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Winch loads and what is happening when you winch

xxdabroxx

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Oh, goddamnit, don't tell me that I finally figured it out and it's STILL wrong... 🤬
It's right when you make a 180D turn at the pulley but it gets weird at anything less than that angle.

I think of this: If you pull 1000lbs of force in a straight line on a winch cable and you walk up to it and pull on it in the middle in any direction you'll be able to move it with less than 1000lbs of force. So the flatter the angle is the less pressure it's going to exert on the turn.
 

Sundowner

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It's right when you make a 180D turn at the pulley but it gets weird at anything less than that angle.

I think of this: If you pull 1000lbs of force in a straight line on a winch cable and you walk up to it and pull on it in the middle in any direction you'll be able to move it with less than 1000lbs of force...
Okay, I can follow that; you're just using leverage to your advantage by grabbing the middle of the rope and pulling perpendicular to the line. Makes sense.

...So the flatter the angle is the less pressure it's going to exert on the turn.
So, if I was to walk up to a winch line and wrap my hand around it loosely but not push or pull or alter the travel in any way, there would be 0 pounds of force on my hand...but the further I pull the line out of a straight path, the more force ends up on my hand? That doesn't seem to work with the examples Blaine posted, because it seems like the most force - the entire weight - would exist at 180°...but that doesn't account for the triangle part and the change-of-direction...so...

...fuck, I'm confused again. I wish I had a better foundation in mathematics, but I don't.
 

xxdabroxx

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You have 2 times the weight that is on the winch line when you are making a 180D turn and 0 times the weight at a 0D turn. Loads being equal on the winch line.

The 90D turn is where it starts getting funky. I believe the math says it should be 1.5 times the weight that is on the winch line but Blaine is seeing less than that for some reason, stretches, resistance at the pulley, etc.
 

xxdabroxx

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As I'm thinking through this, is some of the load going to the tree in this graphic being lost to pushing the two jeeps closer together (trying to make the 180D turn at max load on a pulley)? Or does that not matter if we consider them to be fixed objects?
1565732390210.png



Untitled-1.jpg
 

pagrey

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As I'm thinking through this, is some of the load going to the tree in this graphic being lost to pushing the two jeeps closer together (trying to make the 180D turn at max load on a pulley)? Or does that not matter if we consider them to be fixed objects?
The line can't apply a side force, it will always draw something directly towards the pulley. It doesn't matter if the object is fixed or moving.
 
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pagrey

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...fuck, I'm confused again. I wish I had a better foundation in mathematics, but I don't.
The forces are calculated with trigonometry, which is easy if you've studied it and hieroglyphics if you haven't. Don't wish by the way, pick up a textbook. The important part of this thread is that the trig calculations that everybody always spouts out as accurate aren't what @mrblaine is seeing which is very interesting. Even if you did "know the math" you'd still get the wrong answer in these tests which is the whole point of this in my opinion, figuring out what is really going on.
 
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Jerry Bransford

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Math seldom ever worked for me in school on mechanical shit, it only made sense when I was doing some kind of electrical/electronic work. Which is why I have stayed out of this thread lol.
 
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Steel City 06

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The load applied to the pulley varies sinusoidally, not proportionally, to the angle between the pulley anchor line and whatever ropes are attached to it.

If the angle through the pulley is 180 degrees or straight, the cosine of (180/2) degrees is zero, and no load is applied to the pulley. If the angle through the pulley is zero, meaning the rope doubles back on itself, the cosine of (0/2) degrees is one, meaning each end applies all of its force to the pulley, and the load is double. At 90 degrees, the cosine of (90/2) is 0.707, so each rope applies 0.707 times its force, or 1.414 times total. Thus the load on the anchor line is 1.424 times what it is on one line.

