Mounting a Vibration Sensor: Screw, Adhesive or Magnet and Their Influence on Frequency Response

→ Product category: Vibration measurement and vibration sensors

 

A vibration sensor is initially attached to an electric motor using a magnetic base. The measurement shows a clear vibration component at the rotational frequency, but only minor activity at higher frequencies. During a later inspection, a sensor is rigidly mounted at the same bearing location – suddenly, additional higher-frequency signal components appear.

Has the condition of the machine changed?

Not necessarily.

The mounting of the vibration sensor alone can significantly change the usable frequency range of the entire measurement chain. A mechanical connection with its own mass, stiffness and resonance is created between the machine surface and the sensing element. A screw, adhesive layer, magnetic base or hand-held probe therefore does not transmit vibrations identically across the entire frequency range.

This is particularly relevant for condition monitoring. Unbalance and other low-frequency machine movements can often be measured effectively using temporary magnetic mounting. High-frequency components of an incipient rolling-bearing defect, however, can be considerably more sensitive to the mounting method.

For reliable trend measurements, not only the sensor type and measuring range should therefore remain constant, but also, as far as possible:

• measuring point,
• mounting method,
• mounting surface,
• measuring direction,
• cable routing,
• operating condition of the machine.

Suitable sensors can be found at ICS Schneider under vibration measurement and vibration sensors. Further sensors for mechanical measured variables can be found under displacement, force and motion sensors.

Why sensor mounting is part of the measurement chain

An accelerometer should detect the movement of a machine surface with as little distortion as possible.

Ideally, the sensor base moves exactly in the same way as the surface on which it is mounted.

In reality, however, several mechanical elements are located between the machine and the sensing element:

• machine surface,
• contact surface,
• mounting screw or threaded stud,
• adhesive layer or magnetic base,
• sensor housing,
• actual sensing element.

Together, these components form a mechanical system.

The stiffer and more direct the connection between the machine and the sensor, the better particularly fast vibrations can be transmitted.

If, on the other hand, one of the following is introduced between machine and sensor:

• soft adhesive layer,
• thick intermediate layer,
• poorly seated magnetic base,
• movable hand-held probe

the transmission path changes.

The measurement therefore always represents the combination of machine, measuring point, mounting method and sensor.

What does frequency response mean for a vibration sensor?

Frequency response describes how uniformly a sensor transmits or measures vibrations at different frequencies.

An ideal sensor would, for example, measure an acceleration of 1 g at:

• 10 Hz,
• 100 Hz,
• 1,000 Hz,
• 5,000 Hz

with exactly the same sensitivity in every case.

Real sensors, however, have a limited usable frequency range.

Outside this range, the signal can:

• be attenuated,
• be amplified,
• be distorted by resonances.

The frequency range specified in the data sheet also only applies under the mounting conditions defined by the manufacturer.

If a sensor is mounted differently from the way it was characterized or calibrated, the practically usable frequency range may become smaller.

The question:

“The sensor can measure up to 10 kHz – is that sufficient?”

is therefore not sufficient on its own.

An additional question must be asked:

“Up to which frequency does my actual mounting arrangement still transmit the machine vibration to the sensor with sufficient accuracy?”

Mounting resonance: why mass and stiffness are decisive

A simplified representation of sensor mounting is a mass-spring system.

Its resonance frequency can in principle be described by:

fr = 1 / (2 · π) · √(k / m)

where:

• f_r = resonance frequency,
• k = effective stiffness of the mechanical connection,
• m = effective mass of the sensor and mounting components.

The formula illustrates two fundamental relationships.

Higher stiffness

If stiffness k increases, the resonance tends to shift toward higher frequencies.

A:

• flat metal surface,
• direct screw connection,
• correctly tightened connection

is therefore advantageous for high frequencies.

Additional mass

If additional mass is introduced between the machine and the sensor, for example by a heavy magnetic base, the resonance frequency of the mounted system tends to decrease.

A compliant intermediate layer also reduces the mechanical stiffness.

The result can be:

The sensor itself has a wide frequency range, but the mounted combination no longer does.

The simplified formula does not replace manufacturer specifications for the specific mounting method, but it explains why the mounting method is so important for high-frequency vibration measurements.

