Friction Saver Friction -- some measurements

[moray]

...the pressure applied per unit of area would have been correspondingly greater...
Somewhere there MUST be a consideration for the radius of the object being wrapped. I suspect that the coefficient of friction must vary acording to the radius that the rope is bending over, because we all know that a rope wrapped around a square bar has much more resistance than a round bar. (it's a lot more destructive to the rope, too!) The only difference between the two objects is the miniscule radius for each of the 4 corners and the deformation of the rope as it passes over those tiny arcs.

You've nailed it--we didn't miss it. The practical effect of smaller radius is greater pressure per unit area. The smaller radius is harder on the rope. It's also harder on the tree. In two otherwise identical heavy-load setups, one with big limb and the other with a small limb, you could have a safe and normal operation with the big limb, but the small limb would get stripped of its bark and the rope surface would start melting.

My buddy Jack with 35 years in the business likes to tell me about a huge (1 1/2 in?) nylon bull rope he used to have. It was used rarely, mostly on highway projects where heavy equipment was available to pull a whole tree out of the way. On one such job the rope passed over a small crotch. The load was far smaller than the rope capacity, but Jack was shocked to find later that the rope had undergone some serious melting where it had passed over the crotch. The intense friction around the tight radius meant intense local heating, like holding a torch to the surface of the rope as it passed by. A much bigger radius would have lowered the intensity and given the rope time to absorb the heat, more like heating it with an electric blanket. Nowadays Jack always uses blocks and steel capstans to deal with big loads.

This is one of those cases where things don't scale together as one might expect. That big rope would have been almost 10 times as strong as a little 1/2 in rope, and able to do a lot more work per minute than the little rope. But it couldn't absorb heat through a square inch of surface at 10 times the rate of the little rope. Maximum heat absorption rate doesn't scale up at all, and this led to an early grave for the huge bull rope.

 

[pdqdl]

I still believe that the coeffecient will actually change according to the radius of the curvature, especially (as you pointed out) relative to the diameter of the rope. Granted, we understand the contact surface area is not a theoretical variable in calculating the friction. It is also well known that the coefficient of friction often changes according to the forces applied.

A good example might be steel on concrete. At a high enough pressure, the steel is no longer "rubbing" against the concrete, it begins to deform it's shape into the molecular crevasses of the concrete, and the coefficient changes.

I suspect that there may be similar changes for the coefficient of friction for a rope around an axis of varying radius. Unfortunately, I have not been able to find ANY information on "coefficient of friction" relative to ropes and wood.

I think I'll e-mail a couple of rope manufacturers. I'll bet their engineers have all this figured out already.

 

[pdqdl]

rope melting is NOT a determined by just the force applied.

..
My buddy Jack with 35 years in the business likes to tell me about a huge (1 1/2 in?) nylon bull rope he used to have. It was used rarely, mostly on highway projects where heavy equipment was available to pull a whole tree out of the way. On one such job the rope passed over a small crotch. The load was far smaller than the rope capacity, but Jack was shocked to find later that the rope had undergone some serious melting where it had passed over the crotch. The intense friction around the tight radius meant intense local heating, like holding a torch to the surface of the rope as it passed by. A much bigger radius would have lowered the intensity and given the rope time to absorb the heat, more like heating it with an electric blanket. Nowadays Jack always uses blocks and steel capstans to deal with big loads.

This is one of those cases where things don't scale together as one might expect. That big rope would have been almost 10 times as strong as a little 1/2 in rope, and able to do a lot more work per minute than the little rope. But it couldn't absorb heat through a square inch of surface at 10 times the rate of the little rope. Maximum heat absorption rate doesn't scale up at all, and this led to an early grave for the huge bull rope.

Rope only melts because it is exposed to more energy at a given point than it can dissipate. So far, our equations have not been used to relate the energy applied to any situation, only forces. Energy + friction= heat

Static friction can be infinite, the force applied to overcome it can be almost infinite. Until something moves, there is no heat generated. This is why injection molding and aluminum extrusion work so well.

