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Old 20th December 2005, 12:14 AM   #41
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by valveitude
[B
My next attempt will be a hydraulic dampener of some sort. The ideal fluid ( to my mind anyway) would be mercury, but being hazardous would require extra care to make sure its sealed. Second choice would be a heavy oil. I will probably try oil first.

Another thing that became painfully obvious playing with the belt drive, is how freakin’ critical alignment is. I found that tilting the motor a couple thousandths of an inch with shims would cause the belt to “walk” up or down the pulley/belt groove. I believe the larger diameter of my low rpm pulley exasperates this problem relative to a high rpm setup running the belt directly on the motor shaft. So, I will be modifying my motor mount, and plinth, to accommodate a fine level adjustment.

Casey [/B]
If other dampening attempts don't work, and before you even think about mercury, buy a motor built for the job. Sometimes doing something, just because you can, ain't wise. This would be one of those times. Or replace the bearings with something that doesn't have enough play to vibrate.

Maybe a little concave on the face of your motor pully would make vertical adjustments much less critical.

Sheldon
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Old 20th December 2005, 12:50 AM   #42
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Quote:
If other dampening attempts don't work, and before you even think about mercury, buy a motor built for the job.
But just think of the added excitement of the listening experience waiting for the seal to fail.

I didn't express myself very well in the above post. I was mentioning mercury as a "baseline" as to what an optimum fluid would be..re-reading what I wrote, I seem to be saying I would use it, I won't.

As for the motor, it was built for the job. I robbed it out of an old H/K turntable. The vibration I'm talking about isn't extreme like a loose bearing would cause, I just have given myself a very high bar. If you wraped mylar tape around any ac motor under tension )like on my setup) I suspect you would see it vibrate the tape to. Its not going crazy, but if you look very close you can see the tape edges blur from vibration.

After thinking about it today, I am going to try a dry dampning approach first. I am going to partially fill the pully with a mix of iron powder and fine silica sand.

Later,
Casey
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Old 20th December 2005, 01:06 AM   #43
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by valveitude


But just think of the added excitement of the listening experience waiting for the seal to fail.

I didn't express myself very well in the above post. I was mentioning mercury as a "baseline" as to what an optimum fluid would be..re-reading what I wrote, I seem to be saying I would use it, I won't.

As for the motor, it was built for the job. I robbed it out of an old H/K turntable. The vibration I'm talking about isn't extreme like a loose bearing would cause, I just have given myself a very high bar. If you wraped mylar tape around any ac motor under tension )like on my setup) I suspect you would see it vibrate the tape to. Its not going crazy, but if you look very close you can see the tape edges blur from vibration.

After thinking about it today, I am going to try a dry dampning approach first. I am going to partially fill the pully with a mix of iron powder and fine silica sand.

Later,
Casey
Maybe a couple of idler pulleys with dampened mountings, or idler pulleys made with naturally dampening materials (felt, etc.).

Sheldon
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Old 20th December 2005, 01:34 AM   #44
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Quote:
Maybe a couple of idler pulleys with dampened mountings, or idler pulleys made with naturally dampening materials (felt, etc.).
That is worth considering. Possibly spring loaded to the pulley itself, dampening the pulley directly as well as the belt...hmmm.

I'll try to keep to the KISS principle first, failing that I may just persue this.

Thanx for the grist for the inspiration mill

Casey
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Old 22nd December 2005, 07:12 PM   #45
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The way to make the belt stay on the pulley is to make the middle of the pulley the widest point.
The top and bottom of the pulley taper away very slightly from the center.
The belt will automatically 'walk' to the middle of the pulley.

Jim.
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Old 22nd December 2005, 07:17 PM   #46
SCD is offline SCD  Canada
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Hello Jiiim:
I may be a little dislexic but I think the opposite would work here with the taper leading to the centre of the pully. Also make the flat spot just a little bigger than the width of the belt.
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Old 22nd December 2005, 07:30 PM   #47
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Hi

I know it doesnt sound right, but the 'fattest' part of the pulley needs to be in the middle.
The belt will 'walk' to the widest point.
Try it - it's a mad thing to watch!

I'm hunting for a picture of such a tapered pully on the net.
I'll post a link when I find one.

Jim.
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Old 22nd December 2005, 07:40 PM   #48
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From sheldon;

Quote:
Maybe a little concave on the face of your motor pully would make vertical adjustments much less critical

from jiiim:


Quote:
The way to make the belt stay on the pulley is to make the middle of the pulley the widest point.
from SCD:


Quote:
I may be a little dislexic but I think the opposite would work here with the taper leading to the centre of the pully. Also make the flat spot just a little bigger than the width of the belt.
I believe all three of you guys are, in fact, describing the same thing..differently.

I agree that this is the "easiest" method for a self centering belt. I was turned off to it at first because of the lathe operation required to do this, and still have a high tolerance dimension. The "right way(s)" are either to make a shaping tool with the desired profile that cuts the whole surface at once, or, use a "ball" attachment on the tool post. Making a shaping tool would be a pain, and I don't have a ball attachment.

A third way is easy but diminsional accuracy suffers..a file. This will be my approach, and I'll tweak any speed deviation with the motor speed control, or just keep making pulleys until I get good at it

Casey
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Old 22nd December 2005, 07:42 PM   #49
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Here we go.

A rounded one rather than the tapered one I was talking about, but it works on the same principle.

A tapered one would be easier to make at home, as it just requires 2 angled cuts on a lathe.


Jim
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Old 22nd December 2005, 07:55 PM   #50
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by valveitude
From sheldon;




from jiiim:




from SCD:




I believe all three of you guys are, in fact, describing the same thing..differently.

I agree that this is the "easiest" method for a self centering belt. I was turned off to it at first because of the lathe operation required to do this, and still have a high tolerance dimension. The "right way(s)" are either to make a shaping tool with the desired profile that cuts the whole surface at once, or, use a "ball" attachment on the tool post. Making a shaping tool would be a pain, and I don't have a ball attachment.

A third way is easy but diminsional accuracy suffers..a file. This will be my approach, and I'll tweak any speed deviation with the motor speed control, or just keep making pulleys until I get good at it

Casey

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.

Sheldon
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