Comparison of screw, adhesive, magnet and hand-held probe mounting

Mounting method	Stiffness / coupling	High-frequency behavior	Repeatability	Typical application
Direct screw or stud mounting	Very high	Generally best transmission	Very high	Permanent condition monitoring, reference measurements, broadband frequency analysis
Rigid adhesive mounting base	High with a thin, hard adhesive layer	Good to very good, depending on adhesive and mounting arrangement	High	When drilling is not possible or permitted
Magnetic base	Medium to high, depending on magnet and surface	Upper frequency range may be reduced	Good at a defined measuring point	Mobile measurement routes and temporary measurements
Hand-held probe / probe tip	Low and operator-dependent	Significantly limited	Low	Quick indicative checks

This ranking is deliberately qualitative.

There is no universal frequency limit that applies to every sensor and every mounting method.

The actual frequency range depends, among other things, on:

• sensor mass,
• sensor design,
• magnet size,
• adhesive,
• adhesive layer thickness,
• surface quality,
• mounting surface,
• measuring direction.

Screw and stud mounting

Direct screw or stud mounting normally provides the stiffest mechanical connection between sensor and machine.

It is therefore the preferred solution when:

• the widest possible frequency range is required,
• high-frequency signal components are relevant,
• the sensor is permanently installed,
• trend values must remain reproducible over long periods.

Good screw mounting, however, requires a suitable mounting surface.

Important factors include:

• smooth seating surface,
• sufficient flatness,
• clean contact surface,
• threaded hole perpendicular to the mounting surface,
• correct thread engagement depth,
• manufacturer-specified torque.

The screw or threaded stud must not bottom out in the threaded hole before the sensor base is fully seated on the mounting surface.

Otherwise, a clean mechanical coupling is not achieved.

Insufficient tightening torque can also be problematic.

A loose connection can:

• generate its own resonances,
• alter high-frequency signals,
• reduce repeatability.

Excessive torque is also unsuitable and can damage the thread or sensor.

The mounting torque specified for the respective sensor should therefore be observed.

Adhesive mounting

Adhesive mounting is a useful alternative when:

• a threaded hole must not be made,
• the structure is very thin,
• the sensor is only to be installed semi-permanently,
• the surface must not be mechanically modified.

The design of the adhesive joint is decisive, however.

For good high-frequency transmission, the adhesive layer should generally be:

• thin,
• rigid,
• uniform

.

A thick, elastic adhesive layer, on the other hand, acts like an additional spring between the machine and the sensor.

This can reduce the mounted resonance frequency and therefore reduce the upper usable frequency range.

Use an adhesive mounting base instead of bonding the sensor directly

For accelerometers designed for this purpose, a separate adhesive mounting base is often advantageous.

The base is bonded to the machine, while the sensor itself is attached to the base using its intended thread.

This offers several advantages:

• adhesive does not enter the sensor thread,
• the sensor base is subjected to less stress when removed,
• the sensor can be replaced without recreating the measuring point,
• the measuring point remains reproducible.

An adhesive pad is not the same as a rigid adhesive joint

Soft double-sided adhesive pads can behave very differently from a thin, rigid adhesive joint.

Foam or elastic pads in particular have significantly greater compliance.

They can therefore strongly attenuate high frequencies.

For demanding vibration analysis, the documentation should therefore not merely state:

“Sensor bonded”

but, where possible, also include:

• type of adhesive mounting base,
• adhesive used,
• surface preparation,
• position.

Magnetic mounting

Magnetic bases are particularly practical for mobile vibration measurements.

Within just a few seconds, the sensor can be:

• mounted,
• used for measurement,
• removed,
• mounted at the next measuring point

.

This makes magnetic mounting particularly useful for:

• maintenance routes,
• comparative measurements on several machines,
• temporary diagnostics,
• mobile inspections.

However, this convenience has a metrological cost.

The magnetic base:

• increases the mounted mass,
• introduces an additional mechanical interface,
• is more sensitive to surface quality,
• can reduce the upper usable frequency range.

Magnet on a smooth surface

A magnetic base performs best on a surface that is:

• clean,
• smooth,
• sufficiently large,
• ferromagnetic

.

Problematic conditions include:

• thick paint layers,
• rust,
• dirt,
• metal chips,
• strongly curved housings,
• contact surfaces that are too small.

Flat magnet and dual-rail magnet

On strongly curved surfaces, dual-rail or other special magnetic bases are sometimes used.

They provide better mechanical adaptation to curved surfaces, but may have a different frequency response from direct flat magnetic mounting.

For trend measurements, it is therefore advisable not to switch arbitrarily between different magnet types.

Do not allow the magnet to snap onto the machine

A powerful magnet should not be allowed to snap uncontrollably onto the machine housing.

