Myford Super 7 and ML7 motors - modification from plain to ball bearings

I needed to restore 2 motors, one for a Myford Super 7 and another for a Myford ML7. The former a 3 phase and the other a 1 phase, both 4-pole ie. 1440'ish rpm at 50Hz/230VAC, resilient mounted and fitted with plain bearings. These are lubricated by an arrangement where a so-called "wicking material" holds oil that is fed to the bearings. The bespoken material in my specimen were felt in the 1ph motor and something called Permawick in the 3 phase motor. The latter was in need of replacment, so I decided to get rid of that and instead use wool felt. This turned out OK, and I could probably have used it in this condition, but it was a little thirsty (for oil), so after looking at the Internet I saw that some had modified their motors to ball bearings which looked like a good idea. Not so much to lose anyway, the motor being quite old. In the 1ph motor, the wicking material (felt) was OK, so i just cleaned the motor parts and re-assembled. Then I though it would be interesting to measure the noise on these motors, ie. making a comparison as the claims were that a plain bearing motor would be "silent" as opposed to a ball bearing equipped motor. Well, I can confirm that at least the motors I tested, this proved true, but the difference is not that significant.. and less so for a probably somewhat worn plain bearing when under load vs. a ball bearing version.

PS! Just so that is said.. the noise measurements are done with professional Class 1 equipment and with the microphone at a random distance from the noise source = motor(s). The parameter is one of sound pressure ie. in measuring the sound level at the point of the microphone, for this purpose OK. The background level in this case being insignificant. Should we want to measure how much noise the motor(s) is(are) producing we would need to measure sound power, LwA, for example as you would find labelled on power tools, but for this comparative measurement it is not needed. The difference in noise levels between the motors were quite large, but still quite low levels anyway, so no drama.