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Techniques Winching - Recoveries - Chainsaw Safety - Proper aproach to 4wding etc

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  #1  
Old 05-06-17
Kumabear Kumabear is offline
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Default Advanced winching physics

Could someone check over the attached picture.

Is my maths correct for this hypothetical recovery situation.
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File Type: jpg Advanced Winching.jpg (146.8 KB, 138 views)
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Old 05-06-17
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Chainsaw Chainsaw is offline
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All ropes must shorten to gain a mechanical advantage. All your doing is an indirect pull.
If you want to get into it....you can get the basics here.............. http://www.bits.de/NRANEU/others/amd...22%2862%29.pdf

Last edited by Chainsaw; 05-06-17 at 03:08 PM.
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Old 05-06-17
dhula dhula is offline
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To me it looks like you have set up a 3:1 pull
Depending on which way you want to look at it and in very basic terms:
* The winch will work 1/3 as hard to do the same work
* The winch is able to exert 3 times it's normal pulling power
This does not equate to exerting 3 times the load on the stuck car as it will take what it takes to get it out of the bog and there are other formula/rule of thumb for that depending on what you are stuck in and the gradient (if any) of the hill.
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Last edited by dhula; 05-06-17 at 07:27 PM.
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Old 05-06-17
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Your diagram has me confused as there are 2 vehicles involved and an anchor point.

Easiest way to work out mechanical advantage is to compare how much winch cable is spooled in for the amount of movement of the bogged vehicle.

1 metre of cable/rope spooled in and 1 metre movement of the bogged vehicle is a 1:1 ratio. 100% effort on the winch. Winch cable connected directly to anchor point.

1 metre of cable/rope spooled in and 0.5 metre movement of the bogged vehicle is a 2:1 ratio. 50% effort on the winch. Single pulley block attached to anchor point and the winch cable/rope connected back onto the vehicle.

1 metre of cable/rope spooled in and 0.333 metre movement of the bogged vehicle is a 3:1 ratio. 33% effort on the winch. Single pulley block attached to anchor point, cable/rope returned through another single pulley block attached to the bogged vehicle and then the cable/rope returned back and attached to the anchor point.

The above calculations are a general "rule of thumb" because there are also added frictional forces of the pulley blocks which require small amount of winch power to over come.

OJ.
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Old 05-06-17
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No, your maths isn't correct.

Ignoring the losses in the pulleys (as mentioned by OJ) the winch is applying force X to the rope - the tension in the rope is X Newtons.

Because we're ignoring the losses required to turn the pulleys, each length of rope has X Newtons of tension, is applying X Newtons of force. Three ropes pulling on the stuck car = 3X Newtons.

Two ropes pulling on the tree = 2X Newtons. Add the 1X Newtons from the winch car = 3X Newtons. Equal & opposite forces etc.
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Old 05-06-17
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i think it is only a 2:1 leverage.The "load" is only supported on two "ropes" ,you dont count the one you are pulling

http://www.explainthatstuff.com/pulleys.html
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Old 05-06-17
twisted32 twisted32 is offline
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OJ and nj are correct.
Think about it this way, the force applied by the winch, can be regarded as tension in the cable (X Newtons). A pulley does not increase the tension in the cable, it enables the cable on both sides of the pulley to impart the tension force on the pulley attachment and hence the pulley on the top car by pulley 1, is 2X Newtons. The second pulley is simply changing the cable direction, back to the top car, where the cable imparts a X Newton force on said car. Therefore total imparted force due to mechanical advantage of the pulleys is 3X Newtons on the top car. ie negating frictional forces of the pulleys, a 3:1 mechanical advantage is achieved.
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Last edited by twisted32; 05-06-17 at 07:40 PM.
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Old 05-06-17
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Quote:
Originally Posted by KiwiNTPajero View Post
i think it is only a 2:1 leverage.The "load" is only supported on two "ropes" ,you dont count the one you are pulling

http://www.explainthatstuff.com/pulleys.html
Misread the diagram.... It is a 3:1 mech adv. I mistook the snatch block 1 pulley as anchor point. The vehicle together with the snatch block moves towards the winch vehicle. As I said to gain mech adv "each rope" must shorten. Because each rope is becoming shorter they each are carrying one third of the load (forgetting friction losses , as mentioned).

Last edited by Chainsaw; 05-06-17 at 07:24 PM.
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Old 05-06-17
Kumabear Kumabear is offline
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Well thanks for the pointers.

My goal was to think about how to recover a car that is very stuck and requires more than a single line pull but without putting all the load on the front of my car.

From the responses this is still achieved by this setup although I did not have the forces correct.

And from the sound of it would also be effective without the second snatch block if you wanted a 2 line pull using my vehicle and the tree to anchor?
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Old 05-06-17
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Quote:
Originally Posted by Kumabear View Post
Well thanks for the pointers.

My goal was to think about how to recover a car that is very stuck and requires more than a single line pull but without putting all the load on the front of my car.

From the responses this is still achieved by this setup although I did not have the forces correct.

And from the sound of it would also be effective without the second snatch block if you wanted a 2 line pull using my vehicle and the tree to anchor?
Yep.
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Project: NJ SWB. 285/75R16 ST Maxx, 2" OME suspension, 2" body lift, ARB 110, 120l tank, bullbar, scratches, no major dents. Fully engineered in SA. NW DiD & auto in place - a long way to go....

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