Measuring forces in tree-work with a dynamometer?

[27RMT0N]

This is something I've spent a fair amount of time thinking about recently. Partly because a lot of my work involves pulling over trees and partly because COVID-19 has given me a lot of time, in which I've found the YouTube channel HowNOTtoHIGHLINE ( https://www.youtube.com/channel/UCQvq-0fss4lNrmIz7gcPLtQ ) where they do a great deal of dyno testing of slings, carabiners, bolts and so on to find their breaking point, as well as testing the forces when taking huge drops and swings on ropes. It isn't tree work, but it is some pretty cool content.

To the best of my knowledge, the only tree-specific YouTube videos I've found regarding actual force-testing has come from Patrick Brandt ( https://www.youtube.com/channel/UC-X5KWesbmpDr8-w0Jcn_7g ), one or two videos from August Hunicke doing some dynamic testing while lowering some pieces and one other person who I'm struggling to find right now, but those were mostly static tests of slings on a bench-test. Am I missing anyone?

What interests me is knowing the amount of force a person can apply by hand, with a 2:1, 3:1, 4:1, with a Maasdam Rope Puller, With a Wyeth-Scott More Power Puller, with a truck, etc. How do knots compare to a prusick in terms of holding strength? Does throwing a pulley into the mix really double the force and does the type of rope make a difference? I'd love to see numbers and so far haven't found them at the level of detail I'd like.

Right now the two dynos I know about that may work for learning more about this are the Rock Exotica enForcer Load Cell and the lineGrip lineScale 2. Are there others I'm missing? The lineGrip is both cheaper and measures greater forces, but the enForcer has a higher refresh rate. I'm just about curious enough to spend the $650/$875 to buy one of these tools and find out, and of course make some videos of what I find.

Is anyone else interested in this as well?

 

[Tom Dunlap]

There’s a rule of thumb for rope/pulley mechanical advantage or even pulling 1:1 on a rope. You can generate about 60% of body weight pull. ie—->200# person pulls 120#. I found this in many search n rescue teachings.

 

[JeffGu]

@Richard Mumford-yoyoman has a RE Enforcer... or three... or five... and uses them in some of his videos.

 

[39Buick]

@Mark Chisholm did a very interesting video on various anchor points.

 

[27RMT0N]

@Richard Mumford-yoyoman has a RE Enforcer... or three... or five... and uses them in some of his videos.

Thank you, that was the other person I was thinking of but wasn't remembering.

A few interesting videos around for sure, and Marks SRT forces one is very good. I guess I'm still searching for dyno measurements of various pulling methods/tools. Maybe no one has made such a video yet?

 

[surveyor]

I have pulled 2500 lbs with a 3:1 setup with 2 non bearing pulleys and a maasdam rope puller. The maasdam was not slipping, but I was about at my limit for cranking on the lever of the maasdam.

 

[Mark Chisholm]

@Mark Chisholm did a very interesting video on various anchor points.

Thanks. Was just some ideas I was wondering about.

 

[Tony]

Dr Kane at UMass Amherst has done a good bit on climbing system forces, but you’ll have to research his articles as he does not post videos that I know of.

I have worked with the enforcer and it is fine, but the bluetooth range is underwhelming. Also worked a good bit with Dillon Dyno’s. Tough as nails and nice, but big.

Straightpoint makes some seriously nice stuff including a dyno integrated into a DMM impact block. The remote function is awesome. Lot’s of $$ though

Fellow NATS trainer did some solo research on efficiency in MA systems. I’ll see if I can find it. No promises.

Tony

 

[Bart_]

Petzl published a document showing less than 2:1 force from base anchoring. Mark's video base anchors on a pulley tip, completely removing the bollard effect at the tip. In his redirect at least its a biner, but not a natural crotch which has yet more friction. A bit of a bug a boo I'd love to see cleared up.

For safety inline I suggest two inline knots binered to the enforcer, so if the enforcer breaks its not life support. The slack between the knots would take up the load after a small drop. Separate the up and down rope legs measurements, or any other rope segments similarly. Then vector add the numbers.

More on topic, you can estimate tension losses a bit via pulley efficiencies, bushing vs bearings, fat ropes on tiny pulleys or over large diameter pulleys. Rope bend radius messes with the results. Check some rigging or engineering texts then you can know what to expect in your measurements. Also find the rope grab vs prussic sheath damage info that's floating around on the web. There's one quite long video I saw quite a while ago.

 

[Daniel]

 

[Tom Dunlap]

Video unavailable

 

[Daniel]

you need to watch this on youtube as embedding is not allowed.

Just click "watch this video on youtube"

 

[Bart_]

Somewhere or other I did a post kind of like fall factor but not jerk/sudden stopping, instead letting it run to achieve 1 G or 2 G etc deceleration to a stop within a distance, rule of thumb eyeballing type measurements. It yielded a limiting of achieving G deceleration based on CofG fall distance and stopping distance so you could use the G's to multiply the piece's weight and guide yourself to not exceed 20% working load for your rope size and see how far the fall/stop should use up to stay within limit. That calculation could turn into a chart (for any keeners out there).

That said, there is the dynasorb(or similar) x length of rope in system to help out like in Daniel's video. A definite factor. Especially if the groundsman muffs up, not necessarily through their own fault, you never know. Can I say Murphy's law can strike? Don't be offended I just walked into that one and couldn't not say it.


edit: did the figuring again. log falls distance under 1G. If you put "brakes" of 1G on rope log continues same speed no net up or downward force. Put 2 G's on the rope one is used up on gravity and the other gives exactly the same gusto as the free fall. So free fall one log length before rope tension, apply 2G's on rope and log stops in one log length of rope run. So the rope tension is double the log weight. Let it run longer and force drops to less than 2 G but always greater than the original weight. Try to stop the log in roughly half the log length and you probably hit 3 G's, 1 for gravity and two for stopping the log so your rope sees triple the log weight.


Upshot - for moderate rigging, fall one log length, run one log length you put double the log weight into the rope. So 5:1 should really start out 10:1 choosing log weight vs rope strength because you're likely to double up the force off the get go.

This meshes nicely with Daniel's data showing snubbing into plenty of rope peaking at about double the log weight. The plenty o rope can save you from peak snubbing forces. Thumb rules rule.