Winch, rope and winch bar limitation calculation

[Schattenjager]

Perhaps a dumb question related to http://www.expeditionportal.com/for...ttenjager-s-2006-Tacoma?p=1609424#post1609424:

My Warn 8000-S winch is rated to 8K. The synthetic line and bull bar are rated to 10K. I understand that a snatch block will DOUBLE my pulling power and make life much easier on the winch, even if speed suffers (no biggie - I'm stuck anyway) SO, does this mean the line and the bumper are subjected to DOUBLE the pulling force and therefor exceeding their capabilities? I do not think this is right, but during the install of my ARB bull bar I was taken aback by the THIN construction of the winch mount area. Also the Warn rope capacity cautions were printed EVERYWHERE in the packaging, so the subliminal suggestion has been firmly planted.

Any help for the mentally feeble would be much appreciated. :snorkel:

 

[AFBronco235]

Yes and no. What's happening is you're not doubling the force acting on the bumper so much as doubling the MAX load your line and winch can take. If your line can take 10K lbs. using a block will double that to 20K lbs. roughly, since you're literally doubling the lines you have attached between your truck and your anchor. Its slightly less but for rough calculations it works. The maximum force acting on your mounting points will still be equal to the amount of force needed to get your vehicle unstuck.

To give you a better idea, say you're vehicle weighs in a 5K lbs. including your gear when you get stuck. You're stuck in about 6 inches of mud. Because you are having to lift the vehicle up and over the mud, essentially, you'll require a pulling force of around 8K or 9K lbs. With the setup your described, snatch block and line, you'll be able to pull, in theory, about 20K of force, which is more than enough to get you unstuck. This is also the reason why, when choosing a winch, you choose one rated for several times your vehicle weight and then still get a snatch block or three.

Also, you're not so much trading speed for power as you're trading distance for power. Since Work is force applied over distance (W=F/d) you can actually increase the power your winch can pull with by applying the same about of force over a longer distance. Since you're using the snatch block, you're doubling the distance of the line your winch is pulling in. ie. For every two feet of line on the drum, it only moves the truck one foot. But what you loose in distance, you gain in power.

This is why I don't see the point in getting the fastest winch out there to begin with. I can only think of one instance where a speedy winch would be needed, and that's if you're somehow stuck on a beach with the tide coming in, and that's just piss poor planning to me.

If you want a better idea of how to winch safely and properly, look up FM 20-22. Its essentially an old army field manual on how to recover vehicles, for everything form old GPWs to tanks. It does go over what I just explained very well and then some.

 

[lcsodiver]

If you run your line through a block and back to your rig then there are two lines to share the load, so your not doubling the weight applied to the line.... you are however doubling the max potential weight applied to your anchor point at and past the snatch block (shackles, tree straps). And if you come back to your bumper, then yes you are also doubling the max potential weight applied through your bumper also.

Your winch should stall out at ~8000 lbs normally where as your stall force would be 16000 lbs when going through a snatch block and back to the rig. So if you were stuck in concrete and unable to move then yes you are doubling the max load your gear will see..... in the real world, not so much.....

 

[eggman918]

One thing to remember is with a block if you hook the line back to your bumper the stress of the whole load is on the connection point of bumper and truck,but that being said then you are looking at the shear strength of the bolts and that is quit a bit more than their tinsel strength ://nucor-fastener.com/Files/PDFs/TechDataSheets/TDS_013_Shear_Strength.pdf
FM 20-22 is available on line in PDF free many places it is worth the time it takes to read it explains things in simple "solder proof"terms.

 

[emmodg]

You're not "doubling the power" of the winch.(That would be "magic") You're effectively "half-ing" the weight of the load in RELATION to the winch.(whatever load must be moved) This is in a 2:1 pull. But remember, whatever load is being "lessened" by the the snatch block(s), in relation to the winch, that load HAS to go somewhere so it must be born by the snatch block and it's dependent rigging. This is why many-a-man has lost rigging by over- loading their snatch block(s) in hopes of lessening load on the winch. We see this in crazy 3:1, 4:1 rigs.

 

[Schattenjager]

Thanks for the great feedback. The army manual should be required reading.

 

[I Leak Oil]

Just remember that physics and the conservation of energy still apply to winching. You never get more for free!

 

[Schattenjager]

Just remember that physics and the conservation of energy still apply to winching. You never get more for free!

