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So a question to those more failiar with orbit plots than I. Would I be correct in saying that an orbit plot filtered to 1x can be nothing other than a circle or ellipse with only one 'blank-bright'? This would be my assumption as vibration at 1x (or any other specific frequency) can only be a sinosoid in both the x and y axis. The combination of which gives either an ellipse or circle dependant upon the relative amplitudes.

The reason I ask is because I have seen some material online of an orbit with an internal 'loop' claiming to be 'filtered' at 1x and an eliptical orbit with two blank-bright's also claiming to be filtered at 1x. This has left me somehwat confused!



thanks in advance

Gary

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I don't see how either of those plots could be truly filtered to 1X. Did the material state that the plots were filtered specifically to 1X?

The only reason I ask is that filtered orbits, in my experience, make for a much nicer data presentation than unfiltered orbits. However, you must filter them to the frequency range of interest. For example if you are looking at a 60 Hz large generator, you might look at orbits filtered with a 500 Hz low pass to remove the high frequency noise. Filtering orbits in this manner tends to make them look much smoother, more like an oscilloscope display.
RM
quote:
The combination of which gives either an ellipse or circle dependant upon the relative amplitudes.


Two sinusoidal signals of the same amplitude but with phase not +-90 degrees (assumping probes 90 degrees apart) produces an ellipse and not a circle, perhaps a degenerate ellipse like a straight line if the relative phase differenc is 0 or 180 degrees.

As long as you get 1 trigger per shaft revolution, 1X should have 1 bing per cycle.

Inner or outer loops, look up the loop rules. We discussed this once upon a time on this forum.
RM
Doesnt the relative phase also gave you the orientation of the elipse?

Could you not also get an elipse if the sine wave amplitudes are different horiz to veritcal? Hence the generally squashed circle or elipse for a casing measurement on a pedastal bearing due to the greater degeee of freedom in the horizontal plane. (I know this is a slightly different thing but helps to illustrate what I am thinking)

regards

Gary
RM
Gary,
Mike may have hit the nail on the head. The author may have meant to save just 'filtered' and added the '1x' by mistake. If you have a bandpass filter set for only 1x, then you are correct in your thinking that there will be only sinusoidal information coming in from both probes. So with two probes and a key phasor, you will get an ellipse with one 'bing' (Hey Bill, is that the technical term from the Bently Glossary? No joke, I want to know.).
Remember, a circle is a subset of the great world of ellipes, so you can still have a circle if you get an ellipse, it's just equal magnitudes for each.

As Bill stated, the phase matters as well, so that you can end up with an extremely flattened ellipse, almost a flat diagonal line.
So, no inner or outer loops and definitely no more than one 'Bing'.
Ron
RM
Gary is correct that orientation information can be derived from relative phase. If the relative phase differs by exactly 90 deg, then and only then will the major axis of the ellipse be in line with the transducer axis. Someone really familiar with an orbit can readily approximate relative phase. Some of us here are from an old era where this was a normal activity using an oscilloscope.
RM
quote:
Doesnt the relative phase also gave you the orientation of the elipse?


Agree, but the orientation of the transdures matter, too. Sometimes one may have non-orthogonal probe locations; these can be corrected for orbits.

quote:
That would presume some standard relative placement of the tacho to probes or is it the relative phase X/Y? Olov


When properly plotted, the orbit is the orbit, with or without a taco, bing, mark. Relative phase is inherent in the plot if done correctly.

Some may remember Charlie Jackson placing his oscilloscopes on 2x4's (I am sure there is a metric equivalent.) at 45 degrees. This way the orbits and physical vibration aligned (with or without bings) when the probes are mounted at 45 degrees as is recommended by API.

Of course, one needs to follow (when using o-scopes) the probe orientation convention when viewing your measurements on the o-scope. To some peoples surprise this has nothing to do with rotation of the machine. Essentially, you line up your x and y probes with the directions driven on the o-scope. Now, by rotating the o-scope (take a picture - yes people used to have to do this - and rotate the picute or mark it for up) one aligns the measurement to the physical situation for vibration. People had and have reasons for some of these conventions; they were not always just nice to do. Often, they have meaning.

With today's digital devices, the software should, but may not, take care of this for you.

quote:
Gary is correct that orientation information can be derived from relative phase. If the relative phase differs by exactly 90 deg,


Good point and not trivially obvious by most. Having equal amplitudes does not mean you have a circle, and most importantly (perhaps most) is that it means that you have not measured the highest 1X or nX vibration in the plane of mreasurement. One can calculate the highest and lowest measurement locations for 1X or nX in this plane based on amplitudes and phases. A 2-channel analyzer without a trigger can be used to determine relative phase, filtered orbit orientation, the major and minor axis - both amplutude and phase orientation.

If you don't know where your probes are - unlike Oli, who was born with a stinger in his hand - you can only determine the relative major axis orientation, but you still get the amplitude for the major axis. If your probes aren't 90 degrees apart, and you don't know where or how far apart, then you don't know enough, and you should find out to do the job. Ever have redundant probes, and the site can't tell you what is connected?
RM

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