Carabiners! How Strong?

 Super Strong Carabiners! How Strong?

Shown above (from L to R) Singing Rock K408 40KN (using SCM 435 steel) and Kong's Heavy Duty Alu 36KN made with 7075T Aluminium.

Why are some carabiners so strong that they exceed the maximum strength of any rope? What is the purpose of this?

 

To answer these questions, first we need to understand that modern manufacturing processes along with high strength alloys, whether that be aluminium or steel, allow for such high specifications of such carabiners. 

In other words: they are that strong because it's relatively easy due to modern technology to make them by default this strong. 

What distinguishes the "super strong carabiner" from the "regular carabiner"? Usually it is the weight, as in there is simply more material that needs to yield. As material is removed to make carabiner's lighter but still within the safety standards for the particular end applications, so to will the strength decrease. It must be a balance between weight and strength for climbing and caving activities, bearing in mind that the person or persons involved with such activities, absolutely must understand the relationships between fall factors and operating within the safe ratings of the PPE in use. 

In other words one can not rely upon their equipment if dangerous fall factors are involved and excessive mechanical stresses placed upon anchors and anchoring equipment which could cause failure, injury and or fatality.

In the example below for example: we see two aluminium carabiners from the same manufacturer (both made with 7075T Aluminium).

Despite the different overall shapes, both carabiners have about the same internal dimensions of the major axis. Carabiner (A) has an obviously wider end of approx 45mm perpendicular to the spine compared with carabiner (B) for which this dimension is approximately 35mm. With only a 10mm difference between the two carabiners at the widest end, however, their weights vary significantly with carabiner  (A) weighing 74g Vs carabiner (B) at 89g, therefore carabiner (B) is approximately 20.3% heavier than (A). Therefore one can not achieve an equivalent failure rating, for example with carabiner (A), without an increase in either weight/size (impractical for the same material ie Aluminium) or a change of material, ie steel and keep the overall shape and dimensions.

This can also be seen for other personal protection equipment (PPE) devices used in climbing and caving such as a figure 8 descender. 

For example a Petzl Huit (weight 100g) is rated at 100kg maximum working load (not rated in KN, which is a FORCE) and Petzl has an interesting way to rate the safety of this device, since it is not a force but a weight, compared with the mighty Kong Rescue 8, rated at 40KN, device weight of 260g. So from this rating, given by Petzl I'd say that many heavy people (>75Kg) will be approaching the upper limits of safety for this device and hence the HUIT will be unsuitable for such a person. OR are Petzl stating this because anyone who is approaching 100Kg will find it difficult to gain enough friction using this device? Petzl do not state the maximum or minimum breaking Force in KN, therefore it is impossible to determine which is correct.

In amongst this pile of gear (shown above), the mighty KONG Rescue figure 8m special military camouflage version.

According to a study conducted by staff at MIT Centre for Sports  Innovation, on carabiner failure (Analysis of Fatigue Failure in D-Shaped Carabiners 2002). Their findings indicated the following: "Decreasing carabiner weight will likely result   in decreased life forcing the need for fatigue ratings". Intuitively this makes sense as we have seen above.

So why would one need to use such a high strength carabiner, a carabiner whose strength far exceeds that of a brand new, unknotted rope? 

Because humans may not be directly involved with the connectivity of such devices, in other words the use of such devices having little bearing upon the safety of humans as in they (humans) may not be connected directly to these high strength carabiners. 

Such applications where super high strength carabiners are used are for rigging purposes, for example: the film and television industry, where steel cables or other rigging  materials of whose strength will far exceed that of the abseiler's harness or climber's rope ratings . 

High strength aluminium carabiners can also be used on high strength aluminium rigging plates for countless rigging configurations.

Of course the weakest link (no pun intended) will be a static rope and any knots tied within the system of ropes, and tapes used in the rigging. As for the most part these high strength carabiners exceed the strength of brand new ropes and tapes. Much evidence has come about in regards to how certain knots used in rescue, climbing and caving activities can weaken the static strength of a rope considerably (Symmetry 2024, 16, 167.) or electronic  Symmetry 2024, 16, 167. https://doi.org/10.3390/sym16020167.

There are other reasons that may not be so obvious.

I would say that as a long term climber and caver and having put my life and others in the trust of the integrity of climbing ropes and equipment, there is a satisfying confidence in using (not all the time but sometimes) connecting equipment which far exceeds the strength of the rope one is about to abseil on. 

Often phrases like: "bombproof" come to mind in regards to the rigging setup/s required for a multi-pitch descent into the bowels of the earth down raging icy cold waterfalls or at best out of the water but no less exciting an adventure. It is with this added confidence and knowledge that goes a long way for the right "frame of mind" when one is about to embark upon a 24 hour  underground adventure for what many  non cavers and climbers would (quite sensibly) deem "extremely risky" activities!

