In previous articles, the fundamentals of tree climbing techniques and accessories have been explored to enable arborists to appreciate the many recent advances within our field. The next step is a series of articles designed to educate arborists in recent advances specifically in the area of tree and branch removal, now referred to as 'rigging'. This first article presents an appraisal of traditional techniques.

Rigging - what exactly is it?
Rigging is a word recently introduced to practical tree management, with a mixed understanding to the meaning of the term. Essentially, rigging is a term used in reference to various pulleys, lines and connectors applied to accomplish a certain task. This extends to hauling sails in yachting and lifting operations in docks, but typically, in an arboricultural context, it refers to the dismantling of trees with ropes, pulleys and lowering devices. Even in its most simple form, this can be a complex, arduous and hazardous task if the work is not properly planned and the correct equipment is not used. However, although it is still considered to be an essential part of the services provided by a professional company, this doesn't mean that it should be undertaken lightly.

How did rigging technology reach arboriculture?
As suburban populations have grown, the art of arboriculture has grown with it, and also the need to remove large trees and branches in close proximity to people and property. The innovators that began rigging trees in this industry probably used expertise in rigging skills gained from crewing large mast ships, service in the military and navy, handling heavy loads in docks and other heavy industries. The humble rope and pulley has, and still is, used for great effect where such technology as heavy lifting cranes cannot be accessed.

Before the invention of chainsaws, controlling large limbs with ropes and pulleys made perfect sense - it meant only having to make one or two cuts, rather than several, in order to remove a tree limb. This saved energy and speeded up the operation. I was told of a timber man of old in Ireland, who could dismantle large trees and branches with a horse, ropes and pulleys. These rigging principles are still of merit today, even with the advent of light and powerful chainsaws.

In the USA since the 1970's, certain arborists have made a name for themselves by undertaking extreme tree removal operations. With their knowledge of mechanics and tree removal they improved upon and invented new equipment to suit their needs.

Since this specialised arboricultural rigging equipment went on general sale in the UK some six years ago (some specialist items of equipment have been available in the USA since the late seventies), there have been many new improvements and more than a few instances of equipment failure and abuse! That is the reason for this series of articles looking again at fundamentals - to identify the need for training, specialised equipment and to improve the safety and efficiency of rigging operations.

Why should I invest in the new equipment?
It depends on the nature of the operations you are likely to undertake. In the UK if you are still using the traditional method of a 25mm diameter 3-ply polypropylene rope, and if you are unlikely to snatch (top down) anything heavier than a 16inch diameter by 4 foot long piece of Beech, then fine. However, I wouldn't recommend the use of a 'topping strop' in place of a tree block when snatching such pieces, as the impact load and tight bending radius will weaken the lowering line to the point where a game of 'Russian Roulette' is being played. If the rope breaks and causes damage, (in the UK) you could find yourself liable to prosecution under the 'Lifting Operations and Lifting Equipment Regulations '98' (LOLER 98).

The advantages of such a system are simplicity and low initial cost, but these are cancelled by the many disadvantages: -

. The bulky rope is less easy to handle than a smaller diameter braided rope.
. Polypropylene melts easily, reducing strength by up to 50%, and has poor energy absorption.
. Repeatedly snatching on 'topping strops' with heavy pieces further risks rope failure.
. 3-ply construction only lends itself to trunk wraps or a heavy duty friction bollard for holding a load, and it has inherent stretch.
. Trunk wraps are time consuming and tiring, and it is difficult to reduce shock loads by letting the work run.
. Large trees cannot be dismantled without the required safety factors(SF) - this would be to risk rope failure and consequent accident and potential prosecution under LOLER '98.
. Competitors may be presenting a more professional, pro-active image and be operating more efficiently and safely.

All that said, a pulley and lowering device is not always the most suitable option. 'Person-sized' pieces e.g. sections of Beech 12 inch in diameter by 4 foot in length, can be easily handled when anchored from above by a polypropylene 3-ply rope of 16mm diameter. Just using a polyester 3-ply rope immediately gives better performance and is safer than using the same diameter polypropylene rope. Because of the disadvantages of Polypropylene, there is little to recommend it except on water.

How do I know what equipment to use where?
Selecting appropriate pulleys, ropes and lowering devices is becoming a bit of a minefield, with some suppliers offering the products but not always the necessary underpinning knowledge. As a result it is becoming more frequent to see karabiners, pulleys and lowering devices used incorrectly, with some interesting examples of catastrophic abuse. The best way to insure yourself against mis-using equipment is to be trained by an experienced and up to date instructor. This way you should have hands-on experience of various types of equipment and be fully aware of their limitations before purchasing them, and possibly becoming dis-enchanted with them.

