1/4 " phono belt

there are reasons to use magnetic tape as belt.

it is slippery
it is utterly stretchless ( the base film usually is pre-strechtred mylar)
it is easy to splice and the splicing does ot change the neutral line (compare the knot of a knotted string)

Consider the rubber belt as a worst case solution, hurting the Einstein law (make everything as simple as possible, but not simpler):
both platter and motor have a considerable momentum of interia (J). Moreover the motor has about 10x the rotational speed/angular velocity of the platter. And the stored energy of a rotary mass is (J*w^2)/2, w being the angular velocity. So even if the motor has only
1% of the platter intertia, it can store about the same amount of energy (10% squared is 1%).

If now both rotary masses are connected via an elastic rope (the rubber belt), they can exchange energy. Worse, they can do so at frequencies well in the audible range; the result are rotational oscillations. If the motor is a synchronous AC motor, an additional rotary spring constant is added. So, the more equal the stored energies are, the lower the exchange frequency, the more probable you hear the influence.

There is a remedy against this situation:
make the belt as non-stretching and slippery as possible. A string will do but it will have a knot insisting on running between belt and pulley on the long run and driving you slowly crazy. No way to persuade the knot eternally. It may run properly outside the pulley for weeks (provided you managed to knot the string untwisted) and then start to fool you.

Magnetic tape also is non-stretching and slippery enough. No knot, no insanity.

But it has its price: the motor has to sit on a ball-beared sled. If your TT is a subchassis design, the sled has to be pulled away from the platter spindle by string and weight (spring constant: zero, no additional oscillation source). Otherwise a spring will do.
Thanks Bernard for your reply.
I agree with all, that you say here, and keen to try it sometime.
In the past I was told that the best way to drive a platter is with one driven pulley and two idle pulleys so that the belt is contacting the rotating platter in three places equidistant around the perimeter.
Any comments on this from your knowlege and experience ?
BTW - a good cheap source of small bearing races is VCR pinch rollers.
Remove a retaining sleeve and the bearing ***'y will slide out.
3mm and 4mm are the internal diameters.
If you were to ask at a VCR repair shop, I'm sure they would give them to you. :)

Regards, Eric.
for lightweight subchassis TT as the Linn, Pink Triangle and the like, the three- pulley configuration certaibly is a good idea as resulting belt force vector is zero.

However, if magnetic tape is used, all pulleys have to sit on sleds. And all pulleys have to have as low friction and as low inertia as possible.

Going to be expensivish :)

One thing should be clear: the belt slips on the platter, not on the pulleys then as the "wrapping" angle of the belt around the platter is very small. Eytelwein formula says the ratio of two rope forces is exp(µ*angle), µ being the friction koefficient.

I better avoid lightweight subchassis TT, too complex for my simple mind :)

Pinch roller:
is this the roller pressing the tape on the capstan? They use ball race bearings meanwhile? For R2R tape recorders they used to use plain bearings.

I always have use for tiny ball bearings, thanxalot!
Hiya Bernard, Yes indeed I speak of the roller pressing the tape against the rotating capstan shaft.
The rubber wears where the roller contacts the capstan (protected by the tape) and are discarded (changed) for this reason, and most repair technicians just throw them into the rubbish bin.
The rubber wears out long before the bearing does !.

As I mentioned the bearing assembly internal diameter is 3mm or 4mm, and outside diameter 6mm-8mm.
A single ball race is used to give the roller assembly the property of self alignment when pressed against the capstan shaft.

To remove the bearing I use just the right size drill that will bite into the plastic retaining sleeve, and rotate and pull slightly at the same time, withdrawing the sleeve and then allowing the bearing to easily slide out.
DO NOT try to force or hammer the bearing and sleeve out together, or you will ruin the bearing surfaces and balls.

Another source of even smaller ball bearing races are the bearings on dentists' air turbine drills - the turbine and bearings are changed as an assembly, but probably not so easy to get for free.

Bernard, I have been fixing all sorts of stuff for 25 years, so please feel free to ask any questions you might have.
I hope this can help you,

Regards, Eric.

DO NOT try to force or hammer the bearing ...
wanna teach your grandma how to steal horses? :)

Anyway, thanks for your hints how to gain small ball bearings. I will have to ask our local VCR technican. I will have to search one, i do neither own a TV set nor a VCR. Not my world.

But i have slight doubts the bearings last considerably longer than the rubber. That they do not yet go rough does not mean they are not already worn, it only means there is no dirt or shred inside polluting the grease.

I have designed stuff for the mass market. It is very usual to carefully choose exactly the manufacturing quality just even sufficient.

For e.g. automotive components it is not only prescribed what it may cost, it also is prescribed how long it has to last, not shorter, not longer than a specified tolerance. In some cases however it is impossible to degrade quality below a certain level because the primary function of that component is degraded to an unbearable extent. I would expect this with those ball bearings. Let's hope it. :)

But believe me, ball bearings in a car are a hidden sandclock. And so is the sheet metal, during production there is a specified percentage of junkmetal brought into the melt to ensure the produced sheetmetal starts to corrode from inside after a period of, say, 4 to 10 years, depending on the car brand.
ball bearings

Hiya Bernard, what I meant was that the bearings typically still run smooth.
These bearings seem to be of high quality.
I have only found three that ran roughly and they came from north-west WA mine sites, and the rubber was totally had it too.
Those guys are in a gritty, dusty environment and they run their vcrs till they lierally grind to a halt.
Also I didn't mean to insult you regarding removing the bearings.
Some people need advice like this though !.

i do neither own a TV set nor a VCR. Not my world.
That is a pity Bernard, as there is much interesting documentary info on TV, more especially on satellite channels.

Re automotive, yes, this is called planned obsolescence.
I have a mechanic friend who tells me that for a number of japanese cars, he can quite closely predict which parts wear out and in what order, and this happens repeatably and reliably according to the make and model.
Maybe this is testimony to modern manufacturing precision and close tolerances !.
And poisoning the sheet metal melt is a nasty trick !.

Regards, Eric.

BTW - did you see my tip regarding VCR capstan motors as phono motor ?