Aerial Friction in Top-Down Rigging Scenarios

If we reduce aerial friction at the rigging point, where does that friction go? The author ran a series of tests to try to find out. This drill was used in the tests to run rope through the system at load. Photo courtesy of the author.

Arborist technical rigging is often as simple as a rope, a branch union and a few trunk wraps, or it can grow to complex systems with multiple lines converging from multiple points to lower large and small tree parts. Of all the tools and techniques we apply, rigging setup and load management are often the most impactful. The weight and forces generated can build quickly, and, if not managed correctly, can lead to system or tree failure.

The concept of friction is universal in tree-rigging operations. It is at once friend and foe. No matter which, it must be managed for safety and efficiency. From our humble beginnings, friction inundated our rigging systems. Using branch unions added friction that varied depending on rope type, union size and tree species. Trunk wraps added more friction as necessary, but again were variable and at times unpredictable.

Arborist rigging blocks entered the picture and allowed less-restrictive placement, including placement of rigging points almost anywhere in the tree. Trunk-mounted, or basal-mounted, friction devices became necessary as aerial friction decreased, and thus our systems changed.

New horizons

In the last few years, some rigging devices have emerged that once again introduce friction into the ring system at the rigging point. While varied in form, the function is much the same; absorb some of the force generated in release of potential energy at the rigging point through added friction. Often in training courses, students will claim that the added friction lessens force in the system. This statement, while on the surface appearing true, is not entirely accurate.

We know through measurement that aerial friction does decrease the amount of force at the rigging point. However, as the laws of thermodynamics tell us, force is neither created nor destroyed, only changed in form. Therefore, while less force is generated at the rigging point, it must go elsewhere. This was the genesis of the rigging trials and tests conducted in Kent, Ohio, in the fall of 2018 and presented in my session on this topic at TCI EXPO 2018.

Questions

In looking at rigging and aerial friction aloft, I decided to use top-down or rigging-point-below-the-load scenarios. I did this mainly because these are the rigging scenarios that can and do generate the most force on the tree and the system.

I wanted to see if these systems cause a significant amount of rope abrasion. I also wanted to look at the temperature generated and see if it was a factor of concern for stability and/or efficiency. Lastly, I wanted to see if the increased friction at the rigging point altered the way the tree was loaded during the fall of the load. To do this, I devised a series of tests and executed them with the help of two excellent fellow arborists, Rick Denbeau and Alan Kraus, both CTSPs.

Device tested

While I had intentions of using a number of the devices currently on the market, due to time and some logistical considerations, I was only able to run the X-Rigging SafeBloc through my trials. A big thanks to Sherrill Tree and Brandon Nance, Sherrill’s technical advisor/trainer, for the help and support.

Rick Denbeau with the drop-test setup. Photo courtesy of the author.

Abrasion

To test abrasion, we set up a fixed loop of rope through the device, then tensioned the system to 200 pounds with a mechanical-advantage system. A GRCS (Good Rigging Control System) powered by a three-quarter-inch electric drill was used to run the rope. In all, we cycled 1,000 feet of rope through the system. As expected, while some abrasion happened, we did not feel it was at a level to compromise system integrity. Other factors in the rigging system would damage rope more readily than abrasion at the device.

Temperature

Temperature tests were performed two ways. First, we checked the temperature at the device as we cycled the rope through for the abrasion test. For this, we also increased the load on the system to 300 pounds for the last two cycles. Second, we checked the temperature during the 10 rigging cycles of our drop tests. Temperature was tested with an inexpensive infrared thermometer.

Again, as we expected, we found the temperature did increase. It slowly built throughout the tests. We stopped the testing due to time constraints well before any temperatures that would compromise rope strength were reached. Generally speaking, while heat is generated, reasonable loads for the system, along with a reasonable rigging-cycle time, should negate the adverse effects of heat generation. If ambient temperatures are high, this may be a greater factor. It is also wise to conclude that the device can heat up enough to burn when touched, so caution should be taken when hauling the device between cycles.

When we increased the load by 100 pounds, the temperature generated jumped accordingly. Again, we found neither excessive force nor excessive heating in reasonable rigging scenarios. However, caution is warranted with extremely heavy loads, as rope damage is a very real possibility.

Infrared thermometer.
The temperature of the X-Rigging SafeBloc was checked with an infrared thermometer Photo courtesy of the author.

Force

We wanted to try to determine when load came to bear on the system as a whole and whether the force was different when an arborist block was used with the same load. To test, we repeatedly dropped a 200-pound load into both systems and videoed the stem movement and rigging-system response. We did not measure load at the rigging point. This has been done in the past, and we did not want to increase the distance of fall by adding a dynamometer under the block or aerial-friction device.

While there was some difference in stem deflection between the two systems, we were unable to draw any substantive conclusions with our limited capabilities. Further studies to look at this vital aspect are being pursued. The system did load differently, as observed by the amount of “shake” the climber experienced. Again, further testing is required before drawing any conclusions.

We noticed another factor that affected system load. When the load was pushed into the system with the arborist block, the rope backed through the block. The slack generated was taken up by the ground worker. When the load was pushed into the SafeBloc, the slack generated by distance of fall could not be managed by the ground worker. The friction at the rigging point prevented it. Therefore, the load hit the whole system harder and appeared to pull the stem forward more than with the arborist block. The arborist block tended to load the stem more in compression, while the SafeBloc pulled the stem forward, creating greater bending movement.

Conclusions

In general, we found the use of aerial friction to not excessively increase rope abrasion. While increased temperature is to be expected, it can be managed with reasonable measures and good judgment, especially when it comes to load-weight management.

In rigging-point-above scenarios, aerial friction offers benefits in friction management as well as decreased kit and clutter on the ground for moderate loads. If speed and large swings are a necessary part of the rigging plan, less friction may be better if rigging-cycle times are short.

SafeBloc.
This was the setup for the abrasion and temperature testing. The rope showed no significant wear, but had some coloring from the SafeBloc. Photo courtesy of the author.

In rigging-point-below scenarios, our trial indicated stem loading and slack management are issues to be addressed. There is also a slight chance the rope can become caught between stem and device in these scenarios. While rare, it has been reported and should be taken into consideration. As stated earlier, more research is needed and has been proposed before any firm conclusions can be drawn. Having said that, there seem to be no real gains in aerial friction in top-down or rigging-point-below scenarios, and there may be a few possibly significant disadvantages.

As always, choose your systems wisely, plan accordingly and work safely.

Load applied with mechanical-advantage system. Photo courtesy of the author.

Tony Tresselt, CTSP, is director of safety and training for Arborist Enterprises, Inc., an accredited TCIA member company based in Lancaster, Pennsylvania. His travels and training can be followed at http://gravitationalanarchy.wordpress.com/

This article was based on his presentation on the same subject at TCI EXPO 2018 in Charlotte, North Carolina. To listen to an audio recording of that presentation, go to this page in the digital version of this issue of TCI Magazine online, under the Publications tab at http://www.tcia.org/, and click here

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