Vibration sensors for pumps and motors: Detecting bearing condition and imbalance at an early stage

→ Product category: Vibration sensors

 

Pumps, electric motors, fans, compressors and gearboxes generate mechanical vibration during normal operation. Completely vibration-free operation is neither realistic nor necessary for rotating machinery. What matters is whether the vibration changes compared with the known normal condition.

Increasing imbalance, incorrect coupling alignment, a loose machine foot or an emerging bearing defect changes the amplitude, frequency and direction of the vibration. These changes often occur before an increase in temperature, noise or loss of performance becomes clearly noticeable.

Vibration sensors make such changes in condition measurable. Depending on their design, they record vibration acceleration, vibration velocity or vibration displacement and provide the values to a portable measuring instrument, data logger, PLC or condition-monitoring system.

However, a single vibration measurement does not automatically permit an unambiguous fault diagnosis. The same overall value can be caused by different problems. For a reliable assessment, the measured variable, frequency range, sensor position, mounting method, machine condition and development over time must be considered together.

It is particularly important to distinguish between general machine-condition monitoring and detailed bearing diagnostics. Vibration velocity is well suited to assessing imbalance, misalignment and mechanical looseness. Very early rolling-bearing defects, however, often generate high-frequency impulses that must be investigated using suitable acceleration measurements, spectral analysis or envelope methods.

This article explains how vibration sensors are used on pumps and motors, which measured variables are suitable for different fault patterns and why the measuring position and mounting method determine the quality of the results. It also shows how repeated measurements can be developed into a practical condition-based maintenance strategy.

Table of contents

• Why pumps and motors should be monitored
• What mechanical vibration means
• Vibration displacement, vibration velocity and acceleration
• Distinguishing overall values from frequency spectra
• Detecting imbalance
• Misalignment of motor and pump
• Detecting rolling-bearing defects at an early stage
• Mechanical looseness and soft foot
• Resonance and critical speeds
• Detecting cavitation in pumps
• Electrical motor faults and vibration
• Which types of vibration sensors are available
• Selecting the appropriate frequency range
• Measuring range, sensitivity and overloading
• The correct measuring position on pumps and motors
• Measuring radially, horizontally, vertically and axially
• Comparing threaded, adhesive and magnetic mounting
• Using magnetic mounting correctly
• Mounting surface and mechanical coupling
• Cables, strain relief and EMC
• Operating condition and comparability
• Speed as a reference variable
• Correctly interpreting limit values and ISO 20816
• Why the trend is often more important than an individual value
• Periodic measurement routes
• Continuous vibration monitoring
• Data loggers and measuring intervals
• Pre-alarm, main alarm and shutdown
• From measurement to a maintenance strategy
• Combining vibration with temperature and process data
• Systematic diagnosis of abnormal vibration
• Typical errors in vibration measurement
• Practical example: Increasing vibration on a centrifugal pump
• Which information is required for sensor selection
• Which measuring instruments / products are suitable?
• Conclusion
• Frequently asked questions about vibration sensors

Why pumps and motors should be monitored

Rotating machinery is among the operationally critical components in many production installations. If a central cooling-water pump, process fan or drive motor fails, the entire installation may come to a standstill.

Many faults do not occur suddenly from a completely normal condition. They develop over a longer period. A small imbalance increases the bearing load, incorrect alignment places additional stress on the coupling and seals, and insufficient lubrication can gradually lead to bearing damage.

During this phase, the machine may continue to provide the required flow rate or drive power. Only when the problem becomes more severe do noticeable noise, heating, leakage or a loss of performance occur.

Vibration monitoring is intended to make this development visible at an earlier stage. It makes it possible to:

• detect deviations from the known normal condition
• plan maintenance according to the actual machine condition
• reduce unplanned failures
• prepare repairs during a scheduled shutdown
• verify the effect of alignment, balancing or bearing replacement

However, the measurement does not replace a technically sound mechanical diagnosis. It provides an objective signal from which a cause can be derived together with speed, process condition, temperature and machine design.

What mechanical vibration means

Mechanical vibration is a movement that changes over time around an equilibrium position. In rotating machinery, it is caused, among other things, by changing forces from the rotor, shaft, coupling, bearings, electromagnetism and the conveyed medium.

Vibration has three essential characteristics:

• Amplitude: How strong is the movement?
• Frequency: How often does the movement repeat per second?
• Phase: What is the timing relationship between the movement and a reference?

Frequency is specified in hertz. An event at 25 Hz repeats 25 times per second.

For a machine operating at 1,500 revolutions per minute, the rotational frequency is:

1,500 min⁻¹ ÷ 60 = 25 Hz

A pronounced vibration at 25 Hz is referred to as the 1× rotational frequency. Peaks at 50 or 75 Hz correspond in this example to twice or three times the rotational frequency.

This relationship is important for diagnosis because certain faults frequently produce characteristic frequency patterns.

Vibration displacement, vibration velocity and acceleration

The same vibratory movement can be described as displacement, velocity or acceleration. These measured variables are mathematically related, but emphasise different frequency ranges.

