a tachometer or an encoder providing at least one pulse per revolution. Some systems can derive speed from the vibration signal itself or from a drive's reported speed, but a dedicated once-per-revolution pulse is the cleanest reference. Without a speed reference of some kind there is no shaft angle to resample against, so true order tracking is not possible.
No, it complements it. On a genuinely constant-speed machine a standard fixed-Hz spectrum is perfectly adequate and simpler to set up. Order tracking earns its keep on variable-speed equipment and during transients, where fixed-Hz analysis smears the very peaks you need to see. Many analysts use both, choosing order tracking whenever speed is changing during the measurement.
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