4:1 MA double sheeve pulleys

[MichaelXYZ]

I was rigging a 4:1 MA double sheave pulley system when the training officer walked by and said use the double loop. I told him that I was not familiar with that so he coiled the rope in two loops and said, now you can't make a mistake and he walked off. Well as he left I felt dumb cause I did not see it and rigged it the way I knew would work, but I really wanted to understand his method but was afraid of looking dumb so I did not ask.

Can anyone explain this method if you know what I am talking about?

BTW we were rigging a system like in image.


Thanks

[FiremanLyman]

Coiling the rope into a double loop will prevent you from crossing the lines over each other, adding unwanted friction into the system. With the rope into two loops then you place the duble pullys into the system, opening the bottom sheeves first then the top sheeves. It just prevents you from crossing the rope.

Another tip, build the 4:1/5:1 small, only inches apart from each pully. Leave it small (collapsed) until you need to add it into the system, it will reduce one trip hazard and add to a "clean kitchen" effect that will increase effeciency of most operations.

[MichaelXYZ]

Thanks a lot for taking your time to answer my remedial questions. It is appreciated. I think I will give that a try on the stuff I have at home.

[MichaelXYZ]

I think I got the idea, I tried it at home, so if I am correct, the inside loop goes on the top sheave, and the outside loop to the bottom. One thing I am unable to try on my setup id that at work, we have some RSI CSR pulleys with a rope guide and break cam. I will see if I can attach some photos.

[servantleader]

I was rigging a 4:1 MA double sheave pulley system when the training officer walked by and said use the double loop. I told him that I was not familiar with that so he coiled the rope in two loops and said, now you can't make a mistake and he walked off. Well as he left I felt dumb cause I did not see it and rigged it the way I knew would work, but I really wanted to understand his method but was afraid of looking dumb so I did not ask.

Can anyone explain this method if you know what I am talking about?

BTW we were rigging a system like in image.
Attachment 22225

Thanks

Your training officer needs some training on how to train.

[MichaelXYZ]

I forgot to show the completed setup.

[MichaelXYZ]

Your training officer needs some training on how to train.

I take it, I did not do it right.

[Rescue 2 Training]

I could be mistaken, but I think it was more a statement that a good training officer should actually TRAIN you how to do something, not make a coil of some rope on the floor and walk away without further explanation.

When reeving a B & T like that, I find it easiest to orient the pulleys on the floor like you have them, and then reeve them "Bottom to bottom, Top to top".

I run the rope through both bottom sheaves first and the through both top sheaves; all in the same direction. Meaning, I will just run the rope counter clockwise for the entire process.

[servantleader]

I take it, I did not do it right.

It looks fine to me. It's correct.

What I meant was that there was no reason that your training guy needed to make you feel dumb just because you were taking your time learning something new. There's a big difference between showing a student you know how to do something versus teaching that student so that learning takes place. Perhaps I jumped the gun being critical of him since I wasn't there.

For the completed setup you show above, I would use a double overhand loop (AKA scaffold knot) instead of the figure-of-eight bight. It will give you a much more compact unit.

[servantleader]

I could be mistaken, but I think it was more a statement that a good training officer should actually TRAIN you how to do something, not make a coil of some rope on the floor and walk away without further explanation.

When reeving a B & T like that, I find it easiest to orient the pulleys on the floor like you have them, and then reeve them "Bottom to bottom, Top to top".

I run the rope through both bottom sheaves first and the through both top sheaves; all in the same direction. Meaning, I will just run the rope counter clockwise for the entire process.

You are not mistaken, that was what I was getting at.

[MichaelXYZ]

That double overhand loop does sound like a better way, I wonder how it will tie with 1/2" static kernmantle. I will give it a try.

Thanks

[FiremanLyman]

One thing I am unable to try on my setup id that at work, we have some RSI CSR pulleys with a rope guide and break cam. I will see if I can attach some photos.

Those style of pulleys are great for confined space work, can get them up high on a fully extended tripod and easily operate the progress capture with a laynard.

When reeving a B & T like that, I find it easiest to orient the pulleys on the floor like you have them, and then reeve them "Bottom to bottom, Top to top".

I run the rope through both bottom sheaves first and the through both top sheaves; all in the same direction. Meaning, I will just run the rope counter clockwise for the entire process.

Second this, go from bottom to top. Less chance of twisting up stuff.

