Mechanical advantage of a moving pulley vs an anchored pulley

[Rolacycle]

I believe that this will be the limit to the effective MA of P6:
When the angle of the ropes running through a pulley are at 0 degrees, the force will be doubled.
When the angle of the running lines is not 0 degrees, the effectiveness of the pulleys is diminished.
When the angle of the running lines reaches 120 degrees, there is no more mechanical advantage in that pulley.

Look for "120-degree rule" here:
https://www.frostburg.edu/faculty/r...s_preppers_chapters/Ch15-PulleySystems_v2.pdf

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Yes closer to lifting limit height the MA will completely disappear. But once it’s raised and off ground all I have to do is stop pulling it up when it gets heavier

 

[hutchphd]

What you don't seem to appreciate is that (if the pulleys are "frictionless") as soon as the load is off the ground (and static) the tension in all segments of the rope is the same. So your initial diagram is impossible...in particular pulley 6 will be pulled about 5 ft lower when the forces equilibrate (and you will need >30lbs tension to lift it initially and more as you go up). If you need more MA then you need to change the design. What are your requirements?

 

[FactChecker]

Slack like unused rope used to raise load. Well it’s a deer feeder and it doesn’t refill itself so load has to be dropped and doing so at 6:1 rope length the amount of rope to lift load makes a lot of slack when is laying on ground. And when u re raise load slack with be pulled out of pulleys in 6,5,4,3,2,1 order thus lifting the p6/3 (1st load) combo before raising the p4/2 (2nd and primary load)

and yes the top doesn’t have to be at perfect 180° plain. But the separated anchor is to keep wind at bay and maintain centeral vertical lifting point. MA is nice but honestly 100lb I could life with no advantage but 4:1 my 12yr old daughter can refill it

I noticed that the pulley system in post #27 can only raise the weight 1/6 of the total height.

 

[Rolacycle]

What you don't seem to appreciate is that (if the pulleys are "frictionless") as soon as the load is off the ground (and static) the tension in all segments of the rope is the same. So your initial diagram is impossible...in particular pulley 6 will be pulled about 5 ft lower when the forces equilibrate (and you will need >30lbs tension to lift it initially and more as you go up). If you need more MA then you need to change the design. What are your requirements?

Yes correct. But that only happens after all loads are suspended. And as I pointed out p6/3 is a load and raises1st. Right?

And my requirements are it be stable lifted, withstand moderate to high winds and raise and lower in the middle of poles. I had same 6 pulleys on single pole version but without offset from pole wind and it rubbing pole KO’d rope within 3weeks.

 

[hutchphd]

I don't understand what you mean. Assuming "massless" rope and pulleys and "frictionless" pulleys and that you are lifting slowly: as soon as there is any lifting force on the load (i.e. no slack) the rope geometry is fixed. The other way to say this is that the geometry is the same for any sized load. Obviously as the load moves the geometry changes. But at that initial point the top line is bowed by ~5 ft and the MA suffers as per my estimate. If that MA is sufficient then OK. So what are the requirements? If your design is sufficient (with added stability requirements which this will meet I think) then what is our purpose here?

If you just want to understand better may I suggest you rig this with 10 lbs load and lightweight ropes and play...the physics is the same. Will be glad to provide analysis.

 

[Rolacycle]

I don't understand what you mean. Assuming "massless" rope and pulleys and "frictionless" pulleys and that you are lifting slowly: as soon as there is any lifting force on the load (i.e. no slack) the rope geometry is fixed. The other way to say this is that the geometry is the same for any sized load. Obviously as the load moves the geometry changes. But at that initial point the top line is bowed by ~5 ft and the MA suffers as per my estimate. If that MA is sufficient then OK. So what are the requirements? If your design is sufficient (with added stability requirements which this will meet I think) then what is our purpose here?

If you just want to understand better may I suggest you rig this with 10 lbs load and lightweight ropes and play...the physics is the same. Will be glad to provide analysis.

