what is terminal slack? does it have to do with slack electrical cables?  Or loose connection?

Either of the above would not be detected by vibration?

The Entek diagnostic wall chart that I have indicates that a loose connection on a motor supply lead will produce 1/3 line frequency sidebands around the 2Xline frequency vibration peak. In my experience I have never nailed one of these.

Petri - I agree with you 100%. I have seen that wall chart and I have never been able to come up with any thoeretical explanation whatsoever for why a loose connection would cause F_Line/3 sidebands or any real-world example of that pattern (never seen anything at these frequencies even on a log scale).  Personally I believe that a loose connection will have no detectable vibration effect whatsoever until it becomes very severe (severe enough to change the voltage to the motor), at which point twice line frequency vibration may show up due to the voltage imbalance.

Mechanical "Looseness" often causes third order harmonics of the fundamental. Maybe that has something to do with it?

Cheers

Greg Locock

3X line frequency harmonics I could by.

1/3 times line frequency?  I have scratched my head until there is barely any hair left and I can't imagine it.

More importantly, after reviewing several hundred motor spectra I have never seen the slightest bump at those frequencies, even on log scale.