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Purchase calculations

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Sam.Spoons View Drop Down
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 3:59pm
Thanks gents Thumbs Up I will now go away and rethink the Cunningham and Kicker on the Blaze as they both lack travel but could do with a bit more advantage too.
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mozzy View Drop Down
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 4:28pm
Thanks for making a mock up! I'll use those pictures for the class if anyone asks! 

Also, top makes to oatsandbeans, got it right in a matter of minutes. 

The system is what's used on most 800s. But class rules say 4:1. But the whole thing is stupid as it means having a moving cleat-block thing (trapeze adjuster) rubbing up against the mast. There's metal chafe pads but they're always being ripped off by the cleat moving around.

It seems much more sensible to have a 4:1 cascade the traditional way up, and a cleat on the mast. Then you only have two very small blocks (or friction rings) moving around. 
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pondlife1736 View Drop Down
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Post Options Post Options   Quote pondlife1736 Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 5:23pm
Easy way to think about it is draw horizontal line through the cascade and sum the tensions. Above the middle pulley you get 3xblack + 1xgreen. Below you get 1xblack+2xgreen. Green =2xblack so in both cases you get 5:1 (assuming fall is vertical, take cosine of angle if not).
Don't think it matters which end is fixed.
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Post Options Post Options   Quote Peaky Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 5:30pm
Originally posted by mozzy





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. 


Ah yes, of course. Thanks!



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Sam.Spoons View Drop Down
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 6:49pm
What 'e said  Wink

On my mock up the bottom was fixed and the top on a bungy. And you're welcome to use the pics.
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mozzy View Drop Down
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 6:50pm
Originally posted by pondlife1736

Don't think it matters which end is fixed.

I think the reason it matters is because if the top was fixed, then the 10kg you pull on the fall line would not count in the system.  
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Do Different View Drop Down
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Post Options Post Options   Quote Do Different Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 8:23pm
After all that I'm inclined to think the system purchase is 4:1, that is to say the tension being applied to the shrouds is four times the effort applied to the black line.

Surely: to be in balance forces are opposite and equal. Therefore your later diagrams showing 50kg of tension above the system and 40kg below the system cannot exist.




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mozzy View Drop Down
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 8:59pm
When you have hold of the tail, and are pulling it with 10kg of force, there is only 40kg of force on the fixed block at the bottom, but that is still 50kg of force pulling down on the cap shrouds. That 50kg is equalled by 50kg of tension in the cap shrouds.

You then let the line you are pulling slip back in to the cleat and the block at the bottom carries the extra load, so there is now no load on the black tail line, and 50kg on the fixed orange line, equalled to 50kg on the purple cap shroud line.  
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423zero View Drop Down
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Post Options Post Options   Quote 423zero Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 9:00pm
I was always told number of strings not counting one in your hand and cascade doubles, so 4to1 under that system.
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Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 14 Jan 19 at 9:19pm
It is quite simple if you think that the load is being taken by the various lines of rope. If you put a known force into one, by pulling the black rope, you can work out the loads in the others and add them up to get the total force produced by the system.

The loads in all the balck ropes must be the same. So that is x3 the applied load. The load in the green one is confusing some, but that must be double that of the black as it is conneced to the black by a simple 2:1 arrangement. So that makes 3+2. That is it
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