 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
203.214.31.244
Further down the page I posted with a link to an article on belt creep.I've been through the article now and have found some interesting results.
Executive summary : Belt creep limits the speed stability of a belt drive system. Typical speed stability on-load for a belt sytem running open loop will be 0.2%.
The part of the article that really got me sitting up was this: "if the pulley is assumed to be rigid and the belt stretching is assumed to be isothermal, the adhesion condition implies (that) the belt must maintain a constant strain in the adhesion zone. This further implies that belt tension is also constant through this region, and thus (that) no frictional forces are supported by, or exerted on, the belt."
In other words if the belt didn't creep it could not transmit the driving force. This means that belt creep is an essential part of the belt drive system which cannot be engineered out. The authors go on and prove mathematically that in fact the supposed adhesion zone cannot exist, only creep zones do. Given this, the creep can be approximated as
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.
Mark Kelly
Follow Ups:
When I first read this post I said to myself, "What a bunch of nerds! Why worry about it, just play music!"Then, utilizing the one intact brain cell I own I realized that it's just another part of this hobby!
Really, think of it!!! It's not that there aren't a huge amount of ideas and theories to discuss in any other hobby, but since this is OUR hobby, it's nice to know these brain ticklers occur amongst those interested in the math and physics of phono play.
After all, it's that type of thinking that created the medium...and for that matter, anything we mortals create!
The point is, ain't it great to belong to a hobby with folks that can THINK?
REMEMBER! June 3, 2006 is Ribfest! All who read this (and even if you don't) are invited! E-mail for details.
I wonder why belt drive turntable systems have never implemented toothed gears as a way of controlling belt creep. I would imagine it's part of the adhesion factor of the materials involved. Interesting stuff, and it does infer the quality of the belt composition is very important.
I knew I saw one somewhere, I've spent half an hour looking for it, so here you go.Best wishes, Craig
"I prefer a bit of tape hiss actually"
-George Harrison
I didn't inquire what the price might be, but I have bookmarked the page. It was kind of you to spend the time you did to let me know someone has usurped my idea :-)Again, thanks Craig.
You're very welcome.I guess he's still working on the arm? My EAR dealer knows nothing of this turntable.
Best wishes, Craig
Its been a long time since I studied mechanics, but I can't see any accounting for the interia of the driven pulley (platter) in these calculations - so they do not reflect the real world, even in power transmission applications where there is generally a considerable amount of rotating mass which steadies things.
The inertia of the platter acts as a dampener of the belt creep phenomenon. Platter inertia will ensure that when momentary drag increases, the platter will plough on with negligable speed variation. This reduces the belt stretch and the belt creep.
I suppose one could work out % energy of 0.5mNm v/s say a 5kg platter at 33 1./3 rpm. I reckon it would be pretty microscopic.That said, it sure shows why belt drive with light weight platters might not be a good idea.
Cheers,
John K
An increase in drag removes energy from the system as heat. That energy must be replaced for the system to maintain speed. The only source of energy is the motor, so the torque from the motor increases. That increased torque creates belt creep.The rotational kinetic energy stored in the platter can only be released by a change of speed, so its only effect is to reduce the rate of change of speed. Accordingly, a high moment of inertia will mean that the rate at which the platter slows down is reduced, so short term variations in torque requirement (eg modulation drag) will have less short term effect. Long term effects (like the difference in net stylus drag torque from the outside grooves to the inside) can only be minimally affected by rational values of moment of inertia.
A typical platter in a medium mass system has an Icm of about 0.2 kgm 2 , so a torque of 1mNm will cause a deceleration of .002 s 2 so we are talking of period of a few seconds for a speed change of less than 1%.
Mark Kelly
So is this another advantage [other than noise] for the lowest torque motor possible for the given application [platter weight]?Slight possibility I understood some of your post-I may be doing some of that "accidental learning" I referred to earlier!
No it has nothing to do with the maximal torque from the motor.
Mark Kelly
why Alison Krauss sounds like Bob Dylan when I first lower the stylus onto the album.Tom
had to do it
~Slainte,
The OMalley
nt
Can't say Thing #1 about the technical assertions there, but if what this means is.... that periodic micro-variations in stability are endemic to compliant belt-drive ...... then that confirms my empirical observations re belt drive.Nothing is perfect, and BD certainly has it's undeniable charms, but like a slight aftertaste or a deja-vu moment, the underlying characteristic speed concerns are noticeably evident once you're familiar with them.
J.
The equation above is only an approximation, I'm still trying to come up with appropriate values for some of the terms for the eqs in the paper to derive a better solution. As an example, I have completely ignored any belt creep effect due to the platter itself, which is bound to mean that the real value is greater than my approximation.As you can see from my response to your near namesake above, with reaonable values of moment of inertia for the platter the response time to these speed variations is in the seconds, so I'm not sure how much variation due to modulation drag would actually be detectable.
Mark Kelly
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
