Tables That Feature Bearing Friction

I would think that an "air drive" platter would not opnly be difficult to get started, but if it got spinning too fast would also be hard to slow down to the proper speed. The platter would have to have a wee bit of friction I would think.

A push with the hand is really not an option to start off unless there was some minor friction. You would want to have the speed build up and stabilize, so low to proper speed is the goal rather than high to proper + friction.

It is a rather intruiging proposition though, although the "friction" of the air on some form of wing under the platter could cause a rumble of sorts and a cogging effect I would assume. Not without its flaws, but probably the next $100,000 turntable idea. Anyone want to come up with a business plan?

Bob

Berlinta, T_bone, Raul, Dave, et al: Do you all except Dertonarm's contention that belt drive is the inherently superior way to motivate a platter, compared to all forms of idler- or direct-drive and notwithstanding theoretical air propulsion? It seems as though you all do. I don't, yet. T_bone, I think it's very significant that Chris Brady reports that his belt drive tt, and his rim drive motor when applied to his heretofore belt-drive tts, outshine any of his belt-drive creations, which are themselves held in very high esteem among end-users. I don't think his real-world experience should be dismissed BECAUSE it is anecdotal, if I understood you correctly. Rather, these are good data that have to be explained. The escape clause for anyone who wants to hang onto his or her own bias, is that perhaps none of us has heard the "ultimate" belt-drive tt.

Hi again,
Mark Kelly is correct about the tension differential, provided the thread/belt does stretch and that there are frictional losses in the platter/bearing system. I don't think he said that slippage was neccessary, but rather that it was unavoidable in real life(maybe I got that wrong...)
Slippage can be avoided, idler, belt or tape creep can't.
My comment on the counterpulley symmetry relates to Dertonarm asking for a debate in the theoretical domain. I made it the sake of debating idealized concepts rather than compromised implementations(called reality), please take that into account.

A single constant airstream working against "wings" will cause cogging. Two(or more), 180°(x°) perfectly out of phase air vents with proper orientation of the wings will cancel the cogging(don't want to be responsible for the maschining of that one...). But this isn't all that different from a conventional multipole motor.
A single leaf and a one impulse(or 0,5, ...) per revolution is different. One could achieve the same without the complexity of an air supply, the turbulence issues, etc. by using a very heavy platter, direct driven with the motor being switched on only infrequently , be it for a very short duration or ramped up, then down again, but only when the speed drops below a set deviation threshold. Oops, I see the word feedback on the horizon, mmh...
The inertia of the platter will be an intergrator again, the constant motor noise/mechanical jitter will be exchanged for an occasional pulse and the bearing losses ought to be zero ideally.
Low frequency pitch stability will once again be less than perfect :-(
There again, it contains compromises... I'm still tempted to build such a device(and sell it for --- ONE GAZILLION DOLLARS, buahahaha...!).

A pulsed air supply with transfer function matched "wings" won't cause cogging IF you solve the problem of syncronisation(and if there was no such thing as turbulence, a.k.a. CHAOS).

Now back to "real" work :-)

Frank

P.S.: Related to the original post: There are at least 6 different currently manufactered turntables that partially rely on creating additional drag on the platter for maximum speed stability.

Please take into consideration in all discussion about what drive and platter weight that is is not alone about constant speed.
There is the "2nd system" (mentioned before ) and its energy transfer into the platter: - the act of extracting information from the groove by the stylus. There is more to the platter than (high) torque, inertia and constant speed.
It has to handle complex energy transfer (and/or damping) provided by the stylus demodulating the groove.
Bringing this into consideration will clarify some points which came up in the last posts by Berlinta and T_bone. The high mass platter plus the low tension thread made of aramide or similar will too give some answers, why there is no problem with vibration or resonance being transmitted or initiated by the thread.
As for the hypothesis of the high inertia (which is fully backed and put into "half-commercial" product by Jean Constant Verdier in his Magnum ) providing close to ideal constant speed, it would be helpful if Kirkus - who is respected by us all - could clarify or illustrate the point.
If I try it might soon become too dogmatic again.
I am on holiday now and will - randomly and infrequently - follow this thread, but I will make no personal remark to anyone anymore and will not answer to any directed to me either

Hello Lewm,

No, I don't agree with Dertonarm's contention. If you reread my posts, you'll find that I'm trying to point out that there are several ways of approaching the challenges to run a platter at constant speed, ALL of which are flawed in one or several ways, as soon as we are dealing with reality.

I am very much familiar with Chris's table, in fact I suggested the use of an eddy current brake acting on the platter as soon as he started making platters with brass or aluminum bottom layers and I heard both a prototype and the first production model in comparison to his then top of the line tape driven tt. His direct drive turntable offers exceptional speed stability(significantly better than his tape driven decks) and is proof that it can be done(and, again, look at one of the last generation cutting lathes).

I'd prefer to discuss real world experiences, but Dertonarm made such a big deal out of the supposed mediocrity of most, if not all contemporary turntables that there simply was no point in "getting real".

Strangely, I have not gotten a response from him...

Best wishes,

Frank