Measured variable	Typical unit	Typical application
Vibration displacement	µm or mm	Slow movements, shaft vibration, sleeve bearings and large machines
Vibration velocity	mm/s	General machine condition, imbalance, misalignment and looseness
Vibration acceleration	m/s² or g	Higher-frequency events, impacts and rolling-bearing diagnostics

Vibration velocity is frequently specified as an RMS value. It represents a wide range of typical mechanical machine faults well and is therefore often used for general condition assessment.

Acceleration reacts more strongly to rapid changes and short impulses. Damage to raceways or rolling elements generates an impact each time the damaged point is passed. These impulses can become visible in the acceleration signal at an early stage, while the general vibration-velocity value remains comparatively normal.

Vibration displacement is particularly meaningful at low frequencies and large movement amplitudes. For small, high-speed pumps and motors, it is generally less suitable for overall housing monitoring than velocity or acceleration.

A single measured variable therefore does not cover every fault pattern equally well. For basic monitoring, an overall vibration-velocity value is often sufficient. More detailed diagnosis additionally requires acceleration, a frequency spectrum and, where applicable, envelope-analysis values.

Distinguishing overall values from frequency spectra

An overall vibration value combines the vibration energy within a defined frequency range into one number. It is easy to monitor and is well suited to trends and alarm limits.

If the overall value increases, this indicates a change in the machine condition. However, the value alone does not explain the cause.

A frequency spectrum divides the signal into individual frequency components. This makes it possible to determine whether the vibration occurs mainly at the rotational frequency, at multiples of that frequency, at a blade-passing frequency or within a higher-frequency bearing range.

In simplified terms:

• A dominant 1× component may indicate imbalance.
• Pronounced axial components and several rotational harmonics may indicate misalignment.
• Numerous harmonics may occur with mechanical looseness.
• High-frequency components that are not synchronous with the speed may indicate rolling-bearing or gear-tooth problems.

These patterns are indications rather than conclusive proof. The machine design, load, foundation, resonances and measuring position can strongly influence the spectrum.

Detecting imbalance

With imbalance, the centre of mass of the rotating component is not located precisely on the rotational axis. This produces a centrifugal force with every revolution.

Possible causes include:

• deposits on the impeller
• uneven wear
• damaged fan or pump blades
• installation errors
• loss of material
• inaccurate balancing after a repair

Imbalance frequently appears as pronounced radial vibration at the fundamental rotational frequency. The amplitude increases significantly as the speed rises.

With a variable-speed pump, the vibration may therefore remain inconspicuous at low speed but become clearly pronounced at high speed.

However, a single 1× peak does not prove that imbalance is present. A bent shaft, eccentricity or structural resonance can also produce a strong speed-synchronous component.

Misalignment of motor and pump

With coupled machines, the motor and pump shafts must be aligned within the permissible limits. A distinction is made between parallel offset, angular offset and combinations of the two.

Misalignment produces additional forces in the coupling, shafts and bearings. It can lead to increased wear, seal damage, heating and energy losses.

Typical indications may include:

• increased axial vibration
• pronounced components at 1× and 2× rotational frequency
• different vibration values at the coupled bearing positions
• increased coupling or bearing temperature
• recurring damage to seals or coupling elements

Alignment should be performed with the actual operating condition in mind. Thermal expansion, pipe forces and foundation movement can mean that a machine aligned correctly at standstill is no longer optimally aligned when warm.

A new vibration measurement is useful after laser alignment. It shows whether the measure has had a positive effect under actual operating conditions.

Detecting rolling-bearing defects at an early stage

Rolling bearings consist of an inner ring, outer ring, rolling elements and a cage. Local damage to these components produces small mechanical impulses each time the damaged area is passed.

The associated fault frequencies depend on:

• speed
• number and diameter of the rolling elements
• pitch diameter
• contact angle

They therefore frequently do not correspond precisely to the fundamental rotational frequency.

During an early stage of damage, the released energy is low and often occurs within a higher frequency range. The general vibration-velocity value may still remain inconspicuous at this point.

Suitable methods for early bearing diagnostics include:

• high-frequency acceleration measurement
• envelope analysis
• shock-pulse or comparable bearing-condition values
• frequency spectra using known bearing geometry data
• trend comparisons under consistent operating conditions

As the damage progresses, broadband acceleration, vibration velocity, noise and temperature frequently increase as well.

A sensor with a frequency range of only 1 to 100 Hz can record low-frequency machine movement and imbalance well, but may not fully detect the high-frequency impulses of a very early rolling-bearing defect. The required frequency range must therefore be derived from the diagnostic objective.

Mechanical looseness and soft foot

Loose foundation bolts, inadequately secured bearing housings or play in mechanical connections can produce non-linear vibration patterns.

Several harmonics of the rotational frequency frequently occur. The measured value may also depend strongly on the load, direction and measuring position.

A soft foot condition exists when one machine foot does not rest flat on the foundation. Tightening the bolts then distorts the machine housing. This can alter the alignment, bearing load and air gap in the motor.

Abnormal vibration should therefore not immediately be corrected by balancing. The foundation, bolts, shims, pipe forces and coupling should be checked first.

If structural looseness is concealed through additional balancing, the machine may run more smoothly for a short period while the actual cause remains present.

Resonance and critical speeds

Every mechanical structure has natural frequencies. If a natural frequency is excited by the speed or another periodic force, the vibration amplitude can increase sharply.

