Twisting up a thread shortens it, but this is not normally an efficient mechanism for use in mechanical engineering because most of the work of twisting goes into elastic stretching of the thread. However, when the ratio of twisting torque to thread tension (a ratio having dimensions of length) is large enough in proportion to the diameter of the thread, twist instabilities (solenoids and plectonemes) appear which cause further twisting to act against the external load that maintains the thread tension. This has been quantitatively demonstrated for the plectoneme instability of DNA. In effect, a thread containing twist instabilities becomes a very light and finely-threaded screw.
Behavior of the mechanism is quite non-linear at the onset of instability. For example, in the absence of non-linearity there is no advantage to pre-twisting a spanish windlass: turning the peg increases twist on one half of the other, but decreases twist on the other half to no net effect. However, if the spanish windlass is pre-twisted to near solenoidal transition, solenoid turns will add to one half, but not be subtracted from the other. A net contraction results with little wasted twisting.
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