Help with snatch block calculations

[Keith_J]

Most rigging calcs are fudge factors. As are winch ratings but you should count on at least 12k of pull bare drum. Each block must be rated for twice that or 24k, more important is sheave diameter which should be equal or greater than the winch bare drum diameter.

Sure, calcs may show a little over 8k of pull is needed but that is for smooth ramps, most aren't. So you might need 2 part.

 

[73m819]

The block attached to the truck being pulled needs to be handle the rolling resistance of the truck in the dirt, the up hill pull of the ramps, dragging brakes, ect., in another words DO NOT SKIMP on the snatch block, the load on the snatch block adds up quick, on a single snatch block, two part pull, the block takes ALL the load, figure your load then add 25%, this is the size of the snatch block you need.

 

[Csm Davis]

I hope that answered your tackle block question. Kind'a got side tracked.

Slope or uphill pull shouldn't have any effect on calculations, just like water.
Mire factor(how deep the truck is stuck in the slop) friction is the only load increasing factor. TLR will decrease if it's a rolling load (on wheels on smooth hard level ground) vs non rolling/mired.

Thanks for the refresher on the blocks, but I must disagree with you on slope, any increase from °1-°90 will increase the load, from just a little bit to 100%.

 

[Csm Davis]

The block attached to the truck being pulled needs to be handle the rolling resistance of the truck in the dirt, the up hill pull of the ramps, dragging brakes, ect., in another words DO NOT SKIMP on the snatch block, the load on the snatch block adds up quick, on a single snatch block, two part pull, the block takes ALL the load, figure your load then add 25%, this is the size of the snatch block you need.

Okay Ron on a 2-1 pull you need up to double the winch rating? So if I want to be safe with a 20,000 lbs winch I need a 40,000 lbs snatch block?

 

[zebedee]

Okay Ron on a 2-1 pull you need up to double the winch rating?

Jeff - No, you are a bit backwards.. Did you mean "double the snatch block"? if you are using a 2 part pull (2:1), it's because you are pulling more than the winch can handle, so in effect, you can get away with a half size winch for the given load.

So if I want to be safe with a 20,000 lbs winch I need a 40,000 lbs snatch block?

Re sizing snatch blocks - I think the load calc and winch rating are the starting points. Whatever winch you are using, it has a rating and a rope size. The rope should be stronger than the winch capacity (by quite a margin) and after that, the snatch blocks are designed to handle up to a max line size (3/4" for the rear drag winch)... so if your rope goes through the snatch block - it is assumed (and here come the "you know what you get when you assume" cracks) that it has the capacity to do what ever the rope is good for multiplied by the effect of the pulley: 2:1, 3:1, 4:1 etc., depending on the #of sheaves.

The BII snatchblocks on the 936/816 will deal with 3/4" steel rope in either 2:1 or 4:1 setup even though they have a lbs value on them.

Try using a winch with 3/4" rope through a 1" or 7/8" snatch block to give you a theoretically bigger rated snatchblock will allow the rope to crush as it fills the bottom of the sheave. Not exactly the intention of the snatch block making it easier for the winch.

Clear as mud eh?

 

[Csm Davis]

Okay Ron on a 2-1 pull you need up to double the winch rating? So if I want to be safe with a 20,000 lbs winch I need a 40,000 lbs snatch block?

So I guess that was not clear so Zebedee, let me clear it up, the front winch on the 816 is rated at 20,000 lbs. So if I want to be safe I need a 40,000 lbs rated snatch block, correct?

