Z-Drag Pulley System Question

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?

are you trying to minimize travel on the haul line or the object line?

Haul, I'd think - and does placement of the larger pulley have any bearing on efficiency?

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

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).

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.

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?

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?

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?

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

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.

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

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

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.