I’ll start out with a disclaimer: I am not an engineer or physicist! That said, I have a logical mind and can apply it practically…and there’s Google. I started thinking about rope breaks and it seemed to me that friction is a big factor. It has been speculated, not unreasonably, that the ‘sawing’ action of rope against rope, especially at the bight, could cause undue wear and possible failure. However, I believe that what is apparently called ‘static friction’ might be a a big factor.
Although I really need a device to measure peak weight, some rough and ready experiments lifting a chair with a spring balance in-line showed this effect. The peak load required to overcome the static friction was very much much higher when I ran the rope up over the ring and back through the bight compared to using a karabiner instead of the bight. Whilst the static friction provided by the chair remained constant, the kinetic friction of rope on rope is far greater than rope on a karabiner. Due to the inaccuracy of my test equipment, I’d be reluctant to give measurements as to how much more load was applied but I’d hazard a guess that it was quite a bit more. I would imagine that the suddenness of the acceleration would also be a factor as more force is required to accelerate a given mass faster. In other words, a heavier load, more friction and a faster lift will increase the chance of over-loading your rope. I would be interested in hearing from those better qualified than I as to whether this effect is linear or exponential. No doubt, if I could overcome my allergy to figures and formulae I would be able to work it out.
In short, reducing friction and sudden loading is the best way to minimise the chance of rope breaks. How to reduce kinetic friction? Use karabiners, not the bight or rope against rope, and keep your rope well lubricated by regular waxing and oiling. Not only does the latter reduce friction against the load but it also reduces wear caused by the fibres rubbing against each other which will weaken the rope. Be aware that dramatic fast lifts put extra stress on the rope. I have noticed how much difference in effort needed in attempting lifts with Nina’s 40 or so kilos and a model of a more typical 60kgs. It should also be appreciated that most pro Japanese models are nearer Nina’s weight, so if you use a light 5mm jute with a larger western model you are quite possibly exceeding the capabilities of your suspension rope if you subject it to sudden loads with excessive friction. I suspect this was a significant contributing factor in Bob Ropemarks’ rope break as Dutch Dame, whilst being delightfully proportioned, is considerably heavier than say Nina. I should clarify that Bob doesn’t trust thinner lines for suspensions and was using a 6mm jute at the time as he says in the comments below. The problem with natural fibre is that there are so many variables that can affect strength from batch to batch or even within a single length. Where the fibre was grown, the variations in crop from year to year, quality, joins in the yarns, number of yarns, lay of the rope etc. will all influence the strngth of new rope. Even more variables come into play when it is used: wear, damage, friction, knots, rot, attack by vermin, contact with substances that might weaken the fibre, UV, humidity, the way load is applied and so on. As we are not working with the sort of safety margins that climbers or industry expect as a matter of course, we need to be very careful with our equipment, taking great care how we use and treat it.
Some of the light 5mm I have tested is not strong and will easily snap if it is shock loaded with my mere 60kg. My highly scientific test consisted of tying a loop that would take my foot and suspending it not far off the ground. Then supporting my weight in the loop and keeping my balance by holding the suspension point, I jumped. It broke easily at a point between the two knots. Admittedly, this was using a single strand but it is easy for a line to snag and put more load on just one side of a doubled rope.
Lastest update:
I have conducted some more very approximate tests. I tried lifting a 12kg load running the rope over the ring and back through the bight of the end tied to my load (a box of printer paper). It took around 12kg of force to get it moving! With a ‘biner, this fell to around 8kg. Once up, it only needed about 7kg to support it. I used rope that had developed a smooth patina through use and treatment, so it is reasonable that a more grippy rope would produce higher peak loads due to the extra kinetic friction. You could expect maximum resistance using an old style rig where the suspension point is another rope loop, thus giving an extra point of high kinetic friction. One also needs to condsider the effect of twists in the lines or friction on other ropes, e.g. at ring or ‘biner. There is good reason to pay close attention to how you run your lines and reducing friction.
These are rough and ready measurements as a) the spring balances are crude measures b) the readings were not neccessarily maximums as catching the peak visually is not reliable. Thinks…maybe if I videoed it and slowed the footage down?