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How strong is my climbing gear?

I'm often asked how strong climbing gear is.  The short answer is much stronger than usually need.  But, to expand on that, here's some more to think about.....

Force, Weight and KiloNewtons

 

When buying all that shiny new gear you may ask yourself how strong it actually is.  Or, and more likely, you’ve asked yourself that question already while staring at a skinny bit of nylon tied to your waist as you dangle above a precipitous drop onto jaggedy rocks!

 

Well, to my mind at least, there are two things to consider in this issue.  Firstly, how strong is your gear and secondly, how strong does it actually need to be in every day climbing situations (and this is where it gets a bit more complicated).  I’ll attempt to answer both but, whilst I’m no professor of physics, be warned that there are some unavoidable references to the kind of things you probably hated at school.  That said, you’ll hopefully be pleased to hear that this is my simplified version of it all…

 

In climbing, our equipment is subjected to forces that do not always equate to simple measurements of weight, i.e. I think we all intuitively understand that a falling climber will exert more force on his equipment than just his weight (something to do with gravity!).  Gravity is something close to all climbers’ hearts and minds, since it is that, and not the fact that we’ve not done any training, which causes our forearms to get pumped!  For those of you who remember your physics classes, you may remember that gravity was defined as exerting a force on everything on earth of 9.8 metres per second per second (no that isn’t a typo), or 9.8m/s2.  So, as long as we’re climbing on earth, we are constantly subjected to a downward force of 9.8m/s2 (anyone fancy putting up a few E14s on the moon at the weekend?).

 

To measure these forces we use something called Newtons.  One Newton (named after that very clever chap Sir Isaac Newton, he of apple falling on head fame) is the force required to accelerate a mass of 1kg at a rate of 1 metre per second per second, or 1m/s2.

 

So, coming back to all that shiny kit you’ve just spent your hard earned cash on, you’ve probably noticed that it’s all marked with a number, followed by the letters kN, this stands for kiloNewton.  Each piece of climbing equipment on the market today has its strength rated in kiloNewtons; one kiloNewton is 1,000 Newtons.

 

It may help to be able to understand these Newton thingys a bit more easily and, to help with this, science does allow us to convert Newtons directly into Pounds.  This is because, technically speaking, pounds are also a measure of ‘force’ (but kilograms are a measure of ‘mass’ and so it is not a simple matter to convert kiloNewtons directly into a weight in kilograms, but I will later anyway, so read on!).  A key thing to remember is that a falling climber exerts force on his equipment, which will always equate to more than the measurement of his mass.  Anyway, suffice to say that 1kN is equivalent to a force of 224lb (bending the rules of science a little, this is very roughly equivalent to 100kg).

 

So, now that we hopefully have a rough idea of how the strength of our gear is measured, lets firstly look at the strength ratings of its main components …

 
Ropes

The UIAA (the Union Internationale des Associations D’Alpinisme) certifies the strength of all dynamic mountaineering ropes.  To measure the strength of ropes, the UIAA tests their impact force i.e. the force that is exerted (or impacted) upon the anchors and climber when the rope holds a dynamic fall.  These tests also establish a rope’s fall rating, i.e. the number of falls a rope can hold before it breaks.

The impact force is measured in kN by subjecting the rope to a drop-test that simulates a violent high-impact force fall.  The drop tests are repeated (using the exact same length of rope for each fall) until the rope breaks.  The test subjects ropes to tremendous forces that are rarely duplicated in actual climbing to ensure that ropes are safe during normal climbing activities.

The exact nature of the test is dependent on the type of rope being tested.  Dynamic mountaineering ropes are labelled into three different rope systems; single, half and twin (twin ropes are rarely used by British climbers but are common elsewhere in Europe).  UIAA drop tests are established for each rope system as follows (http://www.uiaa.ch/web.test/visual/Safety/SafComPictorials/PictUIAA101-EN892DynamicRopes.pdf):

For a single-rope, an 80kg weight is attached to it and it and must hold five consecutive test drops with an impact force not exceeding 12kn on the first test drop.

Half-ropes are tested individually using a 55kg weight and must hold five consecutive test drops with impact forces not exceeding 8kn during the first test drop.

Twin-ropes are tested as a pair and must hold twelve consecutive test drops with the same maximum impact force as a single-rope (12kn on the first test drop).

