Audiophoolery?

Sure, but the diameter of the idler wheel when it is in contact with platte r is less than the diameter you'd measure when you weren't compressing the idler wheel to get a finite contact area with the platter.

Belt drives have the same kind of problem - the belt is longer when it come s out of the pulley it is driving than it is when it goes in. There's a bui lt-in "slip" even if the belt doesn't slip when it is contact with the driv en pulley or the drive pulley - tension pulleys don't matter.

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stepped motor shaft. It would be difficult to guarantee good speed accurac y with the poor dimensional stability of rubber.

ce flutter.

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What are you disagreeing with?

By constant diameter, I mean around its circumference, not over time - if t he radius of the idler at the point where it touches the motor shaft is dif ferent from the point where it touches the turntable the rotational speed o f the turntable will tend to vary cyclically as the idler rotates. That wil l be at a fairly high speed for a small idler so the mass of the turntable will filter it out but it is undesirable.

I've also had old idlers develop divots through contact with motor shaft th at can cause audible noise.

kevin

You missed my further posting... old belt-drive turntables didn't have an "idler", they had a variable diameter "spindle". ...Jim Thompson

Thanks everyone for the corrections. After reading a few of the comments, I realized that I was wrong. However, determining why I was wrong took me most of the afternoon. The problem is the gear ratio has to be calculated individually between two adjacent gears, not multiplied together as I incorrectly stated. When I increased the number of teeth in the idler gear, the gear ratio between the drive gear and the idler increased, and the gear ratio between the idler and the output gear decreased by the same amount, resulting in no change in overally gear ratio.

Using the first example of 10, 30, and 100 teeth: 30 / 10 = 3:1 gear ratio and 100 / 30 = 3.333:1 gear ratio with a combined gear ratio of: 3 * 3.333 = 10:1

If replace the idler gear with a larger 50 tooth gear, I get: 50 / 10 = 5:1 gear ratio and 100 / 50 = 2:1 gear ratio with a combined gear ratio of: 5 * 2 = 10 = 10:1 which is the same as with the 30 tooth idler.

If I totally removed the idler gear, I get: 100 / 10 = 10:1 gear ratio which is also the same as the 30 and 50 tooth gear ratios.

So, the diameter of the idler doesn't matter in a turntable. My apologies for screwing up (again).

There was the odd design that had both belt & rumbler wheel. Why I've no idea.

NT

platter speed. "

I caught you wrong. I admit it doesn't happen but it just did.

Even using the gear aspect for example, say your motor shaft has 20 teeth a nd the ultimate driven has 400.

If there is an interim gear, which would be analogous to the idler, if it h as 30 teeth or 50 teeth matters not. Only the 20 teeth and the 400 teeth.

I scored in the 99th percentile in mechanical reasoning. You can take my wo rd for it, but do experiment if you like. I'm sure you can find something a round to prove it. I know it sounds illogical, but it is true. I'll try to find an easy way to prove it with junk laying around.

A smaller idler or idler gear will simply run faster. But i ts ratio to the input and output remains the same. This is not true of car tires for examp le so that analogy does not work because the diameter of the tire only affe cts the output, not the input. An actual idler wheel or gear affects both p roportionately.

Now deformation of the idler if it is rubber brings in a ton of math nobody wants to do. Because the deformation is different between input and output it does affect speed because the circumferance is not constant.

diameters partly determines the speed. "

Yes of course, but that is not what we were talking about.