If you add friction to the pulley, then you have to start adding the components separately, and the above simplified math does not apply. A pulley with friction will simply allow the ropes to change angles, and potentially loads depending upon how they are fixed, until it reaches equilibrium. Thus each end of the rope through the pulley may not make the same angle, and then must be calculated independently. In order to calculate the load at the anchor line, the loads on each rope and their angles must be used to calculate their contributions to the load on the anchor line independently, with an equation like L3 = L2*sin(A23) + L4sin(A43) where L2 and L4 are the loads at 2 and 4 respectively, and A23 and A24 are the angles the ropes from 2 and 4 make with the anchor rope at 3.
 

Steel City 06

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In addition to the friction in the pulley, the measuring devices themselves likely have a role to play in the values we see. As Blaine increases the load in the system, the crane scales act as springs and stretch. This is mostly inconsequential for 1, 2, and 4 but this means that the line holding the pulley gets longer with added force. Thus, the movement of the pulley changes the angle between the ropes, and changing the forces we see. The more load he applies, the larger the angle from 2 to 4 gets, and the less load we see on 3. Blaine probably started at 90 degrees but the angle likely changed as the system was loaded.

In addition, the crane scales themselves aren't terribly accurate at measuring linear rope forces in a static system. They have a decent amount of internal friction, which isn't necessarily a bad thing for crane use (prevents the load from bobbing, or oscillating up and down on the spring), but that means when there isn't enough vibration or movement in a system they can settle on a less accurate value. However, this is likely a smaller contribution to the differences we see.
 

bobthetj03

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Sure makes you want to think about how to rig up a pull safely and at what angles provide the safest, or not so safest pull.
 

LukesfirstJeep

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mrblaine,

Thanks for taking the time to publish the results of your experiment. Real world numbers supports my understanding of whats going on. When I was taught physics, in the last century, we used "mass-less ropes" and "friction-less pulleys". Out results were similar, but only existed on paper :)
 
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bobthetj03

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Sometimes we don't have a choice to pick the safer angle so we need some idea of what is going on and the potential for things to go well.
If one can learn from your findings and apply them, one can at least try to minimize the danger involved with a pull, knowing what potential forces are at play at certain rigging situations. Thank you for taking the effort to post your findings.
 
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Goatman

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The shaking of the loaded system is intended to remove binding, sticking and settle undistributed loads from things like the casters and extra slack in the line/connections.

A familiar situation for many here is using an engine hoist. The hoist is on casters. As the engine is lifted, the hoist will move and shift as the force of the engine load attempts to normalize. Giving the hoist an occasional small shake during the lift will help further equalize the load and prevent unexpected shifts and jumps.

The point being that the resistance and binding from the cart ought to be inconsequential for the purposes here. And moreso after the cart and other components in the system have been normalized after a good shake.
Blaine said that 1 1/2 hours earlier. And I am guessing that either you or I don't quite get what that winking emogee means.🤔 This (I thought) was supposed to be a fun little "game" to play. Let's not start getting too worked up over it. My math was so far off that I now know to never guess how many jellybeans are in the jar.😁
 
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Mr. Bills

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@mrblaine ,

Thanks for this thread. I resisted reading it until today because I wanted to let the discussion develop and then read it all at once.

I won't pretend to have an answer that adequately explains why your test results vary from the mathematical model, but your experiment takes me back 52 years to one of my first full time jobs - as a research assistant at Harvey Mudd College assigned to work on a National Science Foundation grant to design and develop laboratory experiments for high school and college chemistry courses.

Some of my "take aways" from that job: (1) Experiments that do not provide consistent results have limited utility as teaching tools, hence the NSF grant; (2) Designing experiments that actually test what you think you are testing can sometimes be a daunting task, and (3) Refining experiments so they can be performed by many different people with consistent results can be even tougher.

Still, it was a fun job. Sometimes we got to blow up stuff.
 

Ericict

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Thanks for posting Blane. Good conversation on a topic that isnt completely understand by many. The actual pull tests are very helpful.
 

FireJeep

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That is a very well thought out explanation/diagram of forces exerted. Thanks for posting
 
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