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Z-Drag Pulley System Question #276357
11/21/14 07:52 AM
11/21/14 07:52 AM
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tshan Offline OP
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I probably should know this, but in a z-drag pulley system where I have pulleys of different sizes, which pulley should be the larger diameter of the two? Pulley 1 or Pulley 2? Why?

[Linked Image]


Tom
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Re: Z-Drag Pulley System Question [Re: tshan] #276360
11/21/14 09:20 AM
11/21/14 09:20 AM
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Naples, FL
waterbug_wpb Offline
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are you trying to minimize travel on the haul line or the object line?


Jay

Re: Z-Drag Pulley System Question [Re: tshan] #276362
11/21/14 09:25 AM
11/21/14 09:25 AM
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tshan Offline OP
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Haul, I'd think - and does placement of the larger pulley have any bearing on efficiency?


Tom
Re: Z-Drag Pulley System Question [Re: tshan] #276364
11/21/14 09:29 AM
11/21/14 09:29 AM
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waterbug_wpb Offline
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and the prussiks are there to limit overall travel?

Seems way more complicated than it has to be unless you need to limit overall object movement


Jay

Re: Z-Drag Pulley System Question [Re: tshan] #276365
11/21/14 10:12 AM
11/21/14 10:12 AM
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Jake Offline
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what the heck is that for? I'm guessin that this is an incremental hauling system where you can pull the "pull here", reset "prusik 2" manually to keep the load from sliding backwards, reset "prusik 1", and pull again.

If I understand that correctly, you won't actually be pulling any load through pulley 2. All of your pulling load is transmitted through pulley 1 to prusik 1 - so make that sheave diameter on pulley1 as large as possible to reduce friction...you'll get no purchase advantage by sizing the sheaves differently - just a little less friction. Pulley 2 is just taking up the slack in the system as you pull at the "pull here" point - the load is moving by the load on "prusik 1" (it will be pulling with twice the force of the line you are pulling on).


Jake Kohl
Re: Z-Drag Pulley System Question [Re: Jake] #276366
11/21/14 10:18 AM
11/21/14 10:18 AM
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well...wait a minute...that was tricky...Both sheaves are loaded equally in that system. and because you are splitting the forces on the main line tied to the object, this system results in a 3:1 purchase.

So I am changing my earlier assessment....both sheave diameters will affect the system friction equally. Increasing either one will have an equal reduction of system friction. Different sheave diameters will not affect the purchase of the system...larger sheave diameters only reduce the friction because of the larger radius the line turns, more bearing surface, etc. That's my final answer.


Jake Kohl
Re: Z-Drag Pulley System Question [Re: tshan] #276367
11/21/14 11:11 AM
11/21/14 11:11 AM
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dave mosley Offline
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what is the purpose of Prusik 2? Unless its a bungee cant see the mechanical advantage in it


The men were amazed, and said, "What kind of a man is this, that even the winds and the sea obey Him?" Matthew 8:27





Re: Z-Drag Pulley System Question [Re: dave mosley] #276369
11/21/14 11:52 AM
11/21/14 11:52 AM
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Jake Offline
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Originally Posted by dave mosley
what is the purpose of Prusik 2? Unless its a bungee cant see the mechanical advantage in it


It parks the load while you reset prusik 1. I guess you would use this if you are trying to haul something a significant distance. You pull the line "pull here" until you can't go anymore (presumably because of some dimensional/distance limitation). While you hold the load at "pull here", you slide prusik 2 forward and it will hold the load while you reset prusik 1 for another pull. It lets you ratchet the load up in steps....or prusik 2 is some kind of safety to keep it the load from going too far backwards.

tshan...what are you working on over there?


Jake Kohl
Re: Z-Drag Pulley System Question [Re: Jake] #276370
11/21/14 12:01 PM
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waterbug_wpb Offline
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Keeping it from running backward makes sense... likely if you're a single person trying to perform the whole operation.

surely there must be a better way?


Jay

Re: Z-Drag Pulley System Question [Re: tshan] #276371
11/21/14 12:21 PM
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[Linked Image]

If I'm looking at this right, it's for easing a boat down a river?

The brake prusik would be adjusted as tension is applied to the haul line?

so the brake prusik holds the line while pulley 1 goes slack so you can slide that prusik up towards the boat. You then tension the "haul" line again and work the brake prusik toward the boat end?


Last edited by waterbug_wpb; 11/21/14 12:24 PM.

Jay

Re: Z-Drag Pulley System Question [Re: tshan] #276374
11/21/14 03:55 PM
11/21/14 03:55 PM
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tshan Offline OP
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The use I was reading about was removing a jammed boat in whitewater that had become stuck against a rock in the current. Someone asked the question of where to put the large diameter pulley, if it would make a difference. I figured you people would know grin


Tom
Re: Z-Drag Pulley System Question [Re: tshan] #276375
11/21/14 03:57 PM
11/21/14 03:57 PM
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tshan Offline OP
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It is called a "poor man's come-along", so it is an incremental hauler where it has to get "reset". It is 3:1.


Tom
Re: Z-Drag Pulley System Question [Re: tshan] #276376
11/21/14 05:04 PM
11/21/14 05:04 PM
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Isotope235 Offline
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Originally Posted by tshan
Someone asked the question of where to put the large diameter pulley, if it would make a difference.

Sheave size is dictated by line construction and diameter. Blocks with larger sheaves often (but not always) have higher load ratings.

Since the load is the same on both blocks, and the same line is running through both, it doesn't matter which block goes where.

Regards,
Eric

Re: Z-Drag Pulley System Question [Re: tshan] #276391
11/24/14 09:31 AM
11/24/14 09:31 AM
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Columbia South Carolina, USA
dave mosley Offline
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I dont know if thats a "poor mans come-along", I can get a come along at Harbor frieght for 16.99, Harken doesnt even sell anything for 16.99 anymore, except maybe the keyring shackle smile


The men were amazed, and said, "What kind of a man is this, that even the winds and the sea obey Him?" Matthew 8:27





Re: Z-Drag Pulley System Question [Re: tshan] #276398
11/24/14 01:45 PM
11/24/14 01:45 PM
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tshan Offline OP
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It is typically put together using climbing carabiners with the sheave/thimble slipped on them to turn the carabiner into a pulley. You can do this very low-tech and it be light weight (and fairly soft-sided) for your canoe or kayak. It is also used in mountaineering settings and most people have all these bits in their climbing gear.

You are correct - you could put a very expensive version of this together, if you wanted to.


Tom

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