Help me out here- 5:1 design factor unreasonably low?

[peteinjp]

I have a PTO winch going on my bj44. Intended use is to move rocks and trees off the trails. Unintended use- you guessed it- get my truck out of what ever trouble I get into. I go out exploring old logging roads- a single vehicle most of the time. Now just to get things straight I can hike out of these places and I am very very careful because I do not have much experience off road- I walk just about anything that isn't dry first. I'm exaggerating but the point is that I know that I don't know and there is no one for me to learn from. So I have to be sure I can do it (and do it on the way back) or it just isn't worth the risk.

So I have the winch and I'm working on getting it set up so worst case I need to pull my truck off or out or what have you. Based on the reading I have done recently on this forum and the references that were posted in the 'jeep rolls over silverado' thread- I have been able to begin getting set up.

So first math problem- wire rope rating.

Line rating: 8mm(5/16) 7x19 line 4445kg) breaking @95% efficiency (swaged thimble) 4223kg

5:1 = 844kg(1860lbs)

According to the same site Pirate4x4.Com - Extreme Four Wheel Drive a truck stuck down to its axles needs a force appox 3 times the GVW. My factory rated GVW is just over 2355kg. That means with 5/16 cable I would need to have 8 pulley blocks to extract my truck at 3 times the GVW (stuck to the frame in mud) and achive a 5:1 safety factor. That just aint right.....

Now my truck will never be stuck to the rails in mud but I'm just throwing the numbers around because every ones advice "know your equipment and what its good for" Now that I do the math I know exactly why the stock shear pin is so weak- they did the math too.

Now I'll just take a guess that 99% of the time a winch gets used to extract there is no more than one pulley involved. So is my math off somewhere or is the 5:1 thing just unrealistic?

Pete

 

[Cruiserdrew]

You are not confused at all. The 5:1 safety factor is for OVERHEAD lifts. That's where all the standards come from.

For winching and recovery, there are no standards at all. That's why you see all kinds of inadequate equipment being used when off road.


A 5/16 cable in perfect condition, pulled by an 8000 pound winch is very close to it's breaking strength (which is near 10000 pounds), so with your math skillz you can calculate the "safety factor" and realize it isn't there. Fortunately, most stucks do not need 8000 pound of force to overcome, but they can when deeply stuck in the mud.

That's where all the precautions come from like protecting yourself from a snapped winch cable, putting a blanket on the cable and the like.

I think the factory PTO winch has a shear pin that is supposed to shear befor the cable breaks, but I don't know how reliable it is.

Good luck.

 

[peteinjp]

Ok- well what is a reasonable figure in terms of a design factor for winching. I don't want to have to break something to find out- ok that is too much. But on the other hand I sure as hell am not going to go out and buy an extra 300ft of cable and 8 pulley blocks. Given that I have no experience and will be self taught- I'm thinking that 3:1 is probably a good margin for the cable. But there seems to be no harm in keeping the higher 5:1 on the pulley, D- rings, strap.

Ultimately I know the shear pin will break before anything else- but I want a matched set of equipment. At 3:1 on the cable if I went with a 5:1 on the pulley, D- rings, strap I would be looking at a minimum WLL of 3 tons ( 2 times the 3:1 factor of the cable.) Seem reasonable?

Pete

 

[peteinjp]

I should have stated that my reasoning for using a lower safety rating ratio for the cable than the other stuff is that the cable breaking is preferable to the block or sling snapping.

Pete

 

[lshobie]

Buy some masterpull synthetic rope with a breaking strength of 26.5K pounds. Run a hawse fairlead and always use a snatch block - that will be more than enough to get your truck out of anything you will encounter. If you don't use a snatch block you will be busting shear pins in big pulls.

This is mine, straight pull, mod winch, 36 inch tires, very heavy - in clay, 7/16's masterpull line. Something tells me you won't even be close to this.

 

[opie74]

Ok- well what is a reasonable figure in terms of a design factor for winching. I don't want to have to break something to find out- ok that is too much. But on the other hand I sure as hell am not going to go out and buy an extra 300ft of cable and 8 pulley blocks. Given that I have no experience and will be self taught- I'm thinking that 3:1 is probably a good margin for the cable. But there seems to be no harm in keeping the higher 5:1 on the pulley, D- rings, strap.

