Knotty question - settle an argument ?

[Observer]

Following on from this post, can somebody clarify whether the truckers' hitch provides a 3:1 purchase or 2:1 purchase?

I am discussing the merits of this hitch with the website owner - I believe there is a better way of doing it but I also mentioned that I thought the hitch has a 2:1 mechanical advantage not 3:1. Am I wrong?

 

[Dave_Seager]

3 to 1, provided that you are standing on the ground and not riding on the cover that is being tightened. If you shorten the line you are pulling down by 3" the cover will move down 1".

If you simply pulled a single line attached to the cover, that would give 1 to 1.

 

[eyehavit]

There are variants of a truckies hitch and you can check the ratio yourself. When the rope is being tensioned, measure the rope movement on the tensioning side compared to the tensioned side. If 2:1 the tensioning distance will be twice that of the tensioned distance and so on.

 

[Observer]

[ QUOTE ]
If you simply pulled a single line attached to the cover, that would give 1 to 1.

[/ QUOTE ]

Yep. So if you pull the line at the point it emerges from the loop, that's 2:1 innit?

Say at the starting position, the bitter end is just touching the ground. Pull the cover down 1" there will be 1" of the bitter end on the ground before you tighten the line. Then you have to pull 2" through the loop in order to tigten the line. Therefore 2:1.

 

[stevebrassett]

But the bitter end has moved 3" for a movement at the cover of 1" - therefore 3:1. To look at it another way, if you held the bottom of the loop nearest you, and pulled on the bitter end, you would have 2:1, as it passes through the top loop. Having the running part go through 2 loops gives you 3:1.

Did that make any sense?

 

[Observer]

[ QUOTE ]
Did that make any sense?

[/ QUOTE ]

Nope - still struggling. In my example, the 1" of the bitter end on the ground is simply the 1" downward movement in the cover. You can't count it twice.

If I've placed a £1 bet and get back £3, that's 2:1 odds, not 3:1. Same sort of thing innit?

 

[Deleted User YDKXO]

Agree with you, observer. I can only see 2 falls of rope, you cant count the end bit after the loop. 2:1 advantage as far as I can see
In fact I cant see how you can get a 3:1 advantage. If the rope when down and around the hook again, the advantage would be 4:1

 

[Observer]

[ QUOTE ]
Agree with you, observer. I can only see 2 falls of rope, you cant count the end bit after the loop. 2:1 advantage as far as I can see
In fact I cant see how you can get a 3:1 advantage. If the rope when down and around the hook again, the advantage would be 4:1

[/ QUOTE ]

Thanks Mike and I've found this which confirms it. I'd forgotten all the pulley stuff but it's quite clear with the refresher.

 

[Observer]

[ QUOTE ]
[ QUOTE ]
Agree with you, observer. I can only see 2 falls of rope, you cant count the end bit after the loop. 2:1 advantage as far as I can see
In fact I cant see how you can get a 3:1 advantage. If the rope when down and around the hook again, the advantage would be 4:1

[/ QUOTE ]

Thanks Mike and I've found this which confirms it. I'd forgotten all the pulley stuff but it's quite clear with the refresher.

[/ QUOTE ]

But there again, thinking a bit more, the 'load' in this case is acting upwards and the direction of pull is downwards so that would make it 3:1, according to the link I posted - erk!

 

[Riggy]

I can't settle the argument as that seems to have been done by the more experienced....but when I was tying down lorry sheets years ago I was only taught one and that was called a "Dog(s) Leg".....would that have been a variation of of the Truckers Hitch??
Riggy.

 

[stevebrassett]

Yep, that's it. What you have is an iverted figure 4.

 

[stevebrassett]

Or even inverted.

 

[Deleted User YDKXO]

Yes, I was thinking the same thing after looking at the website but it depends which is the load and which is the fixed point. Time for some experimentation I think

 

[hlb]

With a Dolly as we have always called it. You can make as many dolly's as you want on a rope, each time doubling the force fore equal pulling power. Course you will loose some through friction. Cant remember how many trucks I've sheeted and roped in the past. Never looked into the science of it though!!

 

[aztec]

sure it's 2:1 "rove to advantage" as you're pulling the same way as the standing part.

remember though it's not exact as the friction needed to hold the turns, also works againt you in terms of force V's load.

could be wrong.. wouldn't be the first time.

steve.