On a variable-speed machine, resonance often becomes apparent because the vibration is particularly high only within a limited speed range.

A machine may run smoothly at 1,200 and 1,800 min⁻¹, but vibrate strongly at 1,500 min⁻¹. This indicates resonance rather than a simple speed-independent imbalance.

Possible countermeasures include:

• changing the structural stiffness
• improving the foundation
• changing the mass
• avoiding a critical speed range
• eliminating the exciting force

Strong resonance can significantly amplify small excitation forces. High vibration does not therefore automatically mean that a major rotor fault is present.

Detecting cavitation in pumps

Cavitation occurs when the local pressure in the medium falls below the vapour pressure. Vapour bubbles form and collapse again in areas of higher pressure.

The collapse of these bubbles generates pressure impacts, noise and broadband vibration. Over time, the impeller, housing and seals can be damaged.

Possible causes include:

• insufficient inlet pressure
• blocked suction line or filter
• excessive medium temperature
• excessive pump speed
• unsuitable operating point
• air ingress

Vibration measurement alone can indicate cavitation, but should be combined with process values. Suction pressure, discharge pressure, flow rate, medium temperature and valve position help to narrow down the cause.

Cavitation must not be treated exclusively by stronger damping or an alarm threshold. The hydraulic operating conditions of the pump must be corrected.

Electrical motor faults and vibration

Electrical problems within a motor can also cause mechanical vibration. Examples include magnetic asymmetry, broken rotor bars, voltage fluctuations or an uneven air gap.

Certain faults produce frequency components associated with the mains frequency, number of poles and slip.

An unambiguous diagnosis may require a combination of:

• vibration analysis
• motor-current analysis
• voltage and power-quality measurement
• temperature measurement
• speed and load information

A purely mechanical repair is not effective if the vibration is caused by the electrical supply or an internal motor fault.

Which types of vibration sensors are available

Different sensor principles are used for machine monitoring.

Piezoelectric acceleration sensors

Piezoelectric sensors have a wide usable frequency range and are suitable for dynamic acceleration measurements. They are frequently used for spectral and bearing diagnostics.

Many versions use an IEPE- or ICP-compatible power supply. The evaluation instrument must be compatible with it.

Capacitive and MEMS sensors

Capacitive or MEMS-based sensors can measure down to very low frequencies and, in some cases, static acceleration. They are suitable for slow movements, mobile applications and robust machine measurements.

The actually usable frequency range depends on the specific model.

Vibration-velocity sensors

Velocity sensors provide a value directly in mm/s or through integrated electronics. They are easy to use for general machine-condition monitoring.

Vibration transmitters

A transmitter processes the sensor signal inside the instrument and provides, for example, a 4–20 mA signal. This can be connected directly to a PLC, display or control system.

The advantage is straightforward integration. The disadvantage is that a single overall value contains less diagnostic information than a complete raw signal or frequency spectrum.

Selecting the appropriate frequency range

The frequency range must include the fault frequencies relevant to the application.

Low frequencies may be decisive for a slowly rotating pump. Significantly higher frequencies are required for a high-speed motor and early bearing diagnostics.

The lower frequency limit must be sufficiently low to record the rotational frequency and, where applicable, slower structural movements.

The upper frequency limit must be sufficiently high for:

• bearing impulses
• gear-mesh frequencies
• cavitation
• blade-passing frequencies
• high-frequency resonances

A wide frequency range alone does not guarantee a good measurement. The sensor, mounting method, cable, sampling rate and evaluation instrument must all transmit the required range.

Magnetic mounting can limit the upper usable frequency compared with rigid threaded mounting. This influence must be considered in high-frequency diagnostics.

Measuring range, sensitivity and overloading

A sensitive sensor can record small vibrations well, but may reach its measuring limit during strong impacts.

A sensor with a very large measuring range is robust against overloading, but may offer less usable resolution for small signals.

Both the normal machine condition and possible start-up events, load changes and impacts must therefore be considered during selection.

An overloaded sensor can generate clipped signal peaks and incorrect spectra. An apparently constant maximum amplitude may therefore mean that the measuring chain is already operating outside its permissible range.

The correct measuring position on pumps and motors

The measuring position determines how directly the forces generated by the bearing and rotor are recorded.

The sensor should be mounted as close as possible to a load-bearing bearing position. Thin covers, cooling fins, protective guards or distant pipelines transmit vibration in a modified form and are generally unsuitable for reproducible machine assessment.

Typical measuring points on a coupled unit are:

• motor bearing on the coupling side
• motor bearing on the fan side
• pump bearing on the coupling side
• pump bearing on the impeller side

On small close-coupled pumps, not all bearing positions are separately accessible. A permanently defined point on a rigid part of the pump or motor housing must then be selected.

The position should be permanently marked or documented in a measuring-point drawing. A difference of only a few centimetres can produce significantly different values in the presence of structural resonances.

Measuring radially, horizontally, vertically and axially

Vibration occurs in different directions. For a complete assessment, horizontal, vertical and axial measurements are frequently performed at the bearing positions.

Radial measurements record movement perpendicular to the shaft axis. They often respond clearly to imbalance, bearing forces and foundation stiffness.

Axial measurements are taken parallel to the shaft. They are particularly relevant to misalignment, axial forces, coupling problems and certain bearing defects.

