There are several causes of this including: Fibre abrasion; where the rope fibres rub on each other as the rope bends. Compression fatigue aramids ; where the fibres on the inside of a bend go into compression and form kinks, Aramids are particularly susceptible to this. Differential creep; where the fibres on the outside of a bend are under higher load and creep more than the fibres on the inside.
Thermal degradation; in extreme cases rope will heat up when repeatedly bent, this can cause damage to the fibres, UHMPE in large sizes is particularly susceptible to this. The life of a rope when repeatedly bent is exceptionally difficult to predict because of the interaction of a large number of variables that can all have a significant affect on the fatigue.
These variables include:. Rope material: different fibres and even fibre grades have different resistance to fatigue and are affected by different mechanisms. Rope construction: Some constructions are more resistant to fatigue then others, for example 3 strand ropes have less fibre crossings and so are resistant to fibre abrasion while short rope pitches are more resistant to differential creep and compression at the expense of strength.
Degree of bending: Sheave diameter and amount of wrap has a significant affect on life. Rope size: The thermal properties of a rope do not scale linearly; large ropes are more affected by these fatigue mechanisms.
Fibre coatings: there are many coatings that can enhance or reduce! Temperature: the temperature of the environment the rope is in can affect fatigue. Water: whether the rope is cycled is a wet or dry condition can affect the life.
Contamination: Dirt and other materials that get into the rope can affect the life. Some of these factors can have a huge affect; for instance in one test performed by DSM a change in period cycle speed from 10 sec to 12 sec doubled the life of the rope when all others conditions remained the same. Some coatings can increase the fibre abrasion resistance by a factor of Similarly contaminants, including salt crystals, can rapidly abrade fibres reducing the life by orders of magnitude.
For these reasons it is normally impractical to attempt to predict the fatigue life of a rope in a specific application as even apparently insignificant details can hugely affect the results.
If fatigue is a concern in an application then replacing the ropes early and testing for residual strength is the best way to build a picture of the life in the specific conditions this rope sees. Read our recent article on flex fatigue or contact the technical team for more information. Heat setting is the process where a rope is heated to remove the residual stress in the fibres.
Generally a heat set rope is easier and nicer to handle. Pre-stretching pulls the initial elongation out of a rope, both in terms of yarn elongation and constructional elongation. Pre-stretching is far more effective when the rope is heated. Most lifting items are now stamped with a WLL, or at least they should be!
Share this post Link to post Share on other sites. Great post! Arctostaphylos uva-ursi. Just so. Create an account or sign in to comment You need to be a member in order to leave a comment Create an account Sign up for a new account in our community. Register a new account. Sign in Already have an account? Sign In Now. Go To Topic Listing. Alamo Group successfully acquire Timberwolf Ltd Read more New GreenMech website - packed full of products and new innovative features!
Read more Isuzu Grafter N Looking for arborist. Pollards, the forgotten art-discussion. Top 10 most popular wood-burning and multifuel stoves. Tip site reviews. Or even better your safe working load remains the same and your safety factor then gets doubled Just as you can increase strength by certain configurations, you can also weaken it. In this situation your WLL is an overestimate and must be reduced.
A minimum breaking strain is usually stated in kilonewtons which is a unit of force not weight. For most comparisons, 1 is a unit of force not weight. For most comparisons, 1 kilonewton is estimated at kg actually The WLL is usually stated in kilogrammes or tonnes. Even though the WLL looks like it is a 10kn equivalent sling and therefore looks weaker than the 25kN sling.
This confusion is because of the difference between what each rating it measuring. One already has a margin of safety , the other does not. Even more confusion. Where a WLL is in t it can actually mean 3 different weights. These issues don't matter if you use a metric ton and calculate a suitable SWL but it's good to be aware of them. This small difference can add up in very big lifts. It will depend on where in the world your equipment came from as to which unit it will use.
From this, you must decide if it is suitable after you have calculated its safe working load. Important note for lifting applications. The WLL is the also the operational maximum that equipment is permitted to carry. Do you have any questions? If you have any questions, please send us a message and we will be happy to help.
Lifting KnowHow - we have the KnowHow required. Learn more. We are experts in both standard and customized products and solutions and we offer everything from service and inspection to installation and testing. We look forward to solving your next lifting challenge! Maximum load that the non-fixed lifting attachment is designed to lift under the conditions specified by the manufacturer.
Formerly a common term but has today been replaced by WLL for all lifting equipment under the crane hook.
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