No.10 Smooth Lines
Roger Barnett, an infamous Pacific North West rigger once said that in all his years of roped rigging he has never really known the weights of wood that he lifts, pulls and drops.
Roger has a huge understanding of his tools and can creatively manipulate situations to his advantage because of it.
When rigging trees tool knowledge is extremely important, as is an understanding of basic physical laws, as is an understanding of tree morphology and risk assessment, as is an understanding of component configuration.
The list goes on, there are many connected 'parts' to successful rigging.
Knowledge of tree weights is indispensable too.
A green log chart and measuring tape is one way but through crane-works we can learn to guess the weight of tree sections quickly and surprisingly accurately.
Crane work is 'static', a 300kg load will always be 300kg, assuming the work is done correctly there will be no peak loading.
That same 300kg load in a roped scenario can be made to exert more or less weight on our rigging and anchor points.
Weight reductions are made by not hanging or catching the load but by letting it move at speed to the ground.
The ground will take the weight impact and the kinetic movement will be changed to thermal at the rope's connecting points, the pulley and friction device.
The diametric opposite is a 'snatching' scenario when our 300kg load will exponentially increase in weight in relation to its distance of fall.
'Running rigging' and 'snatching rigging' are two poles of loading examples with many possibilities in-between.
Before I go any further I must say that rigging is a deep and complex subject and knowledge of it will improve your style and safety while being immensely enjoyable.
I encourage everyone to seek out training alongside their own study and practice.
This article is about Friction and an ability to control it through the use of a lowering device and its subsequent change to thermal energy and the affects that it has on synthetic components.
Friction force in a rope depends on three things:
i)The load on the rope.
This is whatever the rigger decides is suitable for the strength of the equipment, strength of the tree, type of drop zone and skill level of the groundsmen.
ii)The coefficient of friction.
This is the relationship between the surface area of the rope against the surface area of the lowering device.
The frictional coefficient isn't affected by the size of the device.
With one wrap of rope around a device the estimated ratio is 10:1.
Each additional wrap increases the holding power exponentially, so that two wraps will generate a 50:1 holding power and three wraps generates 250:1.
For a 300kg load 30kg of holding force is required with one wrap, 6kg with two wraps.
For anyone that has guessed the amounts of wraps incorrectly and shock loaded a climber off his spikes, this is an intuitive act.
iii)The angle that the rope turns through.
'Mechanical Rope Advantage' and 'Rope Angles' should be studied to achieve optimum power and strength.
The same three characteristics can be used to think about kinetic and thermal energy in relationship to rope.
i)The load on the rope.
A light load lowered quickly may cause more thermal damage than a heavy load lowered slowly.
A heavy load that is 'snatched' will cause thermal damage in specific areas of the rope and possibly overload a Safe Working Load.
The scenarios are endless.
ii)The coefficient of friction.
Too few wraps may cause thermal damage due to speed.
Too many wraps may cause thermal damage in specific areas of the rope and possibly overload a Safe Working Load.
iii)The angle that the rope turns through.
A larger Bend Radius spreads the amount of rope exposed to thermal energy.
It encourages MA to be added due to its ease of use.
It enables a larger diameter of rope to be used.
A pulley and friction device potentially makes a 'frictionless system'.
A ground worker can add friction at the device on a scale from 0%-100%.
He can add power into the system for pretensioning and lifting.
This is what we aim for in a typical rigging system.
Buckingham 'Porta-wrap'
A simple device that is usually tied to the bottom of the tree like this.
It is available in three sizes and three coatings. The picture shows Small with 12mm rope, Medium with 16mm rope and Large with 18mm rope.
It comes in steel with powder coating (black), steel with nickel plating (silver) and aluminium (grey).
Aluminium is a heat sinking material and works positively to draw thermal energy away from the rope.
Aluminium is lightweight but will show wear much faster than steel.
It can be tied to the tree in different ways. It always pivots and this is accommodating for difficult shaped trees and the consequential rope angles.
For me the great thing about the porta-wrap is that it is small enough to carry on my harness and use it for various techniques in the crown.
