Testing the strength of heavy duty caving belts

A Method of Testing the Strength of Heavy Duty Caving Belts

The aim of this little test was to establish a method to test the strength of heavy duty caving belts that did not rely on having access to a load cell. I hoped to produce a simple system that needed very little equipment and that would deliver a test load to a belt that exceeded the minimum strength requirement for its use.

What strength does a belt need to be?

Well, this one is a potential can of worms…. Let’s be clear, the manufacturers do not condone the use of their heavy duty belts for taking any load at all. There is a historical use in cave and mine exploration that involves using the belt for the purpose of slip prevention and security on steep ground. If you were intending to use it for this purpose, you’d need to be sure that the belt was strong enough for that, something that the manufacturers will not say.
We take the figure for a heavy person with kit that is used for some load ratings on PPE equipment: 120kg.
If a caver has a short lanyard which they climb above, ignoring all stretch or slack in a system, we will assume a possible fall of 1 metre onto the belt. The person will then be travelling 9.81 m/s (acceleration due to gravity).

velocity = √ (distance x gravity x 2)

v = √ (1 x 9.81 x 2)
v = 4.43 m/s

Kinetic energy = (v²m)/2

Ke =  (4.43² x 120)/2
Ke = 117.72 J

Impact force = Kinetic energy / Impact distance

iF = 117.72 / 0.1 (arbitrary minimum for complete stop)
iF = 1177.2 N

Impact Force = 1.177 kN

Confirmation:

Force = Mass x Acceleration

F = 120 x 9.81
F = 1177.2 N / 1.177kN

So a 1m drop of a 120kg caver onto a belt, not taking into account any stretch or bounce, produces a force of nearly 1.2kN.

Apply to this any safety factor you wish. Worse case? Twice the force falling onto half the strength of kit due to wear and age: 2 x 1.2kN = 2.4kN force will need to be held for a worst case fall and we halve the strength for old, worn or wet kit so 2 x 2.4kN = 4.8kN.

So as long as we can apply a test force of 4.8kN or more to the belt, we can be assured that the item can hold the greatest possible force we can apply to it even if not in new condition. The only remaining factor of concern is that would applying this force in test render the belt unsafe to use again, in essence, are these tests destructive? Only 1 way find out…..

You could easily argue that this figure is seriously overkill for a belt so you should undertake your own discussion as to the correct safety factor to apply and then make sure you test to that, e.g. 2.4kN using 2 or 3 people on the 3:1 MA system.

Testing

Using 1 very large Corsican Pine and a good sized Birch tree, we set up a pull testing rig with a simple 3:1 theoretical configuration. I used a Rock Exotica load cell to get live feedback on the testing here but if you copy the method, you would not need to use one.

For the estimation of test force we regarded each person capable of pulling 50kg (see Gethin Thomas’ work on tyroleans). Through a theoretical 3:1 MA system that would be 150kg per person. With 5 undertaking the pull reaching 750kg and 6 equalling 900kg or approximately 7.5kn and 9kN respectively.

Kit used (minus load cell): Petzl rescue pulley, Petzl Basic jammer, Petzl Partner pulley, Lyon wire sling for tree, assorted karabiners, 20m rope.

Due to the force expected to be placed on the rope, I did not anticipate that I would be able to untie the end knot (fig 8 loop). This was accurate and the knot had to be cut from the rope end. Bare this in mind with your own rope!

We also used a Petzl Rollclip to redirect the angle of pull to make it easier to stand on the tarmac of the road alongside the trees.

Initially we had 5 people pulling the first test on a Lyon roller-buckle belt (brand new).
This produced a force of 5.9kN with no damage or slippage. This is lower than expected but there was a lot of tightening in the knot and stretch in the rope coupled with a general timidness of the pulling team.

The remaining tests used 6 people to pull. This one was conducted on my 10 year old Caving Supplies square buckle belt (already retired). This belt has nicks, fluff and rust  and comfortably took a force of 7.74kN showing no damage or slippage. Next came my current AV belt, with it’s central maillon removed and directly attached to the pull line. This belt held 7.7kN without failure or slippage.Finally, the pulling team seemed at their most confident that nothing was going to break and send shards of metal and wood at them so they really gave the last belt some pain. This Warmbac square buckle belt was subjected to 8.64kN with no damage or slippage noted at the time.It is not surprising that the force exerted by the pulling team was less than the theoretical 3:1 system implied. In practice with the loss of friction due to bearings and turns in the rope a 2.5:1 is a more real world figure and so our 5 x 50kg pulling average adults could be expected to make 500kg/5kN using this system. Add a 6th person if the ground is poor or your team are small!

Conclusions

Using a system like the one shown here, with 5 people pulling at average strengths, you can apply a force greater than 4.8kN to your test belt.

Once the test is complete you should thoroughly examine the belt to see if any damage or slippage has occurred. Any that do show signs of damage should be retired. Any slippage may be down to the buckle tightening down so consider testing again. If a belt has taken the test load and shows no damage or deformity then you can be comfortably sure that the belt will be fit for its intended use.

Final inspection of belts:
Lyon roller buckle                                5.9kN            No damage, continuing to use.
Caving Supplies square buckle           7.74kN          No damage, already retired.
AV maillon closed harness buckle       7.7kN            No damage, continuing to use.
Warmbac square buckle                      8.64kN          No damage, slight curvature to webbing now when hung vertically which indicates over stretching or broken fibres down one side. Retired from service.

As a side note, I maintain that the Caving Supplies belts are the tanks of the heavy duty caving belt world and, if kept very clean, will ultimately outperform every other type or brand available. I think this test shows that well as the CS belt had at least 5 more years of abuse over the other belts. I will dispose of the Warmbac belt just in case but don’t tend to use these anyway, but that’s another blog post!