Happy New Year! I have a few questions related to vibration testing instruments
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Dear sea friends, Happy New Year! I take this opportunity to wish you all a happy New Year, hoping that you will enjoy good health, success in your work, and happiness within your families in the coming year. Working abroad, the New Year can be quite boring, but it gives me the opportunity to work on some technical issues that I don’t have time to tackle on a regular basis. Today we covered vibration analysis, with two questions: 1. What does the term \"channel\" refer to in the technical specifications of a vibration tester? Is it the maximum number of vibration points that can be measured simultaneously, or is it the number of vibration directions that can be measured for the same measurement point? I used a Fluke 810 years ago; it was a three-channel model, which meant that it could measure vibration levels in three different directions at the same time. 2. In order to measure the vibration phase difference at different points, for example, to determine or rule out whether the high axial vibration velocity is caused by misalignment, is it necessary to have the same vibration tester capable of measuring the vibration conditions at two different measurement points simultaneously? This question was raised after reading an article that mentioned the vibration frequency of the induced draft fan to be a fundamental frequency; “measurement of the phase revealed a 171-degree phase difference in the axial direction,” which led to the conclusion that the vibration was caused by shaft bending or improper installation of the bearing housings, and the problem was ultimately resolved. At the same time, I noticed that on the photo of that vibration meter, it showed “channel phase difference”.
This post was last edited by “The Cat God Who Can’t Be Undone” on February 13, 2016, at 22:08. In my personal understanding, a “channel” refers to an integration circuit; if there are two channels, then there are two integration circuits. This means that two vibration measurement probes can be connected to measure the vibration values at two different locations simultaneously. There are two methods for simply measuring the phase difference: 1. Place a cursor (zero point) on the rotating shaft, connect a laser trigger to the instrument, measure the phases of the two measurement points separately, and then subtract them to obtain the absolute phase difference. Each measured phase is the absolute phase (relative to the cursor). This method requires only a single-channel instrument, and an external trigger can be used (the trigger provides only the trigger signal without occupying a channel). A 2-channel instrument can measure the vibration at two measurement points simultaneously, or the vibration in two directions at one measurement point. It usually has the capability to measure the relative phase difference; in this case, no external trigger is needed. One only has to manually enter the rotational speed, and then measure the vibration at both locations at the same time. By comparing the time differences when the vibrations reach their peaks, the relative phase difference can be determined, as the starting point for data collection is the same (which is the function of the trigger). This method can only yield the phase difference, not the absolute phase at a certain position. In fact, the phase difference of the channel that you’re looking at, as the original poster mentioned, should be the second type – it measures the relative phase difference.
When measuring the vibration of rotating machinery, two probes are usually placed at each point, in an X-Y configuration, which requires two channels.