Reeving a Tackle to Advantage -- What am I Missing?

[cal40john]

Originally Posted by Lodesman

Both images have two parts at the moving block; both only have 2 lines being shortened; both have the same line pull at the hauling part. See image, and tell me where the third line is being shortened?

The second picture you point at the block that is not moving, point at the block that is moving, there are 3 lines sharing the load.

[Seaworthy Lass]

Originally Posted by Lodesman

Both images have two parts at the moving block; both only have 2 lines being shortened; both have the same line pull at the hauling part. See image, and tell me where the third line is being shortened?

In the second image there are three parts at the moving block. You have drawn in the third part at the block, but are choosing to ignore it .

[conachair]

Originally Posted by Departing2017

Lass,
Could you show a sample of this calculation? I would love to be able to add this into my lectures. Currently, i just use some sample values that I memorized, e.g. when the line enters/exits the pulley at a 45 degree angle (rather than parallel) the MA of the pulley is reduced from 2:1 to around 1.4:1 (which may or may not actually be correct).

If the the exit angle is A then the force is cos(A). So if the rope is pulling dead in line (0deg) you gain 1. If it's off by 45deg you gain 0.707 and pull the load sideways by the same force.
You can do it by drawing and measuring as well.

[Seaworthy Lass]

Originally Posted by Seaworthy Lass

In the second image there are three parts at the moving block. You have drawn in the third part at the block, but are choosing to ignore it .

Edited to add: Conachair was quicker typing and has said the same thing .

[Bill Seal]

Keerist on a Krutch!!! You are over analyzing this thing.

If the becket is on the block that doesn't move, relative to the standing still puller, that is the "standing block". Otherwise, if it moves, that is the "moving block".
Hence the name.

In the first illustration, if the puller were on the boat, it would be a 3:1 purchase.

[Lodesman]

Originally Posted by conachair

If the the exit angle is A then the force is cos(A). So if the rope is pulling dead in line (0deg) you gain 1. If it's off by 45deg you gain 0.707 and pull the load sideways by the same force.
You can do it by drawing and measuring as well.

The force is applied in drawing the two blocks together - the angle at which the hauling part leaves the block plays no part in it at all the line pull remains the same.

[Departing2017]

Originally Posted by conachair

If the the exit angle is A then the force is cos(A). So if the rope is pulling dead in line (0deg) you gain 1. If it's off by 45deg you gain 0.707 and pull the load sideways by the same force.
You can do it by drawing and measuring as well.

Does this look right?

[charliehows]

Originally Posted by Bill Seal

Keerist on a Krutch!!! You are over analyzing this thing.

If the becket is on the block that doesn't move, relative to the standing still puller, that is the "standing block". Otherwise, if it moves, that is the "moving block".
Hence the name.

In the first illustration, if the puller were on the boat, it would be a 3:1 purchase.

yeah +1

[conachair]

Originally Posted by Lodesman

The force is applied in drawing the two blocks together - the angle at which the hauling part leaves the block plays no part in it at all the line pull remains the same.

We must be talking about different things here. If the block the hauling part goes into first is attached to the piece your trying to move then yes, of course it makes a difference.

[Seaworthy Lass]

Originally Posted by Lodesman

The force is applied in drawing the two blocks together - the angle at which the hauling part leaves the block plays no part in it at all the line pull remains the same.

It certainly does play a part. See diagram below (poor ipad snapshot, tell me if you need it photographed better).

Lodesman, in the first example I have shown, do you really think the purchase is 1:1 not 2:1?

[conachair]

Originally Posted by Departing2017

Attachment 100287

Does this look right?

Looks right to me, but I've been wrong many times Anyone else care to join in?

[Seaworthy Lass]

Originally Posted by Departing2017

Attachment 100287

Does this look right?

Yep, that is right, but it is easier to think of A as being the angle of pull off parallel as shown here rather than a remote spot:

Total purchase in this example = 1 + cosA

When the pull is parallel to the other lines A=0 and cos0=1, so 1 is added to the purchase.
When the pull is perpendicular to the other lines, A=90° And cos90°=0 so nothing is added to the purchase.

[Lodesman]

I'll add another picture. In the drawing top left, a simple overhead pulley is shown - you need to exert the equivalent force of the weight of 'w' to the line in order to lift it. I assume we can all agree to that?

So that is no different to the drawing below it, other than that force is being applied straight up. Therefore the pulley at the standing block offers no mechanical advantage in itself - it just redirects the direction of force W. I assume we can all agree to that?

By the same measure if you add a "moving block" regardless of whether it is on the object being moved or attached to the fixed point, the "standing" pulley adds no mechanical advantage to the rig - see the middle and right images. Whether pulled up or through the standing pulley and down, the force required at the hauling end is 1/2W.

If you go back to the OP's image - or my picture of it - the green lines indicate the total rope that can be pulled out through before the blocks are chock'a'block. In either case you can only get the mechanical advantage from the green ropes - the hauling end cannot add to advantage unless is rove again through the moving block.

The mechanical advantage is derived entirely from spreading the work over a greater distance, thus dividing the force by the same factor the distance has been increased - distance in this case is the amount of rope that can be pulled through the tackle to effect the work.

[Lodesman]

Originally Posted by conachair

We must be talking about different things here. If the block the hauling part goes into first is attached to the piece your trying to move then yes, of course it makes a difference.

The third drawing is only 2:1

[Lodesman]

Originally Posted by Seaworthy Lass

It certainly does play a part. See diagram below (poor ipad snapshot, tell me if you need it photographed better).

Lodesman, in the first example I have shown, do you really think the purchase is 1:1 not 2:1?

Lass, you have a special way of misinterpreting what I write and passing that creation off as my opinion.

In the image you reference there are two parts at the moving block - so it is 2:1. I don't see how that is so hard to understand.