Direct drive vs belt vs rim vs idler arm

As to Dover's post, belt creep is not caused from stretching the belt. Find Mark's writing on the subject because he explains it far better than I ever could, and he backs it all up with the proper math. It is a physical limitation of a belt drive system that generates an inherent tracking error.

As far as I know, Mark doesn't address tape drives or string drives, however. I don't look at a string drive the same way as a belt drive, but tape may be a viable way to avoid the problem. String drives have a limited slip that has always intrigued me.

Syntax discounts the idler drive out of hand, but I assure you that all the problems that he mentions have been addressed. It has a lower tracking error than a belt at around one part per million, and it has fewer maintenance worries than a string. Not only that, but with a proper controller it is more speed accurate. However, that's not to discount the string because I believe similar outcomes can be reached with a string when the system is designed correctly and built well. It can be fiddly, though.

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Mosin - please re-read my post.
I did not say that elasticity causes the belt creep.
I said it is an assumption and that if the belt doesn't stretch then there is no creep.

Mark Kelly's calculation of belt creep is :

Creep = T/r x A/E

Where T is torque transmitted, r is pulley radius, A is belt cross sectional area and E is the elastic modulus of the belt material for small strains. Thus a torque of 1mNm on a 10mm diameter pulley using a belt of 10 mm2 and made of rubber with a modulus of 50MPa will display 0.4% creep. If the torque reduces by half so does the creep so the speed change on load for a 0.5mNm load variation is around 0.2%. The 0.5mNm load variation is pretty typical of the stylus drag changes found on turntables.

If the belt has no elasticity, then the Elastic Modulus (E in the above calculation ) is ∞ ( infinity ), and Belt Creep will be effectovely 0 ( zero ) exactly as I said.

To give some examples of tensile strength :
Rubber 15mpa
Human Hair 380mpa
Silk 1000mpa
Aramid fiber 2557mpa

These are single fibres only.
Here are the calculations using Mark's example, and assuming the cross sectional area is arbitrarily 1/10th the size of the rubber belt.

T R A E Creep
1 5 10 50 0.04000
1 5 1 380 0.00053
1 5 1 1000 0.00020
1 5 1 2257 0.00009

On the subject of thread drive TT's, they do have to be designed properly, as in the Final Audio. The Final Audio uses an AC synchronous motor with precisely controlled regenerated sine/cosine waves for the motor and variable voltage regulation to optimise the torque applied to the moving high mass/high inertia platter. In addition to this the pulley profile must be designed for a thread rather than a belt. If I recall correctly the pulley should present a concave hemisphere to the thread.

Hi Dover,

I stand corrected, I think. This is where I got tripped up on your post with the word "and" being key. "I said it is an assumption and that if the belt doesn't stretch then there is no creep."

More from Mark Kelly:

"Some form of tensioner (always on the "non-drive" side) will eliminate slip, but it won't touch creep.
Creep is necessary for a belt drive to work. The mechanism of creep is how the belt transmits force from the pulley to the platter, so no creep = no rotation."

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Mosin - no worries. The silk thread I use only seems to slip fractionally on start up, but the 20kg platter can be up to speed in less than 1 rotation, depending on how I tension it, which I think is pretty good considering the pulley is only about 10mm versus the 20kg platter. This suggests the silk has very good grip. With the high mass platter it does drive the motor pulley as well. If I turn the motor off, the platter/thread will drive the motor for a while. The designers original intent was that the silk should be "chalked", I presume for a controlled slip, but I prefer direct. The silk lasts a long time as in can go for a year or two. The only breakage I've had in 20yrs is when I've moved the deck and once when I went on holiday for 6 weeks and left the TT running ( I think high humidity weakened the silk fibre ).
Timeline speed checked on playing records inside/out etc and it's rock solid. These things all come down to the quality of the implementation, whatever direction is chosen.

"These things all come down to the quality of the implementation, whatever direction is chosen."

Exactly. :)