There are plenty of different friction devices for rigging. The most commonly used one in New Zealand is the Port-A-Wrap, but there is a big variety of friction devices available and all of them have pros and cons.
Choosing the right friction device for a rigging system is an essential part of efficient rigging operations.
In the last few months I was often asked if there was a way to know how many wraps you need to apply for controlling a specific load weight with an friction device, but unfortunately there is no simple rule of thumb. The amount of wraps needed to control a load depends on the friction device, the type and condition of the rope and the rigging system in use. The weight of the load is only one of many factors that have to be considered.
Examples of important factors:
- A wet rope tends to produce more friction than a dry rope, so on a rainy day I may have to use less wraps than on a dry day for the same load with the same friction device.
- Ropes that are dirty with gum and mud will produce more friction.
- A rope running through a natural crotch produces more friction than a rope running through a pulley or block.
- The bend ratio of the rope around the friction device will influence the friction majorly. The smaller the bend ratio the greater the friction.
- The position of the load in means to the last rope redirecting point in the tree will influence the force of the load.
When belaying the friction device it is important to look at the whole rigging system – only when all components of a rigging system line up you will achieve a smoothly running rigging operation. In a modern rigging system we use pulley and arborist blocks to reduce the friction everywhere in the system and concentrate the friction at the friction device. By concentrating the friction at the friction device we can better control the load. All friction devices work in the same way; the more wraps and the greater the holding force (ground staff holding the rope), the more weight can be controlled.
The best way to learn how to control a load with an friction device is to use it. Starting with lowering light loads will demonstrate in which way the friction device reacts to the load. It will improve our skills as a tree rigger to lower every load with the friction device, regardless how light the load is. By holding the rope slightly between thumb and index finger, you can feel the movement of the rope better than when holding the rope in clinched fists. To increase the friction on the friction device you can squeeze the rope between your palm and your remaining three fingers. If we notice while lowering that we have applied too many wraps to the friction device we can flick these off by doing a step forward to create slack in the rigging rope and this slack can then be flicked off the friction drum with a circular movement of the wrist. When getting prepared to lower a load, do not lean back into the rope as this would slow down your ability to react quickly. Stand straight with slightly bend knees and feel the movement of the rope with your fingers. This will allow you to put more force towards the friction device or move forward to slack the rope to reduce the friction on the friction device.
Lowering with an friction device is a skill that requires training and practice. Only through a team effort between climbers and ground workers can a safe tree rigging operation be ensured.
Some simple rules to follow when rigging with an friction device:
- All friction devices (bollards, pipe wraps, winches) used at the base of the tree should be used with a sufficient safety factor*.
- All lowering of loads are done by the ground workers (not the climber in the tree).
- Before lowering a load the ground worker should look for good footing and assure the rope can run freely through the friction device.
- The ground worker who is lowering the load with the rope should wear leather gloves to avoid rope burn on the hands.
- All loads are to be lowered dynamically to minimise shock loading of the tree.
To dynamically lower a load we let the rope run and decelerate the load while it is falling. We can use all the space between the last redirecting point of the rope in the tree and the object that we are protecting for the slowing down. Increasing the rope in the system and decelerating the load slowly is the best way to minimise the negative forces in a tree rigging system.
- The lowering rope in a rigging system should always run through a friction device and not be just hand held.
- The ground worker on the rigging rope should leave at least 3m of rope between him and the friction device.
- Never wrap the lowering rope around body parts.
*The general safety factor for arboricultural tree rigging components in New Zealand is 10/1.
Published: (Tree Matters, 2010,12/3)
Author: Andreas Ross