This can generate a very high short-duration acceleration.

Controlled mounting reduces the risk of unnecessary shock loading of the sensor.

Hand-held probe and probe tip

The quickest method is to press a sensor against the machine surface using a probe tip or hand-held probe.

This method can be sufficient for an indicative check.

For reproducible frequency analysis, however, it has significant disadvantages.

The measurement result depends, among other things, on:

• how much force the operator applies,
• the angle at which the probe is held,
• whether the operator’s hand moves during the measurement,
• how hard the probe tip is,
• how flat the surface is.

The hand-held probe therefore forms a comparatively soft and poorly defined mechanical connection.

High frequencies in particular may be transmitted significantly less effectively.

For quantitative trend measurements, a hand-held probe should therefore only be used if the measurement method is explicitly designed for it and can be reproduced sufficiently consistently across all measurements.

Preparing the mounting surface correctly

Even the best mounting method performs poorly on an unsuitable surface.

The contact surface should ideally be:

• flat,
• smooth,
• clean,
• free of rust,
• free of metal chips,
• free of loose paint layers

.

For permanent high-frequency measurements, the mounting surface is often prepared or machined accordingly.

The sensor base should make contact as completely as possible.

Even small burrs can cause the sensor to rest only at individual points.

The intended rigid connection then becomes a mechanically less defined coupling.

Painted machine surfaces

For magnetic measurements, measurements are often taken directly on painted motor or pump housings.

A thin, firmly adhering coating does not necessarily prevent a low-frequency comparative measurement.

For demanding high-frequency measurements, however, the additional layer becomes part of the mechanical connection.

Particularly problematic are:

• thick paint,
• multiple coating layers,
• corrosion beneath the coating,
• uneven coatings.

For recurring measurement routes, a defined measuring point or permanently installed measurement pad may therefore be useful.

Selecting the correct measuring point on the machine

Not only the mounting method but also the position of the sensor determines the measurement result.

For bearing diagnostics, the sensor should be mounted as close as possible to the relevant mechanical force path or bearing housing.

Unsuitable locations include:

• thin protective covers,
• fan guards,
• loose sheet metal,
• cladding,
• brackets located far away from the bearing.

Such components have their own resonances.

The sensor may then primarily measure the vibration of the cover rather than the actual bearing or machine vibration.

Typical measuring points on an electric motor are therefore more likely to be:

• drive-end bearing housing,
• non-drive-end bearing housing,
• rigid housing areas immediately adjacent to the bearings.

The exact selection of the measuring point depends on the machine design and diagnostic objective.

Considering measuring direction and sensor axis

A single-axis accelerometer primarily measures along its defined sensitive axis.

Different directions are commonly investigated on machines:

• horizontal radial,
• vertical radial,
• axial.

The same machine can exhibit very different vibration levels in these directions.

The statement:

“The motor has 3 mm/s.”

is therefore incomplete without specifying the measuring direction.

For trend measurements, the following should be documented:

Measuring point + measuring direction + mounting method.

Example:

Motor M1 – DE bearing – horizontal radial – magnetic base

Only then can a later measurement be compared meaningfully.

Apparent cross-axis sensitivity caused by tilted mounting

If a single-axis sensor is mounted at an angle, its measuring axis is no longer exactly aligned with the required machine direction.

The sensor then detects components of several actual directions of motion.

For an angular deviation α, the component of a vibration along the sensor axis can be represented in simplified form by a projection:

ameasured ≈ a · cos(α)

In addition, vibration components from other directions can enter the signal.

This effect must not be confused with the internal cross-axis sensitivity of the sensing element.

Incorrect mechanical alignment can cause a directional error even before the vibration reaches the actual sensing element.

Influence of sensor mass on the structure

The sensor should influence the structure being measured as little as possible.

On a massive motor bearing housing, a small accelerometer generally has considerably less influence than on:

• thin sheet metal,
• small printed circuit boards,
• lightweight brackets,
• thin-walled pipes,
• small components.

If the mass of the sensor or magnet becomes too large relative to the local structure, it can alter the structure’s natural frequency and vibration amplitude.

The sensor then no longer measures exactly the original structural condition.

For lightweight test objects, the mass of the following should therefore also be considered in addition to the sensor itself:

• magnetic base,
• mounting adapter,
• cable.

Avoiding cable movement and tensile forces

After mechanical mounting, the sensor cable is often overlooked.