Darn physics getting in the way of my good ideas yet again

 

[Scoutn79]

It is going to depend on where you attach the hook end of the winch cable when using the snatch block.
If you winch will pull 8k lbs and you double it back and attach the cable to the bumper (assuming zero loss through the snatch-block and zero gain from pulling on a lower cable wrap on the drum) you are effectively doubling the force applied to the bumper/mounting points. If however you attach the cable to a separate point on your vehicle to load will be shared between the bumper mounts and the other attachment point.

Darrell

 

[emmodg]

It is going to depend on where you attach the hook end of the winch cable when using the snatch block.
If you winch will pull 8k lbs and you double it back and attach the cable to the bumper (assuming zero loss through the snatch-block and zero gain from pulling on a lower cable wrap on the drum) you are effectively doubling the force applied to the bumper/mounting points. If however you attach the cable to a separate point on your vehicle to load will be shared between the bumper mounts and the other attachment point.



Darrell

?

A 2:1 - with incoming leg and outgoing leg of line at 0 degrees to block sheave and hook at vehicle - results in a traveling block. The lines share the load equally. The block will of course see a multiplier of 2 or a doubling of the load. The block and mounts on the truck - no matter where they may be - will not share the load. Look up a factor table for blocks somewhere - Crosby has one - the multiplier applies to the block and block alone.

Snatch blocks are a must have for a winching kit and they do some great things but they do get a little tricky when we start getting "geeky" about them in detail.

 

[lcsodiver]

?

A 2:1 - with incoming leg and outgoing leg of line at 0 degrees to block sheave and hook at vehicle - results in a traveling block. The lines share the load equally. The block will of course see a multiplier of 2 or a doubling of the load. The block and mounts on the truck - no matter where they may be - will not share the load. Look up a factor table for blocks somewhere - Crosby has one - the multiplier applies to the block and block alone.

Snatch blocks are a must have for a winching kit and they do some great things but they do get a little tricky when we start getting "geeky" about them in detail.

Read what he said again, he never said the block shared the load with the truck mounting points..... the lines share equally until they tie back to the vehicle. If both lines, the one from the winch and the one at the bitter end of the cable attach to the bumper then the bumper may see 16,000 lbs of force applied to it. But if you take the bitter end and attach it somewhere else on the vehicle say a frame mounted recovery point then the bumper only sees 8,000 lbs of force and the frame sees 8,000 lbs of force. Total applied to the vehicle is still 16,000 lbs max. And yes the block will see 16,000 lbs max loading.

 

[Scoutn79]

?

A 2:1 - with incoming leg and outgoing leg of line at 0 degrees to block sheave and hook at vehicle - results in a traveling block. The lines share the load equally. The block will of course see a multiplier of 2 or a doubling of the load. The block and mounts on the truck - no matter where they may be - will not share the load. Look up a factor table for blocks somewhere - Crosby has one - the multiplier applies to the block and block alone.

Snatch blocks are a must have for a winching kit and they do some great things but they do get a little tricky when we start getting "geeky" about them in detail.

So are you saying if I double up a 10k winch putting the hook back to the truck and put 20k on the snatchblock that only 10k is applied to the truck?

 

[AFBronco235]

So are you saying if I double up a 10k winch putting the hook back to the truck and put 20k on the snatchblock that only 10k is applied to the truck?

Sort of. 20K is still applied to the truck, but at two points that share and spread the load across the bumper. Each point, one being the winch and the other being a shackle point, will have a load of 10K, making a total load of 20K. And yes, the snatch block also holds 20K.
Of course, that's the MAX load, not the actual load. The actual load is going to be much less if you're doing this right. We just always use MAX load as a safety.


Hopefully, this image helps clear anything up. Its from the FM 20-22 that we keep telling you to read.

 

[Scoutn79]

Sort of. 20K is still applied to the truck, but at two points that share and spread the load across the bumper. Each point, one being the winch and the other being a shackle point, will have a load of 10K, making a total load of 20K. And yes, the snatch block also holds 20K.
Of course, that's the MAX load, not the actual load. The actual load is going to be much less if you're doing this right. We just always use MAX load as a safety.


Hopefully, this image helps clear anything up. Its from the FM 20-22 that we keep telling you to read.
View attachment 234168

That's precisely what I was saying....

Darrell

 

[Inyo_man]

This is exactly the reason that I use a 7/8'' shackle on the snatch block when needed.