If the same situation described had been pulled at a slow enough rate, the rope would never have melted. The applied forces would have been exactly the same, but the velocity would have been different. Increase velocity, keep the forces the same, and the energy applied to the rope increases.

The rope melted because they pulled too hard, too fast. It probably didn't help that they smoked it going over a small crotch.

 

[moray]

...A good example might be steel on concrete. At a high enough pressure, the steel is no longer "rubbing" against the concrete, it begins to deform it's shape into the molecular crevasses of the concrete, and the coefficient changes.

I suspect that there may be similar changes for the coefficient of friction for a rope around an axis of varying radius...

I am sure this is true; the question is when does the ideal equation start to break down and how badly does it break down?

The rope is non-ideal from the get-go. The internal fibers move and slide with respect to each other every time the rope goes around a bend or is crushed against a surface. The sharper the bend, the more severe the effect. This internal friction shows up in the tension difference between the two legs of the rope, but it is not accounted for by the ideal equation. Is it a big deal? For light loads and gentle bends certainly not. I don't know at what point internal rope friction becomes a significant part of the total, if ever.

Your comment about surface deformation identifies the issue that seems most significant to me, but again, it is just guesswork on my part, as I have no evidence. I could (and would) measure this myself if I had two things: a convenient way to apply large known loads, say 1000 lbs, to a rope; and a reasonably accurate way to measure pulling force, up to say 3000 lbs, in the other leg of the rope. This would be fun!

 

[pdqdl]

This could turn into a huge time-killer for me. I'm thinking about loading up some logs of varying sizes & types of wood for rope wrapping points, getting a standardized weight of say 500-1000 lbs, and seeing how many wraps (in radians!) that it takes to get a 50lb draw force (spring scale, I have one) to hold in a slow descent. Then we could do the math, and establish an experimental coefficient of friction for OUR application, and establish an experimental (not theoretical) ratio of wraps to load reduction.

I have a small crane, I could do that. Unfortunately, it is the spring rush, and I don't have time for that.
Perhaps in the summer doldrums ?

 

[moray]

Hmmm... I have some time... Wouldn't wanna loan a feller yer crane, would ya?:lifter:

 

[pdqdl]

Maine ? ......!!!

You could probably BUY my old beat-up chipper truck with crane for the fuel expense just to get it there & back. About 5-6 mpg: gas. Only goes about 50 MPH.

Pretty cool old RR service truck, it has a 12,000lb knuckle boom crane (35' height) behind the "man-cab", and a dump body behind the crane. We built a removeable top for the dump bed: we can go to a job, chip the brush, remove the 1000 lb "lid", load the logs, and drive away with the whole job done. Ugly as all get out, we never have painted it.

It's the best toy I own for fixing broken stuff. Pick up dead equipment, pulls engines, and it is the absolute thing to have when something metal gets bent.

Not bad for a truck I bought for $7500 in early 2001. Been working fine, all this time, except for what we have broken by abuse or normal wear. Kills us on gas, though.

 

[moray]

I like it!

 

[pdqdl]

Don't unsubscribe from this thread yet. When I get some more data, I will post it. Right now, I'm trying to find some relevant coefficients, I just haven't found any yet.

I'm avoiding the experimental route.

 

[moray]

...The saw on the winch is a Shindaiwa 488. Pretty good little saw, but I don't even know the cc on it. Just guessing.

Pretty damn good guess. 47.9 cc, 3.5 HP (2600 W)

...I think the 3/8th amsteel will work much better, but I will be very watchful for meltdowns.

By the way, the capstan is aluminum, about 2 1/4" diameter, with a drum width of 2.75". Four wraps fits well for the 9/16" stable braid, 5 wraps is overlapping a bit, but really pulls then.