When tied next to a climber it can be climber controlled or the right amount of wraps make it a hand's free device.
It doesn't have a 100% frictionless potential but for many rigging scenarios this is not necessary.
Hands free rigging is an interesting area and can change loading forces dramatically.
Two fixed bollards: Treeworker FRICTION TUBE & Stein RC3001.
Bigger and faster rigging calls for a more substantial bend radius and both of these devices are aimed at heavy rigging.
One man is able to quickly set up the TWFT.
it is mounted with two ratchet straps that give the single plated device a very secure mount on a variety of trunk profiles.
The back plate has a slim profile which gives the impression that the designer focused their ideas on a hugely strong device that performs well where it matters....the tube.
There are two mounting methods in-built with the added flexibility of 'cutting in' for the most secure and heavy rigging.
The rubber mounts can be removed with a socket wrench to expose the top visor plate, which can also be removed so as not to damage bark and cambium
Buckingham recommend 19mm maximum on the Large portawrap.
Look at the bend radius on the TWFT, it is easy to see how thermal energy will be more intense with a smaller bend radius. The picture shows the result of heavy rigging on too small a device, 15m of heavily glazed rope.
one man is able to quickly set up the rc-3001.
It is mounted with one ratchet strap. Stein recommend the use of an additional sling at the bottom of the device.
The device can be easily 'cut-in'.
Stein recommend a maximum rope diameter of 19mm.
It has two rubber mounts. The blue powder coating chips away instantly but the whole device is zinc plated. The blue colour is for aesthetics only.
Mechanical Advantage
All these devices can add power into a rigging system due to their frictionless potential and the larger the bend radius on the friction device and pulley, the easier this is.
Good Rigging Control System/GRCS
The GRCS has an articulated back plate made up of three sections.
One plate holds a ratchet strap.
The middle plate houses a Harken sailing winch or an aluminium tube.
The third plate is a catch for the hooked end of the ratchet strap.
several additional items are available at extra cost.
A visor plate can be added to make it very secure for heavy rigging.
It can be mounted on a tow hitch and a gas powered drill can drive the powerful sailing winch.
Tree felling, stump pulling, skidding and of course tree rigging are all possible with the GRCS.
The massive in-built mechanical advantage can make very easy work of large tree removals. It can also get a novice into huge difficulty with potentially fatal consequences.
It is common for those new to the GRCS to exert so much power into a rigging system that anchors are snapped out.
Training should be sought out.
A proficient rigger with a GRCS system are only limited by their imagination.
The aluminium bollard can be filled with ice packs to add even more heat sinking properties when very big or fast rigging is needed
Statistics
Mike Popham, an equipment inspection specialist, recalls watching a steel karabiner being pull tested to destruction at the ISC factory in Wales, UK.
As he watched the obvious deformations as it approached Maximum Breaking Load there was a CRACK like sound.
Mike tells the story that a part of the karabiner broke the speed of sound, wether this is true or not, it is certainly a reality that an overloaded metal rigging component made to fly through the air from an over loaded work scenario will have such momentum to pass right through the soft fleshy body of a climber!
Ropes should break first! The is a rule and don't forget it!!
The table shows that the mini, normal and large porta-wrap will all break before their maximum rope diameter will, or rather on paper it does.
The statistic for Yale's Polydyne is of a static straight pull, a rigging rope instantly loses strength through deforming knots and bends.
With this in mind I would say that perhaps only the large should be downgraded a size to 16mm.
It is a personal choice at the end of the day and I hope that all riggers make safer ones when deviating from manufacturers recommendations.
I aim for smooth lines for my ropes by using splices instead of knots and large bend radius' at the pulley and friction device, with my style of rigging I would use a maximum diameter of 14mm on the large porta-wrap.
Either way you choose it it is imperative to know your tools intimately when undertaking large rigging.
Glossary
Bend Radius: the ratio of rope size to object it passes around.
Cutting-in: cutting a slice of the trunk away to fit the lowering device into the tree.
Mechanical Advantage: rope passing through a series of pulleys amplifies the force applied