Saturday, February 18, 2012

Science, spinning, and illusion

For a long time, one of my jobs was to evaluate technologies, first for Bechtel and its clients, and later for the US Department of Energy.  Large capital investments and the health and safety of large numbers of people were on the line, so the reports had to be correct.

As I started spinning, I looked at the technology technology of spindle spinning with all of the rigor that years before I had applied to technologies related to disposal of hazardous oil refinery wastes and zinc refineries.

One of my conclusions was that the use of a half hitch as taught by Abby Franquenmont (and as she learned as a child in South America) limited the fineness of the yarn that could be spun with such spindles and/or limited the the size of the copp that could be managed.  I reported my conclusions below and somebody asked on a thread on Ravelry, if those conclusions were correct.  There was a lot of back and forth.

Abby had to defend her long held style of  spinning, so she set up an illusion, purportedly to test the physics, in the same way that 19th century snake oil salesmen purported to test the laws of medicine.

Her little test did prove two things.  Unless one takes special care, hand spun yarn is not uniform.  She should have taken Amos's class on spinning to a standard back in the days when it was offered.  The second is that when loosely spun wool singles are subjected to tensile stress to the point of failure, first the single stretches, then the fibers drift apart.  Every spinner that looks at what they are doing knows this.  Abby used this to present an illusion that pretended to be science.

Physics says that a half hitch will reduce the strength of a half hitch by about 40% in an  axial load, and this is based on the difference in load on the inside and the outside of the radius as the yarn wraps around itself  in the knot.  Abby choose a load direction that minimized the difference between the inside and the outside radius.  However, this is not the direction of the load that occurs during spinning.  This is physics, and in physics, the direction of the forces matters.  That is why we learn to use force vectors. Abby should  have paid more attention in physics and calculus.

If you have a fairly uniform wool yarn, tie an overhand knot in if and pull on both sides, the yarn will break at the knot.  However, with a half hitch the force on the spindle side of the line is reduced by friction as the yarn spirals down to the copp.  In the case of the half hitch  the failure starts at the knot and runs between fibers away from the spindle (and half hitch.)  The actual break occurs a few fiber lengths away from the knot.  Thus, the failure of the yarn appears to be 1-6 inches away from the knot depending on the twist, ratchet, and  staple length. This is indeed what Abby found.

I would say that Abby's little illusion absolutely demonstrates that a half hitch reduces the strength of  a woolen yarn, despite her attempt to minimize the effect by changing the geometry of the stress to minimize the knot effect.

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