Thursday, April 5, 2012 10:42:15 AM
...I always try to get a bit too much where items are rare. The cable is available, but in just a little more than twice the amount required to wire the Argent's central frame. I'll have to be more careful not making further mistakes. What's good is that the 13 available meters might reportedly be delivered as soon as tomorrow. The mail mentions DSK75 as an alternative. DSK75 might be equivalent to Amsteel blue, which in fact was what I've been using so far. However, I always had some doubts on the safe working load limit for the 75, which breaking load is sometimes reported as low as 2500 kg (versus the 3850 kg of the quarter-inch-thick DSK78). Now, 2500 kg would still be about 6.5 times the maximum force exerted by one of the motors in downstrokes, but then most sites suggest using a working load 10 times less than the breaking load for such cables to prevent premature failures. That's why I already had some doubts about the DSK75, and this may as well explain why I had apparently stumbled into higher-than-normal elongation ratios in the recent past. The diameter can't be reasonably more than 6 millimeters (especially because of splicing: once spliced, a 6-mm cable gets as thick as nearly 10 mm, and more than that would surely collide with one of the bolts holding the pulley at the end of the downstroke bar), so the DSK78 seems much more ideal. But as happened with those long 5/16" UNF bolts last summer, the component seems rarely used and therefore held in limited amounts in stocks. I guess I might blame that on the veeery particular features and requirements that the Argent must meet, so I shouldn't be surprised. The mail explains it very kindly and sincerely: "that item isn't much requested in that exact diameter, so our supply is short and it won't be back-ordered". However, they have 13 meters, and once correctly installed, DSK78 might be tough enough to make replacement unlikely in the short term. I'll make them suffice. Oh well, and at least I'll be spending a little less.
Um... upon closer inspection of DSK78 specifications as reported by the cable's original manufacturer, it looks like elongation I had witnessed was indeed just normal. And it won't be much different with DSK78. See here if curious. So, I'll have to consider shortening the cable by a good 2%. (update: what the heck I'm saying? the load for 2% elongation on the ordinate was in decanewtons (1 daN ~ 0.98 kgF), so the end of the line is close to the cable's breaking load. Elongation would be that for nearly 400 daN, much closer to the origin of the graph and around 0.3%, meh, I was probably still sleeping, and that comes back to mean the anomalous variation in length was really due to the excessively short splice already about to give up). ANYWAY, fine-tuning to account for elongation could be easily done without re-splicing, because of how the cable is fixed on the side of the motor's shaft, i.e. by looping inside the massive aluminum pulley (that was an early attempt which was later refined by excavating a round channel before the cable's exit point to avoid the "knee", but no worry there: it demonstrated to be as strong as splicing, except it can be fine-tuned more easily). The specs also report the exact breaking load: over 4 metric tons, so we'll be using it at less than 10% of that.