(For more detailed information on ropes, see here: Technical Information on Ropes)

 
Harnesses

Harnesses conforming to UIAA and/or European (CE) safety standards are actually made to be much stronger than they are ever likely to need to be (the CE standard for a harness belay loop is basically that it should hold a 15kN force for 3 minutes – that’s the weight of 1.7 tons for 3 minutes – equivalent to the weight of a 2.3L Ford Mondeo automatic - considerably more than even the keenest pie-eater!).  Notwithstanding this minimum requirement of 15kN, many belay loops on harnesses today are capable of withstanding forces of over 20kN.

 
Slings and other nylon webbing

Simple one this, since all sewn slings and tape bought off a reel are rated to a minimum of 22kN.  That includes the tape that is sewn into cams and hexes.

 
Nuts / Wires

Nuts come in all different shapes and sizes, from your micro-wires that you would do well to only really trust when aid climbing, to your No 9 cowbell Hex that you’d be happy to hang your granny off.  We all commonly carry a rack of DDM Wallnuts, or Wild Country rocks, numbered 1 to 10.  The number 1 in these sets is rated at 7kN when placed with its wide faces in contact with the rock and 4kn when placed with only its thin edges in contact with the rock.  Contrast that with the number 10 Wallnut/Rock, which is rated to 12kN and also your number 9 Hex, which is rated to 14kN.  My simple advice here is, if you can get a bigger nut to fit, use it!

 
Cams
This is a little more tricky since the same nominal size of cam made by different manufacturers will not always be comparable, for example, the number ‘0C3’ Camalot will be a slightly different size compared to, say, a size ‘0’ Wild Country Friend.  Each will therefore have a slightly different strength rating.  However, let’s consider a few examples[1]:
 

Wild Country Technical Friends

Black Diamond Camalots

DMM 3CUs

Nominal Size

Strength kN

Nominal Size

Strength kN

Nominal Size

Strength kN

0

14

0C3

7kN

0

14

1.0

14

1

14kN

0.5

14

2.0

14

2

14kN

1

14

4.0

14

4

14kN

1.5

14

 
Karabiners

When we consider the strength ratings of Karabiners we need to consider several things:

-          the material used in its manufacture – some karabiners are made of steel so as to be extra hard wearing and these will often be stronger than comparably designed karabiners made of lighter weight alloys

-          karabiners are designed to be used with the load being taken in line with their spine (their longest side) and with their gate closed.  If a load is taken by a karabiner with its gate open it will be significantly weaker, as it will be if a load is taken at 90o to its spine i.e. across its minor axis.

 

So, HMS karabiners are commonly rated to 24kN or 25kN when loaded along their spines, but this may reduce to around 7kN or 8kN when the gate is open or 8kN to 10kN along their minor axis.  Modern snap link karabiners are commonly rated to very similar levels as HMS karabiners.  Finally, should you be paranoid enough to be bothered and also strong enough to carry them, steel screw gate karabiners are commonly rated at around 45kN.

 

So, finally, let’s put this all together….

Provided I still have your attention after all that technical stuff (?), you may now be wondering what it all means for you as a climber.…

Well, for starters, let’s say that the HMS karabiner used to belay with and all other karabiners in the system are rated to 24kN.  That equates to being able to hold the force of 5,395lbs, or 2.7 (imperial short, or net) tons.  Now, since it’s possible to convert imperial tons into metric tonnes, for ease of understanding within my pea sized post 1970’s brain, I am going to do something very naughty and say that the same karabiners will therefore hold the equivalent of 2427kg (1lb is 0.45kg), or just under 2.5 metric tonnes.  Which is apparently equivalent to the weight of a Volkswagen Touareg, the Soyuz TM-26 space descent capsule, a newborn humpback whale or the twin-engine EC 135 helicopter!  So, pretty strong then!

 

But, what about dynamic loads exerted during a fall?  Well, let’s firstly consider a fairly moderate fall.  The fall involves a climber weighing 75kg (11stone & 11lbs) who has climbed 20m out from the belay and falls at a point which is 5m above his last piece of protection.  The fall factor in such a fall is 0.5 (20m of rope and a 10m fall, but see here for a full explanation of fall factors: Technical Information on Ropes) and the force exerted on the climber is 5.3kN (you may well be impressed by my ability to calculate this complicated physics, but actually I cheated and used this!: http://www.getbeta.com/fall_factor.asp).  The 5.3kN force exerted on the climber is well within the strength of the harness being used so he/she is fine, or are they?  Well, because of something called the ‘pulley effect’, the force exerted on the top runner in any fall is roughly equal to 166% of the force exerted on the falling climber.  So, the force on the top runner, because of the pulley effect, is calculated as 5.3kN times 1.66, which is 8.8kN.  Consequently, the force on the belayer is calculated as the force on the top-runner less the force on the climber, so 3.5kN.

