Vibration Meter

Vibration meters from PCE Instruments are suitable for the regular recording of physical parameters such as vibration displacement, vibration velocity and vibration acceleration. Some vibration meters also have other measurement options, such as temperature measurement, speed measurement or determination of the bearing parameter. A vibration meter can be used to check the general condition as well as the condition of individual components (roller bearings, gears, etc.) of a machine. Trend developments of these parameters provide information about the impending damage. In this way, maintenance schedules or machine downtimes can be planned according to the condition of the machine. With this concept, a vibration meter increases the operational safety on the one hand, and at the same time increases the machine efficiency due to optimized machine running times and reduced downtimes. As mobile devices, vibration meters from PCE Instruments can be used autonomously, even in the difficult-to-access places. Depending on the vibration meter model, the measurement data can also be stored and subsequently evaluated with the help of the software.

When selecting a suitable vibration meter, it is important to pay attention to various aspects because of a large number of applications. These include measuring range, frequency range, the possibility of an FFT analysis, determination of the bearing parameter, memory capacity and other measuring functions. A frequent application is machine monitoring and machine evaluation according to DIN EN ISO 10816. This standard classifies various machine sizes according to drive power and assigns limit values to the individual classes, compliance with which guarantees safe, trouble-free operation. The standard also defines a number of specifications which vibration meters must comply with in order to ensure the measurement in conformity with the standard.

On the one hand, the standard specifies that the vibration velocity in mm/s is the relevant unit for machine monitoring. Furthermore, the frequency range to be covered is specified, which is between 10 Hz and 1 kHz. This means that a suitable vibration meter should be equipped with a corresponding frequency filter. All PCE Instruments vibration meters have these frequency filters, and depending on the vibration meter model the other useful machine monitoring functions are available.

Another important parameter in vibration monitoring is the speed, as comparable measurements and trend determinations must always be taken under the same operating conditions. Some vibration meters therefore have integrated, contacting or non-contacting speed sensors with which the speed can be determined directly on site. Since the defective components or bearings often generate increased friction values during the rotation, the surface temperature of a machine in the direct vicinity of the critical components can also provide valuable information on the machine condition. Some measurement data collectors therefore also have temperature sensors for determining the surface temperature. Another useful function is the FFT analysis. With this frequency evaluation, the vibration behavior of a machine can be analyzed in terms of vibration frequencies.

The vibration meter with the corresponding function shows the frequency spectrum of the machine on the display. Frequency peaks within the spectrum can indicate damage in rolling bearings. In order to be able to localize the errors more precisely, damage frequencies of the components at a given speed are required. Damage frequencies can be calculated using formulas, and information on this can often be received from the component manufacturer. Vibration meters with the functions described above can be found at PCE Instruments.

Machine condition monitoring is a key manufacturing maintenance task performed in production facilities around the globe. By identifying potentially problematic equipment based on machine diagnostics data, factories can take action to fix issues before equipment failures lead to costly production downtimes.

The PCE-VT 2700, PCE-VT 1100 and PCE-VT 204 vibration meter models primarily are used to measure vibration acceleration, vibration velocity and vibration displacement (also referred to as three-axis or X,Y,Z vibration) in manufacturing production machines and equipment. Using one of these PCE-branded vibration meter devices, a machine maintenance technician can assess the current condition of the equipment and identify the cause of any undesirable vibrations.

During machine vibration analysis, a graph of the vibration frequency spectrum is a useful visual aid. Increasing vibration is usually an indication that something is going wrong with the machine. Since vibration is connected to frequency, a frequency spectrum graph will reflect any increases or decreases in vibration.

The PCE-VM 25 vibration meter has a special function used for Fast Fourier Transform (FFT) analysis. For this reason, the PCE-VM 25 vibration meter is ideal for detailed investigations of machine bearing frequencies. The image on the right shows the PCE-VM 25 vibration meter in use to assess the vibration of a bearing in an industrial motor.

