Sorry double post, Regards Frank.
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Sorry double post, Regards Frank.
See you live in Balmain, I was born and bred in Balmain, still have a soft spot for tiger territory.Quote:
Originally Posted by RR P38
Now by what law of physics do you double energy by using a snatch block in the way you describe.
FYI to gain a Mechanical Advantage (MA) using a single sheave block you have to first be pulling to "Advantage", i.e. pulling in the same direction that the load is moving.
Second the single sheave block must be moving with the load.
The formula for working out the MA achieved using single/multiple sheave blocks is quite simple, MA= number of parts of rope SUPPORTING the MOVING BLOCK.
If your statement Quote: " Correction here.
If you put the snatch block on a tree and run your winch wire off and through the snatch block and back to the recovery point on your vehicle you gain a 2 x reduction. ie if you have a 5000lb winch in theory you have 10000lb pulling power."
is correct then if you sat your arse on the ground, tied one end of a rope around your waist (you representing the towed vehicle). Then ran the rope to a snatch block anchored to a tree (anchor point) and back down to your hands(representing the winch) and you weighed 100kilos. then you are saying that you would only require 50kilos of pulling power from your arms (winch) to drag your arse along the ground.
How does that compute, regards Frank.
I just fitted a TJM 9500 to my car last week, I'm not sure how much it weighs, but I reckon it's light enough to build a cradle for it and have it very portable,, you could make it so it fits either end of the car, or attatched it to a tree for use as well,Quote:
Originally Posted by Stuart02
Chops
It seems to compute simply to me..Quote:
Originally Posted by Tank
there are 3 forces:
Rope A - between vehicle and pulley
Rope B - between winch and pulley
Rope C - between pulley and anchor
By simple physics the forces must balance. A and B both have force of 50kg on them, so C must have 100kg to balance.
Steve
Steve
Steve somewhere on the forum there is an OH&S guide for riggers, go to P.30 and read all about it, then get hold of the writers and tell them they are wrong, or if you wish send me your email address and I will email it to you, Frank.Stanton@bigpond.com Regards Frank.Quote:
Originally Posted by steveG
The winch is still only seeing half the load was the point i was trying to make.Quote:
Originally Posted by steveG
Half of the load is being supported by the tree and the other half is the vehicle.
So a 5000lb winch single line pull is 5000lbs
Double the parts (line) and it can pull 10000lbs
Sorry my mistake i should have made it more clear YES you need 2 snatch blocks, one on the recovery point on your vehicle and one on the tree.
Balmain aint what it used to be.
Yeh, the coffee latte set have taken over, I wish I still owned my old house though, the garage and back yard was subdivided and sold for $1mill., the front half would be worth $2mill. by now, right on the water, sigh, Regards Frank.
I don't get it either, I know Frank would be right and even after watching the first part of this Video which explains it for the simpleton (me)
In my head I still think If I reel my winch cable out to a tree thru a snatch block and back to the car then the winch has to reel in twice the cable so it takes twice as long to get to the tree, isn't it going to be easier on the winch and the rope, Even if the MA =1 ???
or am I just wearing the winch out by making it go twice the distance
what am I missing ??
[ame=http://www.youtube.com/watch?v=vSsK7Rfa3yA&feature=relmfu]Mechanical Advantage (part 3) - YouTube[/ame]
funny how the guy is talking about pulleys but he dosent know what an actual pulley is called so he refers to it as a disk :)
Goingbush, the simple formula is that if the block (snatch) is moving with the load then the MA = the number of parts of rope supporting the moving block, in the video drawings above where he went round the world to work out the MA the number of parts of ropes supporting the moving block was 2 therefore the MA is 2. In the first scenario the MA is one, because the block is doing no work, other than changing the direction of pull. The only advantage is that as you are retrieving twice as much rope it takes twice as long and you have less layers of rope on the winch drum, if you are using one, but you are pulling in the opposite direction that you want the load to move so you are pulling to disadvantage.Quote:
Originally Posted by goingbush
A simple way to remember, is that if the block is moving with the load it is doing work, how much is governed by the number of parts of rope supporting that moving block.
In rigging you would normally use multi sheaved blocks that have more than one pulley wheel, just look at large cranes with many parts of rope supporting the moving block.
In theory you could move a load weighing 100's of tonnes with a bit of string, if you had enough multi sheave blocks. But that theory in real life is not possible because of up 20% friction on each pulley wheel so the more pulley wheels the more friction.
For instance if you attatched a single sheave block (snatch block) from a beam in your garage and put a piece of rope say 3 metres long through the pulley and tied a bucket full of sand to each end and each bucket weighed 10kgs, they would balance out equally and support each other. Therefore you have 10kgs on each part of rope, to move one bucket up you would need to apply pressure of around 2kgs to overcome the friction generated (up to 20% of load) in the pulley bearing/shaft.
That means to lift 10kgs through a fixed single pulley you need to apply 12kgs to the opposite side, which means that the rope holding the pulley to the roof beam 10+10+2=22kgs, it is important that the rope holding the snatch block can support the weight which is double the original load, in this case no net gain and no mechanical advantage so it is MA=1, hope this helps, Regards Frank.
Your example of lifting a weight from a single pulley on a beam is a different scenario than a winch on a vehicle with the line anchor point attached to the same vehicle (which effectively is pulling in the direction of travel).
If you took the winch off the vehicle and anchored it to a tree - then you would have the same scenario as your beam lift. But that's not the case.
The winch on the front of the vehicle IS pulling to advantage - if you stopped the snatch block pulley from rotating the winch would pull the vehicle forwards.
With the pulley free, the tension from the winch is felt on the vehicle via the fixed end of the line - the same as it would if the winch was attached to another anchor point (not the vehicle).
So there are 2 forces pulling the vehicle forward. The winch pull, and the pull from the line coming back from the block.
Is that not correct?
Steve