 

The HMS karabiner used by the belayer and all other karabiners in the chain were correctly aligned and are therefore rated to 24kN, so they are fine.  The rope was new, UIAA certified (see above) and was not running over any sharp edges.  It was therefore more than capable of the withstanding the forces arising in such a fall.  But what about that top runner that actually held the fall?  Well, provided it was well placed and rated at more than 9kN (equivalent to a No. 2 Wallnut or bigger), it should have held.

 

OK, now that we’re building a bit more confidence in the strength of our gear, what about a more serious fall?  So, consider the same climber (so again weighing 75kg) who has climbed the same 20m above the belay, but this time his last piece of protection was 15m below him (so, a 30m fall on 20m of rope) giving a fall factor of 1.5 – this is a very BIG fall!  This time the force exerted on the fallen climber is 7.4kN.  The force on the top runner is 7.4kN times 166%, so 12.3kN and the force on the belayer was 12.3kN minus 7.4kN, so 4.9kN.  With a force on the top runner of 12.3kN, it calls into question whether even a No.10 wire (typically rated to 12kN) would have held this fall.

 

But remember, the UIAA drop test certifies that all new single ropes must not exert forces of more than 12kN in their first 5 falls (and that test is actually calculated with a fall factor of 1.7 – that’s an even more violent fall than in this example).  So, even in our example, provided the climber had used a wire of size 4 or above, his top runner should have held.  Added to the security of this, is the fact that in a real life climbing situation, the impact force on each part of the chain is likely to be less than that which we have calculated with these simple (?) techniques, that is because there are many more variables than those which we have considered here, most of which are acting to reduce the impact force, but more of that in the next example.

 

We’re pretty close to seeing how far to push it now, so let’s see if we can break something!  This time we have the pie eating champion of the Lakes, weighing a whopping 100kg!  He’s climbed 5m out from the belay and, after stopping for a quick bite to eat, he’s fallen off 3m above his last piece of gear.  The fall factor here is 1.2, not too high you might think and lower than our previous faller.  The distance fallen is also pretty small (just 6m compared to 30m in the previous example) but because of his weight and the fact that there is much less rope out to absorb the fall, the impact force on him amounts to 9kN.  Let’s assume that the rope is a single rope and is again UIAA certified but that it has seen a bit of action during our pie eater’s attempts to red-point his latest F5+ project.  As a result, the rope is no longer capable of limiting impact force in the system to 12kN.  The impact force on the top runner is therefore nearly 15kN and the impact force on the belayer is 6kN.  Suffice to say that, even if the gear holds, this is a pretty painful fall for both climber and belayer.

 

But would the gear have held?  Well, even the biggest pieces of protection we’ve considered are only rated to 14kN, as opposed to the 15kN exerted during this fall, so our pie eater may actually for once be grateful for his extra cushioning!  Having said that though, there is still a small possibility that if his top runner had been a No.9 Hex or even a No 1 friend (each rated to 14kN), they may have held him.  This is because in a real life climbing scenario an anchor system is used that consists of a harness, knots, slings, cord etc. and two human bodies.  The tightening of knots, slippage of rope through the belay device (whether intentionally or unavoidably) and the body’s flexing during a fall will all act to reduce the total impact force on each part of the security chain.  This is all getting a bit marginal though and so I would not advise our champion pie eater to be that confident!

 

Some Practical Tips

As a climber it is wise to have some understanding of these somewhat complex issues, even if it’s only a limited understanding.  A healthy respect for issues such as fall factor, impact force and the strength rating of your gear could well be the difference between you having a long and happy climbing career or, well, not having one!  Having said all of that, if physics is really not your strong point, just remember a few practical points that should stop you getting into the danger zone:

 

  1. always retire your gear if it is getting weary, or has suffered a single high impact fall (see: How long will this gear last)
  2. on multi-pitch routes, always construct a bombproof belay anchor, with preferably 3 pieces of well placed protection, each of which is independently tied off with angles between each piece of protection of preferably less than 60o
  3. when the leader leaves a multi-pitch belay stance, he/she should always place a good piece of protection as quickly as possible and continue to place good regular protection where possible as he/she climbs
  4. always wear a helmet!


[1] Taken from manufacturers’ respective web-sites on 26 October 2007.

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