Friction Saver Friction -- some measurements

[moray]

Anyone who has climbed DRT with a friction saver knows it reduces friction dramatically compared to tree bark. A pulley likewise is dramatically better than a friction saver. A couple of days ago I decided to actually measure the friction of a few devices.

Partly I was motivated by the diagram below, from Petzl, part of a large amount of info they supply if you buy one of their pulleys. The pulley in this case is a so-called rescue pulley, a model with ball bearings and low friction.

From the diagram it appears that 1.1 lbs are needed to lift 1 lb of load, which is an efficiency of about 91%. It is interesting that they show the efficiency of a carabiner in the same setup to be only 50%.

I used the same setup for my measurements. The load was a 20-lb dumbbell, the rope was a 1/2 in climbing rope, and I was standing on a pretty good bathroom scale to take the measurements. To get more bang per setup, I did the Petzl-style measurement while pulling down on the rope, but then took another measurement while slowly lowering the weight. It was the difference between these two measurements, in lbs., that I recorded; later I combined them mathematically to get the efficiency.

The measurements turned out to be more reproducible than I expected: for a given device I could take measurements 4 or 5 times and record the same difference every time within a pound or less for high-efficiency devices, and within 2 or 3 pounds for the low-efficiency devices; the averages in the table should be a bit better than that.

The results are shown in the table below.

The results surprised me. The first surprise was the poor performance of the micro pulley, which I had climbed under a number of times. The other pulleys waste 1/3 as much energy. Second, the aluminum rings, at under 50% efficiency, were much worse than I imagined. This would mean that footlocking or hip-thrusting under the rings would cost you 4 feet of work for every 3 feet of actual height gained. No wonder SRT feels so clean and efficient.

 

[Adkpk]

I have climbed on the rescue pulley and I went immediately back to the friction saver. I was footlocking and with every upward advance there was close to equal descent. The friction saver did some of the work as to hold a little to allow me to advance my hitch without slipping back down. Of course I have learned a few tricks by now and should try again using a different hitch and maybe a slack tender. With the spring coming it time to dust off the climbing sack and get up some tree. Can't wait.

 

[Mikecutstrees]

thats cool, I have noticed myself the huge difference a smaller smooth TIP makes compared to a larger rougher TIP.... Thanks for the experement and results....

 

[D Mc]

Moray, thanks for posting the numbers. Interesting information. I have been climbing with a pulley as my tie-in point a little over a year now and absolutely love it. I am reminded just how well it works when I can't use it and have to use the friction or rope saver.

I think it is the wave of the future.

D Mc

 

[ATH]

Wow...makes the pully look pretty attractive!

It'd be interesting to try the same measurement with the rope over a few different species and diameters of limbs (and even some crotches that the rope ends up in). Obviously harder to have the scale on a good solid/level surface when the rope is in a tree...

So, if the assumption (unmeasured at this point??) is that a friction saver reduces the friction by 50%, that means if you use nothing, you are lifting 40 lbs to move 20lbs, right?

Even if the friction saver saved no work, I still think they are better than climbing on the bark alone (cambium saving, and reduced wear/tear on the rope for those who aren't interested in the tree's bark...)

 

[moray]

...It'd be interesting to try the same measurement ...

So, if the assumption (unmeasured at this point??) is that a friction saver reduces the friction by 50%, that means if you use nothing, you are lifting 40 lbs to move 20lbs, right?

I hope I am understanding your question correctly... The measured friction of the aluminum rings is such that it takes 40 lbs to lift 20 lbs. The rough bark of a tree limb will be much worse--maybe it would take 80 lbs to lift 20. I do intend to measure that, and do some experiments with a Port-a-Wrap as well. Stay tuned...

 

[ATH]

Yeah...poor wording on my part. I'll look forward to seeing more!

 

[tomtrees58]

yes works good but every tree is different tom treesopcorn:

 

[pdqdl]

Rethink your analysis !

Anyone who has climbed DRT with a friction saver knows it reduces friction dramatically compared to tree bark. A pulley likewise is dramatically better than a friction saver. A couple of days ago I decided to actually measure the friction of a few devices.

[parts deleted for brevity]

From the diagram it appears that 1.1 lbs are needed to lift 1 lb of load, which is an efficiency of about 91%. It is interesting that they show the efficiency of a carabiner in the same setup to be only 50%.

I used the same setup for my measurements. The load was a 20-lb dumbbell, the rope was a 1/2 in climbing rope, and I was standing on a pretty good bathroom scale to take the measurements. To get more bang per setup, I did the Petzl-style measurement while pulling down on the rope, but then took another measurement while slowly lowering the weight. It was the difference between these two measurements, in lbs., that I recorded; later I combined them mathematically to get the efficiency.

