Corian Turntable Fun

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valveitude said:
[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
 
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. :D

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
 
valveitude said:


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

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
 
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
 
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.
 
From sheldon;

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


from jiiim:


The way to make the belt stay on the pulley is to make the middle of the pulley the widest point.

from SCD:


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 :smash:

Casey
 
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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valveitude said:
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 :smash:

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
Sheldon
 
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.


Sheldon
 
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…

pulley-parts.jpg


…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…

pulley-fill.jpg


…. And glued the cap on…

pulley-seal.jpg


…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…

pulley-profile.jpg


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

pulley-track.jpg


…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
 
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.
 
Hello Nordic,

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.

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 :D

Casey
 
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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
 
Hi choky,

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

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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Joined 2004
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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.. :D

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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