Static LSK: A Design of Compromises
February 8, 2019 | posted by Nichola Bushnell
Like many things the design of a good quality static rope is an exercise in compromises. Many of the requirements the rope must meet are met at the expense of another desirable property. For this reason, the performance figures published don’t tell the whole story and it’s necessary to look a little deeper into the design of the rope to really understand what makes a good product and why these compromises are necessary.
Materials: Static ropes are normally made from Polyamide (Nylon) in order to maximise the energy absorbed in the event of a fall. It is possible to use polyester in some designs but the dynamic performance is often very poor. Nylon has unparalleled ability to stretch and soak up a fall but this comes at a price with respect to some of its other properties, most noticeably nylon is hygroscopic and is affected by water. This will cause the rope to shrink along its length and become stiffer after soaking. Some users will actually pre-shrink their ropes by soaking and drying before use. It is possible to make ropes with mixtures of materials such as polyester covers and Nylon cores, but this must be done with caution. If the cores shrink and the cover does not, then sheath slip can become a problem.
Low melt point materials such as polypropylene are not allowed by EN 1891.
The cores: The standards allow the designer considerable flexibility with respect to the cores, which can be any size and construction. In most static rope designs the cores are the primary load bearing part of the rope. Larger cores are faster and therefore cheaper to produce but they don’t give the same fine control over the diameter as smaller cores. Also, more smaller cores tend to give a rope that is rounder, more supple and less prone to lumps when bending.
Twisted cores are normally in the form of tiny 3 strand sub-ropes. Like a rope, they should be balanced and should not have a tendency to twist or hockle. It is possible to make a static rope product with single twist cores but this rope would likely lose performance over time, the only obvious benefit to doing this would be price! The twist level of the cores is another compromise, as highly twisted cores aid drop performance and give higher elongation, while lower twist is stronger and has less elongation. The ideal twist is something for the designer to establish! A good rope will have an equal ( or thereabouts) number of cores twisted in either direction, this ensures the rope has no tendency to twist over its lifetime.
The cover: For most designs of static rope the purpose of the cover is to protect the core. There is a balance to be achieved between a very thin cover that provides minimal protection but allows for more load bearing core material, and a thick protective cover that leaves little space for the core. Most static rope designs opt for a 16 plait cover made on a 32 carrier braiding machine, (this is sometimes erroneously referred to as 32 plait). The same machine can also make a true 32 plait cover if the carriers are run in a different configuration. It is possible to make a rope with a thicker cover and less carriers, Marlow’s Diablo is an example of a 24 plait rope made on 24 carrier machines. 40 or 48 carrier machines can be used to achieve thinner covers.
To get the best durability from the rope the cover yarns should be twisted, this will ensure that any filament breaks that occur from abrasion form a protective ‘fuzz’ on the rope surface. One common way to save money in economy ropes is to skip this process and use flat yarns. A rope like this can have the same performance figures on paper but it’s durability will be significantly reduced.
Performance: To pass EN 1891 the rope must meet 9 tested parameters, often these are a compromise, to perform well on one parameter may result in poor performance on another. For example, the static elongation from 50kg to 150kg must be less than 5%, a low elongation here results in a low stretch rope that’s efficient to climb on, however it’s may also result in a high impact force and poor dynamic performance!
The parameters tested for in EN 1891 are:
- Diameter:Measured at 10kg ref tension, Average of 6 measurements. Must be between 8.5mm and 16mm diameter.
- Knotability:Must be less than 1.2 – This is a test to ensure the rope is flexible enough to be used with normal hardware and can be knotted.
- Sheath Slippage: Measurement of relative movement between core and cover, ensures cover won’t bunch up in use.
- Elongation:Measured from 50kg to 150kg load – this is designed to simulate the loads when ascending the rope. This figure gives an indication of how bouncy the rope will be in use. This must not be more than 5%.
- Core/cover ratio: This requirement is designed to ensure the rope has a load bearing core and that the cover is sufficiently thick to provide protection.
- Impact force: Peak load seen when 100kg (80 kg for type B) is dropped through a fall factor of 0.3 (distance dropped divided by the length of rope). This must not exceed 6kN.
- Dynamic performance: The rope must survive at least 5 fall factor 1 drops with 100kg drop mass (80kg for type B).
- Static strength without terminations: The rope must have a strength exceeding 22kN (18kN for type B).
- Static strength with terminations: The rope must have a strength exceeding 15kN (12kN for type B), this test is done with a fig 8 knot in as ropes would typically be used. If a rope is supplied with other terminations such as sewn or spliced this is the strength requirement.
In addition to these the following are also tested but there’s no requirement to pass.
- Shrinkage:Gives an indication of how much a rope will shrink in service.
- Mass: Rope mass is measured at reference tension.
It would often be possible for the rope designer to optimise the rope’s design to perform better in one or other of these properties but that will be at the detriment of other desirable properties. Part of the skill is in achieving the right balance at the right price. This includes hidden factors that don’t appear in the figures like the ‘feel’ of the rope, the durability of the rope and it’s compatibility with other hardware.
The published figures can only ever tell part of the story……