Vibration measurements can and should be taken not only to monitor the condition of the machine, but also to determine the effect of the machine's vibration on the machine operator. Vibrations may be permanent and continuous, or they may occur only from time to time, like impulses. Vibrations may be inertial, forced (i.e., a force acting on an object), relative or free (when displacement is involved). Depending on the position of the operator and his / her proximity to the equipment, he / she may experience whole-body or hand-arm vibrations.

If exposure exceeds the permissible limits, the operator may experience long-term problems with his / her fingers, hands, back and other body parts. Therefore, no matter what type of vibration is in question, the vibration should be evaluated according to its impact on the health and safety of the machine operator. If possible, changes to the machine itself (or to the amount of time the operator spends using the machine) should be made immediately to prevent or limit the operator's exposure to negative vibrations.

Vibration meters also are used to measure vibration in buildings. There are many factors that can contribute to vibration in buildings, such as the building's proximity to high-traffic roadways, railroad tracks, airports, quarries, construction sites and other heavy industrial areas. In addition, buildings can vibrate due to natural causes such as wind, weather and earthquakes, or due to the operation of internal building components like elevators and heating, ventilation and air conditioning (HVAC) systems. Vibration testing helps to determine what areas in a building are at the higher risk for vibration. Regular vibration monitoring in high-risk areas can provide property managers with advance notice of vibration-related structural issues.

Permissible exposure limits for vibrations are established by entities such as the International Standards Organization (ISO). ISO vibration regulations include:

- DIN ISO 10816 (vibration severity in machines)
- DIN ISO 7919 (mechanical vibration)
- DIN 4150 (structural vibration, buildings)
- DIN ISO 10816-3 (machine condition, machine vibrations)
- DIN ISO 2631 (whole-body exposure)
- DIN ISO 5349-1 and DIN ISO 5349-2 (hand-arm vibration)

External calibration in a calibration laboratory
To ensure that a vibration meter works with the accuracy stated in the device specification, the devices should be checked regularly. Checking the measurement accuracy according to the specified rules is also called calibration. It can be done, for example, in particularly for this specialized external calibration laboratories. There are two different standards for calibrations, which differ, among other things, in the effort required and in the number of the measuring points. These are the ISO calibration according to the requirements of DIN EN ISO 9001 and the more demanding DAkkS calibration according to DIN EN ISO 17025.

The parameters that can be calibrated in a vibration measuring device include the vibration acceleration, the vibration velocity and the vibration displacement. The vibration frequency is also tested. When commissioning the calibration, it is important to ensure that the target values correspond to the range in which the operational measured values lie. A certificate is issued about the calibration carried out, which contains all the important information about the calibration procedure and values. Many calibration laboratories also offer to adjust the vibration meters if the accuracy specifications are not met, at a separate charge.

On-site calibration with a vibration calibrator
When calibrations are carried out in external laboratories, the vibration meter is often not available for the measurements for several weeks. Therefore, these calibrations should be planned and organized in advance. An alternative to this is to check the vibration meters in your own company with a vibration calibrator. For this purpose, the vibration sensor is attached to the coupling surface of the calibrator and set in defined vibrations. These target values of the vibrations generated by the calibrator are then compared with the measurement values from the vibration meter. The purchase of the calibrator can be particularly useful if several vibration meters are to be checked regularly on site.

An important criterion for selecting the vibration calibrator is the values of the vibrations generated. In addition to the calibrators, with which only one fixed value for frequency, acceleration, velocity and displacement can be generated at a time, there are also calibrators with which several vibration frequencies and vibration amplitudes can be set. With the calibrator capable of generating seven vibration frequencies between 15.92 Hertz and 1,280 Hertz and vibration accelerations between 1 m/s² and 20 m/s², special vibration meters for low- or high-frequency vibrations can also be tested for their accuracy at several measuring points. To ensure that the vibrations generated correspond to the defined range, the vibration calibrator must also be calibrated at regular intervals. Some of the devices have an internal test function that warns visually and acoustically in the event of deviations from the set value.

Use of the vibration measurements to classify machine faults
Often, the unusual vibration of the machines is an indication of uneven loading or bearing. The vibrations can be measured non-destructively with the vibration meter while the machine is running. By measuring and evaluating regularly, it is possible to detect irregularities early and take countermeasures before the major damage occurs. Many of the possible faults on the machines generate characteristic vibration components, so that the search can be narrowed down to these areas in the event of anomalies.

