Purchase calculations

   This is 6:1 surely?  3:1 on the black line which itself is pulling the 2:1 green cascade. 

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Guests Send Private Message Find Member's Posts Guest Group	Post Options Post Reply Quote Guests Quote Reply Topic: Purchase calculations Posted: 14 Jan 19 at 1:46pm
	This is 6:1 surely? 3:1 on the black line which itself is pulling the 2:1 green cascade.

Guests Send Private Message Find Member's Posts Guest Group	Post Options Post Reply Quote Guests Quote Reply Posted: 14 Jan 19 at 1:58pm
	The black line does make up three purchases, but only two of those purchase are pulling on the green line. I.e. if you pull 10kg of tension into the black line. You now have 30kg of load pulling the caps on. Two of those 10kg loads also pull on the green line, so that must have a 20kg load. So, total load when you pull 10kg on the black line is 50kg (30kg on the black, 20kg on green).

Guests Send Private Message Find Member's Posts Guest Group	Post Options Post Reply Quote Guests Quote Reply Posted: 14 Jan 19 at 2:08pm
	That would give 20kg on both green lines (40kg total in the orange).

Guests Send Private Message Find Member's Posts Guest Group	Post Options Post Reply Quote Guests Quote Reply Posted: 14 Jan 19 at 2:22pm
	Yes, there is 40kg load on orange. Made up of 20kg on either green line exiting the sheave. 20 kg on one green line is going directly to working block, and the other 20kg is being distributed evenly between the two black lines exiting the cascading sheave. You can't double count those 2x10kgs on the black line coming from the cascading block as they are the same 20kg that is on the green line. So, you either say for a 10 kg pull on black you get 20kg on green + 20kg on green + 10 on black = 50kg on purple. But that's the same as saying 10kg on black, plus 10kg on black, plus 10kg on black, plus 20kg on green = 50kg. But whichever way you add them, it's still 50kg of load that gets pulled on purple, for 10kg of load pulled on black. Edited by mozzy - 14 Jan 19 at 2:48pm

Sam.Spoons Send Private Message Find Member's Posts Really should get out more Joined: 07 Mar 12 Location: Manchester UK Online Status: Offline Posts: 3398	Post Options Post Reply Quote Sam.Spoons Report Post Quote Reply Posted: 14 Jan 19 at 2:49pm
	Which end is fixed? If the top is fixed the black fall is irrelevant so it's 4:1 if the bottom is fixed the green line is irrelevant and it's 3:1
	Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish"

Guests Send Private Message Find Member's Posts Guest Group	Post Options Post Reply Quote Guests Quote Reply Posted: 14 Jan 19 at 2:52pm
	Bottom is fixed to mast, top is cap shrouds (did say in OP). I get that the black fall would be irrelevant if the top was fixed (which its' not). That would make it a 2x2 (4:1) cascade) But why would green be irrelevant if it's the bottom that's fixed (which is the case)? Edited by mozzy - 14 Jan 19 at 2:54pm

Sam.Spoons Send Private Message Find Member's Posts Really should get out more Joined: 07 Mar 12 Location: Manchester UK Online Status: Offline Posts: 3398	Post Options Post Reply Quote Sam.Spoons Report Post Quote Reply Posted: 14 Jan 19 at 3:04pm
	Because it is fixed it can only act as a turning block not as an additional purchase (I think).
	Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish"

jeffers Send Private Message Find Member's Posts Visit Member's Homepage Really should get out more Joined: 29 Mar 04 Location: United Kingdom Online Status: Offline Posts: 3048	Post Options Post Reply Quote jeffers Report Post Quote Reply Posted: 14 Jan 19 at 3:26pm
	Something like this in real life. Yellow rope is fixed to the mast, forged hook is the equivalent of the purple rope. If the cascade was after the 3:1 secondary it would be 6:1 but because it is the other way around it is only 5:1 because you add them together rather than multiply: Edited by jeffers - 14 Jan 19 at 3:39pm
	Paul ---------------------- D-Zero GBR 74

Sam.Spoons Send Private Message Find Member's Posts Really should get out more Joined: 07 Mar 12 Location: Manchester UK Online Status: Offline Posts: 3398	Post Options Post Reply Quote Sam.Spoons Report Post Quote Reply Posted: 14 Jan 19 at 3:32pm
	Ok if you said 5:1 you were right (can't quite work out how yet) edit :- @Jeffers got it now I do like being retired Edited by Sam.Spoons - 14 Jan 19 at 3:54pm
	Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish"

Guests Send Private Message Find Member's Posts Guest Group	Post Options Post Reply Quote Guests Quote Reply Posted: 14 Jan 19 at 3:51pm
	Exactly like that Jeffers. Sam, yes, it is a fixed block. But it's not the green line rounding the fixed block that is creating purchase. It is the green line rounding the top block that creates purchase. But as this is a cascade, the green line doesn't round the top block itself, but the force that it's carrying does round the top block, via the black line. In a simple block and tackle system you can can count moving block and double to get the purchase. But in a cascade you have to follow the forces / /velocities as obviously the lines isn't continuous through the system. Case example... moths have huge purchase cascades for Cunningham in the boom or under the deck, with the cascade going around a fixed block at the end of the boom. They do this to get a long enough run on their cascade system to give sufficient range. Edited by mozzy - 14 Jan 19 at 3:53pm