Also you can get a few more inches of throw out of the MA if you either tie the figure 8 into the becket itself (delete that carabiener) or tie it into the carabiener that will go onto the top of the pulley. Those few inches add up; a few for that carabiener and knot + few for the bridal on the Fernno/Stokes/SKED = not clearing the patient up out of the hole.

[MichaelXYZ]

Also you can get a few more inches of throw out of the MA if you either tie the figure 8 into the becket itself (delete that carabiener) or tie it into the carabiener that will go onto the top of the pulley. Those few inches add up; a few for that carabiener and knot + few for the bridal on the Fernno/Stokes/SKED = not clearing the patient up out of the hole.

Thanks Drew. I just noticed I need to learn how to spell carabiner If you don't mind all the questions, I have one more. Would not the Loop directly on the becket cut into the rope, or put a kink in it?

Thanks

[Rescue 2 Training]

Thanks Drew. I just noticed I need to learn how to spell carabiner If you don't mind all the questions, I have one more. Would not the Loop directly on the becket cut into the rope, or put a kink in it?

Thanks

The loop directly to the becket will be fine. Remember, that end of the rope is only seeing 1/5 of the load. The rest of the load is divided amongst the other strands of the block and tackle.

[MichaelXYZ]

The loop directly to the becket will be fine. Remember, that end of the rope is only seeing 1/5 of the load. The rest of the load is divided amongst the other strands of the block and tackle.

I don't know why, but I have trouble with the math of pulley systems. To me I see 1/4 load on the becket. Is my drawing correct? What I am thinking is we have a compound 2:1 x 2 system here.
-------------------edit--------------
oh wait, I think I see it now. The top two anchors share the load, the bottom to also share the load leaving the becket to have the remainder of the load making five points or 1/5. Is that it? But wait, that does not work out either, final load should be 25 pound so um..dang still confused.

[MichaelXYZ]

In my need to figure this out, I found this online pulley simulator.
http://www.compassproject.net/sims/pulley.html It is pretty neat. Well, anyways according to the simulation, the load on the becket would be 1/4 total load. So with the NFPA 1983 two rescuer load number of 600 lbf there would be 150 lbf on the becket. To me, I would not feel comfortable with the rope directly on the becket. I would go with the double overhand loop and a rope thimble.
Thanks for playing

Sim results: Load = 10N Force = 2.5N

[EricUlner]

I don't know why, but I have trouble with the math of pulley systems. To me I see 1/4 load on the becket. Is my drawing correct? What I am thinking is we have a compound 2:1 x 2 system here.
-------------------edit--------------
oh wait, I think I see it now. The top two anchors share the load, the bottom to also share the load leaving the becket to have the remainder of the load making five points or 1/5. Is that it? But wait, that does not work out either, final load should be 25 pound so um..dang still confused.

Attachment 22249

Michael, what's of great importance for you to see that I think you're maybe missing is... What are you doing to your anchors? In your gif image, you say that the 2 anchors are sharing the load. Yes, they are sharing, but not equally. The anchor on the left is seeing 50 lbs. The anchor on the right is seeing 75 lbs. This assuming the following:
- ideal mechanical advantage, ie., zero friction pulleys.
- hauler is simply holding the load in stasis. Nothing moving.

I would also refer to the pulley system in your gif image as 4:1cd (change of direction).

Also, in order to have a compound pulley system you need to have a simple pulley system pulling on the end of another simple pulley system.

[MichaelXYZ]

Okay, I think I got it. The top pulley where the force is being applied has 25 on each side for 50lb on anchor. The other top pulley has 25 on each side plus 25 at the becket for 75. Overall weight at the anchor points is 125 lbf.
So it seems all the MA is from the bottom two pulleys 2:1 x 2, and the top pulleys are just change of direction.

Thanks.

[EricUlner]

Okay, I think I got it. The top pulley where the force is being applied has 25 on each side for 50lb on anchor. The other top pulley has 25 on each side plus 25 at the becket for 75. Overall weight at the anchor points is 125 lbf.
So it seems all the MA is from the bottom two pulleys 2:1 x 2, and the top pulleys are just change of direction.

Pulleys that are moving/traveling are either class 2 or 3 levers, giving either mechanical advantage or disadvantage. Anchored pulleys function as class 1 levers, giving you a 1:1 mechanical advantage against the load.

Stay psyched...

[EricUlner]

Otherwise, regarding your initial question about reeving... Consider a reverse reeve. Set your two double sheave pulleys on the floor, beckets facing each other. Orient one vertically on edge and the other horizontally/flat. Reeve back and forth same side top hole to same side top hole and bottom likewise. Very clean...