A freely moving cable can:

• transmit mechanical forces to the sensor,
• load the sensor mounting,
• generate additional interference signals,
• reduce repeatability.

Especially with small, lightweight accelerometers, cable force can become significant in relation to the sensor mass.

The cable should therefore:

• be strain-relieved close to the sensor,
• not be pulled tight,
• not strike loosely against the machine housing

.

For mobile measurements, care should also be taken to keep the cable as still as possible during the measurement period.

Why bearing faults are particularly sensitive to mounting

An incipient rolling-bearing defect often generates short mechanical impulses.

These impulses can excite structural resonances in the bearing housing and the machine.

This produces signal components that can be significantly higher in frequency than the actual rotational frequency of the machine.

For the corresponding diagnostic methods, the entire measurement chain must be able to transmit these frequencies.

This includes:

• sensor,
• mounting,
• cable,
• signal conditioning,
• measuring instrument,
• sampling rate,
• analysis software.

A sensor with sufficiently high bandwidth is therefore of little benefit if a soft mounting arrangement mechanically attenuates the relevant high-frequency components before they reach the sensor.

This is exactly why one mounting method can be completely adequate for an unbalance measurement but unsuitable for early bearing diagnostics.

Rotational frequency and relevant vibration frequencies

The rotational speed of a machine can be converted into rotational frequency:

f = n / 60

where:

• f = rotational frequency in Hz,
• n = rotational speed in rpm.

Example:

n = 1,500 min-1

gives:

f = 1,500 / 60 = 25 Hz

Classic unbalance often produces a significant signal component at or close to this fundamental rotational frequency.

Other faults can generate additional frequency components.

Typical diagnostic ranges can include, for example:

• 1× rotational frequency,
• 2× or higher harmonics,
• gear mesh frequencies,
• blade-passing frequencies,
• rolling-bearing fault frequencies,
• high-frequency resonance ranges.

The higher the frequency required for the diagnosis, the more important the following become:

• rigid mounting,
• clean surface,
• suitable sensor,
• sufficient sampling rate.

Which mounting method is suitable for trend measurements?

In condition monitoring, the focus is often less on a single absolute measurement than on the question:

“Has the machine changed compared with its previous normal condition?”

Repeatability is decisive for this trend comparison.

A permanently screwed measuring point normally provides the best conditions for this.

It ensures that:

• the position is always the same,
• the alignment is always the same,
• the mechanical coupling is always the same

.

A permanently installed adhesive mounting base can also provide a good reproducible measuring point.

For mobile measurement routes, a magnetic base can also be used effectively if:

• the measuring point is marked,
• the same magnet is always used,
• the surface remains clean,
• the sensor is always aligned in the same way.

The following comparison, on the other hand, would be problematic:

• Month 1: magnetic base on bare bearing housing,
• Month 2: hand-held probe on painted surface,
• Month 3: bonded sensor 50 mm away.

If the measured value changes, it is then difficult to determine whether the machine itself has changed or only the measurement method.

Repeatability in mobile measurement routes

For mobile condition-monitoring routes, clear identification of the measuring points is recommended.

For example:

Measuring point	Position	Direction	Mounting
M1-DE-H	Motor drive-end bearing	Horizontal radial	Magnet
M1-DE-V	Motor drive-end bearing	Vertical radial	Magnet
M1-NDE-H	Motor non-drive-end bearing	Horizontal radial	Magnet
P1-DE-H	Pump drive-end bearing	Horizontal radial	Magnet

Relevant operating data should also be documented:

• rotational speed,
• load,
• flow rate or process condition,
• temperature,
• operating mode, where applicable.

A vibration value measured at 30% machine load may not be directly comparable with a value measured at 100% load.

Trend measurement therefore means: same measuring point, same mounting method, same direction and, as far as possible, a comparable operating condition.

Why different mounting methods should not be compared directly

Assume that a motor is initially measured using a magnetic base.

The acceleration is:

a = 1.8 g

One week later, a sensor is screwed into place at the same location and the measured acceleration is:

a = 2.4 g

.

The conclusion:

“The vibration has increased by 33%.”

may be incorrect.

The screw mounting may simply transmit a frequency component more effectively that was previously attenuated by the magnetic mounting.

The reverse effect is also possible if a mounting arrangement close to resonance creates a local amplification.

After changing the mounting method, a new reference or baseline should therefore be recorded.

Old and new trend values should not be continued as one identical measurement series without first verifying comparability.

