Vibration Analysis

In measuring vibration there are three different types of probes (measuring device) used as show below. The device used to read the inputs from the probes and perform the vibration analysis to determine root cause of vibration is the vector analyzer.

• Displacement probe which is used with a Proximetor. The Proximetor converts RF (eddy current) into a voltage output. The gap voltage will represent the distance between the end of the probe and the shaft. Used in the Bentley Nevada process below. Also known as a fixed proximity prode, measures the peak to peak movement of the rotor. It can also be used for axial (back and forth) movement, for thrust measurement of the rotor.
• Velocity probe where the output signal is proportional to the speed of movement. Used in the IRD analyzer process below.
• Acceleration probe where the output signal is proportional rate of change of movement. This signal can be integrated to produce a velocity output.

Vibration Analysis with the Bentley Nevada process analyzer.

We are concerned about vibrations and vibration analysis because vibration causes wear and damage to equipment which in turn causes an inefficiency. Some examples for the cause of vibration are imbalance, misalignment, looseness, unbalanced weight system, wear and tear on equipment, low oil temperature and misalignment of couplings. The most important element to be aware of to optimize and reduce vibration is shaft alignment. Proper alignment saves on coupling elements, bearings and seals. Your equipment will run longer with fewer emergency repairs.

Critical speed is an important factor to look at during design. During this speed the vibrations are amplified due to frequency's being in sync. At a machine's critical speed the vibration is considered to be at far excessive levels, which can cause damage to components very easily. Critical speed is when the frequency or rpm's of the machine become equal to or in sync with the resonant (natural) frequencies of the machine and its system components. ORBIT and DVF (Digital Vector Filter) oscilloscope digital readings are used to determine critical speed.

Different materials are prone to vibration in different ways. This is compensated for within the Bentley Nevada Vibration Analyzer. The gap spacing is changed with the micrometer based on the different materials. Different materials also change the calibration procedures. If a vibration analysis was done on a compressor with a shaft material of type 'x' the target material should also be type 'x". Using different materials will most definitely affect the output and would not be a proper practice.

A probe is used to measure the vibration using distance. It's a gap to voltage transducer to measure vibration. Also, a key phasor can be used to determine rotor RPM and the location of the maximum peak movement. The relationship between the sensor gap and the resulting voltage is as the gap increases the voltage increases. The output from a vibration probe is not completely linear. The probes output will be linear for part of the operational output range but not at the very low or very high ends of the range. The gap voltage for the X and Y probes on the rotor kit should be approximately 9.5 volts for the zero setting.

The calculations used for this process are:

• If the time measured from the oscilloscope waveform is 26 ms, the Speed (RPM) would be calculated as follows:
  Frequency = 1/ time = 1/26ms = 38.46 Hz
  Speed (RPM) = Frequency (Hz) x 60 = 38.46 x 60 = 2307.6 RPM
• A mil is equal to 1/1000 th of an inch
• When using a digital voltmeter (DVM) to determine the peak to peak voltage of the proximetor reading, the formula that must be applied to the signal is:
  (Measured Voltage / .707) * 2 = Peak to peak voltage

Vibration Analysis with IRD

The IRD analyzer is used for spot measurement. This means data may be collected daily or weekly depending on the procedure developed for preventative maintenance to observe any variations on the concerned equipment. The IRD analyzer uses a velocity probe.

It important to monitor vibration because servicing is performed only when needed. Use of this analysis method prevents catastrophic failures and allows more effective planning and scheduling of maintenance work. These analyses' often can tell when, for example, bearings need to be replaced. Because of these early warnings, repairs can be scheduled before the pump completely fails.

When measuring and producing vibration signatures the IRD "amplitude range" should be checked and set before recording any vibration signature. It important to turn the frequency knob slowly when a vibration peak is detected to ensure that the maximum peak value is recorded by the X-Y plotter. A filter circuit is designed to pass or reject a specific frequency band.

When troubleshooting, the strobe light can be used to determine balancing, verifying alignment, checking for loose bearings and other connections. The most likely cause when frequency is in terms of RPM is 1x RPM, 2 x RPM and 3 x RPM

• 1x RPM: unbalance
• 2 x RPM: mechanical looseness
• 3 x RPM: misalignment

It is desired to have the shaft appear as though it is standing still while the strobe light is shining on it. If an image seems to appear more that once, but also seems to be standing still the shaft is not spinning at the same frequency as the strobe light is shining. This produces a false positive and is not an accurate reading of RPM. If two images appear, while there should only be one, generally it means that the shaft is spinning twice as fast as the strobe light is flashing.When the strobe light frequency and the shaft rotational speed are the same the shaft will appear to stand still (And be a clear image). At this point the light frequency equals the RPM.

The relationship between unbalance and amplitude is proportion. The most common cause of vibration is unbalance. Amplitude is proportional to unbalance, largest in radial direction.