Hi,

I am interested to know the behavior of a shaft rotating in a journal bearing. i know that polat plots will show us how the shaft moves with respect to journal during start up. please see the attachment and pics on the left hand side. can someone guide me how i can draw these polar plots. my instrument can take phase and displacement signals from the panel.

thanks in advance.

Madhu

polar_plot.pdf (46 Kb, 73 downloads)

quote:

i know that polat plots will show us how the shaft moves with respect to journal during start up. please see the attachment and pics on the left hand side.

The plots on the left hand side are clearly labelled as shaft centreline plots - nothing at all to do with polar plots.

Shaft centreline plots are constructed from the dc gap volts from a pair of proximity probes mounted at 90 degrees to each other. You can assemble a shaft CL plot using data obtained with a multimeter

To emphasize what Buzz stated, simply record the DC voltage from each of the two prox probes (90 deg apart) as a function of speed. Since prox probes have a linear range (assuming they are gapped properly), you can plot out the actual position of the shaft centerline during a startup. Normalize the data (i.e. measure the delta change in voltage) and use vector addition to plot out the change in shaft centerline; starting from the 6 o'clock position (since you will assume the shaft is a bottom of the journal while at rest).

This is a great troubleshooting technique. It helped me once to confirm abnormal side loading from an incorrectly designed bearing. It also helps define the stability of the shaft, eccentricity, sufficient loading, etc.

Regards
Jim P

Just to clarify, a polar plot is typically a plot of vibration amplitude and phase ploted as a function of speed. The vibration is filtered to an order of running speed, usually 1X. This plot is used for balancing.

As stated above you are talking about shaft centerline plots which is simply a plot of gap volts from the X and Y probes plotted in XY coordinates.

Thanks for the updates. ok i understood that this data can be taken by using multimeter.

my first question is can i use my multimeter and pickup the voltage from the bently vibration monitoring panel which normally will be provided at power plants.

my 2nd question is if we observe both the shaft center line plots (1 and 2) in the attached pdf file the first plot falls in the 4th quarter, while the 2nd plot goes to 2nd first, then to 4th and finally moving to 1st.

how is this happening...?? normally for any graph to plot we need both X and Y values. we are not considering any phase or angle here. in this case how are we plotting the shaft center line diagram. what are those concentric rings shown at equidistant from the center,

is this a plot taken from one probe say X or a combination of both X and Y probes.please advice.

Madhu

It sounds like you may be confused with a shaft centerline plot, which plots position from the D.C. values of the probe.

In the old days we did this with an x-y plotter, and if need by used capacitors and resistors to make a filter for the d.c. from the probes.

The circle represents bearing clearance.. you do not need the circles for the plot. You can use a volt meter to do this, just as long as you stop the roll up or down long enough to get the data.

You do need a reference voltage for both the x and y probes with the shaft at rest. That point represents 0,0 then all the rest of the points are plotted as voltage changes relative to that reference voltage.

If I get a minute later this weekend I will do an example for you..

I remembered I did a simple spread sheet to plot position versus time and relative to a diametral clearence circle..

The greenish blocks are inputs.

SCl.xls (48 Kb, 49 downloads)

My friend John from PA has already found a problem with the spread sheet above. I think the table is ok, but the graphic not working.

Consider it a learning tool rather than a practical application.