What causes turntable rumble

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.

Disagree with Nsgarch. Eldartford is correct. IF ANY lubicant is in the shaft (ie: not dry) it will remain 'properly' lubricated.. to the point at least that no chatter will occur. There is definitely not enough side thrust to 'wipe' the contact area dry. (Go ahead, try.. push as hard as you can sideways.. see if you can push that oil film out of the way (heh heh your gonna need thousands of pounds of side thrust..) In an automobile engine certainly, but not in a plain bearing TT.

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

Neil

rumble is there on all records and only when a record is playing. had increased VTF as recommended on a prior thread because the cart didn't sound very good and someone suggested to increase VTF to max recommended to sound better. cart has 50 hours and sounds better probably due to more break-in, but with rumble. I'll drop the VTF back today and see/hear what happens
added a few drops of sewing machine oil to bearing which was almost dry, thought not completely

Nsgarch..."Wiffle grooves" are usually used on air bearings such as those in the gyros of the missile guidance systems I worked with. However, as Elizabeth says, regular journal bearings without grooves do pump oil quite nicely. Go look at any mechanical engineering text book.

By the way, your MIT connections don't impress me. Been there, done that.