How much mechanical advantage from this pulley setup

[lumberjack]

Stop being difficult, Scott.

 

[NickfromWI]

I have to take off to go look at a dying pine tree, but when I get back I'm gonna set up that 8:1 (or at least a 4:1) compounded system (the third one in burnhams's pic). With scraps and equipment I already have, I should be able to create the same MA as the $100 fiddle block setup I'm looking at getting.

The fiddle block setup from Sherrill is over $200!! You mean to tell me you can get the same MA with $20 in rope, 3 carabiners and 2 micropulleys? (that's less that $100 total)!!!

love
nick

 

[Bounce]

Yup, you sure can. On the other hand though, you have to custom make that kind of system every time you use it. You cannot have it pre-set up, ready to go the way a fiddle block can be. You have to figure out what lengths each strand should be, tie the knots, attach the pulleys, etc. rather than just get it out of the bag and use it.

 

[NickfromWI]

Once set, it couldn't be bagged and reused?

 

[Bounce]

Not really. I mean, what are the chances that the length of the line you need from load to anchor point will be exactly the same distance every time?

 

[Marc-Antoine]

If you connect the rigging line to the MA system with a prussik, you can really use a pre-set compound system, because you put the prussik at the right place to match the MA's length.

You have to set it each time if you try to lift loads with only the MA system. One day pull out the engine off the car, second day, lift rounds over a wall. It's surely different lengths..

 

[NickfromWI]

If my anchor point is a prusik hanging off the rigging line that is hung at least 20-30 feet up, it seems that it would be a known distance every time.

I'll probably figure out what you're talking about once I set it up...

 

[TheTreeSpyder]

"Nature to be commanded, must be obeyed" - sir Francis Bacon​

i always picture an aquarium, with move able end. You can pass a volume of distance x power = workforce into the aquarium and let it spread far, and it would have low pressure on glass. But, if could push end of aquarium inwards towards opposing wall, it would take more and more pressure as water rose vertically, but the length of water would be shorter (horizontally). The Amount of work force input/ poured in as liquid into aquarium, would be the same, just could choose hi tension on glass or far spread combinations. All machines do same, manipulate the power x distance multipliers of the total force x distance input. There is always some 'tax' / loss, this 'births' other things. Besides the loss of efficiencies, all other losses or gains are illusionary,as they are just trade offs, for the system is always in balance (or then moves until it is so).

Potential, is the sum total of force passed to the system.

If 100% efficient (impossible) then potential = power/distance

But, potential - inefficiencies (friction) = power/distance achieved.

Burnham's drawing is great, but the 1st 3 pix limit pulley travel considerably; limiting utility.

we kept a 3:1 and 5:1 and prussics to deploy piggy backed as 'compression jigs' to tighten another line for rigging down, lifting. Mostly used vertically, so can only push up so far, so yes consistent size. But, in pulling over trees, then travel can be different, so would want to build more to suit.

Another way to look at all this, is if 3 equally loaded lines are pulling 600#, then each 1 is of course at 200# tension, so takes 200# to empower system, to hold 600#. To move that 600# 1' though, you'd have to shorten each supporting/pulling line by 1', so would pull 3' of rope thru system at 200# tension pull, to move 600# 1'.

So, really all are equal is the point:
input = 3' travel x 200# tension = 600# foot/pounds work force
output = 1' travel x 600# tension = 600# foot/ pounds work force
so input work force= output work force (potentially), so rule is no free ride, energy is neither created nor destroyed only held or passed/ converted.. only get back what you put in, the machine just converts the same volume of force to different distance x tension combinations!

In reality it is input work force - inefficiency = output work force; a machine can't create more work force than input, in fact can't even create the same volume, all ways and always sum loss. There is no perpetual motion machine, the Kennedy bullet did eventually stop.

Lifting one self while climbing is more of a closed/ self contained system, conserving even more force to target than usual

Same MA in cheap stuff potentially. But, expensive might be more efficient/ less friction system for higher yield of that potential. If pulley is 85% efficient, it might co$t 2x as much for same @95% efficient. That 10% might not seem worth it, but, in a multi pulley system, the inefficiencies start eating up so much power, efficiency really starts to matter. Another factor in efficiency is the pulley diameter, larger is more efficient over same axle, as has more leverage. Leverage over axle friction is calc'd from axle/bearing to the part of the line carrying the tension. So, micro pulley that is efficient can b e real expensive, as has size set against efficiency.

Also, Banjos allow for more direct inline pull, so less efficiency loss there. Rope only works on the inline axis and only in tension direction; so there would b e some inefficiency in line deflection from straight as ran thru sheaves/pulleys, banjos minimize this. If rope is deflected from inline, then not all of it's force can be transferred (so slings spread from center crane hook 3' wide are less leveraged that if same slings are spread to 4' to support same log. Also, in some pulley lay outs, lines rubbing/scrubbing can be an issue, dropping efficiency, but, banjos tend to eliminate this.

Rope choice would be maybe double braid that was flexible enough for pulleys, but low-no stretch, so don't fight to stretch jig line, just devices altered outside of rig.

Old Drawing on pulleys, have much more towards multiple pulls of pulley systems on pulley systems, '2handing' etc. for higher outputs:

http://www.mytreelessons.com/images/Force%20Patterns%20of%20Pulley%20Systems.GIF​