Thread: Hand vs electric winch: solo travelling

Originally Posted by Tank

You posted this while I was typing my last post, all I ask is do some research before you make a statement and don't take it out on me, I'm only telling you what I learned from doing the Riggers ticket through a year at TAFE and the official books I had to read to pass my exam for my ticket, if you have a beef call them, regards Frank.

I looked at the riggers guides, and none of them showed anything about the situation of a winch recovery using a snatch block - or anything close.

I didnt mean any of this to **** on your experience, career or whatever so don't take it that way - but if you can't see the fact there are 2 pulleys in this situation and not the one because you aren't taking into account the winch drum, ain't my fault.

And while we are on the subject, the force involved in this you need to measure is not a pull, its a turning force. The measurement is the torque required for the winch drum (pulley) to turn. If it was a pull, then there would be an open end. It may be that the pull on the line may be the same, which is clearly shown by all your riggers knowledge, and clearly shown on the guide you have thoughtfully provided and I read, but the force we are talking about is not the pulling capacity of rope or wire, its the turning force required to reel in the line.

If you pull on your winch rope, straight pull. Stand near your car and pull on the line threaded through a snatch block, still a straight pull, you are just changing the angle, and putting added load on the snatch block. Stand near the snatch block, pull through the snatch block and back around the winch drum and that is what the winch is doing. Half the strength required. It is just doing the turning at the pulley.

I never said your information was wrong mate, you just didn't have the right equation.



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Here is another way of looking at the situation.

Setup A is straight from the Rigger's Guide and is exactly as Frank says. It is pulling to disadvantage and all that happens is that the direction of the pull is changed and the frictional losses increase the effort required.

Setup B, which does not appear in the Rigger's Guide is the way some people use a pulley in a 4WD recovery.

Frank's statement about how to calculate theoretical MA is correct and again is taken from the Rigger's guide.

I am telling you and anyone else interested, that to get a Mechanical Advantage (MA) from using pulleys or Snatch Blocks the formula is and has always been worked out by*the number of parts of rope supporting the Moving block,*the operative word here is the moving block.

So the important question is how many parts of rope (are) supporting the Moving block?

The rectangle at the bottom, representing the vehicle surely has two ropes supporting it, therefore a theoretical MA of 2.

If you tie the end of the rope to another tree instead of back to the vehicle, you have setup A which as the riggers guide shows has a MA of 1.



Some of the things making it difficult for me to head bush recently are no longer a problem, so I'm hoping to be able to get out soon.

... what fun...

Originally Posted by vnx205

Have a look at page 31 of the 1995 version of Work Cover's Riggers guide.

http://www.riggingtraining.com.au/Ri...ide_part_1.pdf

I think the reason for the disagreement is that none of the images there represent what normally happens with a 4WD recovery.

In fact I can't imagine a rigger needing to use the setup that a 4WD recovery uses, which I suppose is the reason it is not illustrated in the Riggers' Guide.

The example of the man in the seat on this page is close to a lot of 4WD recoveries.

Simple Machines -- Mechanical Advantage

three examples from the top of my head are gyn pole backstays, overhead gantry cranes and manual boom slew lines.. (I forget the current name for them)

the biggest issue as you've identified is that it is rare in the rigging world to have your winching line also be the same point as the anchor for the standing part of the rope.b

at a rough guess 80% of rigging world work is done at disadvantage. (im not counting gantry crane or pre-rigged tensioners that use a inbult rigged to advantage 2:! )

Allan, your "Quote is totally wrong and the easy way to prove it is to try it. Stick a snatch block on a tree branch, loop a rope through it, put a bowline loop in one end, put your legs through the loop and sit down on the ground. Using only your arms lift yourself up to the block.

I bet I can.

In fact I think I could do it with one hand. Next time we are in Bendethera, you supply the pulley and I will supply the rope and the effort.