The values usually recorded as total vibrations result from several superimposed vibration frequencies. From the values of these broadband vibrations, the conclusions can already be drawn about imbalances, alignment errors and electromagnetic components. Often the manufacturers of machines also specify vibration values that must not be exceeded. Some vibration meters offer special evaluation functions for machine monitoring, for example, for rolling bearing control, for evaluation of machine vibrations according to DIN ISO 10816 and for FFT analysis.

FFT analysis as a special function in the vibration meter
The abbreviation FFT in FFT analysis stands for "fast Fourier transform". With this method, vibrations can be broken down into different frequencies that can be evaluated separately. The time-dependent measurement values are converted into frequency ranges for this purpose. To do this, a certain number of measurement data must be recorded at a constant sampling rate. These values are then used to determine how strongly the different frequencies occur in the measured vibration. The selected sampling rate and the measurement duration are decisive for which frequency range can be mapped and in which resolution. The frequency range of an FFT analysis includes the frequencies between zero and half the value of the sampling rate. A sampling rate of 800 Hz means that only the frequencies up to 400 Hz are represented, at 1,600 Hz sampling rate the frequency range increases to frequencies up to 800 Hz.

The vibration measuring device for FFT analyses must therefore be set to high sampling rates. The measurement duration, together with the sampling rate, determines the number of the measured values. The higher the number of measured values, the more individual frequencies can be represented. For example, 1,000 measured values become 500 individual frequencies in the frequency range determined through the sampling rate. The vibration meters with FFT function can offer different functionalities for the evaluation of the individual frequencies. As a rule, the FFT analysis always includes the graphic display of the recorded frequencies in a diagram. In this graph, the individual peak values of acceleration or velocity can be recognized very quickly. In most cases, the numerical values for the individual frequencies can also be tapped directly from this display. Depending on the vibration meter, 125 or even more lines for the numerical values of the individual frequencies are displayed in such an FFT diagram.

Vibrations are periodic deflections of materials or objects around a central rest position. Vibrations from nature and technology constantly affect us and our environment. Some of them we hardly notice, the others are disturbing or even dangerous. Vibrations can be generated and transmitted in different ways and superimposed with other vibrations. Vibration measurement can be used to determine precisely how strong a vibration is and whether it is displaced, amplified or weakened because of the changes of certain conditions.

Why we measure natural and purposefully generated vibrations
We ourselves, other living beings, wind and weather are sources of more or less strong natural vibrations. For example, we generate vibrations when we shake, when we speak, sing or play music, or when we run or jump over a yielding surface. Professionals can measure many natural and engineered vibrations to obtain meaningful data. Vibration measuring technology helps in the prediction of earthquakes, for example, but also in the design and construction of safe structures, machinery and vehicles.

Some technical devices specifically generate vibrations for special applications. Vibrations can be used, for example, for acoustic or silent alarms, compaction, transport or separation of materials. Vibration alarms, vibratory rammers, hammer drills, vibratory plates, vibrating screens, and ultrasonic cleaning baths are widely used. The vibrations transmitted by the equipment should meet defined values to fulfil the intended purpose. To test the values, the equipment manufacturers use special vibration measuring technology in the development and quality assurance.

Types of vibration measuring technology
Vibration measuring technology can be used to study, measure and analyze natural and technical vibrations. The measurands of the vibration measurement include vibration frequency, vibration displacement, vibration velocity and vibration acceleration. Many devices for measuring vibrations offer extensive storage and evaluation options.

Some device types can record the measured values for the three spatial axes x, y and z separately. The other models offer connections for several external vibration sensors. Multi-channel devices offer the advantage that several points of a machine or a structure can be recorded simultaneously. This allows, for example, measures for vibration damping to be checked directly.

But also with a technical stethoscope, which only amplifies the existing machine noises, abnormalities can be detected and assigned. Through regular vibration measurement, the problems on important components can be detected at an early stage with a relatively little effort and eliminated.

