Thursday, January 9, 2014

Spinning Speed Limits

Spinning wheels have built-in speed limits. There is a limit to how fast one can make the flyer (and bobbin) spin. The limit is set by physics and involves things like:
  • wind resistance of the flyer
  • the amount of power that can be delivered to the flyer / bobbin by the drive band
  • the speed at which one treadles
  • the amount of friction in the bearings of the flyer

The standard, single-drive, scotch tensionTraddy, for instance, can't be run much beyond 1800-2000 RPM. While I have not done research on other wheels, it seems to me that most of them will have rather similar limits (since they are made with much the same sized parts and the same materials, and are meant to be spun in the same way: single drive, scotch tension).

If you want to spin faster than 2000 RPM, you need to make fairly radical changes to the wheel itself, changes that are typically not possible using "stock" parts. In particular, you need to do two things:
  1. you will need to decrease the wind resistance and bearing friction of the flyer (which means: a custom-made one, with metal bearings or "bushings" along its axle and a much slimmer, more aerodynamic shape)
  2. you will need to increase the power delivered to the  flyer/bobbin. The latter is usually done by switching to a double-drive system (where both the flyer and the bobbin are driven independently, which is much more efficient than having one "pull along" the other), and using a better drive band with "friction dressing" (ie. coating it with pine pitch to make it stickier).
Most "production wheels" also feature a larger drive wheel, which makes it possible to get to higher RPMs without treadling faster and actually increases the contact area between drive band and the wheel (increases power efficiency).

Such changes will probably bring you another 1000 RPM or so, but beyond that I suspect that an "e-spinner" or direct-drive motor spinner is required. Of course, each increase in speed will require adjustments to the way in which you draft.

It seems to me that this built-in "speed limit" of modern spinning wheels is not widely known. The speed limit has consequences, which are not appreciated. Most spinners, for instance, know that a "higher ratio" whorl can let them spin faster. So many spinners will buy systems that feature "high ratios", not realizing that these are actually not useful.

And here's why that is so. I've made up a little chart which calculates the RPM of the flyer given a certain treadling speed and whorl-to-drive wheel ratio. From the chart, you can see that a ratio of 17:1 is about the maximum sensible ratio to build, because if you treadle at high speed on this ratio, you hit the "speed limit" of the wheel. And this magic ratio is what the standard Traddy ships with as top ratio. Makes sense to me! The last two entries in the table are in red, because these RPMs are not actually achievable. Power limitations, friction and wind resistance will limit the RPM to 2000, and the rest of the energy you are putting in by your fierce treadling is going into heat and vibration.

[relationship between treadling speed, whorl "ratios" and flyer RPM]

So, a "lace flyer" such as is available for the Traddy, is not actually going to let one spin faster than 2000 RPM. It will let one relax when spinning at 2000 RPM (because treadling will be nice and slow). Vibration and noise will be minimal also, because of the lightweight flyer and its bearings. But because of the crappy power delivery (tiny whorl = small surface area for drive band to deliver power), you still cannot get over that 2000 RPM speed limit!

One can achieve 2000 RPM also on a double-drive system with high-speed whorls. Such a system is less effort to treadle than the equivalent single-drive setup. This is because there is less friction (there is no scotch tension pulling back on the bobbin), and also because there are effectively 2 drive bands: one for the bobbin, and one for the flyer, so the power is delivered much more efficiently. This is the right platform for busting the 2000 RPM barrier: you'll need to get an aerodynamic and balanced flyer, with bearings, for a double-drive system. Ashford doesn't sell these, unfortunately. I think Alden Amos used to, but I don't know if he still makes them.

I suspect this is why old "production wheels" you find in antique stores are mostly of this type: double-drive, with large drive wheels. Those guys knew what they were doing.

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