Ultimately I know the shear pin will break before anything else- but I want a matched set of equipment. At 3:1 on the cable if I went with a 5:1 on the pulley, D- rings, strap I would be looking at a minimum WLL of 3 tons ( 2 times the 3:1 factor of the cable.) Seem reasonable?

Pete

Ive found that a solid 3:1 seems ideal. It also falls in line with having your recovery gear 3 times stronger than the weight of your rig.

Also keep in mind that everytime you add a block into the equation, you halve the load on the line. If youre running 1 block, the 8000 pounds generated by the winch will halve to 4000 on each leg. Throw in a second block, you know have 3 legs and each leg will carry 2666.6666666 pounds. However, the connection points for the block will carry full load. Using a block is a great way to increase the pulling power of your winch, but you can also quickly move above the safety limits of attachment points, so keep that in mind.

Also, an 8000 pound winch will only generate 8000 pounds on the first wrap. Each subsequent wrap reduces the rating of the winch.

You are right on with the thinking that you want the line/cable to be the weak link. Its safer to have just a cable coming at you than a cable with anything still attached to the end of it.

Its not difficult to keep everything in your kit around a WLL of 4.75 - 6 tons, keep it manageable and realistic. Keep in mind, a 4.75WLL shackle has a tensile of 38,000 pounds. Far above even the 7/16 line mentioned above.

 

[peteinjp]

Louis- Your right- I'm not going to get as stuck as you. I'm trying not to get stuck!

After seeing what your doing with straight pulls on that rope I think I'll be fine with a line rated at 2/5 the strength (5/16 wire rope.) Using a snatch block or 2 would get me in the same area rating wise. I'm going to use what I have for now. The fact is that the 26K# rope is way over rated what my winch will do with the stock shear pin- though I'm thinking of upgrading that as well.... Looking for a larger shear pin to use.


The 8mm rope I have will pull 1500kg at a 3:1 ratio. Doubling the line will double the force on the block and straps etc. equaling 3 tons. In reality this 3:1 ratio stands the risk of being over loaded so instead of 3 ton hardware and straps I think I'll go with 4.5t WLL or better. Essentially that would be in the 20,000# breaking strength range.

There is so much incredible stuff offered for recovery these days. Here in Japan I'd have to order every single thing in. I'm going with what the rigging industry uses here because it is in my price range and readily available hence the need to do my own math.

Thanks,

Pete

 

[lshobie]

You can't use a bigger shear pin, that would mean you'd break something else as the shear pin is spec'd to break before the winch components do. In any event I think your are slightly overthinking this one Buy a snatch block and you are good to go.

A high lift jack works wonder too - jack up and place something under the tires etc. There is a huge market for 4x4 stuff in japan - you can get everything there.

Have fun!

 

[peteinjp]

Your right about the over-thinking- but like you I'll be having my kids along so I'd rather over than under. The shear pin in the winch is almost always replaced with a roll pin or cap head screw due to its puniness. I think a larger shear pin is a good compromise- stronger but still designed for shear- unlike a roll pin or cap head. Trying to find info on boat prop shear pin diameters.

ARB and big name brands are here- a 2X retail in the U.S. Name brand value is very high in Japan.

Kris- very cool YO-YO! What size rope size you use for that? Good advice there- thanks. For the sling (tree) I'll use it doubled so I'm thinking 1.5 times the rated WLL. So a 3 ton seems adequate- a conservative 4.5t when doubled. The block- I'll have to see what ratings I can find on blocks with appropriate sized diameter/grove width pulleys. Probably in the 5t range. 5T or so D rings as well.

Pete

 

[opie74]

Pete, that was 7/8" Amsteel Blue.

 

[roalco]

Also keep in mind that everytime you add a block into the equation, you halve the load on the line. If youre running 1 block, the 8000 pounds generated by the winch will halve to 4000 on each leg. Throw in a second block, you know have 3 legs and each leg will carry 2666.6666666 pounds. .

Hmmm, IMHO, I think you may want to re-think this...

 

[peteinjp]

Hmmm, IMHO, I think you may want to re-think this...

As long as the lines are from the winch to the pulley and back to the truck again the math seems good to me....

How about an explanation.

Pete