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Karabiners

By: Ted Howard, manufacturing adviser and R.S. King C.Eng. Aerospace structural specialist.
January 1999

This summary presents facts about the design of karabiners

Introduction

It is worth reminding ourselves about the function of karabiners used with ropes.

A karabiner is a much used link in a chain of components intended to provide a life support system either potential, to guard against inadvertent fall, or direct employed in a rigged system intended for rescue, access or industrial use.

The potential life support system is only intended to be used in case of a fall. The gear and its placement is more of a hindrance than a benefit apart from its moral support once placed.  Some risk is accepted.

The direct life support system is gear used dynamically for support as a controlled method with full knowledge of its characteristics.  Risk is not acceptable.

The acceptance of risk is the reason that recreational users are content with less robust and lighter gear than the industrial user.

Fitness for Purpose

The choice of a karabiner in a given situation must be its fitness for purpose.  When it is needed it must work.  It must have adequate strength and stiffness and continue to provide these in the working environment.

It is not the purpose of this summary to give the statistical results of tests readily available through the U.I.A.A., nor to arouse the wrath of manufacturers, but some salient points are given in the hope that they will aid selection of karabiners for specific purposes.

Aluminium

1.                  Many karabiners are made of aluminium alloy by user demand in the pursuit of lightness and also to gain a competitive retailing edge.  They have become lighter and lighter.  Advances in manufacturing and materials technology allow this but gradually the load carrying ability of specific karabiners has decreased.

2.                  Given their working conditions, aluminium alloy is probably one of the worst materials that karabiners could be made from.  Weight advantage has been gained by using increasingly higher strength alloys. These alloys are one of the strongest materials on a weight to strength basis that can be utilised currently for their manufacture.  However in the compromise required to achieve higher strengths these alloys also have reduced ductility and tend to crack more readily, with higher crack propagation rates.  This has the consequence that once a small crack appears then because of the concentration of stress and the nature of the material then the high tech. alloys can rupture easily, even under working loads.

3.                  All aluminium alloys have approximately the same low modulus of elasticity so that improvements to stiffness can only be made by careful detail design of the overall shape and the shape of the local cross-section.  Low stiffness means that the gate can deform requiring a screwed gate to restore some strength by providing a load path.  The size of the pins limits the magnitude of the load that can be carried.

4.                  Aluminium alloys are highly prone to corrosion which has given rise to a huge amount of effort devoted to the problem.  Aircraft in particular use these high tech. alloys and are sometimes grounded for weeks while corroded parts are repaired or replaced.  This means being aware of the various types of corrosion.  These are many.  Only three main types are considered here;

a.                  Surface, is caused by an impurity exposed at the surface making a small electrical cell in the presence of water.  The aluminium becomes an anode and corrodes.  The appearance is a flaky white powder.  The repair is to remove this mechanically and polish the exposed surface until no black pitting shows.  Restore the protective finish.

b.                  Sub-surface is caused by corrosion along the grain boundaries, starting at an edge or a hole. Sometimes called exfoliation corrosion, because the metal flakes, it is hard to detect in the early stages when a simple repair might be possible. The undetected corrosion represents a loss of strength.  Usually the repair is to throw the affected item away.

c.                  Galvanic corrosion is caused by dissimilar metals in contact in the presence of water. For example steel hinge pins in aluminium, unless they are coated with cadmium, will cause corrosion. Unfortunately aluminium will be the anode and will corrode.  It then becomes another throwaway job.  Water, particularly with salts, is damaging.  The simplest repair is to remove all corrosion but please note that both strength and stiffness are reduced by material removal!

Design

5.                  Weight.

Design is usually a compromise between a number of conflicting requirements. The best link might be a closed steel loop tested safely at 100kN.  Anything less is a compromise but we are obliged to compromise.  A gate is required, that adds weight and reduces strength.  Weight is always a consideration if someone has to carry it, but how light should we go? Constructional rigging and rescue work recognises more readily that the strength of the links is paramount and in such situations weight can be tolerated if adequate personnel are available. After all, lightness is of no benefit if it leads to failure!

In climbing or caving the choice becomes blurred, with the decision being biased towards lightness in the interest of success.  How often do we contemplate falling off!  Remember too that a direct belay or an abseil point where one or more lives are at risk can be considered to be a direct life support system which needs higher security.

It is a useful exercise for a climber to weigh a rack of twenty lightweight karabiners say 20kN rated, and compare the difference in weight with a rack of twenty 28kN rated.  With due consideration it may be decided to leave a bar of chocolate behind and take the stronger rack - unless chocolate drives you on!

6.                  Material.

Steel is a much more robust and reliable material than the high tech, aluminium alloys and the red rust of corrosion is more easily spotted with less damaging effects.  Stiffness is a function of the material and also of the detail design.  If we assume that an aluminium and a steel karabiner have the same shape (or volume) the following comparisons can be made. The steel unit will be about twice as strong and three times more stiff but three times more heavy than aluminium.  So, for the same strength the steel karabiner would be about 50% heavier.  You'd get about 18 of these aluminium karabiners to the kilogram or 12 steel ones.  Steel is harder than aluminium and better resists abrasion and impact damage.  It's your choice.

