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  #21  
Old 01-30-2011
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The average male has a pull strength of 8o#. So depending on the mechanical advantage of your tackle, your lifting ability is MA x 80#.
So a two fold rigged to disadvantage has a MA of 4, thus the theorical lifting of one of your crew would be 360#. But you have to add 10% of the weight being lifted for each sheave. Thus you will have a lifting weight of 504# and that requires 126# pull on the standing part. So you may have to have your partner to help you lift said item.

360 + (360 X .1 X 4) = 504#. This if you do proper maintenance on your tackle.

Heave! Heave! Heave!..... Panting hard because I'm by myself.

Two fold has two sheaves on the moving block and on the stationary block each, making 4 total.
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Last edited by Boasun; 01-30-2011 at 12:07 PM.
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  #22  
Old 01-30-2011
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Now lets say you have two pieces of line connected to a link, that link is right at the half way point on the sheave, is not half the load pulling one way and the other half pulling the other.
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  #23  
Old 01-30-2011
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Quote:
Originally Posted by P35juniper View Post
Now lets say you have two pieces of line connected to a link, that link is right at the half way point on the sheave, is not half the load pulling one way and the other half pulling the other.
Never happen because the link will not fit through the block. And the line will part at it weakest point.
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  #24  
Old 01-30-2011
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Quote:
Originally Posted by P35juniper View Post
Now lets say you have two pieces of line connected to a link, that link is right at the half way point on the sheave, is not half the load pulling one way and the other half pulling the other.
This is exactly the issue that made me doubt my original reasoning of just multiplying the breaking strength by the mechanical advantage.

Quote:
Originally Posted by pdqaltair
The tension on every part of the rope MUST be the same. Otherwise there is no balance and the rope will move.
True, but---and maybe my understanding of what "tension" means is faulty here---it seems that in the scenario P35juniper is describing, the tensions do balance, so the rope will not move.

Another way of looking at the same issue. Suppose we consider Boasun's "average male" pulling 80 lbs. Let's give two average males a rope whose breaking strength is 159 lbs. They each pull on one end in the direction opposite the other. No blocks or tackle involved. Shouldn't the rope break?

And finally, I've now realized the answer: the two guys each pulling 80 lbs is no different from one guy pulling 80 lbs and the other end made fast to the wall. In the latter case, the wall is pulling 80 lbs---if it weren't, it would accelerate towards the pulling guy (just like the guy's buddy would if there were two guys on the rope, pulling in opposite directions).

If we now bend the line around a sheave, all that does is change the direction of the forces, but not their magnitude. The tension throughout the line is the same as the tension at either end. Tension must be "bidirectional", in that you don't add tensions from people pulling in opposite directions.

In conclusion, I think the original "divide breaking strength by mechanical advantage" argument is sound.
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Old 01-30-2011
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Quote:
Originally Posted by AdamLein View Post
In conclusion, I think the original "divide breaking strength by mechanical advantage" argument is sound.
I will second that. If you have a 4-1 system and you are lifting 100 lbs, each line will have exactly 25 lbs, assuming your original caveats.
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  #26  
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Note: If the line is anchored to a wall with one man pulling on it, you will have a static load of about 80#. One of the tests required for lifting equipment. The other test is the Dynamic test where said test weight is being lifted by the equipment.
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  #27  
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Originally Posted by Boasun View Post
Note: If the line is anchored to a wall with one man pulling on it, you will have a static load of about 80#. One of the tests required for lifting equipment. The other test is the Dynamic test where said test weight is being lifted by the equipment.
Yeah, that's the next interesting place to go but I don't really know anything about dynamic loading. Other than it's bad.
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You have two situations: the breaking strength (or safe working load, whichever concerns you) of the line through the block and tackle and second, the breaking strength (or safe working load of the mechanical components, if that's what you are working with).

Count the lines on the movable block... that gives you the mechanical advantage. Thus if you have 5 lines to the movable block and you are lifting 100 lbs., you will need to apply 20 lbs. pull to lift the weight. Forget the friction of the bearings, etc. For the accuracy of a practical situation, they will be relatively small if you have lubricated the sheeves (rollers). The load applied to each part of the line is 20lbs. in this example.

For the mechanical side, each component has a breaking strength or safe working load. Without knowing the characteristics of each metal, each plastic, and size of each component, you or I cannot really calculate the strenght of each component. If you will determine the manufacturer and model of the blocks, hooks, etc., you probably can go to the manufacturer's catalog and see what the safe working load or breaking strength is for that component. The weak component is your safe working load for the system. Sometimes blocks, shackles, and the like give breaking strength also. Generally, the breaking strength will be on the order of 2.5 times the safe working load for mechanical components. For line, it is about 15% of breaking strength. These are only approximations, and condition of the components can reduce this considerably, so care is needed not to push the loading to the limits. Also, if you are picking up heavy loads, make sure that your lines are in good shape and that you know the rating for the lines and also inspect the mechanical components for wear or distortion from loading or bending. If the components are not in good shape and you push the loading too far, you may get an unpleasant surprise. Please proceed cautiously, and if you use the above information, you do so at your own risk. (I don't want anyone saying you told me it was safe and therefore my disaster is your fault).

Also, the manner in which the load is applied is critical. If the load is dropped on the block and tackle, or the slack is drawn out quickly, you move into impact loading. When you do that, stress loading goes up rapidly many times over, so a system that would have held together if the load was applied gradually might well fail.

If you think you are getting close to the breaking stength, up size your block and tackle several times over. It will be cheap comparied with the consequences otherwise.

You original question related to a soft shackle. That is not like a block and tackle. As I understand it, a soft shackle is a piece of line with a knot in one end and a small loop (generally spliced) in the other end and you shackle components together by putting one end of the line through/around items to be shackled together, then pass the knot throught the small loop, which is sized so the knot just passes through. When the load is applied, the system holds together because the knot slides up against the end of the loop. The load that such a system will hold far, far less than the breaking or working strength of the line (or multiple lines) in the soft shackle. Personally, I would not use such an arrangement in a high loading situation, except that I had tested the actual soft shackles in actual loading situations. I expect that commercially made ones have been tested and rated accordingly. Generally knots reduce breaking strength by about 50%, but the way a soft shackle is loaded is more likely to fail by shearing or tearing of the shackle. This would be a partial tearing of the shackle and would not approach the breaking (tensile strength) of the line comprising the shackle. Also, allowable shear stress loading of most materials is considerably less than tensile strength. Also, ropes have certain elastic properties and stretching is likely to be heavily involved in the failure of your soft shackle.

Last edited by NCC320; 01-30-2011 at 04:23 PM.
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  #29  
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When the tackle is rigged how many lines are supporting the load? That is your MA. So if 5 lines are supporting each line is holding 1/5 of the load. In theory you should be able to lift 5 X the SWL of the line. With a traveler secured to the deck when you are pulling the sheet up that line is also supporting the load. But using that same tackle and the traveler was on cabin top and you are pulling down that sheet does not support the load. So that tackle maybe 5 to 1 on cabin top but 6 to i when rigged to the deck.
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Quote:
Originally Posted by Boasun View Post
Never happen because the link will not fit through the block. And the line will part at it weakest point.
Ok Boasun, I have a block that is fo 2 inch line, I'm using 1/4 line and a link, the link will fit, how strong does the link need to be?
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