Typical fault patterns caused by vibration sensor mounting

Observation	Possible cause	Recommended check
Low-frequency unbalance is visible, but high frequencies are missing	Mounting limits the upper frequency range	Check mounting method, surface and sensor frequency range
Screw mounting shows more high-frequency components than magnetic mounting	Stiffer mechanical coupling	Compare both mounting methods under identical conditions
Two magnetic measurements on the same motor differ significantly	Different position, surface or magnetic contact	Mark the measuring point and clean the contact surface
Measured value changes when the cable is moved	Cable forces or cable-induced interference	Provide strain relief and keep the cable still during measurement
Large peak immediately after applying the magnet	Mechanical shock when the magnet snaps into place	Apply the magnet in a controlled manner
Sensor moves despite the screw being tightened	Screw bottoms out or surface is uneven	Check thread engagement depth and contact surface
Measured value changes after retightening the sensor	Different preload or contact stiffness	Use the manufacturer’s specified torque
Much higher vibration measured on a protective cover than on the bearing housing	Local resonance of the thin structure	Select a measuring point closer to the mechanical force path
Bonded sensor shows poor high-frequency transmission	Adhesive layer is too thick or too soft	Check adhesive joint and suitable mounting base
Sensor produces different trend values after repositioning	Mounting position or orientation not reproduced	Permanently mark measuring points
Axial and radial readings appear implausibly similar	Sensor mounted at an angle or measuring directions confused	Check sensor axis and measuring-point definition
Bearing diagnosis remains inconspicuous even though noise is increasing	Frequency range of the sensor or mounting may be insufficient	Check the required diagnostic frequency range of the entire measurement chain

Systematic test and mounting procedure

For reliable vibration measurement, the mounting should already be planned before data acquisition begins.

1. Define the diagnostic objective: Unbalance, alignment, bearing condition, general trend or high-frequency impulse diagnostics?
2. Determine the required frequency range: Which frequencies actually need to be transmitted?
3. Check the sensor: Measuring range, frequency range and output signal must be suitable.
4. Select the mounting method: Screw, adhesive base, magnet or indicative hand-held measurement.
5. Select the measuring point: Preferably a rigid area close to the relevant bearing or force path.
6. Check the surface: Assess flatness, paint, rust and contamination.
7. Define the measuring direction: Clearly specify horizontal, vertical or axial.
8. Mount the sensor: Follow the manufacturer’s specifications for mounting and torque.
9. Apply the magnet in a controlled manner: Avoid unnecessary shock loading.
10. Provide cable strain relief: Do not transmit tensile forces to the sensor.
11. Document the operating condition: Record rotational speed, load and process condition.
12. First check the signal for plausibility: Check amplitude and spectrum.
13. Watch for resonance effects: Investigate unusual narrow-band peaks.
14. Repeat the measurement: If necessary, remove and remount the sensor.
15. Evaluate repeatability: Large differences may indicate a mounting problem.
16. Save a trend baseline: Establish reference values only once the measurement method is stable.
17. Document the mounting method: It should form part of the measurement report.

Practical example: magnetic measurement at a motor bearing

Vibration measurements are regularly carried out on an electric motor operating at 1,500 min^-1.

The rotational frequency is:

f = 1,500 / 60 = 25 Hz

An accelerometer with magnetic base is used for the maintenance route.

Step 1: First measurement

The sensor is mounted on the drive-end bearing housing.

A clear component at approximately 25 Hz is visible in the spectrum.

The measurement is plausible for evaluating low-frequency machine vibration.

Step 2: Test on another surface

The same sensor is then mounted a few centimeters away on a painted, slightly curved housing surface.

The 25-Hz component remains visible, but the higher-frequency components differ significantly.

Step 3: Check magnetic contact

The second surface does not provide full-area contact.

The magnet partly rests on the curvature of the housing.

The mechanical coupling has therefore changed.

Step 4: Define a fixed measuring point

For future maintenance routes, a fixed position is marked on the flat bearing-housing surface.

The sensor is always mounted there:

• with the same magnetic base,
• in the same direction,
• on a cleaned surface

.

Step 5: Record a baseline

Several measurements are carried out under comparable machine operating conditions.

The values are now considerably more reproducible.

Step 6: Evaluate high-frequency bearing diagnostics

For an additional early rolling-bearing diagnostic procedure, a higher frequency range is to be investigated later.

It is now checked whether:

• sensor frequency range,
• magnetic mounting,
• sampling rate,
• analysis

are sufficient for this purpose.

If not, a stiffer mounting method and a correspondingly more broadband measurement chain are required for this specific diagnostic task.

