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Synthetic Ropes for Caving

By Roy Bennett

Because of their greater strength and freedom from rot and mildew, synthetic fibre ropes have displaced those of natural fibres almost completely for general caving purposes. Ropes of four materials are generally available: -

Nylon, Terelene, polypropylene and polyethylene.,

These differ in many important respects, and it is convenient first to consider these differences with respect to general underground usage, and then to discuss special applications.  The report “Ropes made from man-made fibre” published by British Roles Ltd.  Gives an up to date coverage of the properties of interest to cavers and is the source of most of the information used in this article.

Strength And Size

For 1¼” circumference ropes, a size commonly used for caving, the minimum breaking loads are: -

Nylon

(BG.S. 3977)

4590 lbs. (dry)

4270 lbs. (wet)

Terelene

(B.S. 3758)

3500 lbs.

(dry or wet)

polypropylene

(Multifilament)

3020 lbs.

(dry or wet)

polyethylene

(B.S. 3912)

2400 lbs.

(dry or wet)

To obtain the same strength as Nylon in the other materials the following circumstances (to the nearest available size) would be required.

Nylon

1¼”

Terelene

1⅜”

polypropylene

1½”

polyethylene

1⅝” - 1¾”

Thus, as regard bulk, Nylon is the best, while Terelene and perhaps polypropylene are acceptable, but polyethylene is getting rather large for ordinary caving purposes.

Weight

As well as the size of his ropes, the caver is also concerned with their weight.  For 100ft. lengths of the above sizes we have: -

Nylon

4.4lbs.

Terelene

6.6lbs.

polypropylene

4.4lbs.

polyethylene

5.7 – 6.5lbs.

Thus the effect of the lower strength of polypropylene as compared with Nylon is cancelled by its lower density, and both these ropes have an advantage over Terelene or polyethylene.  For the usual 100 to 120ft. of rope used on Mendip, this is perhaps not very important, but it is worth considering where the big Yorkshire pitches are concerned.

Knot Weight

This is similar for ropes of all materials.

Resistance To Shock Loads

Because of its greater elasticity Nylon is markedly better at absorbing shock loads than is either Terelene or polypropylene.  The performance of polyethylene ropes in respect is poor and they are not recommended for such applications where such loads are concerned.  Under normal caving practice, where ropes are used for ladder lifelining or for handlines, high shock loadings should not be encountered. On the other hand of ropes are made generally available to members, sooner or later someone will use one for rock climbing either above or below ground, and will expect a satisfactory performance if the leader falls off.  Thus polyethylene ropes present an unnecessary risk and as such should be rejected for general caving purposes.  By the same criterion, Nylon would be preferred to either Terelene or polypropylene.

Extension

Although ability to absorb shock is important, too much elasticity could be embarrassing on a long ladder pitch.  With no slack in the lifeline to begin with, a 200lb. caver at the end of a 300ft. rope will fall the following distances before coming on the rope: -

Nylon

1¼” circ.

55’

Terelene

1⅜” circ.

31’

polypropylene

1½” circ.

33’

polyethylene

1⅝” - 1¾” circ.

23’

Thus in Gaping Gill main shaft (345ft.) the caver will have to climb somewhere near these distances before the lifeline can afford complete protection.  In the case of Nylon, if he falls of say 30ft. up, he will certainly hit the bottom hard enough to sustain injuries.  Persons capable of climbing such pitches are unlikely to come off so near the bottom, but if this extra protection is considered worthwhile, or if any of the much larger overseas pitches are to be attempted, polypropylene or Terelene are to be preferred.  For Mendip caves, where the largest single pitch is some 90ft. there can be little disadvantage in this respect in using Nylon ropes.

Creep

Nylon and Terelene both have good performances when subjected to continued or to repeated high loads of up to 75% of the breaking load.  Polypropylene is less good and polyethylene is relatively poor.  Although general purpose caving ropes are likely to be fairly heavily stressed from time to time, they should only have to cope with such large loads very infrequently so that these differences are not so important as they might appear.  Nevertheless, the above three materials are definitely to be preferred to polyethylene in this respect.

Abrasion Resistance

This is an area in which data comparing all four ropes is rather limited.  Both Nylon and Terelene show fairly good resistance to coarse abrasion in a standard test in sand, markedly superior in this respect to polypropylene and polyethylene.  This is very relevant to caving usage and more comparative data would be useful. On the above evidence Nylon and Terelene are to be much preferred.

Effect Of Heat

Nylon and Terelene retain much of their strength up to temperatures well above the melting points of polypropylene and polyethylene.  These latter materials show a progressive strength loss with rise of temperature, so that at 100oC for example, Nylon and Terelene show no significant change, while polypropylene and polyethylene have lost 60% and 85% respectively of their strength.  Ropes can be heated by accidental contact with carbide lamp flames or during an arrest on a fairly long abseil.  The first hazard can be avoided by using a back position for the lifeline, a good idea with any rope.  The second can only be safely avoided by not doing long abseils on polypropylene or polyethylene ropes.  Thus these ropes, if in general use could be dangerous in this respect to someone unaware of their limitations.

The frictional heating caused by rubbing between a moving and a fixed rope can also cause damage, particularly with Nylon.  This situation should be avoided by, for example, the use of a karabiner.

Effect Of Chemicals

In general ropes should never be exposed to chemicals in any form.  Caving ropes are at risk however from accidental spillages of electrolyte from lead/acid or nickel/alkali accumulators used for lighting. Polypropylene and polyethylene are unaffected by either material.  Nylon can be seriously weakened by sulphuric acid electrolyte, but is only slightly affected by caustic potash, while Terelene the reverse is the case. Nickel/alkali lamp sets are more common than lead/acid ones so that while in this respect polypropylene and polyethylene are better than other fibres, Nylon is to be preferred to Terelene.

Cost

The retail prices per 100ft. of the four ropes (March 1968) are as follows: -

Nylon

1¼” circ.

£4.3.4.

Terelene

1⅜” circ.

£5.14.7.

polypropylene

1½” circ.

£4.7.6.

polyethylene

1⅝” - 1¾” circ.

£3.2.6.

thus showing a clear advantage to polyethylene, with Terelene being rather expensive.

Conclusions

As might be expected, no one rope has all the advantages.  For general purpose caving in areas where big pitches do not occur, Nylon is to be preferred.

Where longer pitches are to be done, the choice as between Nylon, Terelene and polypropylene is much more open.  In the writer’s opinion, the balance of advantage lies with Nylon for the Yorkshire potholes, and with polypropylene for the larger overseas pitches.  If abseiling is required in this latter case, then Terelene would have to be used in spite of increased weight.

For rescue work, there would appear to be no advantage to depart from Nylon for lifelines.  Hauling ropes tend to be quite large to afford a good grip, so that there is an ample strength margin with all fibres.  They are subject to quite severe abrasion however, but usage tends to be fairly low, so that polypropylene has been found satisfactory, at least in the short term.  Monofilament of fibre film polypropylene may be worth considering as they both have better abrasion resistance and are quite a bit cheaper. They may not afford as good a grip however.  Natural fibre or composite natural and synthetic fibre ropes have been used on Mendip. They do present problems of rot prevention however, particularly in the long term, and this tends to cancel the advantage of their increased abrasion resistance.