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What’s the difference between MBL and ABL?
ABL = average break load. MBL = minimum break load
The average is just that, the average of the test results or the predicted strength. The minimum is a calculated value, generally, we work in line with the Cordage Institute standard 1500 which puts the MBL at 2 standard deviations below the ABL. The CI standard says to test 5 samples and do the maths. This takes no account of variations between batches. At Marlow, we use the CI standard, but in addition over months and years, we accumulate additional tests of many different batches. Every so often we’ll review the data to check that our quoted numbers remain accurate. Statistics show that around 1 in 40 tests will fall below the MBL.
What is the SWL and the WLL?
SWL = Safe Working Load. WLL = Working Load Limit
The SWL and WLL are often used interchangeably however there is a difference. All current standards (BS EN ISO etc) specify a WLL which is calculated by applying a coefficient of utilisation to the MBL of the product. For instance, lifting slings to EN 1492-4 have a coefficient of utilisation of 7 so to get the WLL the MBL (x 0.9 for splice if appropriate) is divided by 7. EG a Rope with MBL of 5000 kg (unspliced) will make a sling with WLL of 643kg. (5000×0.9)/7.
The coefficient of utilisation is often referred to as ‘safety factor’ although no current standards or official documents use this term. It’s the responsibility of the manufacturer to specify the WLL. The WLL is the maximum load that could be applied to a rope in use, normally this figure relates to a straight (vertical) load under normal conditions. This isn’t necessarily the maximum load in a specific application. For instance, if a rope is being used under different conditions, in a different configuration with other rated components etc it may be appropriate to de-rate the WLL. This de-rated figure is often referred to as the ‘safe working load’ (SWL).
It’s the responsibility of a ‘competent person’ to specify the SWL in use. Because the WLL and SWL are normally the same they are often used interchangeably, this isn’t strictly correct. Unfortunately, different industries use different coefficients of utilisation. Because many of our ropes are used in multiple applications it’s not always practical to mark them with a WLL. For instance, Dyneema Winch ropes are used with coefficients ranging from 2:1 or 3.5:1 for towing and 5:1, 7:1 or even 10:1 for lifting, for different applications. Similarly, a reel of Doublebraid may be used to make lifting rope by an arborist or sheets by a sailor.
For this reason, we (Marlow) don’t normally recommend a WLL for our ropes as we often don’t know what application they will be used for. If we are asked for a WLL and no other information is provided then we will use a coefficient of utilisation of 7 in line with the current standards for lifting.
How long will my rope last out doors?
The relative UV resistance of rope making fibres is well known. However this information relates to the yarn NOT the rope, a rope will always last longer than the yarn because only the outer fibres are exposed to the full UV intensity. This means larger ropes will last longer even it they are the same construction and material, i.e. 20mm Marlowbraid will last longer than 10mm Marlowbraid.
The amount of UV a rope will be exposed to will vary hugely depending on the geographic location and even its orientation to the sun! Again this means we can’t predict the life of a specific rope. Generally the UV resistance of UHMPE and Polyester is good, Nylon is ok and PP and Aramids are poor. PBO needs to be kept in the dark to retain its strength!
Will my rope be damaged by chemicals?
It’s very difficult to be 100% sure a chemical will not damage a rope and so it rare that we can offer a definite answer. The safest thing to do is normally to do a trial where a rope sample is exposed to the proposed working environment then tested.
In general polypropylene and polyethylene (including UHMPE) ropes are very resistant to most chemicals, nylon is attacked by strong acids, Polyester is attacked by strong alkalis.
What’s the benefit of PU coating ropes?
There are a number of benefits to PU coating ropes, particularly UHMPE ropes.
- Handling; the PU binds the yarn filaments together making them much less prone to being snagged, this also makes the rope stiffer and easier to splice.
- Abrasion resistance; the PU provides a thin protective layer over the surface of the filaments this adds to the abrasion resistance, the increase in stiffness and the reduction in filament snags also improves durability.
