What causes turntable rumble

Could be time to clean and re-lubricate the main bearing ;--) And I have found the best sh!t on the planet (in terms of blackest audio background) is van den Hul's spindle/bearing oil. It contains suspended particles of zirconium oxide which act like microscopic sized ball-bearings to keep the metal surfaces from making contact -- this is especially important in belt-driven TT's where the bearing is always being pulled to one side.
.

A plain journal bearing does not have metal-to-metal contact when the shaft is turning. The turning shaft functions as a pump to keep a film of oil between the metal surfaces.

Not true unless (A.) The bearing material is of the bronze oil-impregnated type (not generally used in TT's because it's too soft to sustain the side thrust of belt drives) or (B.) The turning shaft is grooved to circulate the lubricant up to the top and then somehow bored out to return it to a sump at the bottom. A plain turning shaft doesn't pump anything -- and simple capillary action won't keep the surfaces apart.

Elizabeth, I think A.J. van den Hul and my mechanical engineering colleagues at MIT would strongly disagree with you ;--)

Neil

Eldartford, I didn't know you were an alum! What year did you graduate, what course?

Wiffle grooves are not just a feature of air bearings (and BTW, many air bearings DO NOT use them, opting instead for multiple inlets in the manifold.) Grooves are also used on the Caliburn TT (I think) or some other expensive TT's that use a recirculating lubricant system.

But simple rotation alone creates no pumping action, and besides, what would be the benefit of "pumping action" if the bearing is filled with an adequate amount of lubricant which doesn't require cooling ;--) Metal to metal contact however is a problem. It is virtually assured in belt drive TT's; or whenever the tolerances are a little sloppy. No amount of lubricant will actually keep the metal surfaces from making contact, and that's among the causes of rumble noise.

Colonel "Rusty" Draper, who for those who don't know, developed the Inertial Guidance System for our early guided missiles, was also the faculty advisor for the MIT Flying Club (which still blows my mind!) My favorite story (of his) was when he thought he was going to die, as he was flying a WWI 'Jenny' over Logan Airport, and the entire tail assembly fell off! (and he had no parachute ;--) He cut the engine, and hoped the wind would blow him toward Boston Harbor; instead, the airplane just "slowly maple-leafed to the ground" as he put it, "and I just walked away!" I often wonder (but never thought to ask him) if that's is where he got the idea for the inertial Guidance System!!

As for the TT bearings: if you suspect a little bearing noise is getting into your music ;--) please just try some of v.d.H's spindle oil, OK? Unlike so many what I call "Emperor's New Clothes" audio products, you will definitely hear an improvement in terms of quieter background, especially on old belt drive units. I know this because, besides my guinea-pig self (who of course sprung for a $50 three-lifetime-supply bottle of the stuff;--) I shared some with at least 4 others who said it made a real difference. Of course THEY got it for free, so I guess they wouldn't say it sucked!

"Doc" was what all the kids and faculty called him. I think the "Rusty" must have come from the people out at Hanscom Field, where he arranged for us to use the Link trainer . . . . a very colorful man.