There are three primary types of motion characteristics detected by vibration transducers normally referred to as accelerometers. These are
• velocity, and
The choice between these three different types of motion transducers depends on the frequencies that have to be measured and the levels of the signal involved. (Wilcoxon Research / ND)
Displacement sensors are normally used for low frequency (1 to 100 Hz) measurements and are useful for measuring very low amplitude displacements. They are employed in such applications as shaft motion and clearance measurements. Traditionally displacement transducers have employed non-contacting proximity sensors and eddy probes. Piezoelectric displacement transducers have recently been developed to overcome some of the mounting problems associated with non-contacting probes. (Wilcoxon Research / ND)
Velocity sensors are used for medium to low frequency (1 to 1000 Hz) measurements. Because they filter out high frequency signals, they are less susceptible to amplifier overloads which come from white background noise and which can compromise the fidelity of low amplitude and low frequency signals. Velocity transducers are useful for monitoring and balancing operations on rotating machinery. Velocity sensors normally employ an electromagnetic sensor to pick up the velocity signal. Now piezoelectric velocity sensors are becoming more popular. (Wilcoxon Research / ND)
Piezoelectric acceleration transducers are used to measure acceleration are the most commonly used motion sensors for vibration monitoring applications. They are useful over a wide band of frequencies from very low to very high frequencies (1 to 20,000...
... middle of paper ...
...ure quality products from a dimensional tolerance and surface finish quality standpoint.
When the condition of a machine deteriorates, the following things will generally happen:
1. The dynamic forces generated by the machine will increase in intensity, causing an increase in machine vibration.
2. Wear, corrosion or a build-up of deposits on the rotor may increase unbalance forces.
3. Settling of the foundation may increase misalignment forces or cause distortion, piping strains, etc.
4. The physical integrity (stiffness) of the machine will be reduced, causing an increase in machine vibration.
5. Loosening or stretching of mounting bolts, a broken weld, a crack in the foundation, deterioration of the grouting, increased bearing clearance through wear or a rotor loose on its shaft will result in reduced stiffness to control even normal dynamic forces.
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