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Old 11th April 2008, 10:47 PM   #21
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Quote:
The next section (45 rpm drive) is slightly narrower. It steps down in dia by about 4 mm from the lower section (90 deg edge on the step). This section is a "parallel cylinder", no "high or low spots"

well you might have answered that there. See the homemade pulley shown in the pic below. See the way its fatter in the middle and thinnner top and bottom - the motor angle is meant to be adjusted so that the belt rides on the widest part. But you seem to have that sorted.


Click the image to open in full size.


Last thing I can think of:

Could you measure (accurately with a vernier callipers) the diameters of the drive pulley and the platter (where the belt rides) and work out the ratio of one to the other. So if the motor runs at 250rpm at 50Hz, the ratio of the pulleys should give you 250/33.3=7.507

After that I'm all out of ideas on what to do here....

Fran
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Old 11th April 2008, 11:32 PM   #22
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At the risk of repetition, you cannot leave out the belt in this calculation.

sp
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Old 12th April 2008, 01:42 PM   #23
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Don't quite understand this. Doesn't the calculation depend upon the inside diameter of the belt, which is the same regardless of the outside diameter of the belt?
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Old 13th April 2008, 01:37 AM   #24
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What you have to look at are the effective diameters of the pulley and hub driven by a flexible belt of thickness t.

These can be approximated as actual diameter plus 0.5 t on each side ie actual diameter plus t.

Now if pulley and hub are the same diameter there will be no effect but because the pulley is usually much smaller there is a relative increase in its diameter.

I measured a Thorens TD160 to have pulley approx 13.8 mm and hub approx 159 mm.

Motor speed is 375 rpm so no belt hub speed is

375 x 13.8/159 = 32.5 rpm

I don't have the belt but it is probably 0.5mm thick so with a belt effective hub speed is

375 x 14.3/159.5 = 33.6 rpm

Difference is around 3%

sp
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