- Colour; UHMPE cannot be post melt dyed due to the low surface energy of the polymer, currently is isn’t available melt dyed ether. PU coating provides a means to colour ropes by applying a coating containing a pigment.
- UV resistance; PU coating can increase UV resistance.
There are a number of different PU’s available that can be used to optimise specific properties such as abrasion resistance, stiffness, fatigue etc. The standard Marlow “Amourcoat” is PU selected to give the best all round blend of these properties for most of our ropes.
What coatings are available and what are the reasons for using them?
Marlow have a range of fibre and rope coatings available:
- ArmourCoat; (see PU Coating) this is the ‘standard’ coating applied to Dyneema rope, it improves abrasion resistance, binds the filaments together, increases friction and carries colour. This coating can also be applied to other fibres such as polyester (Raptor and Arb12).
- GripCoat; this is a ‘self healing’ PU that remains slightly tacky. This is used to reduce sheath movement in some ropes used on winches, the self healing nature may also offer benefits with respect to reducing contamination.
- SlickCoat; this is a lubricating coating that reduces fibre friction and increases flex fatigue resistance.
- EnduraCoat; this s a very high performing and premium polyurethane emulsion that significantly increases abrasion resistance whilst maintaining a high coefficient of friction.
- DriCoat; a hydrophobic coating that repels water to reduce the water uptake of the rope and help minimise weight and the adverse affect of water on the rope (mainly nylon ropes).
- XBO; this is a coating applied to Dyneema by DSM at filament level that improves flex fatigue performance.
- Marine finish; this is a lubricating coating applied to Nylon or Polyester fibres to improve fatigue performance in a marine environment.
What is a D:d ratio and what D:d ratio should I use?
D:d is the ratio of the Sheave diameter (D) to the Rope Diameter (d)
We normally recommend a D:d ratio of 8:1 for most ropes including Dyneema – for example, an 8mm rope should be used on a sheave with a minimum diameter of 64mm.
Aramid ropes suffer from compression fatigue so larger ratios are required; 20:1 is typical for this type of rope.
The figure of 8:1 is a compromise between what is good for the rope and what is practical. Testing on D12 in a static condition has shown that above 5:1 the sheave is not a point of weakness in the system, as you go smaller than this some of the samples will break on the sheave rather than in the splice. As you go smaller than 3:1 all the samples break on the sheave and any smaller than this shows significant strength loss.
However the whole picture is more complicated than this as the flex fatigue rate is affected by the sheave diameter, bigger sheaves and the rope lasts longer. Flex fatigue is also affected by load, speed, rope size, amount of wrap, rope construction, fibre coatings, ambient temperature, wet or dry, and so on… The combination of all these factors makes it almost impossible to accurately predict the fatigue life and therefore impractical to isolate the sheave diameter from all of these other factors. However, the guidelines above are a good rule of thumb.
Read more about flex fatigue in our recent article on the subject.
What is flex fatigue and how long will my rope last?
When a rope is flexed the strength will be reduced over time. 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.
Number of cycles
Cycle speed: the speed of cycles affects the heat build up/loss and so can have significant affect.
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!) the fatigue life.
Temperature: the temperature of the environment the rope is in can affect fatigue.
Water: whether the rope is cycled in 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 other conditions remained the same. Some coatings can increase the fibre abrasion resistance by a factor of 10. 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
What is the difference between ‘heat set’ and ‘pre-stretched’?
Heat setting is the process where a rope is heated to remove the residual stress in the fibres. The fibres in a rope start life straight, after braiding and twisting they form a complex helical shape but if allowed they’ll try and straighten, this means that when the end of the rope is cut the fibres will ‘spring’ out. When the rope is heated the fibres soften and when they cool again they set in the rope shape, this means there’s no springiness when they are released. 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. Marlow’s “Max” super pre-stretching process puts additional tension on the rope during the pre-stretching process and takes that rope to a higher temperature.
Most Marlow heat set ropes are also stretched during the setting process, including D12 and the cores of D2 products.
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