Read my last post above yours and see the bit about mechanical advantage.
As far as the winch drum is concerned, that is my point exactly, with a cable reeved through a fixed pulley and back to the point of origin (4wd Vehicle) twice the length of cable is wound off the drum placing the torque generated closer to the centre of the winch drum. Winches are rated for their pulling power (torque) at the closest point to the centre of the drum as they can get. Most winch manufacturers will supply at point of sale the max load for each layer of cable, the top layer usually coming in at around half the rated load of the first layer.
By running out twice the amount of cable over and above a single line pull you are gaining a near 2:1 advantage, but it has nothing to do with the snatch block. The same advantage could be achieved by picking an anchor tree twice as far away as the tree with the snatch block.
This topic was discussed extensively on 4WD Action Forum a number of years back, the topic got so hot that some of the *****s on that Forum got hold of my phone number and I and others that agreed with me were getting death threats, I eventually found out who they were and they were warned and that was the end of that.
But commonsense and Physics explains it all and I don't think Tafe and all the other Gov. depts. got it wrong.
I look at it this way, if you have a cable attached to a load at one end and a winch at the other and it is stretched tight, you place the wheel of a snatch block against the cable, does that halve the load on the winch, or do you have to have the cable being pushed into a 45 degree angle by the snatch block before MA applies, or is it 60 degrees or 90 degrees.
The snatch block does nothing except change the direction of pull when it is anchored and not moving, what if I ran the same cable through 1/2 a dozen snatch blocks anchored all in different locations, are they going to take half of the load each off the winch, of course not. If the theory that a single anchored snatch block will halve the load on a winch, then if I anchored another snatch block, say 6' to the side of the first block then I should require no effort from the winch, Doesn't work, does it?
If you didn't have a snatch block you could just throw your cable around a tree, more friction, but according to the believers in the magic snatch block, the same principle applies to the tree, change of direction only there is nothing inside a snatch block that allows you to pull 3 tonnes by only applying 1500kgs to one side of the pulley, Regards Frank.

Originally Posted by JoeFriend

I looked at the riggers guides, and none of them showed anything about the situation of a winch recovery using a snatch block - or anything close.

I didnt mean any of this to **** on your experience, career or whatever so don't take it that way - but if you can't see the fact there are 2 pulleys in this situation and not the one because you aren't taking into account the winch drum, ain't my fault.

And while we are on the subject, the force involved in this you need to measure is not a pull, its a turning force. The measurement is the torque required for the winch drum (pulley) to turn. If it was a pull, then there would be an open end. It may be that the pull on the line may be the same, which is clearly shown by all your riggers knowledge, and clearly shown on the guide you have thoughtfully provided and I read, but the force we are talking about is not the pulling capacity of rope or wire, its the turning force required to reel in the line.

If you pull on your winch rope, straight pull. Stand near your car and pull on the line threaded through a snatch block, still a straight pull, you are just changing the angle, and putting added load on the snatch block. Stand near the snatch block, pull through the snatch block and back around the winch drum and that is what the winch is doing. Half the strength required. It is just doing the turning at the pulley.

I never said your information was wrong mate, you just didn't have the right equation.



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Last time you tried it without the snatch block you did yourself an injury, the bets on when are we going, still got to get my stainless steel water tank in though, give us a call when you're ready, Regards Frank.

Originally Posted by vnx205

I bet I can.

In fact I think I could do it with one hand. Next time we are in Bendethera, you supply the pulley and I will supply the rope and the effort.

Originally Posted by Tank

Allan, your "Quote is totally wrong and the easy way to prove it is to try it. Stick a snatch block on a tree branch, loop a rope through it, put a bowline loop in one end, put your legs through the loop and sit down on the ground. Using only your arms lift yourself up to the block.
If you can do that you must be very fit cause you are not getting any MA, to make it interesting put say double your weight in the loop and try lifting that, if you weigh say 150lbs you shouldn't have any trouble lifting 300lbs. with your magic snatch block that gives you a 2:1 advantage.
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umm Tank,

Thats exactly how a single sheeve rigged to advantage works. excluding losses most people can only pull down 1/4th their body mass so most bosuns chairs are rigged in 5:1 advantage, 6:1 disadvantage or 6:1 advantage. to give you an effective MA equivalent of at least 4:1

getting someone who cant pull up their body mass in the first place to then attempt to pull up their body mass+100% (150lb body, 150lb load=300lb) with only a 2:1 MA wont work because it means their back to trying to pull 150lb before you have to deal with the extra work to overcome frictional losses