Your right you don’t. It’s not so much about mechanical advantage over the 100lb load. It’s not about having exactly correct diagram of pulleys, the rope tensions or their relations to each other or varying pulley angles created . It’s not about having most MA possible. Its about keep the load at center between the 2 poles no matter what height I raise the system and this isn’t for school it’s for real life. It’s only required to work it doesn’t need to meet any other requirements or MA minimums short of lifting load out of reach (from my cows, wild hogs and deer). Be stable in wind (spinning and swinging) in wind. I was looking for better way to raise load to center of pole (any height above 6’) not why this isn’t perfect system, that it loses MA as load is raised. Forget MA that’s literally least important part of system and is only bonus not requirement

 

[Lnewqban]

When I first read the OP, I believed that a roller hook and a truss between the poles will do the trick, bit resisting the bending of +100 pounds between 15 feet requires something robust.

Now I believe that a strong rope anchored to top of both poles will not stretch/bend much.
A pulley-hook that is free to roll sideways over that horizontal rope, will self-center between both poles when load is applied.
You could limit that lateral translation with some sort of adjustable collars, in such a way that the rig does not slap either pole under high wind conditions.
Because of that, the diameter of the rigging rope, which tension will be up to @30 pounds (considering friction and MA=4) could be much smaller.

 

[Rolacycle]

When I first read the OP, I believed that a roller hook and a truss between the poles will do the trick, bit resisting the bending of +100 pounds between 15 feet requires something robust.

Now I believe that a strong rope anchored to top of both poles will not stretch/bend much.
A pulley-hook that is free to roll sideways over that horizontal rope, will self-center between both poles when load is applied.
You could limit that lateral translation with some sort of adjustable collars, in such a way that the rig does not slap either pole under high wind conditions.
Because of that, the diameter of the rigging rope, which tension will be up to @30 pounds (considering friction and MA=4) could be much smaller.

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I already have 100’ of 3/8 braided polyester cord rated for 600lb of load.

30lb force to lift aint 16:1 MA. But thank goodness I put my big girl panties on every morning (their 15lb with no MA involved)

Collar? I’m not sure I understand? Strap attaching lowest part of load to pole?

 

[hutchphd]

The rig as designed will self-center.
It will not look like your drawings for reasons I have stated.
This is all relatively easy to calculate. But without actual specifications why bother? Good luck.

 

[Lnewqban]

Collar? I’m not sure I understand? Strap attaching lowest part of load to pole?

Sorry, I should have said rope stop instead.
Just something that clamps the heavy horizontal rope, giving the hanger pulley some room to self-adjust, but not so much that the ropes and pulleys could hit the poles.

 

[Rolacycle]

Sorry, I should have said rope stop instead.
Just something that clamps the heavy horizontal rope, giving the hanger pulley some room to self-adjust, but not so much that the ropes and pulleys could hit the poles.

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Oh ok I savvy. I’m a mechanic & roofer by trade and cattle rancher by birth none of which require complex pulley system knowledge.

I appreciate the insight I’ll have to see if cAn source one those locally. I’m otw the beef patch this afternoon to finish rigging whatever it ends up being today. Honestly I’m most excited about adding baby monitor oink oink and solar battery charger. I think I’ll only have to fill it twice a month.

 

[Rolacycle]

Hah I just realized welding rebar to eyelets at top of pole is easy quick and I’ve already got everything. Also minimal bending

 

[Lnewqban]

Hah I just realized welding rebar to eyelets at top of pole is easy quick and I’ve already got everything. Also minimal bending

Additional info about calculating forces and angles for that top horizontal rope:
https://www.ropelab.com.au/highline-tension-calculator/

Best luck with all projects!

 

[sophiecentaur]

I have to bleat about my ongoing problem with the PF loose use of the term "Mechanical Advantage". That term includes all dead-weight and friction. The back of an envelope ratio that people often rely on, using just the geometry of the situation, is (at least was) called the Velocity Ratio. Of course the MA is always less than the VR and the ratio MA/VR is the Efficiency. Many useful machines work, purposely, with a very low efficiency so that the Load doesn't slip back down again when the Effort is removed. (e.g. a wood screw or a car scissor jack)
@Lnewqban I'm late to this party but I like the 120 degree rule. Never thought of it that way before.