Horizontal and vertical vibration can differ because the foundation and machine structure have different stiffnesses in each direction.

A single sensor in only one direction may miss an important change in condition. For basic continuous monitoring, the most critical measuring point is therefore selected or a multi-axis sensor is used.

Comparing threaded, adhesive and magnetic mounting

Mounting method	Advantages	Limitations
Threaded stud	Very rigid and reproducible coupling, good high-frequency range	Prepared mounting surface and threaded hole required
Adhesive adapter	Permanent mounting possible without a threaded hole	Adhesive, temperature and surface preparation must be suitable
Magnetic base	Rapid mobile measurement and simple transfer between measuring points	Only on ferromagnetic surfaces, limited reproducibility and bandwidth
Handheld probe tip	Very rapid indicative check	Strongly dependent on the operator and only of limited suitability for trends

Rigid mounting is generally preferable for continuous monitoring and high-frequency bearing diagnostics.

For periodic measurement routes involving many machines, a magnetic base may be economical and sufficient, provided that the measuring point and orientation are defined reproducibly.

Using magnetic mounting correctly

A magnetic base allows the sensor to be attached to a metal machine without tools. This is particularly practical for service work and comparative measurements.

The surface must be:

• ferromagnetic
• clean
• flat
• free from loose rust
• sufficiently rigid

Thick paint layers, curved surfaces and irregularities impair mechanical coupling.

The sensor must be mounted at the same position and in the same direction for every measurement. If it is positioned horizontally during one route and slightly rotated during the next, the values are only comparable to a limited extent.

A magnet must not be attached to thin fan guards or loose protective sheets. In these positions, the sensor mainly records the resonance of the sheet rather than the actual bearing condition.

For very strong vibration or impacts, it must also be ensured that the magnet remains securely attached.

Mounting surface and mechanical coupling

There must be no unnecessarily soft or loose connection between the machine housing and the sensing element. Every intermediate layer acts as a mechanical filter.

A thick paint layer, an uneven contact surface or a loose adapter can attenuate high frequencies in particular.

For permanently installed sensors, the mounting surface should be prepared according to the manufacturer’s specifications. It must be sufficiently large, flat and free from burrs.

The tightening torque of a threaded sensor is also relevant. Insufficient torque results in poor coupling, while excessive torque can damage the sensor or thread.

Cables, strain relief and EMC

The sensor cable must not transmit mechanical forces to the sensor. A vibrating or tightly tensioned cable can generate additional signals and place stress on the mounting.

The cable should be strain-relieved close to the sensor without restricting movement of the machine.

For sensitive analogue or frequency signals, the cable type, shielding and maximum length must be observed. Motor and variable-frequency-drive cables routed in parallel can couple electrical interference into the sensor line.

For continuous measurement, the cable must additionally be protected against:

• oil
• heat
• moisture
• abrasion
• moving machine parts

Operating condition and comparability

Vibration values depend strongly on speed, load, flow rate and process conditions.

A pump may run smoothly at its optimum operating point but generate significantly higher hydraulic forces when the flow is heavily throttled.

For a reliable trend, comparative measurements should be performed under conditions that are as similar as possible:

• same speed
• comparable load
• same valve position or flow rate
• comparable medium temperature
• same operating condition after a sufficient warm-up period

If the speed changes regularly, the operating data should be stored together with the vibration values.

An increased measured value at a higher speed does not automatically indicate deterioration. Only comparison at the same operating point reveals a genuine change in condition.

Speed as a reference variable

The current speed is particularly important for frequency analysis. It is used to calculate the 1× rotational frequency.

With a fixed mains frequency, the actual motor speed is not always exactly equal to the synchronous speed. Slip and load alter it.

During variable-frequency-drive operation, the speed can change continuously. A speed signal or trigger then makes it easier to assign frequency components.

Without reliable speed information, speed-synchronous faults and non-synchronous bearing frequencies are more difficult to distinguish from one another.

Correctly interpreting limit values and ISO 20816

The ISO 20816 series contains guidelines for assessing machine vibration. Which parts of the standard and which assessment ranges are applicable depend, among other things, on the type of machine, power, speed, bearings and installation.

A limit value must therefore not be taken from a general table without further assessment.

The following should be considered, for example:

• motor and pump power
• rigid or flexible foundation
• speed range
• bearing type
• measured variable and frequency range
• measuring point and direction
• normal operating condition

Even a value within a general standard zone can be abnormal for a particular machine if it has increased significantly compared with the long-term normal value.

Conversely, a machine that generates more vibration because of its design may operate stably over a long period even though its absolute value is higher than that of another machine.

Standard values and individual trends should therefore be assessed together.

Why the trend is often more important than an individual value

An individual measurement shows the current condition. However, it does not indicate whether the value is normal, increasing or decreasing after a repair.

A trend curve shows the development over weeks or months. A slow but continuous increase may become visible at an early stage.

For a reliable trend, the following conditions should remain as consistent as possible:

• measuring point
• measuring direction
• sensor and mounting method
• frequency range
• evaluation parameter
• speed and load

A sudden change in the trend may also result from a change in the measuring method. If a magnetic base is replaced with threaded mounting or the frequency range is altered, this must be recorded in the documentation.

