Monday, January 05, 2015

Drive bands and vibration

My original analysis was that as flyer speed increased, wind resistance increased, resulting in drive band slippage at high speed. Now, I think the issue of drive band slippage at speed is more complex.

At higher speeds, drive bands vibrate raising two issues.

1)    The  vibrating drive band departs from a straight line, increasing tension between the drive wheel and the flyer/bobbin assembly, and massively increasing bearing loads at speed.

2)   The vibration of the drive band does not stop at the flyer/bobbin whorl, and the vibration reduces contact between the drive band and the flyer/bobbin whorls. Powerful screws to tighten the drive against the flyer/bobbin whorls simply increase bearing loads causing more flyer/bobbin assembly resistance and reducing flyer/bobbin assembly speed.

I have to admit that when I was first shopping for wheel, I wanted a powerful, screw based drive belt tensioner, and as I started speeding up my Traddy, one of the first things I did was put a bigger screw tensioner on it.  However, that was a reflex based on "conventional wisdom" and not the result of thinking.  When that approach did not work, I had to go back and think about the system.

Wheel makers must make wheels that sell, or they go out of business.  For my needs, my wheel is much better than any available commercial wheel.  However, as a beginning spinner,  I would never have bought my wheel as it is now - there is too much play in the system.  It does not feel solid, strong,  and durable.  That looseness that you feel in the antique wheels tended to damp vibration, while the modern, precision built wheels that feel strong and stable tend to trap the vibration in the drive band/ flyer/bobbin assembly system ensuring the drive band vibration that allow slippage at the flyer/bobbin assembly whorls.  An elastic drive band does not solve the problem- it stretches and vibrates away from the flyer/bobbin assembly whorls at a slightly different frequency.

To wrap your mind around this think how strong a steel guitar string is, and how tightly it is stretched as it is tuned. And yet, when it is plucked, it vibrates enough to (intermittently)  lose contact with a whorl.  When it is not in contact, it cannot drive the whorl.   Get out your strobe light and watch your drive band vibrate at different speeds.

Modern wheels are made to run at speeds low enough that there is not enough energy in the system to drive the drive band to vibrate at frequencies where it loses contact with the whorl(s).

The vibration of the drive band causes the effective length of the drive band to decrease hugely increasing bearing load, just as contact with the whorl is decreased by the vibration. Thus, as contact decreases, resistance increases and the speed of flyer/bobbin assembly is limited. (In addition, wind resistance is a factor.)

Like, all good problems, there are several solutions.  One is larger whorls.  The Lendrum High Speed Flyer has small whorls and will never give you the productivity you expect on the basis of it's 1:44 ratio.  As folks that have one about their actual productivity using those flyers.

A second solution is inertial damping.

A third solution is spring based damping.

I find the first 2 solutions to be adequate for flyer speeds up to 4,500 rpm.  Bearing loads are low, so bronze and Delrin bearings work very well, although I do oil frequently and use a very high grade oil.
These days, both of my drive belts are tensioned by the weight of the Mother of All and the flyer/bobbin assembly.  The mass of the  Mother of All and the flyer/bobbin assembly is the inertial damping. Putting a "screw tensioner" on the system would increase vibration and reduce my available speed.  These days my whorls are large.

Thus, I now know that any spinner using small whorls and a screw tensioner cannot come close to spinning as fast as I do.  I know that a spinner using large whorls without an accelerator has a much lower spinning ratio than I use and is spinning much slower than I spin.

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