New Aluminum Platter from VPI

Doug-It's a custom pulley milled by a machinist on VA who was doing one for himself-and he kindly doubled the run so I could have one. Well, actually two-I (we) have one each for both 300rpm and 600rpm motors. I'd send a photo of the tape-drive setup on your email but I don't think you can add attachments from here, so I'll send you a mail, you reply back, and I'll send you the photo.

BTW, I agree that an SDS (or equivalent) is a necessity, and I've seen your rig in the past. Outstanding. Interesting, too, your comments re. a copper mat.

Doug-one other thing. It was DD who warned me about the possibility of the 'cogging' of these motors coming through sonically via a tape drive, so when that occurred a bitter bile rose in my throat when I thought of all the work my VA machinist pal went to producing these pulleys. Though that sentence way overstates the level of my pain. (-:

Johnbrown, That is a good experiment, and one that may be worth continuing with a DC motor or other AC motors to compare to Hurst. The question remains as to whether there are other AC motors that behave any better than Hurst w/r to cogging. Another possibility is that my two-phase Kelly drive controller is doing a better job of reducing AC motor vibration than a single-phase SDS. However, with thread drive I've noticed that the tension needs to be relaxed a bit more than one might expect in order to optimize audible speed stability. I'm inclined to conclude that an AC motor needs a small amount of compliance in the belt-- certainly not as much sloppiness as a stock VPI rubber belt, but probably more than your mylar tape.

Perhaps HW was led to combine compliant belts with rim drive in order to deal with AC cogging. One would have thought(on paper at least), that a direct-coupled DC motor like Teres Verus would be a better choice for rim drive. But Teres has its own cross to bear: the O-ring on that small drive wheel needs to be perfectly round, which may be more easily said than done.

BTW, by adding a spring-loaded tensioning capstan to your tape drive, you might be able to vary compliance.

Dave

"Another possibility is that my two-phase Kelly drive controller is doing a better job" - it had better be!

Try tweaking the third harmonic control whilst running the platter, you should be able to find a minimum in the cogging torque transmitted.

"I'm inclined to conclude that an AC motor needs a small amount of compliance in the belt"

But how much? How does one calculate the optimal value?

I have just taken possession of some new belts which are far , far less compliant than standard Mylar tape. I am combining them with those "crappy coggy Hurst motors". I think you will be interested in the result. It will hopefully be at RMAF in October.

Mark Kelly

I've come to the same conclusion about the thread tension. Too tight and it doesn't sound right. It needs to be just tight enough to grip, but still loose enough for the weight and mass of the platter to do its speed stability thing without being hampered by the string.

I'm also using a twisted silk thread, which seems to be able to give me that little extra grip while still remaining a little loose. The thickness of the thread also seems to make a difference, with a thicker one sounding better to me. But it can't be too thinck or the knot size becomes a problem.

I'm using thread from here and recommend size 2 or 3 (in whatever color your heart desires): http://www.artbeads.com/stringing-materials-griffin-silk-bead-cord.html

I'm also using Mark's controller on my Hurst 600RPM motor. When it's dialed in I almost can't feel it running while holding it in my hand. Very quiet, but it does take a bit of tweaking to get to that setting. It also does vary with the clock setting a bit so I'm prone to finding the right thread and sticking with it rather than swapping them out and resetting the clock and "quietness" each time.

Bob