Things are moving ahead. I have a secondhand AP Dillon 15,000lb/6800kg dynamometer arriving in the next day or two. Admittedly, it’s overkill but it was a tiny fraction of its new cost, so was an irresistable bargain. Here’s the brochure which will tell you more than you ever wanted to know about it. In simple terms, it allows a peak load to be recorded, e.g. breaking strain of rope and hardware. The only downside is that it measures in 100lb/45kg divisions, I would have preferred smaller. On the other hand, I suspect that due to the variations in strength in any type, or even batch, of natural fibre rope might make precise measurements somewhat redundant and that broad brushstrokes of 50lb/40kg are close enough. When other variables like knots, how the load is applied, condition of rope etc. are applied, fine measurements become even more meaningless. Only a fool would work right up to the limit without any margin for these imponderables. It will be interesting to see how close to, or even how far we are over, this line we are operating. My suspicions are far from comforting.
For lower loads, a 300kg crane scale with peak load recording seems to just about fit the bill, again for very little money. The HS4-300 hanging scale will weigh in 100g units so will allow quite accurate readings, albeit with a lower maximum that is ideal. However, I think a simple pulley system could be used to introduce different factors via mechanical advantage, e.g. a simple 2:1 advantage doubles the maximum from 300kg to 600kg. Anyway, it will certainly be sufficient to see what loads my model’s 40kg or so (around 90lbs) generates under suspension even assuming the 4x increase in load during ‘drop lifts’ that has been indicated by my rough experiments with a 10kg load can be extrapolated. I was gratified to hear my estimates of around 1.8x increase in load due to friction of running over a karabiner concurred reasonably well with mathematical estimates of 1.6-1.7x. However, I suspect that mathematical estimates based on pulleys don’t take into account the increased friction due to the small radius and greater contact area of a ‘biner as the direction of pull reaches vertical. There seems to be less friction when using a wooden ring due to the increased circumference causing a more gentle curve. I also have every reason to believe are safer to the less destructive curve and a less knife-like edge. This will be a very interesting component to throw into load testing, as will the friction coefficient of the rope due to construction, e.g. profile of the twist, braided v twisted, or lubricants, e.g. wax or oil.
The next challenge will be to build a safe test rig for destruction testing. The rapid dissipation of energy when the rope breaks needs to be considered if one wishes to avoid catching 7kg of dynamometer in the teeth or tangling with a very rapidly travelling bit of rope. As the video belows shows, standing on the right-hand side of that rig might make a full-on bullwhipping seem like a tickling. It is also interesting that the characteristics of the rope can be far more important than simple rated breaking strain.