A good training course will also educate the trainee about the very important relationship between Breaking Strength(BS), Safety Factor(SF), Working Load Limit (WLL) and Cycles to Failure(CTF). Any one of these factors cannot be changed without affecting the others. This is not as complex as it sounds and the following examples for rope should save a thousand words: -





It can be seen that, as the load approaches half the breaking strength, the load that can be sustained before failure is dramatically reduced. It should also be noted that applying a load equal to 40% of the breaking strength is likely to result in permanent stretch damage (to braided rope), thereby destroying the energy absorption of the rope and its ability to cushion further falls.

Subjecting a rope to loads of only 10% of breaking strength will give lower stretch, more control and fast recovery ready to absorb the next load. Such loads will also give the rope an indefinite life span i.e. it is likely to be removed from service because of obvious signs of wear and tear or age before it is likely to fail. Taking just one piece which is over the WLL will drastically reduce the Cycles to Failure(CTF) to a point that low risk of failure can no longer be gauranteed.

For pulleys, the calculation is carried out in a similar manner but with lower figures for Breaking Strength and Safety Factor: -



However, in practice the relationship between BS, SF, WLL and CTF is a little different. The Working Load Limit is the same as a rope of 10,000kg Breaking Strength with the same CTF but with only half the Safety Factor and Breaking Strength. This is because pulleys and shackles wear a lot harder than textiles at higher loadings. You should choose a pulley with a WLL double that of the rope - if the rope is working with a 1000kg load, 2000kg could easily be acting on the pulley because of reaction forces created as the angles between the legs of rope enter and exit the pulley.

Choosing a suitable anchor sling can be a practical problem. In reality the anchor sling is always going to be the weakest link. Bearing this in mind, the best Safety Factor that can often be applied is 7:1 (1000's of CTF). This is in keeping with the European Machinery Directive minimum requirement for low stretch rope, although this must be increased for extreme situations involving high dynamic loading and valuable targets. To achieve this, the rope used to lash the pulley to the tree should be at least the next rope size up from the rope in the pulley (type for type) i.e one and a half times stronger than the rope in the pulley.

Bending radii of pulleys and lowering devices should be at least 4:1 for running ropes and 3:1 for eye splices. This will only reduce the breaking strength by 15%. Pulleys will reduce rope damage by allowing all of the rope payed out to absorb the load, and avoid abrasion damage that a natural crotch can cause to a rope.

Knot tying is essential for efficient, slack free rigging. Knot tying will reduce the breaking strength of a rope by up to 50%. However, by using the rope to only 10% of breaking strength, this eventuality is avoided. Instead, the rope's cycles to failure are reduced slightly, but still not to a point where failure is likely before retirement. To reflect this thinking, Yale Cordage adopts a 'Safe Knotted Working Load Limit' (SKWLL) to its slings, that is only 12% less than the 'Working Load Limit' (WLL) when used with a 'Safety Factor' (SF) of 5:1.

Single and Double Braided ropes commonly used for tree work are known to decrease with age and use. This is in contrast to a typical kernmantle rope that retains strength for much longer periods. Kernmantel has load bearing fibres heavily protected from UV degradation and internal and external abrasion by an outer sheath. Therefore, wash braided ropes regularly and replace after 3 years of regular use. A kernmantle might be a better choice for everyday rigging ropes if one can be found that knots effectively.

So, to recap on a safe, practical rigging system:
1. Apply a 10:1 safety factor to your ropes
2. Apply a 7:1 safety factor to your slings
3. Apply a 5:1 safety factor to your hardware
4. Apply a pulley of the same breaking strength as the rope running through it
5. Apply a sling to the pulley of one and a half times stronger than the line running through the pulley
6. Never exceed 10% of the ropes breaking strength, and premature system failure is avoided
7. Replace your slings every two years of regular use (If in good condition and never over loaded)
8. Replace braided ropes every 3 years (If in good condition and never over loaded)
9. Hardware only needs to be replaced if dysfunctional or damaged.
10. Use specialised arboricultural equipment for heavy dynamic loading

An example of a matched system:
1. 20mm (3/4") polyester double braid rigging rope 10,000Kg BS
2. 22mm (7/8") polyester double braid dead eye sling 15,000Kg BS
3. 20mm arborist rigging pulley with 5:1 bend radius for the running rope and 3:1 for the dead eye sling 10,000Kg BS
4. System working load limit of 1000 Kg

An expansion of these concepts can be found in further articles in this series, that enable a versatile and cost effective approach to rigging. Stay tuned.