Warn aluminum fairleads
- Thread starter mrblaine
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That would make sense. I found some pics after I took it all off. That was April 2022. I was hoping to find some pics before, during, and after, but no luck.
Sorry, I'm at a conference this week and was busy all day and am just getting back to this. I won't tell you why you're wrong because I don't know if you're wrong or if I'm right. I had only briefly thought about the problem myself, so my post was "thinking out loud." The curve I drew was just the start of a theory. My thinking that the peak force was the same for all diameters was maybe flawed (I truly don't know). However, intuition tells me that there's no way the area under the curve is equal. In my racing days, the first thing a roadracing motorcycle rider was told about crashing was to move your contact points around as you are sliding because the prolonged friction, even if the leather doesn't wear through, can cause serious burns. The reason for this is the prolonged frictional forces concentrated in one location. I related this to a point on the winch line moving around the fictional post in my example. At the same line speed, in my mind, there has to be more heat generated with the larger diameter because the frictional forces act on each point of the rope for a much longer time.
{NERD ALERT - continue reading at your own risk}
I gave a little more thought to trying to solve this problem. The capstan equation is used in the maritime world, and it's somewhat related to this problem, but not exactly. It's a way to determine how friction reduces the load on a rope when looped around a capstan. It's derived by integrating the force balance you mentioned. However, it doesn't solve for the normal force, it solves for the axial tension on the rope at the exit from the capstan. Solving for the normal force could certainly be done with a similar approach, but I'm considerably more removed from my college days than you are, and unless someone's paying me (or I have a compelling reason to know the answer - hint, I don't), the hours it would take to get back up to speed isn't worth it to me (sadly - 'cause I love a good technical challenge.)
I think one of the most important features of being an engineer is realizing one doesn't know everything, so I'm not taking for granted how nice it is to be "debating" a topic like this without either party digging in and getting emotional and refusing to accept that possibility.
Same on the time constraints...I'm supposed to be tying up some loose ends before I miss 4 days of work for a wheeling trip, while also tending to a couple of racks of ribs in the smoker to have my parents, brother and in-laws over for dinner, which I guess is for my birthday but I think is really just an excuse for my wife to entertain. And also getting the LJ loaded up and ready to roll out at 7:30am to meet some buddies for dinner in Pueblo, CO.
The advice for the motorcycle rider makes perfect sense. I don't know if I've been clear up to this point but when I say the heat generated is the same, I'm definitely NOT talking about degrees of temperature; I'm talking about Joules or BTU of energy. The area that those Joules are converted in, how they are conducted through the aluminum of the fairlead, the dyneema (which I suspect is not a great conductor), and convected to the air will certainly result in a different peak temperature.
I like to take an energy approach to thinking of this stuff, because that's my bag, so if we say that motorcycle rider weighs 160 and crashes at 80mph he has about 45kJ of kinetic energy that has to be converted to friction to bring him to a stop. It's completely independent of where or how big of an area he uses for ground contact - to go from 80 to 0 he needs to convert 45kJ of KE to heat, period. Rolling around to alternate his contact points is the best way he has to put that 45kJ into a larger mass of protective gear and reduce the temperature reached at any point.
If humans lacked skeletons and could take any shape at will, I think it would be equally effective to advise the motorcycle rider to assume the shape of a pancake and maximize the surface area in contact with the ground. The rider hasn't changed his weight so total Fn would be unchanged, but the Fn at all points would be reduced to the minimum achievable and the total heat energy generated would be distributed along the maximum possible surface area of the gear instead of localized to his elbows, hips, knees, ankles and shoulderblades and therefore the peak temperatures reached would be far less.
{NERD ALERT - continue reading at your own risk}
I gave a little more thought to trying to solve this problem. The capstan equation is used in the maritime world, and it's somewhat related to this problem, but not exactly. It's a way to determine how friction reduces the load on a rope when looped around a capstan. It's derived by integrating the force balance you mentioned. However, it doesn't solve for the normal force, it solves for the axial tension on the rope at the exit from the capstan. Solving for the normal force could certainly be done with a similar approach, but I'm considerably more removed from my college days than you are, and unless someone's paying me (or I have a compelling reason to know the answer - hint, I don't), the hours it would take to get back up to speed isn't worth it to me (sadly - 'cause I love a good technical challenge.)
In the brief amount of time I've spent looking at the capstan equation I notice that the holding force is a function of the radians of contact and the wikipedia article says it's independent of the radius of the capstan. Since we're dealing with a quarter turn for both of these fairleads, φ would be pi/2 in both instances and the capstan equation would produce the same result.
To detail my approach, which again, may be omitting things that I haven't thought of since I don't usually do this sort of stuff...
I think we would agree that the force vector on the hypothetical posts is purely a function of rope tension - it's the sum of the vectors applied by the rope in the x and y direction. Where we might be missing one another is whether this vector represents the peak Fn or the area under the Fn curve. I believe it to represent the area under the curve, because that's the sum total of the force applied to the post by the rope, whereas the height of that curve at any point is actually just the contact pressure (force/area, pounds per sq in) at that point. The total pounds of force don't change, only the area it's applied to.
The next step I take is Ff = µ*Fn. I started to doubt myself yesterday when you questioned my claim and had to do some research to re-learn Amontons-Coulomb second law of friction which states that the force of friction is independent of the area of contact, so there wasn't some mechanism making the coefficient change for each of these situations which would make this step invalid. So if the Fn is unchanged by the radius, the Ff is unchanged as well.