7.                  Detail Design.

Since the advent of competition sports climbing, the availability of longer nosed karabiners, to aid fast clipping of protection points, has increased. This may be why they feature prominently in shops and that there are so many of them.  Are they fit for your purpose?

Karabiners are essentially like crane hooks.  The load is intended to be carried through the back which is shaped for maximum strength.  It is also designed for stiffness so that the hook will not deform and allow the load to slip out.  A karabiner has to have a gate and modem design makes it part of the strength but it is also a weakness.  It should not deform so much under static load that it cannot be opened intentionally, in an emergency.

The loaded rope is reacted by transferring a direct tension force across the link.  The sketch (fig. 1) illustrates this.  Any offset from the line between where the load is applied and where it is reacted will cause additional, weakening, bending forces.  The highest strength is achieved by keeping the line of action of the load as close as possible to the back of the karabiner.  This will give the lowest offset bending.

Pull or tension tests to destruction, are done with a karabiner mounted under ideal conditions.  Tests are done with the karabiner mounted with twelve millimetre rods tucked tight into the corners of the back.  This gives the least offset and consequently, the highest strength.  In practice extra offset bending can arise because the loading geometry is different.  If the same test is done with a twenty-millimetre tape in place of the rod then the point of application of the load is displaced from the ideal position and has an extra offset which significantly reduces the breaking strength (fig.2).  If the karabiner is jammed against something so that it can react force sideways then the rope or tape can slip towards the gate.  The offset is now far more than it was designed to be.  In these circumstances there is a danger of premature failure, (fig.3).

The figures illustrate the fact that the greater the offset from the line of action of the load the less the potential strength of the karabiner because extra bending forces arise from leverage.

8.                  Dynamic Performance of the Gate

A good 'open-gate' strength is difficult to achieve.

Consider the gate design.  It is made so that the latching end of the gate (as opposed to the hinge end) has its latching pin in a design distance clearance to the receiving slot in the nose (fig A ).  This allows the back to build up resistance as it comes under load.  The deflection of the back then allows the gate to engage and start to offload the back.  This device delays the build up of strain in the gate so that this weaker component can add its strength just before the back starts to yield (or permanently deform) on its way to failure.  This gate feature greatly improves the resistance of the karabiner.  However because of the reduced stiffness of aluminium alloy with the resulting deflection, avoid screwing up the gate when under load.  It may be impossible to unscrew it without loading it again.

Dynamic testing, involving a given load dropped through a specific distance, fastened to a rope running through a karabiner gave some very interesting results.  These were analysed in slow motion and it could be seen that as a result of the vibration set up in the karabiner the gate oscillated open with increasing amplitude. This sympathetic response would significantly reduce the unit's strength should the impact occur with the gate open. It might also allow the rope to escape. Without a gate to help, the integrity of the unit can be compromised making a sort of Russian roulette.  This should be enough to encourage the use of screw gates, twistlocks or any other gate locking device.  Again, more fuss but a reduced risk.

Use

9.                  Care in Use

Where karabiners are made for a specific purpose they are not necessarily fit for multi-purpose use.

Avoid linking karabiners together.  They have a way of twisting against each other, especially when on a ledge, getting a back against a gate and opening it.  They can then slip apart.  This has been recorded many times.  It is a very real danger.

Lock the gate - against the rope slipping out - against vibration and to improve strength.

A lot of thought and experience has gone into the design and manufacture of karabiners.  They are made more and more for specialist purposes which may not be compatible with your requirements.

Different features often arise not as a technical improvement but as need to produce a new product and stay commercially ahead. "A karabiner is a karabiner" is not necessarily true.

Maintenance

10.              Will they last forever?

No, they do not last forever.  It is essential that any safety equipment, including karabiners, be treated with respect.  Karabiners need cleaning regularly.

·        After use, especially near salt water, wash in warm water with detergent, rinse in demineralised water, dry and lubricate with a water repellent including the gate hinge pin. Remember that soft waxes (WD 40) evaporate and need regular replacement.

·        Check for distortion, bent gate pins, fractured noses, surface damage such as indentations or cracks.

·        Check that there is a take-up clearance (fig A) at the nose latch, particularly if the karabiner has had a shock load.  Lack of clearance may indicate that the unit has permanently deformed and has a reduced strength.

·        Don't forget that one long abseil on a rope which is wet and dirty or covered in mud can scour a groove so deep that it puts an alloy karabiner beyond safe use. Cut it in two and throw it away.

·        Guard against sympathetic vibration by checking the spring resistance against the gate opening in comparison with a good quality new one.  If in doubt contact your supplier to have a new spring fitted. It is a simple job.

·        Karabiners are like any other mechanical device.  They are prone to failure, need maintenance and eventually are unsafe to use.

Choose your equipment carefully with its purpose in mind.  If it is to be part of a direct life support system where weight is not a problem then it is safer to use properly maintained steel.