Periodic measurement routes

During a periodic measurement route, an employee visits the machines at defined intervals and measures specified points.

This concept is particularly suitable for:

• larger machine inventories
• less critical units
• easily accessible measuring points
• faults that develop over weeks or months

The intervals depend on machine criticality, known fault development and operating experience.

A monthly measurement may be sufficient for a stable auxiliary unit. A critical machine with a known bearing problem may require much more frequent monitoring.

Mobile sensors with magnetic bases are practical for measurement routes. Clearly marked measuring points and reproducible operating conditions are essential.

Continuous vibration monitoring

With continuous monitoring, the sensor remains installed on the machine. The values are recorded continuously or at short intervals.

This is particularly useful for:

• critical pumps and motors
• machines that are difficult to access
• unmanned installations
• rapidly developing faults
• strongly changing operating conditions

A basic 4–20 mA vibration transmitter can provide an overall value to the PLC. Trends, pre-alarms and main alarms can then be implemented there.

For detailed diagnostics, an additional system capable of storing time waveforms and spectra may be required.

Continuous measurement does not automatically create predictive maintenance. A usable condition-monitoring system only results from suitable alarm rules, data evaluation and defined responses.

Data loggers and measuring intervals

A data logger stores vibration values with a timestamp and, where applicable, additional process variables.

The required measuring interval depends on whether a slowly changing overall value or the dynamic raw signal is to be recorded.

For a trend of the overall vibration-velocity value, a measurement every few minutes or during defined operating phases may be sufficient.

For frequency analysis, the raw signal must be recorded at a sampling rate significantly higher than the highest frequency of interest.

A logger that stores only one individual value per minute cannot record a complete high-frequency vibration waveform. It can, however, document an overall value previously calculated by the sensor or measuring instrument.

Memory capacity, recording duration and data volume must therefore be derived from the required diagnostic function.

Pre-alarm, main alarm and shutdown

A condition-monitoring system can use different alarm levels.

Pre-alarm

The pre-alarm indicates an identifiable change. The machine can often continue operating, but should be inspected and monitored more closely.

Main alarm

The main alarm indicates a clear exceedance or rapid deterioration. Prompt maintenance or a controlled shutdown must be prepared.

Emergency shutdown

An automatic shutdown is appropriate only with systems designed for a corresponding safety function. A basic vibration sensor or PLC alarm is not automatically a certified protective function.

Alarm limits should not be set so tightly that every load change generates a message. Too many false alarms frequently lead to alarms being ignored or limits being increased without proper assessment.

At the same time, a long delay must not conceal a rapidly developing fault. Critical machines may additionally be monitored for the rate of change.

From measurement to a maintenance strategy

Vibration measurement only creates operational value if a defined response follows an abnormal result.

A useful procedure includes:

1. identify critical machines and typical fault patterns
2. document measuring points and operating conditions
3. record the initial condition or baseline
4. define measuring intervals and alarm limits
5. evaluate the trend regularly
6. perform detailed diagnostics if an abnormality occurs
7. plan the maintenance measure and verify its effect through new measurements

Condition-based maintenance does not mean operating every component until it fails. Its purpose is to find the optimum time between unnecessary preventive replacement and unplanned failure.

Combining vibration with temperature and process data

Vibration data becomes more meaningful when considered together with additional variables.

Additional measured variable	Possible benefit
Bearing temperature	Indication of friction, lubrication problems or overload
Speed	Assignment of frequency components and comparison of different operating points
Motor current	Assessment of the load and possible electrical faults
Suction and discharge pressure	Detection of hydraulic problems and possible cavitation
Flow rate	Assessment of the pump operating point
Oil condition	Additional indication of bearing and gearbox wear

If vibration increases at the same time as bearing temperature and motor current, for example, this indicates a different fault pattern from an increase in vibration occurring only when the suction pressure decreases.

Combined assessment reduces misinterpretation and makes root-cause analysis easier.

Systematic diagnosis of abnormal vibration

If an abnormal value is detected, a bearing should not immediately be ordered and the machine should not automatically be balanced.

First, it must be checked whether the measurement itself is reliable:

1. Check the measuring point: Was the measurement taken at the same position and in the same direction?
2. Check the sensor mounting: Is the magnet, adapter or threaded connection secure?
3. Compare the operating condition: Are the speed, load and flow rate comparable?
4. Repeat the measurement: Is the value reproducible?
5. Measure additional directions: Is the abnormality radial or axial?
6. Compare adjacent bearing positions: Where is the amplitude highest?
7. Analyse the spectrum: Which frequencies have changed?
8. Check the mechanical system: Are the bolts, foundation, coupling and pipe connections in good condition?
9. Check the process data: Have pressure, flow, temperature or speed changed?

A specific repair measure should only be derived after these checks.

