What is the advantage of round belts vs flat belts
- Thread starter Jim Creek
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arts
Addicted Member
Likely availability more than anything else.
With the initial demise of the turntable industry and many manufacturers carrying on with only their direct-drive models,the supply (and suppliers) of top quality flat belts no doubt became an issue.
Round belts,on the other hand,are essentially o-rings,which are used in almost every industry,and therefore have many manufacturers.The number of material types that they are available in ensures that many are suitable for turntable use.
With the initial demise of the turntable industry and many manufacturers carrying on with only their direct-drive models,the supply (and suppliers) of top quality flat belts no doubt became an issue.
Round belts,on the other hand,are essentially o-rings,which are used in almost every industry,and therefore have many manufacturers.The number of material types that they are available in ensures that many are suitable for turntable use.
BillWojo
Addicted Member
It's real simple. Having a custom made flat belt is going to cost the TT manufacture real money. The belt company is going to need to charge them a custom tooling charge, and there will be a minimum order of 500 pieces or so.
O-ring belt to custom length? Real easy, buy a roll of o-ring cord stock and a splicing kit. Glue the ends together. Belt cost about a buck and 5 minutes of labor.
I do it when I'm rebuild a piece of machinery and it has non stock o-ring sizes. Usually they are big ones, the smaller sizes I can always find.
Personally, I like a flat belt a lot better. Top quality replacements like the one for the PL-41 are precision ground to the proper thickness after they are cut from the drum. O-ring cord stock is extruded and may vary in diameter.
The flat belt has a lot more surface contact like sKiZo pointed out.
BillWojo
O-ring belt to custom length? Real easy, buy a roll of o-ring cord stock and a splicing kit. Glue the ends together. Belt cost about a buck and 5 minutes of labor.
I do it when I'm rebuild a piece of machinery and it has non stock o-ring sizes. Usually they are big ones, the smaller sizes I can always find.
Personally, I like a flat belt a lot better. Top quality replacements like the one for the PL-41 are precision ground to the proper thickness after they are cut from the drum. O-ring cord stock is extruded and may vary in diameter.
The flat belt has a lot more surface contact like sKiZo pointed out.
BillWojo
Jim Creek
Jazz, Wine, Electronics
Makes sense. I thought perhaps there was a technical advantage to a round belt. Apparently it is all about lower cost of production for round vs flat.
eb2jim
Super Member
A lot of early belt drive machines used non-electronic speeds and wide flat belts. If you shifted from 33 to 45, the belt mechanically nudged back and forth on larger or smaller diameter spindle segments. Electronic speed control machines leave the belt in one spot, and the belt doesn't move - sometimes they ride in a groove. The motor does the speed. A small round (or square cut ) belt doesn't lend itself well to moving, and weren't used on too many turntables. I think when electronic speed control more or less took over they used a lot of the belts that were very common, like the 23.6.
lini
just me...
Sorry, but: Wrong. You have to consider that the belt thickness is part of the equation of the transmission ratio - basically (pulley radius 1 plus half the belt thickness) / (pulley radius 2 plus half the belt thickness). That means that belt thickness doesn't only have an impact on absolute speed, but it also has to be pretty constant, especially as the motor pulley usually is pretty small, so that minor deviations in belt thickness can already significantly impact wow & flutter (unless you have one of the very few belt-drive models that have a motor control with actual platter-speed sensing, which hence would be less sensitive in that regard...). So one would usually want a precisely ground flat belt for the job, if one really cares about minimising wow & flutter.
Calculation example: Typical 16-pole synchronous motor, 50 Hz use -> motor-speed thus 375 rpm; desired platter speed is 33 1/3 rpm, so we'd need a transmission ratio of ca. 11.25. Alright, so let's assume we have a motor pulley with radius 9,5 mm and a 1 mm belt, so we have an effective motor pulley radius of 10 mm - then we need a platter pulley with radius 112 mm. Now let's assume our belt varies in thickness by just a tenth of a millimetre, say down to 0.9 mm. Then we could get 112.45 / 10 = 11.245 and 112.5 / 9.95 = ca. 11.3065 or ca. 33.348 rpm and ca. 33.167 rpm or ca. 1.00044 and 0.99501 times 33 1/3 rpm or ca. +0.044 % and -0.499 % peak deviations from our desired speed. Quite a lot...
Greetings from Munich!
Manfred / lini
BillWojo
Addicted Member
Hi Manfred, nice to see it put in numbers. It's the reason I like good vintage tables if I want a belt drive. I have one belt drive table, a Pioneer PL-41D that I rebuilt with a new bearing insert and a fresh $35.00 precision belt. I know I'm going to get the performance out of it that the original engineers designed into it. They did not make compromises back than, they were competing in a big market and at the time the PL-41 was there best table. Sure, the DD tables that came out afterwards had better specs, it was a better technology.