[parts deleted for brevity]

There is a bit of an error here, unless I have misunderstood your test measurement method. When using DRT (Double Rope Technique?), the forces are not applied as shown in the diagram you provided. SINGLE rope technique offers 1:1 lifting "mechanical advantage", but using a pulley (of any efficiency rating) gives a 2:1 theoretical advantage, because the load (the 20 lb dumbell) is not being lifted by an outside source (you, on the scale).

A single pulley arrangement always puts a 2:1 mechanical advantage (theoretical) on a system. As shown in your diagram (using a frictionless pulley), you would be applying 40 lbs of downforce on the pulley anchor point, with matching loads on the lines. Keep that in mind, if you ever have ground men pull on your "down" line to help you up the tree: it MORE than doubles your weight as the force on the tie in, due to your weight, the friction, and their downward pull.

In DRT, as you pull DOWN on the rope with say, 100lbs force, you are reducing your own weight load by the amount you are pulling down with. Using a 100% efficient (frictionless) system means that a 200lb man can climb the tree using only arms strong enough to pull down with +100lbs of force.

A consideration: Since the tree bark adds friction to the DRT, it often enables a fellow to hold the rope with only one hand, while the other hand moves the friction knot (or ascender) up the line to hold position for the next pull. Without that extra friction (Ex: using a pulley), many of us couldn't pull ourselves up the tree.

I use an ascender, because by growing middle is outgrowing my arms, and the force applied lifting the friction knot overcomes my "holding" arm. After I go up farther than I wish to fall from the ascender, I use two hands to slide the friction knot up the line while I hang from the ascender. No footlocking required! Maybe not a good technique for some of you strong guys, but it works for me.

For a better measure of how a pulley saves effort over a friction saver or just the tree bark, rig yourself from some height, and use a spring scale rigged to your pulling line, and re-do the whole test.

 

[ATH]

.......using a pulley (of any efficiency rating) gives a 2:1 theoretical advantage, because the load (the 20 lb dumbell) is not being lifted by an outside source (you, on the scale).

A single pulley arrangement always puts a 2:1 mechanical advantage (theoretical) on a system.........

A fixed pully changes direction only, it offers no mechanical advantage. To get MA from a single pully, the pully needs to move.

 

[moray]

There is a bit of an error here, unless I have misunderstood your test measurement method. When using DRT (Double Rope Technique?), the forces are not applied as shown in the diagram you provided. SINGLE rope technique offers 1:1 lifting "mechanical advantage", but using a pulley (of any efficiency rating) gives a 2:1 theoretical advantage, because the load (the 20 lb dumbell) is not being lifted by an outside source (you, on the scale)...


In DRT, as you pull DOWN on the rope with say, 100lbs force, you are reducing your own weight load by the amount you are pulling down with. Using a 100% efficient (frictionless) system means that a 200lb man can climb the tree using only arms strong enough to pull down with +100lbs of force...

There are two separate ideas here--one is the friction of various devices, and the other is the practical effect of using them in a standard DRT setup. The only thing I measured--indirectly--was friction. The actual specific friction of a device in a particular setup is probably of no interest to anyone, so I calculated efficiency instead. This is the useful parameter to know for understanding all sorts of setups.

How does all this work out in the standard DRT setup? ATH is exactly right to point out there is no mechanical advantage inherent in a pulley: it merely changes the direction of a force. The advantage of a single pulley comes when the load is attached to the pulley and the pulley moves. The reason is that the rope (and the applied force) move twice as far as the load. That's where the "2" in "2:1" comes from. And that's why DRT gives you the 2:1 advantage you describe--it is just a special case of the moving pulley, even if the "pulley" is a tree limb. The "pulley" may not move, but the rope still moves twice as far as the load.

What about a DRT setup if the "pulley" is a friction saver with 2 aluminum rings with an efficiency of 50%? As your 200 lb. climber steadily pulls himself up hand over hand, we know, because the rings are only 50% efficient, that the force on the climber's arms is twice the force on the other leg of the rope, just as my experiments (and the Petzl diagram) show. The weight of the climber is exactly equal to the sum of the tensions in the two legs, so 2/3 of the climber's weight is supported by his arms. If the climber goes up 3 feet, the rope has moved 6 feet. The arms, doing all the work, have raised 2/3 of the weight 6 feet, or, equivalently, all the weight 4 feet. There you have it. Four feet of work for 3 feet of gain. One-quarter of the energy has been wasted.

 

[moray]

More Data

I now have some friction measurements of rope on wood. Since there are still 2 feet of snow on the ground outside, getting the measurements outside was going to be way too hard. Instead I chose to use stove wood rigged up with a couple of sturdy hooks. The rope is the same one used in the earlier set of measurements of rope-on-metal friction, and the method used was the same.

In this set of measurements there was no attempt to be extremely precise. The measurements were not as steady and repeatable as they were with metal, and in any event I just wanted a rough idea of how wood compared to metal. The data clearly show, for the wood I had ready at hand, that the friction is only slightly greater than for a friction saver with 2 aluminum rings.

This surprised me. Even though the data don't lie, I don't think they are a good fit to a real-world DRT situation. Two major reservations occur to me.