Typical errors in vibration measurement

Error	Possible effect	Better approach
Measurement on a thin fan guard	Natural resonance of the sheet instead of the bearing condition	Measure on a rigid bearing or machine housing
Measuring point changes during every route	Trend values are not comparable	Mark and document the measuring points
Magnet positioned on a contaminated or curved surface	Poor coupling and attenuated frequencies	Use a clean, flat and sufficiently large surface
Only one measuring direction considered	Axial or direction-dependent faults remain undetected	Compare at least the relevant radial and axial directions
Measurements at different speeds compared directly	Apparent change in condition caused by the operating point	Document the speed and load together with the values
Only one overall value stored	The cause of the change remains unclear	Record the spectrum and time waveform as well if an abnormality occurs
Low-frequency sensor used for early bearing diagnostics	High-frequency bearing impulses are not recorded	Adapt the frequency range to the diagnostic objective
Frequency range or filter changed	Sudden change in the trend without an actual machine change	Keep the measurement parameters constant and document changes
Sensor cable without strain relief	Cable movement influences the signal	Route the cable securely and without mechanical tension
Alarm limit defined without a baseline	Too many false alarms or a warning that occurs too late	Combine standard guidance with the individual machine condition
High vibration automatically interpreted as a bearing defect	Unnecessary bearing replacement while the actual cause remains	Check imbalance, alignment, looseness, resonance and process conditions
Vibration smoothed using strong damping	Warning signs and rapid changes are concealed	Investigate the cause and configure filters on a justified basis

Practical example: Increasing vibration on a centrifugal pump

In a production installation, a horizontally mounted centrifugal pump conveys cooling water. The motor and pump are connected by a flexible coupling.

As part of a monthly measurement route, vibration-velocity values are recorded horizontally, vertically and axially at the four bearing positions.

The values remain stable over several months. The horizontal vibration at the pump-side bearing then begins to increase gradually. The values at the motor change only slightly.

An initial visual inspection reveals no leakage and no abnormal bearing temperature. The flow rate remains sufficient.

The fundamental rotational frequency dominates the frequency spectrum. The value increases significantly as the speed rises. High-frequency bearing-condition values, however, remain largely unchanged.

This pattern initially indicates imbalance rather than an emerging rolling-bearing defect.

During inspection of the impeller, deposits are found on one side. The pump is cleaned and the impeller is inspected for damage.

After recommissioning, the vibration decreases significantly but does not yet return to the original baseline. A subsequent inspection additionally reveals slight coupling misalignment.

The motor and pump are realigned. The values then return to the original baseline range.

Several months later, the high-frequency acceleration at the same bearing increases while the vibration velocity initially remains almost constant. Detailed envelope analysis shows a bearing-related fault frequency.

The bearing is replaced during a scheduled plant shutdown. An emerging raceway defect is visible during disassembly.

The example shows that different faults can occur one after another. The general vibration-velocity value initially detected the imbalance, while the subsequent bearing defect only became visible at an early stage through higher-frequency analysis.

Which information is required for sensor selection

The statement “vibration sensor for a pump” is not sufficient for a reliable selection.

At least the following information is required:

• machine type and manufacturer
• motor and pump power
• fixed or variable speed
• minimum and maximum speed range
• bearing type
• required fault detection
• overall-value monitoring or frequency diagnostics
• periodic or continuous measurement
• required measured variable
• required frequency and measuring range
• mounting option and surface
• ambient temperature and degree of protection
• exposure to oil, moisture or chemicals
• required output signal
• existing measuring instrument, data logger or PLC system
• required cable length
• hazardous area or other approvals

For existing machines, photographs of the bearing housing, mounting surface and connection environment are helpful.

For bearing diagnostics, the bearing designation, speed and number of rolling elements should also be known so that characteristic fault frequencies can be calculated.

Which measuring instruments / products are suitable?

The vibration measurement / vibration sensors category contains sensors and signal-conditioning solutions for mobile measurements and the integration of vibration signals.

The higher-level displacement, force, speed, torque, inclination and vibration sensors category additionally contains sensors for speed, position, force and torque. These variables can be recorded together with vibration for comprehensive machine-condition monitoring.

The data loggers and universal measuring instruments category contains systems for recording different measured variables over time. Whether a logger can directly record a dynamic vibration signal depends on the input, sampling rate and software.

HySense VB110 capacitive vibration sensor

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

The sensor has a frequency range of 1 to 100 Hz, a measuring range of ±50 g and a frequency output signal. Its integrated magnetic base allows it to be attached quickly to suitable metal machine surfaces.

The VB110 is therefore particularly suitable for:

• periodic measurements on pumps, motors and mobile machinery
• comparative measurements at defined measuring points
• recording imbalance and low-frequency machine movements
• measurement routes using compatible Hydrotechnik measuring instruments

For very early rolling-bearing diagnostics, it must be checked whether the frequency range up to 100 Hz is sufficient for the expected bearing and resonance frequencies. Depending on the machine, a broadband piezoelectric sensor with suitable envelope analysis may be required.

HySense SC100 signal converter

The HySense SC100 processes frequency signals and, depending on the version, provides a 4–20 mA output signal or switching function.

In combination with a suitable frequency sensor, it can simplify integration into a PLC, display or data logger.

The input frequency, scaling, power supply, cable length and required output range must be matched during system design.

A converted 4–20 mA overall value is well suited to trending and alarm functions. For detailed spectral diagnostics, however, the dynamic sensor signal must remain available in a suitable form.

Measuring systems and data loggers

For mobile measurement routes, the vibration sensor can be connected to a compatible handheld instrument or multi-channel measuring system. This allows vibration values to be recorded together with speed, pressure, temperature or other machine parameters.

Depending on the application, stationary transmitters, data-acquisition systems or PLC inputs can be used for continuous monitoring.