Result: Magnetic mounting is well suited to the existing mobile low-frequency trend measurement. However, this does not automatically mean that the same measurement arrangement is also suitable for every form of high-frequency bearing diagnostics.

Suitable ICS products for mobile vibration measurements

HySense VB110 – capacitive vibration sensor with integrated magnetic base

The HySense VB110 is a capacitive accelerometer for mobile vibration measurements under industrial environmental conditions.

The version specified by ICS features:

• measuring principle: capacitive accelerometer,
• frequency range: 1 … 100 Hz,
• measuring range: ±50 g,
• output signal: frequency or square-wave signal,
• mechanical connection via integrated magnetic base,
• ambient temperature: -20 … +85 °C,
• IP66 protection for the sensing element.

The permanent magnet integrated into the base allows quick mounting on suitable metallic surfaces.

This makes the sensor particularly interesting for:

• mobile vibration measurements,
• recurring comparative measurements,
• measurement routes on machinery,
• low-frequency machine movements,
• unbalance and condition monitoring within the specified frequency range.

With the VB110 in particular, the repeatability of magnetic mounting described in this article is important.

For trend measurements, the following should therefore be used as consistently as possible:

• the same machine location,
• the same sensor orientation,
• a clean contact surface

.

Further information can be found under HySense VB110 at ICS Schneider.

Important: correctly assessing the frequency range of the VB110

The frequency range of the HySense VB110 is 1 … 100 Hz.

This makes the sensor useful for many low-frequency machine movements and comparative measurements.

For a machine operating at 1,500 min^-1, for example, the fundamental rotational frequency is 25 Hz and is therefore within this range.

For very early detection of rolling-bearing defects, however, considerably higher frequencies or excited resonance ranges may be relevant.

The VB110 should therefore not automatically be selected for every type of bearing diagnostic application simply because it is described as a “vibration sensor”.

The required frequency range must first be derived from the actual diagnostic task.

HySense SC100 – converting a frequency signal into 4…20 mA

The HySense SC100 is a multifunctional signal converter for frequency signals.

It can convert a frequency signal into an industrial 4…20 mA signal.

Data specified by ICS include:

• measured variable: frequency, optionally two frequencies,
• measuring range: 0 … 5,000 Hz,
• output: 4 … 20 mA ISDS,
• optional switching output,
• measurement accuracy: ±0.1% FS,
• degree of protection: IP67.

The SC100 is not an additional vibration sensor, but a solution for processing or transmitting suitable frequency signals.

When combined with a sensor, the complete signal chain must be matched with regard to:

• input signal,
• frequency range,
• scaling,
• power supply,
• output signal

.

Further information can be found under HySense SC100 at ICS Schneider.

Which mounting method is suitable for the measurement task?

Measurement task	Suitable mounting method
Permanent condition monitoring	Where possible, rigid screw/stud mounting in accordance with the sensor specification
Broadband frequency analysis	As rigid a direct mounting method as possible
Drilling not permitted, permanent measuring point	Suitable rigid adhesive mounting base
Mobile maintenance route	Magnetic base on a defined, suitable surface
Quick indicative comparison	Magnet or, where appropriate, hand-held probe
Reproducible long-term trend	Fixed measuring point with the same mounting method every time
Very early high-frequency bearing diagnostics	Design broadband sensor and rigid mounting together
Low-frequency mobile vibration measurement up to 100 Hz	HySense VB110 with integrated magnetic base may be suitable

An overview of solutions currently available from ICS can be found under vibration measurement and vibration sensors at ICS Schneider.

Conclusion

The mounting of a vibration sensor is not a minor mechanical detail but part of the measurement transmission chain.

The sensor, mounting element and machine surface together form a vibratory mechanical system.

The stiffer and more direct the coupling, the higher the mounted resonance frequency generally is and the wider the usable frequency range can be.

For demanding broadband vibration measurements, correctly implemented screw or stud mounting therefore generally provides the best conditions.

A rigid, thin adhesive joint can be a very good alternative when drilling is not possible.

Magnetic bases, on the other hand, are particularly practical for mobile measurement routes and temporary inspections. However, they introduce additional mass and another mechanical interface into the measurement chain.

Hand-held probes offer the lowest mechanical repeatability and are therefore more suitable for indicative measurements.

For condition monitoring in particular:

A good trend measurement requires not only the same sensor, but also the same measuring point, measuring direction and mounting method.

