Table of Contents
Advertisement
Section 4 — Measurement Types and their Uses
Orbit Plot
An orbit plot shows the motion of the center of mass of a shaft. You can
plot a shaft's vibration in two directions simultaneously (e.g. vertical and
horizontal) to get a trace of the motion of that shaft.
A perfectly balanced shaft with no movement in any direction produces
a dot in the middle of the plot. Unbalanced or misaligned machinery
may exhibit as circular orbits, figure eights, leaning orbits, or flat orbits.
Rub or looseness may display as indents around the circumference of
the orbit plot, or small loops around the larger plot orbit. You can use
orbit plots to detect abnormal movements of the shaft in the bearing
(e.g. unbalance, misalignment, or rubbing against a drive casing. The
shaft movement will give you an indication of its cause. For example, if
there is a lot of up/down movement, the machine feet might not be
bolted down tightly enough.
Two probes are used when taking and analyzing enhanced orbit plot
recordings. These are nominally referred to as X and Y. By convention,
when viewing the machine from the driver to the driven end, the Y probe
is positioned 90 degrees counter-clockwise from the X probe.
The direction of shaft rotation is not important for the X and Y position.
You just need to ensure you view the machine from the driver to the
driven end. For example, the Y probe will be positioned 90 degrees
counter-clockwise from the X probe regardless of the actual angles and
positions used.
83
Advertisement
Table of Contents
