Formidable as our diagnostic sciences may be, we often sublimate statements necessary for a full comprehension of mechanisms at play. (This is likely a wonderful spot for readers to interject “What’s he rambling on about?”)
Everybody recognizes the half-order frequency inherent to a rub phenomenon. Subtle difference from the usual statement one might have heard, the previous sentence stipulates “half-ORDER”, not half-RPM. The reason why should strike us by its evidence and simplicity: analysts link the 1/2X consideration to rotors (and even flexible rotors). Yet, within a gear context, when might we suddenly see the appearance of 1/2X? Apply it to some other mechanism: a gear “contact” (through the lubricant film) can be described as sliding-rotating-sliding, while the presence of looseness at the bearing lodging or the shaft journal will cause a potential rub reaction at the bearing position itself or at the gears. Thus, 1/2X suddenly applies to the gearmesh. And 1/2 GMF manifests a presence in spectral data.
There are a number of other instances where the rub component of a phenomenon will cause a “half” frequency to appear. Looseness left to grow uncontrolled will allow rubbing of sorts, for one.
© 2007 by François Gagnon