Steel carabiners

High strength steel carabiners such as this DMM 40KN big D zinc chromate coated carabiner

can be used anywhere where there is a need to mitigate abrasive forces for which otherwise might cause undue abrasion to an aluminium carabiner, whether that is against rocks or some metal object.

Cold forged high strength 45KN Steel carabiner DMM Wales

Usually, but not always (see later in this article), such high strength carabiners will be made from steels such as C45 (AISI 1045) or C35. These alloys possesses high tensile strength and of course being steel, good resistance to abrasion (C45 possesses good wear resistance and hardness). This resistance to abrasion will far exceed that of any aluminium alloy. 

 What about applications for high strength aluminium carabiners?

Kong 36KN main axis "Heavy Duty Alu" Carabiner (Kong's strongest aluminium carabiner to date made with 7075T Aluminium) 

In the world of rigging, a considerable weight saving is of great importance and where abrasion resistance is not an issue but the requirement for high strength is, then the choice is for high strength aluminium carabiners such as but not limited to Kong's Heavy Duty Alu. Here is what Kong suggest as possible end uses for their high strength carabiner.

So now we can see the need for high strength aluminium carabiners such as carabiners rated at plus 30KN. 

All Kong carabiners are 100% tested robotically  to 70% of the rated breaking strength.

These types of carabiners can be used for rigging situations where high loads might be encountered and where the rigging structure should be made as light as possible. For example, a steel carabiner such as the Kong Heavy Duty Carbon, rated at 60KN will weigh approximately 232g (which incidentally can easily lift 6 cars!) or even a 50KN Big D (Singing Rock) weighing in at 255g 

Singing Rock tensile strength rating of 50KN along the major axis.

Example high strength steel carabiner from Singing Rock, stamping showing date of manufacture along with rated axis breaking strengths.

each versus an equivalent high strength aluminium alloy carabiner at 36KN (Kong Heavy Duty Alu shown below)

Both carabiners exceeding the tensile strength of the standard 11mm static ropes (for example Eldelrid 11mm static minimum breaking strength unknotted 34KN). 

However, one can see that for every 5 carabiners of the steel type used,

Shown above Kong's 60KN kidney shaped  Heavy Duty Carbon carabiner manufactured from C35  steel (weight 232g)*

will weigh more than double that of an equivalent strength number in aluminium alloy, that’s a huge difference!

 

How About for Adventure Activities?

High strength carabiners are the preferred choice when it comes to piton brake bar abseiling since they will be effectively loaded against the gate axis and thus one should use only carabiners whose gate axis loading strength is at a maximum as seen in high strength carabiners. For example: the closed gate horizontal loading is 11KN for the Kong 36KN “Heavy Duty Alu Screw Gate”). At one point, the piton brake bar was the preferred choice for canyoners in Australia.

Rigging of pitches in caving, where steel high strength carabiners are often the choice when affixed to steel bolt brackets (either custom made or commercial anchors).

A note here regarding safety!

All activities  involved in rock climbing and caving can be hazardous under certain situations and thus the use of any techniques using any type of PPE should only be undertaken by trained and or experienced personnel!

 

Does surface scratching decrease the rated strength of (all) carabiners (in general)? 

There are insufficient scientific data in relation to this question, however, roughened surfaces on carabiners where ropes and or tapes/slings can rub/move will only cause those items to degrade more rapidly and hence the latest UIAA Standards 121 for carabiners) recommendations suggest that the use of those metal links should be reserved for placing against items of the same materials, for example, steel carabiners, linked to steel hangers aluminium carabiners to aluminium carabiners.

(ref pg. 9 UIAA Standards 121).

 

Scratched surfaces may also hide possible defects as a result of harsh treatment by the end user, such as corrosion pits and or crack propagation. The only way to examine for this is with the use of a hand lens of at least X10 magnification, again a maintenance procedure as recommended by the UIAA Standards 121 for carabiners.

 

Therefore, steel carabiners will exhibit a greater wear resistance than that of aluminium alloy carabiners. For example, carabiners made from 7075T will be less hard than C45 steel carabiners, Brinell hardness 150 Vs 190 respectively.

 It should be noted that one should NOT allow any corrosion via galvanic reactions to occur, especially when it comes to the use of aluminium carabiners. This type of corrosion can very rapidly render carabiners unsafe to use (pers comm.).

 

Personal note from the author

* This model Kong carabiner has basically remained (shape) unchanged (except for the gate, nose and notch) for the last 40 years and we used them extensively for caving rigging and large canyon pitches.