Wireless vibration sensor on an electric motor: where to mount it and what to measure

A poorly placed vibration sensor measures the vibration of the sheet metal it was stuck to, not the motor. And a well-placed but poorly mounted sensor loses exactly the high frequencies, where the bearing gives its first warning. This article explains where the sensor belongs on an electric motor, how to mount it, which frequency range matters and what shows up in a motor's spectrum when something starts to go wrong.

It is written for anyone about to install wireless sensors in a plant who wants the data to actually be useful.

Where to mount: on the bearing, as close to it as possible

The vibration you care about originates in the bearing and reaches the housing along the stiffest path. ISO 20816-3 calls for measuring on the bearing housing or on the part of the casing that responds directly to the forces it transmits. On a motor, that means the two end shields: the drive end (DE, the coupling side) and the non-drive end (NDE, opposite the coupling).

What to avoid:

• Fan covers and thin sheet-metal guards: they vibrate on their own and amplify what you are not interested in.
• Cooling fins: they are flexible and sit far from the bearing.
• Base or motor feet: they measure the foundation, not the bearing.

If the end shield has a machined boss near the shaft, that is the spot.

Directions: radial and axial

The standard calls for two perpendicular radial measurements per bearing, usually vertical and horizontal, plus axial when there is a thrust bearing or misalignment is suspected. A triaxial sensor mounted on the bearing covers all three directions at once, as long as one of the sensor axes is aligned with the motor shaft. Write down the orientation: a spectrum with an unknown direction cannot be interpreted.

Rules of thumb for those getting started:

• Horizontal is usually the direction with the highest amplitude on a horizontal motor, because the base is less stiff in that direction.
• High axial vibration relative to radial points to angular misalignment or a thrust-bearing problem.

Mounting: stud, adhesive or magnet

The mounting method determines how high in frequency the sensor can see. ISO 5348 lists the methods and their limitations, and the field numbers are well known:

• Threaded stud: best response, reaching the sensor's own limit (20 kHz or more). It requires a tapped hole, a flat surface and the correct torque: too little torque lowers the maximum frequency.
• Structural adhesive (epoxy) or a bonded pad with a tapped hole: response slightly below the stud, with no need to drill the housing. This is the mounting we use for permanent installations without stopping the machine.
• Magnet: the most practical, and the most treacherous. Without surface preparation, a sensor that responds up to 20 kHz on a stud drops to 3 to 4 kHz on a magnet. With a machined, clean surface and a thin film of silicone grease between magnet and housing, field tests got close to 10 kHz. Thick paint, rust and curved surfaces drag the result down.
• Handheld probe: reliable only up to about 1 kHz.

For a sensor that will stay on the machine for years, use a stud or epoxy. A magnet is fine for a comparison measurement, not for continuous monitoring.

Frequency range and sampling rate

Two ranges matter on a motor:

• 10 Hz to 1,000 Hz, the ISO 20816-3 band, where unbalance, misalignment, looseness and electrical problems live. This is the band used to calculate the RMS velocity compared against the standard's zones.
• Above 1 kHz, where a bearing in the early stages of failure excites its own natural frequencies (typically between 500 Hz and 1.5 kHz) and where envelope techniques work.

The sampling rate must be at least twice the highest frequency of interest (Nyquist); in practice, analyzers use 2.56 times to accommodate the anti-aliasing filter. To cover the standard's band, a 2.56 kHz sampling rate is enough; to see a bearing in its early stage, you need to sample above 10 kHz. Check the sensor's usable range on the datasheet and in the proposal: the number that matters is the maximum frequency with the mounting you chose, not the catalog figure measured on a test bench.

A permanently installed wireless sensor usually works in two layers: it periodically sends the overall values (RMS per axis, peak, temperature, dominant frequency), which feed the trend and the alerts, and it delivers the full FFT spectrum on demand, when the engineer wants to look at the detail. That is how our platform works.

What shows up in a motor spectrum

On a 60 Hz grid, a 4-pole motor runs at about 1,750 rpm (29 Hz) and a 2-pole motor at about 3,500 rpm (58 Hz). With the running speed in hand, the spectrum reads like this:

• 1x running speed, radial: unbalance. The force grows with the square of the speed.
• 2x running speed, radial and axial: misalignment under load. High axial vibration reinforces the diagnosis.
• Integer multiples of 1x (3x, 4x...): mechanical looseness, soft foot, loose base.
• 120 Hz (twice the line frequency; 100 Hz on a 50 Hz grid), not exactly related to running speed: electrical origin: air-gap eccentricity, a stator problem, phase unbalance. The classic test: switch the motor off and the peak disappears instantly, while the mechanical peaks fade slowly as the motor coasts down.
• Non-integer multiples of 1x (5.14x, 3.78x...): bearing. Bearing fault frequencies are never exact multiples of the running speed, and that is what gives them away.
• Sidebands around 1x or around the bearing frequencies: modulation, a sign that a defect moves in and out of the load zone on every revolution.

Temperature at the same point

The sensor that measures vibration on the bearing usually measures the housing temperature at that point too. It is not the bearing temperature, but it follows it. Friction, lack of lubrication and overload show up as a temperature rise before or together with a vibration rise; reading both together avoids jumping to conclusions.

What to do now

1. For each critical motor, define the points: DE and NDE end shields, on solid metal.
2. Choose stud or epoxy. Leave the magnet for one-off diagnostics.
3. Note the rated speed, number of poles and, if you can, the bearing model. Without them, the spectrum is guesswork.
4. Record the orientation of the sensor axes.
5. Take the first reading with the machine in good condition and save it as the baseline.

If you want to know what your plant needs, talk to one of our engineers.

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Sources: ISO 20816-3:2022 (clauses 4.2 and 5); ISO 5348:2021, Mechanical vibration and shock — Mechanical mounting of accelerometers; PCB Piezotronics, white paper WPL-46, Magnet Mounting Techniques for Machinery Vibration Monitoring (Praxair); Ludeca, 4 Stages of Bearing Failures (2021).