Advanced winching physics

[Kumabear]

Could someone check over the attached picture.

Is my maths correct for this hypothetical recovery situation.

[Chainsaw]

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

[dhula]

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.

[old Jack]

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.

[nj swb]

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.

[KiwiNTPajero]

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

[twisted32]

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.

[Chainsaw]

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).

[Kumabear]

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?

[nj swb]

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.

[Ent]

One thing I am curious about is the rope strength required. 10mm rope is rated at 17,600lb breaking strain. Standard winch is 9,500lb so doubling the pull using a single block gives 19,000lb potential pull on the vehicle. Is that not above the breaking strain?

Friend on the SES was looking at some winch tables for recovering a vehicle and was rather amazed at the amount of pull required. I bogged DooSo to his bash plates in a bog hole and had to drag him up steep slope. We used a 1.6 tonne lift 3.2 tonne pull Tirfor and we "stalled". Using Maxitraxs and all difflocks we managed to drag him out but using a 9,500lb winch with block would by the sounds of it run the risk of breaking the rope.

[P4J3R0]

Wouldn't the force on both cars be the same? The force exerted through the winch is actually 3 times less than if it were a straight line pull.

To reduce the load on the front of the winching car the winch would need to be restrained by something (tree) to reduce the load to the vehicle.

[twisted32]

One thing I am curious about is the rope strength required. 10mm rope is rated at 17,600lb breaking strain. Standard winch is 9,500lb so doubling the pull using a single block gives 19,000lb potential pull on the vehicle. Is that not above the breaking strain?

Friend on the SES was looking at some winch tables for recovering a vehicle and was rather amazed at the amount of pull required. I bogged DooSo to his bash plates in a bog hole and had to drag him up steep slope. We used a 1.6 tonne lift 3.2 tonne pull Tirfor and we "stalled". Using Maxitraxs and all difflocks we managed to drag him out but using a 9,500lb winch with block would by the sounds of it run the risk of breaking the rope.

The tension in the cable does not increase above the X Newtons of force applied by the winch. The multiplication occurs due to the multiple application of force X on the top vehicle by 3 cable attachment points.

[Ent]

Just trying to my head around this. With a single block you move the vehicle half the distance for the length of rope. So torque is doubled, correct? That means if the winch stalls at 9500lbs then it is pulling 19,000lb of force with a single block.

Ok my head hurts. What you are saying is a strain gauge will still show 9,500lbs of force on the rope, and part of my brain agrees as the winch can not pull more the 9,500lb regardless the number of pulleys. But at the same time you are putting 19,000lbs of force to shift the vehicle.

My brain is stuck in loop. One side says you have doubled the pulling torque but the other is saying you can not increase the tension on the rope. So what am I missing?

Oh hang on I get it now. With a single pulley you actually have "two ropes" pulling 9500lb so one side of the pulley has two forces of 9,500lb and the pulley single attachment point has 19,000 lbs. Finally got there. The more attachment points the more the torque applied. So when tying pullies to a vehicle use different points to spread the load.

[old Jack]

The force required to unbogging the car does not change regardless of a single, double or triple line pull. By using a multi line pull the load on the winch and on the cable/rope will be halved by a double line pull and reduced by 2/3's on a triple line pull. Also you get a similar reduction in amp draw but you are winching for a longer period as the recovery speed is also reduced by the same factors.
Using a different attachment point for each attachment is a a good practice or using a bridle as a central connection point.

OJ.