A good choice of carabiner shape can increase ergonomics and security for certain uses

All carabiners come with a kN, or kiloNewton rating engraved into the spine. If you have carabiners without a kN rating DO NOT use them for a life-load.

A kiloNewton is equal to about 225 lbs., which is a force of gravity rating, not static weight or mass. If you remember back to algebra class, force is equal to mass times acceleration.

Everything you use for technical rope rescue, rope, webbing, carabiners, anchors and protection is designed to absorb the force (or shock) that’s generated by a fall.

All this equipment has a certain rating of force it can withstand, and that rating is typically referred to as a kN rating.

That rating doesn’t take into account wear and tear on your gear, so always check everything before use, and replace anything with excessive wear.

We mentioned previously that the greatest strength of a carabiner is in its spine, and is why kN ratings typically offer two different strength ratings. One if the load is distributed along the spine, and another if the load somehow gets distributed across the gate.

Obviously, distributing a load on the gate of the carabiner isn’t good, and this is evident by the kN rating which will typically be 1/3 of what the spine rating is. For example, the manually locking carabiner in our photos is rated at kN 27 along the spine, and kN 8 to 9 across the gate.

If you really look at the construction of carabiners you’ll see why they’re rated less along this axis. All that’s holding the gate to the carabiner is a pin where the spring portion of the gate is located.

As you can imagine, an aluminum pin of that size can not offer a comparable load rating vs. the spine of the entire carabiner.

Don’t get us wrong, 8 to 9 kN is still almost 2000 lbs. of force that the pin can take, but wouldn’t you feel safer knowing you were protected by 27 kN (6000+ lbs.).

The Minimum Breaking Strength (MBS) is a statistically derived value and is poorly understood.

If 100 carabiners are submitted for destructive testing then the measured strengths will vary considerably.  Inconsistencies in the manufacturing process may result in larger variations.  The batch test results can be plotted on a histogram showing the probability of breaking at particular strengths and this curve should approximate a “normal distribution” or “bell curve”.

The width of the curve is characterised by the “standard deviation” or σ (sigma).  3σ for a normal distribution indicates that 99.7% of samples should lie within the range of  (mean – 3σ) to (mean + 3σ).  Smaller values for σ indicate that samples are more likely to be close to the average, or mean value.

Many carabiner manufacturers state that they use “3-sigma” to determine their MBS values.  This means that the MBS is actually the mean breaking strength less 3 times the standard deviation (3σ). Therefore, 99.7% of test samples should break above the MBS. 

Maximum acceptable load in normal use

Material behaviour under load is normally characterised by stress vs strain curves.  Two terms that are useful in describing this behaviour are Elastic and Plastic Deformation.  When a carabiner is stressed by applying tension along the spine it will begin to ‘stretch’ at a relatively low force.  If the carabiner returns to its ‘normal’ shape once the stress is removed then we can describe this stretching as ‘Elastic Deformation’. As the stress becomes more significant this ‘stretching’ may enter an irreversible range known as ‘Plastic Deformation’. 

So, carabiners should not fail below their MBS however they will undergo irreversible deformation before reaching this point.  Also, it is perfectly reasonable to expect that repeated heavy loading will fatigue the material and result in failure below the MBS.

The question that may come out of this discussion is “How much can we load a carabiner without resulting in plastic deformation or significant fatigue?” Some manufacturers address this explicitly in documentation stating that connector loading should never exceed 1/4 of the MBS.

Design Factor

The Design Factor (DF) is specified by a designer or manufacturer and this defines the factor applied to the MBS to determine maximum load acceptable load for a component.

Safety Factor

Safety Factor (SF) is generally defined by industry rather than manufacturer.  It may be significantly different to Design Factor. For example, a particular connector may have an MBS of 50kN, a manufacturer specified DF of 4, but an industry specified SF of 10 when used for a particular application.

WLL

Working Load Limit (WLL) is a term used by manufacturers to indicate the maximum force that should be applied to a carabiner in normal use, regardless of industry.  The ratio of MBS to WLL is referred to as the Design Factor (DF).  Many carabiner manufacturers specify a DF of 4 which implies a WLL of a quarter of the MBS.  Manufacturers state the WLL to ensure that the carabiner is not subjected to significant fatigue and remains in the range of normal elastic deformation.

SWL

Safe Working Load (SWL) is typically determined by dividing the MBS of a carabiner by the Safety Factor (SF) required for a particular use.  As stated above it possible that an entertainment rigger may calculate a different SWL for a particular use of a carabiner than the value determined by a rescue technician.

Example

A particular steel carabiner has a 3-sigma rated long-axis MBS of 50kN.  The manufacture has specified a DF of 4 for this connector, regardless of use.

This carabiner has a WLL of 12.5kN and this value should never be exceeded in normal use – regardless of industry or application.  If this value is exceeded it should not fail below the MBS however it should then be removed from service and destroyed.

An entertainment rigger, working in a certain country is required by the industry code-of-practice to use a SF of 10 for flying performers and thus, determines the SWL of this carabiner is 50kn/10 = 5kN.

A rescue technician in another country is supposed to apply a SF of 5 to hardware and thus determines a SWL of 50kn/5 = 10kN for an identical connector.