I didn't have any Amsteel Blue when I did my friction experiments, so I just now measured the efficiency of AmB over 2 aluminum rings: 0.6. This is definitely less friction than the same setup with polyester. Since this represents 1/2 wrap, one can calculate that 4 full wraps would give a load-to-control ratio of 60:1. For 5 full wraps, the ratio is 165:1. With your winch setup, using 5 wraps of AmB, you should be able to pull 1000 lbs with just 6 lbs of pull force on the tail.

What is really cool about this powered capstan, which is just a special case of friction around a post, is it can act as a force multiplier. If I had a bigger version of your winch, say one that could pull 10,000 lbs, and I knew that 6 wraps of my winch rope would give me a force ratio of, say, 500:1, I could easily do measured break tests of ropes and splices and knots with reasonable accuracy. Just hook up everything for a destructive pull test, run the control tail over a small pulley and hang a bucket from it. Start the motor and start filling the bucket with water. When the test item finally fails, The weight of bucket plus water, multiplied by the force ratio, derated by the pulley efficiency, gives the force that caused failure.

 

[pdqdl]

165:1 ! That will be cool to use, but I'll bet it's not that good. We were doing 5 wraps with the 9/16 stable braid, and we were pulling WAY harder than that on a load.

When the amsteel comes in, I'll do a test pull. I'll weigh something heavy (at least 500 lbs, and I will rig the amsteel over the pully on my crane. Then I will hook up my tension scale and see how much feed force is required to move the load on however many wraps. THEN we will have a definative measure of the lifting capacity of the winch and the feed force required to move the load.

Then we can do cool things like weigh heavy loads with the feed force scale and we can confirm the coefficients involved. I might even do a scale test on the same heavy load rigged to my Nickle coated port-a-wrap.

You have been pretty enthusiastic about this physics stuff, I can do a little bit of work as well.

 

[moray]

Excellent!

I had forgotten about your crane--you're all set to do the stuff I can only dream about.

Trying to get a good value of the friction coefficient with just half a wrap and a bathroom scale is like trying to measure the speed of a car whizzing by if you only have 1 millisecond to make the measurement. If you can get a good measurement with 3 or 4 wraps, the calculated coefficient would be many times more accurate than what I can do. If you repeat the experiment several times with 3 wraps, and several more times with 4 wraps, we could combine all your data to nail it down closer than anyone could possibly need.

Good work! I look forward to your results.

 

[moray]

I was wrong!

The spiral wrapped rope will have more surface contact with the tree per radian so I would expect there to be some difference. As the rope pulls tight and the circular wraps go spiral due to the overhead loading angle there might be some beneficial shock load dissipation.

True enough, there will be more contact per radian. But the curvature of the rope is less in the spiral case, since it takes a longer piece of rope to make one circuit of the tree, so there is less pressure at each point of contact. A wash.

It is not a wash; I was wrong.

While working on another problem I again came across the issue of a rope under tension wrapped in spiral fashion around a pole, and this time I engaged the brain before engaging the mouth. There are a number of ways of working out the math, but they all give the same answer: a spirally-wrapped rope produces less force against the pole, and less friction, than a straight wrap. The effect is negligible for a tight spiral, but becomes significant for a very long spiral.

We can quickly run through this without any difficult math. First, remember that a rope under tension applies force to the post only because it is curved where it contacts the post. A straight rope cannot apply a sideways force to anything. What's more, the total force applied to the post is proportional to the amount of curvature in the rope. Every degree of curvature is like every other degree of curvature--each one applies the same amount of force.

Now imagine a really extreme case. Assume a tensioned rope is spirally wrapped around a vertical pole and the angle of inclination of the rope is 89 degrees, just one degree off true vertical. Allow the rope to wrap around 180 degrees to the other side of the pole. My error in the shoot-from-the-hip "wash" comment was to imagine this also meant the rope had curved 180 degrees. How much has it actually bent? About 2 degrees! After all, it is inclined 1 degree from vertical at the bottom, and after travelling halfway around the pole, it is now inclined 1 degree the other way.

The exact result, when you work out the formal math, is that the degrees of pole curvature times the cosine of the angle of inclination gives the degrees of rope curvature. A practical example: you start out with two tight wraps around a tree. As the load starts moving, the spiral stretches out to give a 45-degree inclination. You have effectively just lost about 30% of your wraps.

