Negative Rigging in Tree work
Modern equipment has made arborist rigging safer and easier than it once was. The tree rigging equipment we stock at WesSpur is incredibly strong, well-designed, and well-manufactured. But just because tree rigging gear has high tensile strength doesn't mean it's indestructible, or that the user doesn't need to understand how to calculate loads and force vectors. The size of the load, number of wraps on the lowering device, amount of rope in the system, and many other factors can alter the simple calculation of rigging forces. A sound understanding of rigging forces and a good work safety plan remain critical in today's tree work.
This information is presented to inform decision-making when purchasing tree rigging gear. This is not training and is no substitute for the study, knowledge, and experience needed to rig safely.
Negative Rigging
Negative Rigging describes scenarios where the load is dropped onto the block from above. The line angle in relation to the load is 'negative'. A negative line angle indicates that the block and anchor are below the cut material, as opposed to rigging with a "high" or "positive" line angle.
Negative rigging results in much higher impact forces than rigging with a high angle advantage. The cut material has to free-fall before it impacts the anchor (in this case, the block) since the anchor is below the load. Free-fall multiplies forces greatly; a 100-pound piece of wood can easily generate 1,000 lb of force or more falling just a short distance.
Negative rigging is unique to arboriculture. Rescue and other industrial rope professionals avoid these magnified forces because of the increased risks. In tree work there is often no other practical choice for a removal, so arborist have to be smart, careful, and well-equipped to deal with the forces generated.
Basic Equipment for Negative Rigging
A basic gear setup for tree removals requires a quality tree rigging rope, lowering / friction management device, arborist rigging block and slings for attaching both to the tree.
We have put together tree rigging kits for removals that cover the essentials, and are specialized for different rigging scenarios (tall trees, big wood, confined space, etc.) Our kits contain:
- Arborist Rigging Rope
- Arborist Block
- Loopie Sling for attaching the block without tying knots
- Friction management / lowering device
- Dead Eye Sling for attaching the lowering device
- Rope bag for storage and transport of the tree rigging kit
- A healthy discount on all the gear
High Line Advantage
Mounting the rigging anchor (arborist block or pulley) above the load creates a beneficial rigging scenario with a "high line advantage" or positive line angle. With the rigging anchor above the cut material, the system can be tensioned to support the load prior to release. This can reduce peak loads and impact on the system.
The same gear used in negative rigging can be used to gain a high line advantage. A nearby tree may provide the opportunity to install a high anchor, eliminating the need for negative rigging. With a high anchor, satellite pulleys can be installed to lessen the load on the main block, reduce swing, and facilitate taking smaller pieces for reduced forces.
Advantages of Using a High Anchor
- + Reduces forces on rigging system and anchors
- + Positive rope angle relative to load
- + Eliminates free-fall of the load
- + Prolongs equipment life
Disadvantages of Using a High Anchor
- - Suitable anchors may not be near removal tree
- - May require climbing a second tree to set anchors
- - Improper force vectors can be created
Conclusion - Negative Rigging is Part of Tree Work
We hope this overview helps you select the right tree rigging gear for your next job. Negative rigging is risky but sometimes necessary technique in tree care. The proper gear and a solid understanding of rigging forces can reduce risk and improve safety for you, your crew, and the client. For more information on negative rigging, and tree rigging in general, check out our books and videos in the shop. Our pre-built tree rigging kits are a great way to get a full set of rigging gear and save money, and can be customized to your liking. If you have any questions about tree rigging equipment or gear selection, please contact us and we will be happy to help.
Stay safe!