Today it cost to much to have something as simple as a belt manufactured as the market is to small to amortize the tooling. I'm surprised that the new tables just don't use a standard precision belt like a 23.6" belt. Readily available and much better than o-ring stock.
If I was in the market for a new belt drive table I would look at the very best of the vintage units and find a pristine example and either rebuild it or have it rebuilt as new. I know when I took my PL-41D apart I was very impressed with the quality of it's construction, they will never build anything like that again. The cost of the tooling could never be recouped for the limited number of units that they would sell. The price would be crazy high.
BillWojo
Today it cost to much to have something as simple as a belt manufactured as the market is to small to amortize the tooling. I'm surprised that the new tables just don't use a standard precision belt like a 23.6" belt. Readily available and much better than o-ring stock.
If I was in the market for a new belt drive table I would look at the very best of the vintage units and find a pristine example and either rebuild it or have it rebuilt as new. I know when I took my PL-41D apart I was very impressed with the quality of it's construction, they will never build anything like that again. The cost of the tooling could never be recouped for the limited number of units that they would sell. The price would be crazy high.
BillWojo
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Hi,
Flat belts allow you to use a crowned pulley where the belt automatically
centres itself on that section of the pulley, and are really needed for the
small pulley sizes of the myriad of 4 pole synchronous japenese decks.
Round or square require a grooved motor pulley, silly for 4 pole.
Round I guess can be cheap. Philips for one used ground square belts.
Both would not work well with 4 pole motors. Higher pole motors have
proportionately larger pulley's to drive the same driven diameter.
rgds, sreten.
Flat belts allow you to use a crowned pulley where the belt automatically
centres itself on that section of the pulley, and are really needed for the
small pulley sizes of the myriad of 4 pole synchronous japenese decks.
Round or square require a grooved motor pulley, silly for 4 pole.
Round I guess can be cheap. Philips for one used ground square belts.
Both would not work well with 4 pole motors. Higher pole motors have
proportionately larger pulley's to drive the same driven diameter.
rgds, sreten.
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ripblade
Addicted Member
I'm sorry but I don't follow...why would belt thickness factor into the equation when belt length doesn't? I mean, considering the only part of the belt that touches the pulley is the inner surface, that would mean only the pulley ratios matter. Though I can see how a belt of varying thickness can affect speed as the belt is tensioned between the pulleys....changing the thickness would also affect tension, resulting in speed fluctuations.
Back to the OP, I always assumed a round belt was preferable as a flat belt can 'flutter' as it's lifted off the pulley. For this reason some people put a half twist in a flat belt so as to induce a shearing action off the pulley. A round belt avoids this and also has less contact with the pulley (theoretically), which is one reason (among others) some decks use silk thread instead of rubber.
For the record, I don't have a dog in this fight....of my 2 belt drive decks one has a round belt, the other flat. The differences seem to be more theoretical than actual as they both sound very good to me.
Jim Creek
Jazz, Wine, Electronics
No, that is wrong.
I also thought that belt thickness did not matter since only the inner surface of the belt contacts pulley and platter.
I found out different with my PL41. I bought it used. It came with a cheap eBay belt with the wrong thickness. It ran fast. I could hear it in the higher pitch of music I was familiar with. I put a strobe on it. Sure enough, it was running fast.
I bought the proper belt from LP gear ($30) and the speed was dead on. Double checked it with the strobe. The LP gear belt was the same length but noticably thinner than the cheap eBay belt.
Hi,
The pull of a belt is not determined by the inner surface. A bit of mental gymnastics
imagining a very thin belt to to a very thick one with the same inner surface speed,
should lead you to conclude of course belt thickness affects the speed, and that
~ (because its not precise) the belt adds half its thickness to the effective radius,
or its thickness to the effective diameter, with most effect on the small pulley.
rgds, sreten.
ripblade
Addicted Member
Yeah, that makes sense, since the outside of the belt has to move at the same speed as the inside...
lini
just me...
rb: Well, basically the idea is, that the belt, whilst going around the pulleys, is somewhat stretched on the outside and compressed on the inside - so that the effective linear speed would happen/could be observed at roundabout the middle of the belt. Hence the "pulley radius plus half the belt thickness" (or alternatively pulley diameter plus belt thickness) in the equation for the transmission ratio.
In reality things would of course be a bit more complex, as we'd additionally have to consider belt slippage - but that would be beyond my basic physics, and for my intention of just showing the impact of varying belt thickness it also isn't really necessary.
Greetings from Munich!
Manfred / lini
In reality things would of course be a bit more complex, as we'd additionally have to consider belt slippage - but that would be beyond my basic physics, and for my intention of just showing the impact of varying belt thickness it also isn't really necessary.
Greetings from Munich!
Manfred / lini
Belts hell! How 'bout those tables that use what looks like fishing line?
Mike
Mike