When selecting the data logger, it is necessary to distinguish whether only a slowly changing overall value or the complete dynamic signal is to be stored.

Selection according to the diagnostic objective

A low-frequency overall value may be sufficient for general monitoring of imbalance, misalignment and looseness.

Early detection of rolling-bearing or gearbox defects generally requires a wider frequency range, a high sampling rate and suitable analysis methods.

ICS Schneider Messtechnik assists with the selection of the sensor, mounting method, signal converter and data-acquisition system. For a technical design, the machine type, speed range, diagnostic objective, mounting surface, environmental conditions and required interface should be specified.

Conclusion: A vibration sensor only becomes meaningful through the measuring point, frequency range and trend

Vibration sensors can detect changes in pumps and motors before damage becomes apparent through noise, temperature or loss of performance.

Vibration velocity is particularly suitable for the general assessment of imbalance, misalignment and mechanical looseness. High-frequency acceleration and envelope methods, on the other hand, are frequently more suitable for the early detection of rolling-bearing defects.

The sensor must therefore be selected according to the specific diagnostic objective. A frequency range sufficient for the rotor speed may be too low for bearing impulses.

Mechanical mounting is equally decisive. The sensor should be installed as close as possible to a load-bearing bearing position on a rigid and reproducible surface.

A magnetic base simplifies mobile measurement routes, but can limit reproducibility and the usable high-frequency range compared with rigid threaded mounting.

An individual measured value permits only a limited assessment. A trend recorded under comparable operating conditions often shows more quickly and reliably whether the machine condition is changing.

Limit values from the ISO 20816 series can provide guidance, but must be appropriate for the specific machine, measuring direction and operating mode. The individual baseline, trend and standard assessment should be used together.

For reliable diagnosis, vibration, speed, load and process conditions must be considered together. For pumps, the flow rate, suction pressure, discharge pressure and possible cavitation are particularly relevant.

Effective condition-based maintenance only results when the measuring points, intervals, alarm levels and responses are clearly defined and the effect of every maintenance measure is subsequently verified through new measurements.

Frequently asked questions about vibration sensors

What does a vibration sensor measure?

A vibration sensor records mechanical vibration as acceleration, velocity or displacement and converts it into an electrical signal.

Why should a pump be monitored for vibration?

Changes can provide early indications of imbalance, misalignment, bearing problems, looseness, resonance or cavitation.

Why should an electric motor be monitored for vibration?

Vibration provides information about rotor imbalance, bearing condition, alignment, foundation problems and certain electrical faults.

Which measured variable is suitable for the general machine condition?

Vibration velocity in mm/s is frequently used for the general condition assessment of rotating machinery.

Which measured variable is suitable for bearing defects?

High-frequency acceleration measurement with envelope or shock-pulse analysis is frequently more suitable for early rolling-bearing defects.

What is the difference between vibration displacement and vibration velocity?

Vibration displacement describes the amount of movement. Vibration velocity describes how quickly this displacement changes and is particularly meaningful for many general machine faults.

What does g mean for an acceleration sensor?

g is the acceleration due to gravity. A value of 1 g corresponds to approximately 9.81 m/s².

What does RMS mean in vibration measurement?

RMS is the root mean square value. It describes the energy-related effect of a changing signal within a defined time and frequency range.

What is a peak value?

The peak value is the largest instantaneous signal amplitude. It responds more strongly to individual impacts than an RMS value.

What is peak-to-peak?

Peak-to-peak describes the distance between the largest positive and largest negative signal amplitude.

What is an overall vibration value?

It combines the vibration energy within a defined frequency range into a single value.

Is the overall value sufficient for fault diagnosis?

It is sufficient to identify a change in condition, but frequently not to determine the precise cause. A spectrum, measuring direction and additional operating data are required for this.

What does a frequency spectrum show?

It shows at which frequencies the vibration energy occurs and therefore helps to assign it to the speed, bearing, coupling or other components.

What does 1× rotational frequency mean?

This is the frequency corresponding to the current shaft speed. At 1,500 min−1, it is 25 Hz.

Which vibration is typical of imbalance?

Imbalance frequently produces dominant radial vibration at the fundamental rotational frequency.

How does misalignment appear?

Indications may include increased axial vibration and pronounced components at the fundamental rotational frequency and its multiples.

How does mechanical looseness appear?

Typical indications can include several harmonics, strongly direction-dependent values and changing behaviour under load.

How does a bearing defect appear?

Early defects frequently generate high-frequency impulses. At a later stage, overall vibration, noise and temperature often increase as well.

Can a bearing defect be detected before the temperature increases?

Yes. Suitable vibration and envelope methods can detect an emerging defect before a clear temperature increase occurs.

Can every vibration be attributed to a bearing defect?

No. Imbalance, misalignment, looseness, resonance, cavitation and process forces can generate similar overall values.

What is cavitation?

Cavitation is the formation and subsequent collapse of vapour bubbles in a liquid as a result of locally insufficient pressure.

Can cavitation be detected using a vibration sensor?

It can generate broadband vibration and noise. For a reliable assessment, the suction pressure, discharge pressure, flow rate and temperature should also be considered.

What is resonance?

Resonance occurs when an excitation frequency is close to a natural frequency of the machine or structure and the vibration is strongly amplified.

