Wire Rope vs. Aircraft Cable (1 Viewer)

[Jbissell]

To expand upon what UWDave has said... I have some basic logic to add to this, not trying to insult anyones intelligence. Just adding my thoughts to the mix.


The tension in an unbroken line is constant. That is, if there is nothing solidly attached to the line to increase that tension that tension is constant, however, if there is a second weight solidly attached the tension will change. All is exampled here:

This fact does not change if you put a pulley in the mix, because the pulley does not add tension to the line, only redirects it:

It does not matter how you route the cable this does not change.

Extending this to the following case... A 100 lb weight hung from the snatch block here, will result in a 50lb load along the whole line. Because there is no solid attachment, and there must be constant tension on the whole line as shown in the previous arguments:

That constant tension in the whole line includes the part that is in direct contact with the pulley.

In summary, I believe part of the misunderstanding comes from the thought that snatch blocks multiply the force created. This is not true, rather they distribute the sum total load being pulled across multiple cables evenly across those cables.

Thanks,
J.Biz

 

[80t0ylc]

........The tension in an unbroken line is constant. That is, if there is nothing solidly attached to the line to increase that tension that tension is constant, however, if there is a second weight solidly attached the tension will change.........This fact does not change if you put a pulley in the mix, because the pulley does not add tension to the line, only redirects it:..............It does not matter how you route the cable this does not change.

I don't have any problems with this part of your post. Although I believe the angles play a large part on the stress of a line through a pulley.

........Extending this to the following case... A 100 lb weight hung from the snatch block here, will result in a 50lb load along the whole line. Because there is no solid attachment, and there must be constant tension on the whole line as shown in the previous arguments:..........That constant tension in the whole line includes the part that is in direct contact with the pulley.

In summary, I believe part of the misunderstanding comes from the thought that snatch blocks multiply the force created. This is not true, rather they distribute the sum total load being pulled across multiple cables evenly across those cables.......

What is the source for this? It sounds good and may be correct, but I don't follow the physics involved. I understand how the winch will benefit due to the lower "full load line speed" created by the snatch block(s) providing a kind of gear reduction. But the section of line that is in the snatch block that's attached to the full load is what puzzles me. What you are saying in effect is that my 5/16" wire rope that is rated roughly 10.5K lbs in nominal strength on my 8k winch is strong enough to pull any full weight load as long as enough snatch blocks were used and I didn't run out of cable (dis-regarding friction, of course, and making sure rig was securely anchored). Well, I've heard this before, and it may even be true, but I wouldn't want to be the guinea pig. Here's one of your drawings edited (in red) with my concern:

 

[Ebag333]

I don't have any problems with this part of your post. Although I believe the angles play a large part on the stress of a line through a pulley.

What is the source for this? It sounds good and may be correct, but I don't follow the physics involved. I understand how the winch will benefit due to the lower "full load line speed" created by the snatch block(s) providing a kind of gear reduction. But the section of line that is in the snatch block that's attached to the full load is what puzzles me. What you are saying in effect is that my 5/16" wire rope that is rated roughly 10.5K lbs in nominal strength on my 8k winch is strong enough to pull any full weight load as long as enough snatch blocks were used and I didn't run out of cable (dis-regarding friction, of course, and making sure rig was securely anchored). Well, I've heard this before, and it may even be true, but I wouldn't want to be the guinea pig. Here's one of your drawings edited (in red) with my concern:

From:
Introduction to Statics: Internal forces in slender members
(Section 9.3)

1. The tension force in a light rope or cable that passes over a frictionless pulley is constant

So at any point in the cable, the tension is going to be the same. Tension is odd though because it basically works in two directions at once, which is why you feel that at any given point it has to be doubled.

The catch is that the snatch block really isn't frictionless, so there is extra force on the cable as it goes around it. Doubling it would be a great way to provide a safe margin of error, but isn't really necessary.

 

[80t0ylc]

Thanks for the link, Gabe. (I think) The math & the formulas show the logic. It's just hard for a stubborn old fart like me to grasp the concept. It does make it simpler for figuring, though.

 

[Ebag333]

Truth be told I went looking to prove you right. Guess I didn't pay enough attention in Physics classes all those years ago.

 

[UWdave]

The catch is that the snatch block really isn't frictionless, so there is extra force on the cable as it goes around it. Doubling it would be a great way to provide a safe margin of error, but isn't really necessary.

In reality, nothing is frictionless and nothing is adiabatic. Energy is always lost.

The problem is, if you try to include every possible factor, the problem becomes overly complex and difficult to solve. Because of this, you need to decide what is important.

If I recall correctly, there are 3 things to consider when looking at pulleys.
1) Static Friction
2) Energy loss due to kinetic friction
3) Inertia

Static friction of a snatch block is much much much less than winch capability. Ok to neglect.

Energy lost due to kinetic friction = friction x speed. Because winch speed is so slow, this number will be very low. Ok to neglect.

Inertia affects your ability to change speeds. Nothing rapid about winching. Ok to neglect.

 

[73fj40lc]

Scientific fact always trumps opinions. I have always felt that I had a good grasp of what has been being discussed in this thread. Now, I have to admit that I have learned some things and have alot better understanding than I did. Thanks guys, great work. I think that this thread will be required reading for my son.

 

[Jbissell]

Thanks for the link, Gabe. (I think) The math & the formulas show the logic. It's just hard for a stubborn old fart like me to grasp the concept. It does make it simpler for figuring, though.

Truth be told I went looking to prove you right. Guess I didn't pay enough attention in Physics classes all those years ago.

In both of your defense... That fact of pulleys is probably one of the most non-intuitive concepts in statics. I took statics a few years ago, and it was one of the more confusing things in the whole class.


J.Biz

 

[MUDZLLA]

Won't I exceed its load capacity as soon as I throw a snatch block in the mix?[/QUOTE]

If your winch pulls 9000 lbs, every part of the wire rope sees 9000 lbs so if you double it up through a snatch block the tree and your truck feels 18000 lbs but the winch is still only pulling 9000 lbs.