The first can be illustrated with the measurements on the paper birch, which had the lowest friction in the 5 tests. I have 2 birch trees in my back yard. Even though the bark is nice and smooth, there is no place on either tree where I could rig a DRT rope over a smooth horizontal limb. The rope has to go in a crotch, and crotches are invariably very rough. They are rough and hard to the point of grabbing at the rope, producing "friction" that obviously greatly exceeds that of 2 aluminum rings. My simple tests involved a configuration that is probably rare as a DRT setup.

This leads to the second reservation. With a climber's full weight on a DRT rope, the rope flattens and deforms where it goes over the support. If the support is smooth metal, this should have little effect compared to the light loads in my testing, except to increase the friction in proportion to the force. In the case of tree bark, where surface irregularities are typically much larger than the diameter of a rope fiber, the rope may skate over these irregularities in light testing, but under heavier loads fibers will begin snagging and breaking, and the bark promontories will begin abrading away. All of this is extra "friction" that prevents my light-load results from scaling up properly.

Tomorrow I will try to post results of another experiment that investigates the effect of large-scale irregularities.

 

[pdqdl]

You are one dedicated researcher. Keep it up !

I am looking forward to seing your results.

 

[046]

moray... nice work!!! thanks for sharing

 

[reachtreeservi]

Morey, I just go to the saw shop when it snows. But You....

I enjoy reading your posts , keep it up !

 

[moray]

Thanks for the kind comments, everyone!

This last experiment is in some ways the most interesting to me. Even if it doesn't definitively answer any particular question, it has a strong bearing on the question of how friction of a rope around a limb actually works--is the friction related to the amount of contact surface, or to the size of the limb, or to the number of wraps? What is the "contact surface", anyway?

One of my stove logs in the previous experiment was a piece of locust with deeply furrowed bark. I expected a lot of friction from it because of this, but it differed only slightly from much smoother logs. It had been under the snow all winter, and the bark seemed a little soft to me. Perhaps the softer bark produced less friction. There were just too many variables floating around: hardness of bark, species of wood, amount of resin in the bark, presence and size of furrows, etc. To control all this, I decided to make my own "bark".

It was easy to strip the bark off the birch log that had been used earlier. A router was used to cut 6 parallel longitudinal grooves to simulate bark grooves, but a control section was left ungrooved. To remove any sharp edges, a chisel was used to chamfer both edges of each groove. The entire debarked area was then hand sanded with 80 grit paper. Finally, the groove edges were sanded to give them a soft rounded profile that would not interfere with rope movement in any way. The photo shows the log ready for testing.

The standard friction test was peformed on both the grooved and ungrooved sections. Both sections were subjected to 6 or 8 pulls to get the readings to stabilize (The friction declined slightly during this preliminary as the wood took on a slight polish.) The chart shows the results.

After the test I measured the width of all the grooves where the rope had crossed them. The sum of the gaps was 2.6 in; half the circumference of the entire log was exactly 6 in. Just over 43% of the contact surface had been removed to form the grooves, yet the friction, compared to the ungrooved control, was virtually identical. Clearly, the contact area, when everything else is held constant, has little or no relation to friction.

 

[Nailsbeats]

Word. I think that is an interesting experiment with and interesting outcome. I am gonna go out on a limb here, lol, and say that the rope surface has the most effect on friction. The kind of rope you use (I mean style like braid and material, not diameter), not the kind of tree your in, in most cases. Granted your line is over a horizontal limb and not a crotch, or bound up in pine sap. It seems we use a lot of natural crotches and different style ropes "run" differently. They seem to cut a groove and go. Just a bullsh*t guess.

 

[pdqdl]

...yet the friction, compared to the ungrooved control, was virtually identical. Clearly, the contact area, when everything else is held constant, has little or no relation to friction.

That's a pretty remarkable conclusion, given that it flies in the face of how most people think friction works.

When I took physics (many years ago), formulas using coefficients of friction, mass, force, etc were used to demonstrate exactly the the same point that you have found by experimentation. Over the years, I have tried at different times to explain that principle to others less educated, and I think they usually concluded I was off my rocker. My hat is off to you.

But...it might have been easier for you to read a book on physics. I have never tried, but I'll bet some ambitious researcher like yourself has developed a table of some sort for the coefficient of friction for braided rope on different types of bark. It would take some really deep research to find it though, so I think I'll pass.

 

[moray]

...I am gonna go out on a limb here, lol, and say that the rope surface has the most effect on friction. The kind of rope you use (I mean style like braid and material, not diameter), not the kind of tree...

I think you are exactly right. More than that, if you only consider climbing ropes and bull ropes, you can probably simplify even further and say rope on wood friction is roughly the same in all situations.

 

[lumberjack333]

Very interesting moray, personally wouldn't have guessed that contact area has almost no effect on friction... gives me something to think about next time I'm picking up climbing line! Nice work!