How can resonance be detected?

Vibration frequently increases sharply only within a specific speed range and decreases again above or below that range.

Where is a vibration sensor mounted on a pump?

As close as possible to the load-bearing bearing positions on a rigid pump or bearing housing.

Where is the sensor mounted on a motor?

Typical positions are on the bearing housings at the coupling and fan ends.

May measurements be taken on the fan guard?

Normally not for a reliable bearing assessment. Thin guards have their own resonances and distort the signal.

In which direction should the measurement be taken?

Depending on the task, horizontally, vertically and axially. A complete diagnosis frequently requires several directions.

Why is axial measurement important?

It responds particularly to misalignment, axial forces and certain coupling or bearing problems.

Is a magnetic base suitable for vibration measurements?

Yes, particularly for mobile measurement routes. The surface must be clean, flat, rigid and ferromagnetic.

Is magnetic mounting as good as threaded mounting?

It is sufficient for many routine measurements. However, rigid threaded mounting generally provides better reproducibility and high-frequency transmission.

Can measurements be taken on a painted surface?

A thin, firmly adhering paint layer may be acceptable depending on the application. Thick or loose coatings impair the coupling.

Why must measurements always be taken at the same point?

The machine structure transmits vibration differently depending on the position. Different measuring points can therefore provide significantly different values.

How frequently should vibration be measured?

The interval depends on criticality, fault development and operating conditions. It may range from continuous monitoring to monthly or quarterly routes.

When is continuous monitoring useful?

For critical, difficult-to-access or unmanned machines and for faults that can develop rapidly.

When is a mobile measurement route sufficient?

For easily accessible machines with slowly progressing faults and a lower risk of failure.

What is a baseline?

The baseline is the documented vibration condition of an intact machine under defined operating conditions.

Why is a trend important?

It shows whether the condition is changing gradually. An increasing deviation may be more relevant than an individual absolute value.

May measured values at different speeds be compared?

Only with an appropriate assessment. Speed and load influence vibration and should be documented together with the measured value.

What must be considered during variable-frequency-drive operation?

The wide speed range changes the fault frequencies and vibration level. Speed and operating point must therefore be recorded as well.

What role does the machine load play?

The load influences bearing forces, alignment, process excitation and vibration amplitude. Comparative measurements should be performed under similar loads.

What does ISO 20816 describe?

The series of standards contains guidelines for measuring and assessing machine vibration. The applicable part depends on the specific machine.

Does ISO 20816 specify universally applicable limits for every pump?

No. The machine type, power, speed, foundation, measuring point and operating conditions must be considered.

Can a value within the standard still be abnormal?

Yes. A significant increase compared with the individual baseline may indicate an emerging fault.

Can a value above a general table still be normal?

Possibly. The assessment must be appropriate for the specific machine and applicable part of the standard. The cause should nevertheless be investigated.

What is envelope analysis?

It demodulates high-frequency resonances excited by bearing impulses and makes periodic bearing fault frequencies more visible.

Does envelope analysis require a special sensor?

The sensor, mounting method and evaluation instrument must provide a sufficiently high frequency range and the required signal quality.

Can a sensor with a maximum frequency of 100 Hz detect early bearing defects?

Depending on the machine, later or low-frequency effects may become visible. The range may not be sufficient for very early high-frequency bearing impulses.

What is the HySense VB110 suitable for?

It is suitable for mobile vibration measurements and for recording low-frequency machine movements within a range of 1 to 100 Hz.

How is the HySense VB110 mounted?

The sensor has a magnetic base for rapid mounting on suitable metal surfaces.

Which output signal does the HySense VB110 provide?

It provides a square-wave frequency signal and is used with a compatible measuring or evaluation system.

What is the HySense SC100 used for?

The signal converter processes frequency signals and can convert them into an industrial 4–20 mA signal, among other outputs.

Can a 4–20 mA signal be used for vibration monitoring?

Yes. A scaled overall value can be transmitted to a PLC, display or data logger and used for trending and alarm functions.

Can a 4–20 mA signal transmit a frequency spectrum?

A single standard signal normally transmits only one calculated parameter. The dynamic raw signal or digital data transmission is required for a complete spectrum.

Which sampling rate does a vibration logger require?

It must be significantly higher than the highest frequency of interest. The specific value depends on the diagnostic objective, filtering and analysis method.

Can a normal temperature data logger record vibration?

Not automatically. The logger requires a suitable input, a sufficiently high sampling rate and appropriate evaluation software.

Which additional measured variables are useful for a pump?

Suction pressure, discharge pressure, flow rate, speed, bearing temperature and motor current supplement the vibration diagnosis.

Which additional measured variables are useful for a motor?

Speed, current, voltage, power, bearing temperature and winding temperature can support the root-cause analysis.

Can a sensor be connected directly to a PLC?

This depends on the output signal. A 4–20 mA or 0–10 V transmitter is easier to integrate than a dynamic IEPE or frequency sensor.

What should be measured after a repair?

The same measuring points should be recorded again under comparable conditions and compared with the baseline and the condition before the repair.

Which information does ICS Schneider require for selection?

The machine type, speed, power, diagnostic objective, frequency range, mounting method, environment, output signal and existing measuring or automation system are required.