Understanding Sensor Mounting
Sensor mounting is the method and hardware used to attach vibration sensors — akcelerometri and velocity sensors — to the measurement surface on a machine. It is far more than a mechanical detail: the mounting method critically governs measurement quality, usable frequency response and reliability. A good mount creates a rigid mechanical coupling that faithfully transmits vibration from the machine into the sensor without adding resonances or losses; a poor one limits frequency response, introduces amplitude errors, or lets the sensor fall off.
The method must be matched to the job. Permanent monitoring calls for permanent mounting (studs); routine survey work uses magnetic mounts for speed; handheld contact is acceptable only for the quickest screening. Understanding how each mount affects sensor performance is essential for obtaining accurate, repeatable data — and this is formalised in ISO 5348, the standard for the mechanical mounting of accelerometers.
1. Mounting Methods Compared
Stud Mounting — Best Performance
Method: the sensor is bolted to a tapped hole using its integral stud, with a thin film of coupling agent (grease or oil) between the mating faces, torqued to specification (typically 20–40 in-lb).
- Frequency range: the full sensor capability, from DC to 20+ kHz.
- Mounting resonance: typically above 30 kHz, well clear of the measurement range.
- Repeatability: excellent.
- Stability: permanent and secure.
Applications: permanent monitoring installations, bearing-defect detection that needs high frequencies, critical measurements and reference measurements.
Adhesive Mounting — Excellent Performance
Method: the sensor is bonded with cyanoacrylate (super glue), epoxy or a specialised adhesive in a thin, uniform layer, giving a semi-permanent installation.
- Frequency range: to 7–10 kHz (very good).
- Mounting resonance: 15–20 kHz.
- Repeatability: good, provided the adhesive layer is applied consistently.
- Stability: permanent until deliberately removed.
Applications: temporary installations lasting weeks to months, situations where drilling holes is not allowed, lightweight machinery, and most general vibration-analysis work.
Magnetic Mounting — Good for Routine Work
Method: a permanent-magnet base attaches to ferrous surfaces, allowing quick attachment and removal with no surface preparation.
- Frequency range: to 2–3 kHz (adequate for most machinery).
- Mounting resonance: 4–7 kHz, which limits high-frequency measurements.
- Repeatability: fair, depending on surface contact.
- Stability: can detach if vibration is severe or the surface is oily.
Applications: route-based condition-monitoring surveys, general machinery vibration, quick checks and screening, and anywhere convenience matters more than maximum performance.
Handheld / Probe — Qualitative Only
Method: the sensor sits on a probe tip held against the surface by hand. Contact force varies and there is no rigid coupling.
- Frequency range: to 500–1000 Hz maximum.
- Repeatability: poor.
- Accuracy: ±20–50% variation is possible.
- Stability: hand tremor and variable contact force corrupt the reading.
Applications: quick screening only, gross problem identification, and inaccessible locations — not suitable for quantitative analysis or trending.
2. Surface Preparation
For Best Performance
- Clean surface: remove paint, rust, oil and dirt.
- Flat surface: file or grind if necessary to ensure full contact.
- Smooth surface: remove high spots and roughness.
- Coupling agent: apply a thin layer of grease, oil or a specialised couplant.
Why Flatness Matters
- Flatness is critical for a rigid coupling.
- Gaps let the sensor rock, reducing frequency response.
- Air gaps act as springs, which lowers the mounting resonance.
- Flatness within 0.02 mm (0.001 in) is ideal.
3. Choosing the Mounting Location
Ideal Locations
- Bearing housings, as close to the vibration source as possible.
- Structural paths with a stiff, direct coupling to the bearings.
- Avoid flexible covers and sheet metal.
- Avoid nodes or other low-response areas.
Accessibility
- Safe access for technicians.
- Clear line-of-sight or reach.
- Protected from damage and not in walkways.
- Practical cable routing.
Direction
- Radial measurements perpendicular to the shaft.
- Axial measurements parallel to the shaft.
- Typically measure horizontal, vertical and sometimes axial at each point.
4. How Mounting Limits Frequency Response
Every mount behaves like a tiny mass-spring system whose stiffness sets a resonant frequency. Below that resonance the response is flat and trustworthy; near and above it the reading is distorted. The table summarises the practical limits:
| Mounting Method | Usable Frequency (kHz) | Mounting Resonance (kHz) |
|---|---|---|
| Stud (ideal) | To 20+ | >30 |
| Adhesive | To 7–10 | 15–20 |
| Magnetic | To 2–3 | 4–7 |
| Handheld | To 0.5–1 | 2–3 |
Rule of Thumb
- Use frequencies only up to about one-third of the mounting resonance.
- This keeps the response flat across the measurement range.
- Above it, amplitude errors climb quickly.
You can put numbers to this for a given sensor mass and contact stiffness with an accelerometer mounting resonance calculator, which is the quickest way to confirm a chosen mount will not colour a bearing measurement.
5. Best Practices
Match the Method to the Application
- Bearing analysis at high frequency: stud or adhesive only.
- General machinery below 1 kHz: magnetic is acceptable.
- Screening: handheld for speed, then confirm with a better mount.
Permanent Installations
- Drill and tap holes for stud mounting.
- Use a thread-locking compound.
- Protect the threaded holes when the sensor is removed.
- Document every measurement location for repeatable trending.
Temporary Installations
- Adhesive for multi-day or multi-week installations.
- Magnetic for route-based surveys.
- Verify secure attachment before measuring.
- Clean both the magnetic base and the surface for good contact.
The same discipline applies to the sensors on a portable balancer. When field-balancing with an instrument such as the Balanset-1A, repeatable amplitude and phase depend on mounting each accelerometer the same way at the same point on every run — a stud or a clean magnetic base on a prepared bearing housing — so that trial-weight responses can be compared meaningfully. Whatever the sensor type, from a piezoelectric accelerometer to a velometer, the mounting fundamentally determines data quality. Matching technique to requirement, ensuring rigid coupling through surface preparation, and respecting the frequency limits together yield the accurate, reliable measurements that effective diagnostics and condition monitoring depend on.
