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Old 22nd December 2005, 06:58 PM   #51
Sheldon is offline Sheldon  United States
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I think Jiiim's right and I was wrong. It's counterintuitive at first. The system should seek the lowest energy point, and it would seem like that would be the position with the lowest belt tension, which in turn would be the center of a concave pully. But, on second thought. I think Jiiim is right in that with wider part of the pulley will grab the belt tighter and pull it in the direction of increasing pulley thickness. So the energy low point would be driven more by belt friction than belt tension. Makes sense, because tension mainly increases motor bearing friction, which wouldn't feed back on belt position.

If you can get some thin strong tape (polyester?) you can try a little modeling before machining. I would guess that you can leave the center portion it's current diameter, or very close to it. Then gradually taper in the top and bottom of the pulley.


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Old 22nd December 2005, 07:02 PM   #52
SCD is offline SCD  Canada
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I am now thinking about the pulley on the front of my belt sander and sure enough it is concave as discussed. Good for you Jiiim. This sure is counterintuitive for me.
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Old 23rd December 2005, 02:02 AM   #53
maxro is offline maxro  Canada
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Quote:
Originally posted by SCD
I am now thinking about the pulley on the front of my belt sander and sure enough it is concave as discussed. Good for you Jiiim. This sure is counterintuitive for me.

I think you mean "convex" as discussed.

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Old 24th December 2005, 07:05 PM   #54
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I finished pulley V. 1.02….

First I cut some Corian discs, and laminated them with J-B Weld for turning stock, I then made these pieces…

Click the image to open in full size.

…the stuff in the tipped cup is my iron powder. I then filled the pulley body with the iron powder just shy of the top, leaving room for the powder to “dance” around to dissipate the vibration energy…

Click the image to open in full size.

…. And glued the cap on…

Click the image to open in full size.

…I turned a convex profile by making 10 deg. Cuts 25% in from both sides, and then smoothing with sandpaper. Here is the completed pulley against a block to better show the profile…

Click the image to open in full size.

… it works fantabulous. Here is a “action” photo of the belt tracking ( the tilt is the camera, not the motor)…

Click the image to open in full size.

…alignment is still critical..but doable (unlike before). About that vibration…all but disappeared. If you hold your ear an inch or so from the belt, you can hear it, and if you lightly touch it you can feel it , BUT, it is several orders of magnitude lower than before. I will attempt to get rid of the remaining vibration (such as it is) by running a vcr pinch roller up against the pulley (not the belt, the thickness varies due to splicing tape). Even if this doesn’t reduce vibration any further, I think I can live with what I have.

Whew… glad that’s over.

Casey
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Old 25th December 2005, 03:24 AM   #55
Sheldon is offline Sheldon  United States
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Cool.
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Old 25th December 2005, 09:11 AM   #56
Nordic is offline Nordic  South Africa
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Have you experimented with diffirent diameters for the spindle the belt runs around... It kinda reminded me of bicycle gears... and how the size ratio between the 2 would effect the amount of power you had to apply to the pedals... Woud I be right if I said that if you drive the wheel with the wrong ratio of spindle size to wheel size, it would load the engine more and make it more prone to vibrateing...

Another idea would be fitting a bearing to the top of the motor spindle and have an overhanging arm, (kinda like a tonearm) that keeps the top from wanting to wobble.
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Old 25th December 2005, 10:26 PM   #57
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Hello Nordic,

Quote:
Woud I be right if I said that if you drive the wheel with the wrong ratio of spindle size to wheel size, it would load the engine more and make it more prone to vibrateing
Load does play a big role in motor vibration. The ratio of the pulley to platter is dictated by the motor speed (300rpm) to platter speed (33 1/3 rpm). I can vary from this with a variable freq. power supply ( but that is my design center). The only time the motor sees any significant load is during start-up ( it doesn't need a manual boost by the way), after it's spinning, the motor just needs to put back the losses from bearing friction (very low) and the stylus drag. I will in fact drop the drive voltage to the motor after the speed stabilizes, reducing vibration further.

Quote:
Another idea would be fitting a bearing to the top of the motor spindle and have an overhanging arm, (kinda like a tonearm) that keeps the top from wanting to wobble.
Wobble?!? What wobble ? I ain't got no stinkin' wobble

Casey
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Old 25th December 2005, 10:45 PM   #58
Zen Mod is offline Zen Mod  Serbia
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man who made Teres claims that main spindle is sooo long intentionally-to achieve sort of "large oil film area induced platter breaking".
EMT use felt ring below main platter with adjustable pressure for speed adjust and also for same-breaking - purpose
Garrard from same reasons use eddy current disc ditto on motor spindle

think in "A class" versus "C class" domains

motor must be loaded to achieve that styli induced system changes are
miniscule in overall system energy state

and-when you load motor-wobble will be smaller,for sure

that works-at least for me-just because I have EMT and Garrard to check that claims
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Old 25th December 2005, 11:18 PM   #59
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Hi choky,

Quote:
think in "A class" versus "C class" domains
Quote:
motor must be loaded to achieve that styli induced system changes are
Quote:
miniscule in overall system energy state
Since I'm using such a weeney motor, I can load the motor to a halt with a minor belt tension increase. I'll experiment with loading when it's done...thanx for the tip.

Casey
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Old 26th December 2005, 04:51 PM   #60
kevinkr is offline kevinkr  United States
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Hi Casey,
I think the mass of your turntable is so great that small variations in load due to tracing forces will be so heavily integrated that your motor will never seem them..

Incidentally as much as I love the Garrard 301 or 401 the motors in these things are not synchronous types at all but as I recall are actually shaded pole motors, are not fully locked to the line frequency and actually rely on the adjustable loading to get the speed right, this was a cheap way of getting adjustable speed in the days before electronic inverters could be made reasonably economically.

Incidentally Fairchild in the 1950s manufacturered a turntable with synchronous motor and vacuum tube based electronic speed control!
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