The different types of equipment used in vibration measurement include:

- Stethoscopes,
- Stroboscopes,
- Vibration meters with sensor,
- Shock data loggers,
- Vibration sensors with analogue or switching outputs and
- Vibration calibrators.

In terms of application areas, the vibration measuring technology can be divided into measuring devices for machine vibrations, human vibrations, building vibrations. The devices are adapted to the field of application with regard to the measured quantities, the measuring range and the handling. Seismographs and seismometers for measuring earth vibrations are also part of the vibration measuring technology.

Early detection of bearing damage
The bearings of the moving parts are among the most heavily stressed machine parts. Since bearing damage inevitably leads to the loss of efficiency or quality, early detection of the bearing damage ensures quick remedial action and helps to avoid downtime. The regular inspection of the bearings belongs to the scheduled preventive maintenance. One of the widely used methods for early detection of the bearing damage is vibration measurement. The more detailed the applied vibration measuring technology records and evaluates the vibrations, the more accurate statements can be made about the cause of the fault.

Measurement functions for early detection of the bearing damage

RMS (root mean square) of the total vibration level
The RMS value of the total vibration level increases with bearing damage.
The measured value can be compared relatively easily with the reference values for the bearing.

Crest factor
The crest factor is the ratio of the peak value to the effective value.
An increase in this value indicates early damage to the bearing.

Route measurement
Some vibration meters offer a route measuring function.

This makes it easier to compare measurements at the same location with the data from the previous measurements.
As a result, even some slight changes are noticed and the relevant points can be examined more closely at an early stage.

Machine monitoring according to DIN ISO 10816 / DIN ISO 20816
Machine monitoring is one of the most extensive fields of application of vibration measuring technology. Regulations for vibration measurement and the vibration measuring technology to be used can be found, among others, in:

• DIN ISO 20816-1 "Mechanical vibration – Measurement and evaluation of vibrations of machines.
   - Part 1: General instructions"

• DIN ISO 10816-3 "Mechanical vibration – Evaluation of vibration of machines through the measurements on non-rotating parts
   - Part 3: Industrial machines with a rated power exceeding 15 kW and rated speeds between 120 min-¹ and 15000 min-¹ when measured at the installation site".

• DIN ISO 10816-6 "Mechanical vibration – Evaluation of vibration of machines through the measurements on non-rotating parts
   - Part 6: Reciprocating machines with a power greater than 100 kW".

• DIN ISO 10816-21 "Mechanical vibration – Evaluation of vibration of machines through the measurements on non-rotating parts
   - Part 21: Wind turbines with horizontal axis of rotation and gearbox".

• ISO 10817-1 "Measuring equipment for the vibrations of rotating shafts
   - Part 1: Recording of the relative and absolute radial vibrations".

• VDI 3834 "Measurement and assessment of mechanical vibrations from wind turbines and their components".
   Sheet 1 Wind turbines with gearbox,
   Sheet 2 Wind turbines without gearbox,

Machine monitoring according to DIN ISO 20816 Part 1
DIN ISO 20816 Part 1 defines procedures for vibration measurement and assessment for machines. It regulates the measurements on rotating, non-rotating and non-oscillating components of complete machines. It also covers the measurements of absolute and relative radial shaft vibration. The specified evaluation criteria are decisive for both acceptance and operational control. The three basic vibration quantities, vibration displacement, vibration velocity and vibration acceleration, are defined and their limits named. The guide values are given for the evaluation of vibrations of non-rotating components on machines for which no special standard applies.

Machine monitoring according to DIN ISO 10816 Part 3
DIN ISO 10816 Part 3 assigns limit values for four evaluation zones to various machine variables differentiated according to drive power and mobility of the base. For vibration measurement according to this standard, the effective value of the vibration velocity in mm/s was defined as the relevant unit. The measured values that can be classified in zone A correspond to the new condition. Values of zone B are suitable for safe continuous operation, values of zone C only for short-time operation and values of zone D are classified as inadmissible. In addition, the standard defines specifications that the vibration measurement technology used must have. This ensures that measuring vibrations according to this standard leads to comparable and usable results.