This was my take on it: https://www.diyaudio.com/community/threads/rega-p3-drive-belt-size.83145/post-7024015
Belt thickness has no effect on TT speed unless the belt is so thick and unpliable that it drags down the motor speed, which is highly unlikely.
"Careful analysis of flexible drive belts will show that the effective radius of a pulley is the measurement from the center of the pulley to the neutral axis of the flexible belt (the plane in the belt where stress is zero as it is curved around the pulley), which is near, but not exactly at, the center of the belt. Simply stated, the thickness of the belt factors into the calculation of the drive ratio of the pulley and platter."
This analysis was discussed in a previous diyAudio thread that I am currently unable to locate.
This analysis was discussed in a previous diyAudio thread that I am currently unable to locate.
The effective radius of the two pulliey's is the effective dia of the pulley/ 2 + ( 1/2 the dynamic thickness of the belt.) You''ve just defined the neutral axis of the drive.
From the neutral axis you can calculate the the ratio between the two pully's effective diameter IF the drive pulley is very small in relation to the driven pulley and the belt is thick yes it can make a difference.
From the neutral axis you can calculate the the ratio between the two pully's effective diameter IF the drive pulley is very small in relation to the driven pulley and the belt is thick yes it can make a difference.
Agree with the "neutral axis" concept explained above.
In addition: with a compliant belt, tension and grip on the belt also affects speed because the tension and friction affects the position of the effective neutral axis, it is no longer at mid-thickness once the surfaces are constrained against sliding. Typically, increasing the tension slows down the rotation speed due to this effect. Reducing the tension can result in a slight speed increase as the neutral axis is allowed to return towards the unconstrained position. Until the belt loses traction.
Beyond that: adding a non-stretchy layer to the outside or inside of the belt also moves the neutral axis, changing the speed ratio slightly.
Try it and see. It seems to be one of the more misunderstood principles in turntable design, despite being taught as part of Mechanical Engineering degrees. Many machines use belt drives, so the science is well understood.
It's good practice to separate critical functions - speed control is best effected by controlling the speed of the drive pulley. Then the belt characteristics and tension can be optimised for other parameters such as lowest noise, optimum bearing load, startup behaviour, dynamic response, maintainability etc etc.
But back to the original question: YES!
Cheers, Mike
In addition: with a compliant belt, tension and grip on the belt also affects speed because the tension and friction affects the position of the effective neutral axis, it is no longer at mid-thickness once the surfaces are constrained against sliding. Typically, increasing the tension slows down the rotation speed due to this effect. Reducing the tension can result in a slight speed increase as the neutral axis is allowed to return towards the unconstrained position. Until the belt loses traction.
Beyond that: adding a non-stretchy layer to the outside or inside of the belt also moves the neutral axis, changing the speed ratio slightly.
Try it and see. It seems to be one of the more misunderstood principles in turntable design, despite being taught as part of Mechanical Engineering degrees. Many machines use belt drives, so the science is well understood.
It's good practice to separate critical functions - speed control is best effected by controlling the speed of the drive pulley. Then the belt characteristics and tension can be optimised for other parameters such as lowest noise, optimum bearing load, startup behaviour, dynamic response, maintainability etc etc.
But back to the original question: YES!
Cheers, Mike
That's only true if drive pulley and driven pulley are the same radius. However that's definitely not the normal case for a turntable! I suspect in the asymmetrical situation its a complex function of belt compliance, pulley size, belt thickness etc.
The belt thickness and I would say also the belt material can change the speed, how much it depends by many factors.
Rgds
Adelmo
I thought this uTube video might be interesting to those who're pondering the question of the belt thickness versus speed. It's all about a US SATs geometry question where all the answers were wrong... Also you learn why there's the time we use and sidereal time used by astronomers. They're all connected.
Didn't finish watching the video but it looks like they are making the problem appear more mysterious than it really is. Looks a lot like the solution involves the distance center of the coin has to travel, the point it rotates around (not the distance its edge travels).
Well, yes. Hence belt thickness affecting platter rotation speed. I wasnt offering it as anything other than a wider illustration of the same principle...