If the mounting method is changed, a new baseline should be recorded.

For bearing diagnostics, the required frequency range is also decisive. A sensor may be very well suited to low-frequency machine movements and unbalance while still lacking sufficient bandwidth for high-frequency early bearing impulses.

The HySense VB110 available from ICS illustrates this clearly: the integrated magnetic base enables quick mobile installation, while its specified frequency range of 1 … 100 Hz must be taken into account when selecting it for an application.

For practical applications, the recommended sequence is therefore:

Define diagnostic objective → determine required frequency range → select sensor → select mounting method to match the frequency range → prepare a rigid measuring point → align the sensor correctly → provide cable strain relief → document operating condition → check repeatability → establish baseline.

FAQ: Mounting a vibration sensor correctly

How should a vibration sensor be mounted?

The optimum mounting method depends on the required frequency range and the application. For the widest possible frequency range, rigid direct screw or stud mounting is normally the preferred solution. For mobile measurements, a magnetic base may be suitable.

Which mounting method provides the widest frequency range?

Correctly implemented direct screw or stud mounting on a smooth and rigid surface generally provides the widest usable frequency range.

Why does a magnetic base influence the frequency response?

The magnetic base adds extra mass and another mechanical interface between the machine and the sensor. This can lower the mounted resonance frequency and reduce the usable upper frequency range.

Is magnetic mounting bad?

No. Magnetic mounting is very practical for many mobile and recurring vibration measurements. The important factor is whether its usable frequency range is suitable for the measurement task.

When is a magnetic base particularly suitable?

For maintenance routes, temporary measurements and quick comparative measurements on suitable ferromagnetic machine surfaces.

Can I detect bearing faults using a magnetic base?

That depends on the sensor, magnet, surface and the frequencies required for the bearing diagnostic method. Low-frequency effects may be measured effectively, while early high-frequency bearing impulses may require a considerably more rigid and broadband measurement chain.

Why is screw mounting better for high frequencies?

It creates a very rigid and direct mechanical connection between the machine surface and the sensor. As a result, the resonance of the mounted system is normally higher.

Can a vibration sensor be bonded in place?

Yes. A thin and rigid adhesive joint can be a good alternative to screw mounting when drilling is not possible or permitted.

Which adhesive is suitable for an accelerometer?

The adhesive must be suitable for the sensor, temperature, environment and required installation duration. For frequency response, a thin and sufficiently rigid connection is particularly important. The manufacturer’s instructions for the sensor or mounting base should be followed.

Can I use double-sided adhesive tape?

Depending on the sensor, such mounting may be possible for indicative measurements. However, soft or thick adhesive pads can attenuate high frequencies much more strongly than a rigid adhesive joint.

Which is better: adhesive or magnet?

That depends on the application. A rigidly bonded mounting base can provide better coupling at higher frequencies, while a magnet can be repositioned much more quickly.

Why should an adhesive mounting base be used?

A separate base protects the sensor base and thread from adhesive and allows the sensor to be removed or replaced later without completely recreating the measuring point.

How important is the surface for magnetic mounting?

Very important. The surface should be as smooth, clean and sufficiently large as possible so that the magnet sits securely and reproducibly.

Can I measure on paint?

A magnetic measurement on a firmly adhering coating may work for certain low-frequency tasks. Thick or uneven coatings, however, become part of the mechanical measurement chain and can particularly influence higher frequencies.

Can rust influence the measurement result?

Yes. Rust and dirt can impair the contact surface and additionally reduce the holding force during magnetic mounting.

Why must the mounting surface be flat?

A flat surface enables force to be transmitted between machine and sensor across as much of the contact area as possible and with high stiffness. Point contacts increase mechanical compliance.

How tightly should a vibration sensor be installed?

The mounting torque specified by the sensor manufacturer should be used. Both insufficient and excessive tightening can be problematic.

Why must the threaded stud not bottom out?

If the stud bottoms out before the sensor base is fully seated on the mounting surface, a defined full-area coupling cannot be achieved.

Should grease be applied beneath a vibration sensor?

For certain directly mounted accelerometers, manufacturers recommend a very thin suitable coupling layer to bridge microscopic surface irregularities and improve high-frequency transmission. The instructions for the specific sensor must be observed.

Why should a magnet not be allowed to snap onto the machine?

A powerful magnet can generate a high mechanical shock when it snaps into place uncontrollably, unnecessarily loading the sensor.

Can a hand-held probe be used for condition monitoring?

