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Thanks to Mark Kelly's excellent posts I think I have an understanding of belt creep. Mark has stated on several occasions word to the effect:
"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."
A flat film belt made from Mylar or Kapton will have very little belt creep because it it has very little stretch. This seems to be at odds with Mark's statement.
So how do film belts transmit force from the pulley to platter? Is it really done via the small amount of belt creep? Or is it exclusively friction?
As a secondary question do the driven and non-driven sides of a film belt have essentially the same tension?
Follow Ups:
What if you went to CHAIN DRIVE?
I suspect SOME creep even then, though it would take a while to actually measure.
My BICYLCE chain which is under considerable tension on the driving side, has actually stretched over time.
24 links USED to measure 12". 1/2" pitch chain. NOW? 24 Links measures about 12 1/4" and has worn the rear sprockets a little and almost destroyed the FRONT sprockets which are alloy.
I doubt a TT motor develops enough torque to stretch even a tiny chain in anything less than DECADES of constant use.
Too much is never enough
Chain drive would be noisy and create more problem than it would solve.
Why not a less compliant belt like used for fan belts?
Actually, a less compliant belt LIKE a fan belt would be (probably) too stiff to make the turn around the motor shaft.
However, some of the synthetics I've heard mentioned seem well suited to TT drive.
And, I'll ADMIT to certainly being anything BUT current in the TT arena, but whatever happened to Direct Drive? No belt to worry about. BUT, you'll need a LOT of poles to prevent 'cogging' frequenies.
IS there a state of art DD system?
Too much is never enough
.
Belt creep occurs when the belt resiles from the tensile strain created by the torque of the motor.A more resilient material (soft rubber) will exhibit more strain and therefore more belt creep than a stiffer material (mylar).
I must belatedly apologise for the sentence you quoted, it is easily misinterpreted to mean that belt creep is what drives the platter. It would have been better to say "The mechanism which causes belt creep is the mechanism that transmits force from the pulley to the platter"
The belt works by friction: as an example, consider a teflon belt on a teflon platter: it wouldn't work at all.
The answer to your last question is yes, but to be more specific I need to know whether "driven and non-driven sides" means the inside and outside of the belt (eg when viewing the belt as a ring) or the two segments of belt which run between the platter and motor.
It might be a few days before you get a reply, I don't visit this place very often any more.
Mark Kelly
Edits: 08/03/15 08/03/15 08/03/15
Hi Mark,
You explained belt creep here a few years ago; I thought it was a brilliant explanation at the time and still do.
You using the conservation of mass as the starting point.
Let me paraphrase:
The mass per second of belt that passes a point on one side of the pulley must equal the mass per second that passes a point on the other side. If this were not true, the mass would accumulate on one side or the other which is impossible.
A compliant belt will be stretched thinner on one side of the pulley than on the other side because the tension is greater on the side leading the platter. Now, since the mass per second is constant, then speed must be faster on the thinner, more tense side of the belt.
Assuming the torque of the belt is applied approximately evenly around the circumference of the platter, then the platter will rotate at some speed between that of the slower side of the belt and that of the faster side of the belt.
And in the limiting case, where the friction goes to zero, then the belt creep also goes to zero.
This perfectly explains why increasing the friction applied to a record for a belt driven turntable will causes the patter to slow down even when the speed of rotation of the motor is constant.
Best regards,
BK
AIUI, no they don't. (Weeell, it depends what you mean by 'tension'! ;-)) )
The belt goes round the motor pulley and the platter (or sub-platter in the case of LP12s). So the part of the belt that hasn't reached the platter yet ... is under compression - whereas the part of the belt which is after the platter is under tension.
This tension/compression is like a caterpillar moving along a leaf - and it generates belt creep ... I assume the amount of creep is related to the speed of the pulley or the platter?
The motor speed controller which I am using has the ability to power 2 motors - and to make this to work, it needs to deliver the ability to vary the phase difference between the 2 motors. IE. to synchronise the 'creep' cycles!
I have assembled the parts needed to set up a 2nd motor on my TT but I haven't finished implementing this yet. So I can't report yet, on whether 2 motors provides better SQ! :-))
Andy
If you dig further into Mark's work, you will discover that he considers tape to be virtually immune from belt creep. What you read pertains to the more typical rubber belts. It is proof that the implementation of a system is everything.
If you want a really interesting read, find what he wrote about the typical add-on flywheels that you see from time to time. It will amaze you how inefficient they are. An "Ausenlaufer" AKA "electric flywheel", or external rotor type motor is an entirely different story, however. He crunched the maths on those, too.
When belt-drive was still somewhat common in computer drives, they would frequently use a belt with a fabric mesh embedded in it. The belt had a grippy rubber-like surface, but could barely be stretched enough to put it on resulting in a more uniform tension and grip around the entire path. Once it was on it wasn't strum-ably tight, though.
I would imagine that one obstacle to using these belts in audio would be the diameters required.
In sufficiently large quantities and faced with a steady market and demand the prices would drop
Right. I understand that about tape drive. I did say tape has very little belt creep.
I am trying to understand how force is transferred from belt to pulley. Are there two mechanisms depending on how stiff the belt is?
I did a quick search on flywheels and Mark Kelly. Here a couple results:
4/14/2014. There are a few [motors] which could be tweaked to run well when paired with a well designed external flywheel system but that raises the level of complexity and expense.
8/20/2017. One good example: before building the modelling I had deprecated the use of intermediate flywheels, saying that the multiple resonances would cause more trouble than they were worth. After modelling the dual motor / flywheel / rim drive set up HW showed last week I now know that I was completely wrong.
I am sure there more be Mark seems to have an open mind to flywheels.
Just friction.
That is the only mechanism to transfer the force from the pulley to the belt.
Belt the level of belt creep is determined by the level of elasticity of the belt - more compliant -> more creep. If there is no extension of the belt for a given tension per unit area (corresponding to the case when Young's Modulus for the material is infinite) there will be no creep.
However, materials with a high Young's modulus tend to offer less friction. So there us a trade-off between a belt with a low compliance which has less creep and a lower maximum torque compared with a compliant belt which offers a higher maximum torque but more creep.
A good trade off might be a belt by comprising a non-compliant core material (such as nylon strings) surrounded by rubber to increase the friction but without increasing compliance.
BK
There is a good treatise of this Here.
Click on CHP-20.pdf.
Later Gator,
Dave
he even knows what he will say two years from now!
The Teres project came to the same conclusion. They made an optional motor pulley to use simple cassette tape spliced for a drive system. Most users reported marked improvement in sound. Less wow and flutter so greater dynamics and detail.
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