Next I use W = Ff * d (which would be the change in rope length from start to finish of the pull), and then you have the total Joules of energy converted to heat due to the friction between the fairlead and rope over the duration of the pull, or you could integrate the Work equation over the distance of the pull and calculate Watts (J/s) or horsepower or BTU/h of power dissipated over time and compare that to the heat dissipated...basically the fairlead will continue to rise in temperature until it is able to dissipate heat as quickly as it's being converted, and that may mean exceeding the max temperature of the rope.
So here's the big reveal - The rope isn't just having to deal with the heat that is generated in the time that it's rubbing the fairlead. It's also having to deal with a lot of the heat generated in the fairlead by all of the rope that preceded it. By the time you get 60 feet drawn in that fairlead is going to be HOT and the larger radius has potentially doubled or tripled how long the rope is in contact and drawing heat from the aluminum. Now we have a race between how fast the larger mass and surface area of the fairlead can carry heat away from the interface vs how much heat the rope is picking up because of how much longer that contact lasts.
From there are a few additional considerations that will play into the peak temperature reached by the rope/fairlead interface but are difficult to predict:
1. faster line speed (a common marketing differentiator of Warn winches vs cheaper brands, which are less likely to be paired with a Warn fairlead) means the energy is converted more quickly, but that does not change the thermal conductivity of the aluminum so will act as a positive influence on the peak temp of the fairlead, however it also reduces the time the rope has to be in contact with it. I think it would come out in the wash, but can't say one might not dominate the other.
2. coatings such powdercoating will impact the coefficient of friction possibly generating more heat than an anodized or bare fairlead as well as affecting the heat transfer between the fairlead and the rope.
3. the coefficient of friction probably DOES change as both the coating and the rope heat up, creating a positive feedback loop, so once a heat problem reaches a threshold it would accelerate itself to failure.
The problem with the high school physics friction equation that doesn't depend on surface area is that it breaks down in many real-world situations, and almost certainly does here.
I suspect that the buildup in the corner is something of a red herring and not evidence that a smaller radius is better in this application. All the manufacturer guidelines talk about minimum radii. Surface abrasion is normal and accounted for with sizing. @mrblaine, were there any pictures of the break? Anything to suggest it melted through until the cross-section got too small and broke?
Either way, the best fairlead style for rope is roller.
I'm not sure what you mean by static radii? As in the radius doesn't change or it's bending around a non-rotating radius?
None of those rules apply to dragging the line over an edge radius like we do with a hawse fairlead and synthetic line.
It certainly breaks down in engineered-friction applications like tires on a road surface, but I don't know if it can be "certain" that it doesn't work here unless there's good evidence to suggest that it doesn't, which we don't have.
agree.
I would think the rope strength is impacted the same regardless, with possible caveats to what is ideal based on the topics of this thread.
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Alright, we have some highly educated engineers in here, tell me how that will work? What radius will least impact rope strength on a pull at what angle?
My answer is anyone who attempts to define those parameters and put them out there as even loosely followed rules is just not paying attention. We only use smooth hawse type fairleads on synthetic because by and large we mostly get away with it. I know from bending 6061 T6 that this material is a very poor answer to what we should be using which is a very highly polished hard stainless. Gigglepin has one, that would be my answer if it didn't exist and I wanted to sell 3 of something per year.
I've always been good at not paying attention, so I'll go ahead and take a stab at it. The only way to prove it out is with realistic testing, and I'm good luck finding that data.
All the data I've seen from Samson shows that a larger radius is better. Mooring hardware is very similar to hawse fairleads and sometimes even called exactly that, and with wave action is far from static. Even though the rope is tied off there's enough stretch that it slides in and out with cyclical loading, which is far harder on the rope as it works the same area repeatedly.
The friction formula they give is a subset of the capstan formula. Notice that radius is not a factor.
All the available information points to a bigger radius being better, subject to diminishing returns, and never getting worse. I think that's true at least as a practical matter.
That only leaves overheating the fairlead enough to damage the rope as a limiting factor. But if the friction equation is true then the same amount of heat will be generated independent of radius, so MAYBE you could say that a shorter contact would heat the rope up less, but that only applies as it's moving and would increase the risk of melting when stopped. And the tighter the bend the more internal rope movement and heat generation there is, so it's far from clear that there would be any improvement.
Given the limited information, I'm going to guess that the rope was worn and damaged before the pull.
Thanks for calling me a dumbass. There would not be a thread started if the failure was due to a damaged line.
Fair enough, but the rest of the post has value in its assertion that the radius itself is likely not the cause, which points us to the coating or other characteristics of the finish that may be at work here.
Not what I said. And it's perfectly reasonable to suspect the fairlead. I'm sure the 90-degree pull did significantly increase stress on the line and contributed to it's failure. I'm just guessing that it would have parted on one with a smaller radius, too. And it is just a guess. Evaluating from a 2nd-hand account, and no examination of the line makes this very imperfect. I'm just making the best guess I can given the limited information available. If I get better, conflicting information and I'll change my mind.
As I said a while back if the issue was that the fairlead got hot enough to melt the rope that should be observable, either by measuring the temp of the fairlead or by inspecting the rope.
This is just one case. Warn is a big brand and I'm they've been making fairleads for a long time now. Wouldn't you expect to find multiple examples like this?
Weird shit happens. I've read several complaints about synthetic line parting under seemingly light loads. Some choose to go back to steel. Others choose to pay more attention to their rigging. This one case isn't enough evidence for me to choose a smaller radius over a larger one when rigging dyneema.
Last week I did three short ( less than 2' per) relatively hard pulls with a black Warn fairlead.
Im a serial winch rope abuser and have never personally had that.
Im a serial winch rope abuser and have never personally had that.
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