 

[Adkpk]

Speaking of friction savers I went to retrieve mine today out my giant white pine. Left it up there last climb of year, last year. I knew it was going to be a while till I got back up there. Anyhow, after a few tosses with the slick line I noticed a little stickiness on my fingers. I could see sap glissening on the branches. I decided to wait till I don't have to worry about getting my ropes full of sap. I'm sure the fiction saver will be ok. It's on the north side of the tree back in the woods and the tree is monitored for squirrels.

 

[pdqdl]

I kinda thought so, but I didn't say anything...

I have often observed that the rope wrapped in a spiral up the tree failed to hold as well as I had hoped.

It just took your thoughtful effort and insight into the math of the situation to point out why.

 

[moray]

Speaking of friction savers I went to retrieve mine today out my giant white pine. Left it up there last climb of year, last year. I knew it was going to be a while till I got back up there. Anyhow, after a few tosses with the slick line...I'm sure the fiction saver will be ok. It's on the north side of the tree back in the woods and the tree is monitored for squirrels.

Nice to hear from you again, Adrpk. Unless I misunderstand your post, you left your FS up there without a clothesline through it? Did it get stuck up there unintentionally?

I just retrieved mine out of my big pine a week ago. It had been there since last September and was still in perfect shape when I climbed up to it. I have now installed a much better system for this particular tree--a clothesline that goes over a sort of saddle at 62 feet where the tree had been topped about 50 years ago. Now I can pull up my SRT line up on short notice and zip up the clear side of the tree, something I couldn't do before.

As far as pitch goes, summer seems to be the bad season. Here in Maine, in the winter, the flow of pitch seems to virtually stop. I do all my trimming then, and give it up come Spring.

 

[moray]

I have often observed that the rope wrapped in a spiral up the tree failed to hold as well as I had hoped.

It just took your thoughtful effort and insight into the math of the situation to point out why.

Very kind of you. It is nice that your practical experience bears out the calculations.

Have you had a chance to use your Amsteel Blue, yet?

 

[Adkpk]

Nice to hear from you again, Adrpk. Unless I misunderstand your post, you left your FS up there without a clothesline through it? Did it get stuck up there unintentionally?

I just retrieved mine out of my big pine a week ago. It had been there since last September and was still in perfect shape when I climbed up to it. I have now installed a much better system for this particular tree--a clothesline that goes over a sort of saddle at 62 feet where the tree had been topped about 50 years ago. Now I can pull up my SRT line up on short notice and zip up the clear side of the tree, something I couldn't do before.

As far as pitch goes, summer seems to be the bad season. Here in Maine, in the winter, the flow of pitch seems to virtually stop. I do all my trimming then, and give it up come Spring.

Yes, quite unintentionally. I think it was something like wanting to leave it up there with a slick line through it for a future climb but as I untied it from the climbing line I didn't realize the other end had tangled into a ball and when I let go it pulled the end of the slick line out of my reach. I have a spare.

 

[moray]

me too

Yes, on at least 2 occasions I started pulling my rope down, and just as the other end rose out of reach, realized I didn't have a line attached. It's not the end of the world when you have to climb back up there to retrieve your false crotch, but it's nice when no one is around to see how stupid you are.

 

[pdqdl]

Very kind of you. It is nice that your practical experience bears out the calculations.

Have you had a chance to use your Amsteel Blue, yet?

We used it on the capstan winch just yesterday. It worked fine, but I don't like it for that application. The rope winch relies on friction, and the amsteel is slick as glass. With the small diameter of this rope, we can put on enough wraps to compensate, but you still need to pull on the rope with your hands. Difficult to pull hard with hands on the amsteel.

I admit, the rope winch states that it works best with a 3/8th twist rope. I am saving the amsteel for long reaches into a muddy area for retrieving stuck vehicles or logs. It's strong enough for that, and I do like the weight.