Yes, for indicative measurements. For precise long-term trend measurements, however, it is less reproducible because force, position and alignment depend on the operator.

Why do hand-held and magnetic measurements differ?

The two mounting methods have different mechanical stiffness and therefore different transmission characteristics. A hand-held probe is also more operator-dependent.

Why does a sensor measure such high vibration on thin sheet metal?

The sheet metal may have its own local resonance. The sensor may then measure the vibration of the sheet more strongly than that of the actual bearing or machine housing.

Where should a vibration sensor be mounted on a motor?

For typical machine condition measurements, measurements should be taken as close as possible to the bearings on a rigid part of the housing. The exact position depends on the machine and diagnostic objective.

Which measuring directions are commonly used?

On rotating machines, horizontal-radial, vertical-radial and axial measuring directions are commonly evaluated.

Does the sensor axis need to be aligned accurately?

Yes. A single-axis accelerometer primarily measures along its sensitive axis. Tilted mounting causes the actual motion to be projected onto this axis.

What does cross-axis sensitivity mean for a vibration sensor?

Internal cross-axis sensitivity describes the sensor’s response to acceleration perpendicular to its intended measuring axis. This must be distinguished from a geometric measurement error caused by incorrect alignment of the sensor axis.

Does sensor mass influence the measurement result?

On massive machine components, usually only slightly. On very lightweight or thin structures, however, the additional mass of the sensor, magnet and adapter can change the vibration behavior of the structure.

Why is the sensor cable important?

A moving or tensioned cable can transmit mechanical forces to the sensor and, depending on the sensor type and signal chain, generate additional interference.

How should the cable be routed?

It should be strain-relieved close to the sensor, should not pull tightly on the sensor and should not strike loosely against moving or vibrating machine components.

Why does mounting influence high frequencies more strongly than low frequencies?

Soft or relatively massive mounting components form a mechanical system with the sensor and structure. The resonance and transmission behavior of this system have a particularly strong influence in the upper frequency range.

What is mounting resonance?

The sensor and mounting arrangement together have a mechanical natural frequency. Around this frequency, signals may be amplified, while above it they may increasingly be altered or attenuated.

Can mounting hide a bearing fault?

Yes. If high-frequency components relevant to the diagnosis are strongly attenuated by the mounting method, characteristic signal components may become less clearly visible.

How is the rotational frequency of a machine calculated?

The rotational speed in min^-1 is divided by 60. At 1,500 min^-1, for example, the rotational frequency is 25 Hz.

Which frequency is caused by unbalance?

Classic rotating unbalance often produces a strong component at the fundamental rotational frequency. For a complete diagnosis, however, additional signal components and machine operating conditions must also be considered.

Why do bearing diagnostics often require higher frequencies?

Local bearing defects can generate short mechanical impulses that excite higher-frequency structural resonances. For the corresponding diagnostic methods, the entire measurement chain must be able to detect these frequencies.

Can a 100-Hz sensor detect every bearing fault?

No. A frequency range up to 100 Hz may be suitable for low-frequency machine conditions, but may be insufficient for very early high-frequency rolling-bearing diagnostics.

What frequency range does the HySense VB110 have?

The HySense VB110 offered by ICS has a specified frequency range of 1 … 100 Hz.

How is the HySense VB110 mounted?

The HySense VB110 has an integrated magnetic base for quick mounting on suitable metallic surfaces.

What measuring range does the HySense VB110 have?

The specified acceleration measuring range is ±50 g.

Which measurements is the HySense VB110 suitable for?

It is particularly suitable for mobile vibration measurements, recurring comparative measurements and low-frequency machine movements within its frequency range.

What is particularly important for trend measurements with the VB110?

Because of its mobile magnetic mounting, the same measuring point should be used whenever possible, with the same sensor orientation and comparable surface quality.

Can I directly compare measurements taken with magnetic and screw mounting?

Not without verification. The different mounting methods can have different transfer functions. After changing the mounting method, a new reference measurement or baseline should therefore be recorded.

What should be documented in a vibration measurement report?

At minimum, the machine, measuring point, measuring direction, sensor, mounting method, operating condition, rotational speed, date and analysis parameters used should be documented.

Which mounting method is best for long-term condition monitoring?

If the application and machine design permit it, a permanently defined, rigid mounting provides the highest repeatability. However, the specific mounting method must always be suitable for the intended sensor design.

Where can I find vibration sensors at ICS Schneider?

An overview can be found under vibration measurement and vibration sensors at ICS Schneider.