Measurements in accordance to ISO 20816-1
Machines are now being operated at increasingly high rotational speeds and loads, as well as more flexible operation at part and full load, and under increasingly severe operating conditions. Machine vibration measurements are used for a number of purposes, ranging from routine operational monitoring and acceptance tests to advanced experimental testing, as well as diagnostic and analytical investigations. ISO 20816-1 provides guidelines primarily for operational monitoring and acceptance tests providing limits for
three primary vibration quantities: displacement, velocity and acceleration.
It is common practice to measure vibration on non-rotating parts or to measure relative shaft vibration, or both. The measurement type for the protection system is normally based on the experience from the machine manufacturer. The measurement of vibration shall be broad band, so that the frequency spectrum of the machine is adequately covered.
Vibration measurements on non-rotating parts are generally carried out with a seismic transducer which senses the absolute velocity or acceleration of structure parts on which it is mounted (e.g. the bearing housing).
Relative shaft vibration measurements are generally carried out with a non-contacting transducer which senses the vibratory displacement between the shaft and a structural member on which it is mounted (e.g. the bearing housing).
To assess the machine vibrations the following measurement quantities can be used:
a) vibration displacement, measured in micrometres;
b) vibration velocity, measured in millimetres per second;
c) vibration acceleration, measured in metres per square second.
Vibration level meter
The meter measuring vibration acceleration, velocity and displacement with a required frequency weighting.
Parallel to the vibration measurement, meters offers the RPM measurement with laser tachometer.
Fast Fourier transforms are mathematical calculations that transform, or convert, a time domain waveform (amplitude versus time) into a series of discrete sine waves in the frequency domain.
The instrumentation used shall be designed to operate satisfactorily in the environment for which it is to be used, for example, with respect to temperature and humidity. Specification for instruments for measuring vibration severity is given in ISO 2954 and instruments for measuring shaft vibration are specified in ISO 10817-1.
Particular attention shall be given to ensure that the vibration transducer is correctly mounted and that its presence does not affect the vibration response characteristics of the machine. Specifications for mounting accelerometers are given in ISO 5348, which, in principle, are applicable also to velocity transducers.
Modern instrumentation can provide multiple methods to deliver a measurement value. The acceptance criteria here are based on r.m.s. velocity in mm/s for measurements on non-rotating parts and peak-to-peak displacement in micrometres for measurements on rotating parts. The acceptance criteria may be scaled suitably to match the units in common use at the site, using the same assumptions as programmed into the distributed control systems.
It is desirable that the measurement system has provision for on-line calibration of the readout instrumentation and, in addition, has suitable isolated outputs to permit further analysis as required.
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