Jeff - No, you are a bit backwards.. Did you mean "double the snatch block"? if you are using a 2 part pull (2:1), it's because you are pulling more than the winch can handle, so in effect, you can get away with a half size winch for the given load.
Re sizing snatch blocks - I think the load calc and winch rating are the starting points. Whatever winch you are using, it has a rating and a rope size. The rope should be stronger than the winch capacity (by quite a margin) and after that, the snatch blocks are designed to handle up to a max line size (3/4" for the rear drag winch)... so if your rope goes through the snatch block - it is assumed (and here come the "you know what you get when you assume" cracks) that it has the capacity to do what ever the rope is good for multiplied by the effect of the pulley: 2:1, 3:1, 4:1 etc., depending on the #of sheaves.
The BII snatchblocks on the 936/816 will deal with 3/4" steel rope in either 2:1 or 4:1 setup even though they have a lbs value on them.

Try using a winch with 3/4" rope through a 1" or 7/8" snatch block to give you a theoretically bigger rated snatchblock will allow the rope to crush as it fills the bottom of the sheave. Not exactly the intention of the snatch block making it easier for the winch.
Clear as mud eh?

Yeah Mississippi delta gumbo mud!
Zebedee I have snatch blocks in all different sizes not just 3/4 and don't trust the military gear unquestionably.

Ohhhhhh yeah you know calling a redneck backwards is fighting words!

 

[Whiterabbit]

Thanks for the refresher on the blocks, but I must disagree with you on slope, any increase from °1-°90 will increase the load, from just a little bit to 100%.

Your load doesn't increase. Example, if a truck weighs 10tn sitting on flat ground it still weighs 10tn when you pick it straight up 90deg.
If you've factored 1/7th veh weight for rolling resistance on flat grass then yes you will need to add in .50% for a 30deg grade increase... Or you could just do a short cut and not do the 1/7th veh weight in your original calculations because you know you'll be doing an incline pull. 1/7th of 10tn truck=1.43tn, .50% of veh weight=5tn so 1.43+5=6.43tn.
I think we both are thinking the same thing but calling it different things. LOL! I rarely did rolling/mobile recovery. Most all jobs were mired vehicles or worse so I'm a bit rusty/fuzzy with rolling loads. We always started with 100% veh. weight.

My understanding has always been the only block that sees 100% of the winch capacity is a fixed block. The moving blocks only see a percent of the load based on the reduction factor you've rigged up so you always carry a block rated over your winch max cap in case it winds up as the fixed block.
Clear as mud huh? LOL!

 

[Csm Davis]

Okay Whiterabbit we agree on the angle thing. Now for the snatch block the attachment point on the load will see the total weight of the pull mired, rolling or whatever right? So the block will see the complete weight at the hook or shackle or however it is attached so wouldn't the block need to be rated at least twice what the max pull of the winch is?

 

[mikey]

Okay Whiterabbit we agree on the angle thing. Now for the snatch block the attachment point on the load will see the total weight of the pull mired, rolling or whatever right? So the block will see the complete weight at the hook or shackle or however it is attached so wouldn't the block need to be rated at least twice what the max pull of the winch is?

Technically, the block needs to be double the max line pull, not double the winch rating, but to be safe, you should own a block at least double your winch rating plus some.

If you have a 20k winch and you ever max that out with a 20k line pull, you will want a 40k block. But nothing is simple right? That does not account for friction loss, which whiterabbit puts at 10% but I've seen as low as 8%. So now you're at a 45k block for a 20k line pull.

Throw 20k into this calculator and for a straight pull (first section) it tells you that you need a 40k block:

http://www.thecrosbygroup.com/html/en-US/calc/snatchblockrigcalc.htm

Also see the diagram here with a straight pull (0 degree angle):

http://www.iti.com/pro-rigger-mike-parnell/bid/48983/Block-Tackle-Rigging-Snatch-Blocks

Of course, neither of those include friction loss.

Mikey

 

[zebedee]

So I guess that was not clear so Zebedee, let me clear it up, the front winch on the 816 is rated at 20,000 lbs. So if I want to be safe I need a 40,000 lbs rated snatch block, correct?

Well I kind of figured you were a little word tied! Yes, quite correct... assuming you are using a single shieve snatch block that is designed for the 10/16" rope....

Yeah Mississippi delta gumbo mud!
Zebedee I have snatch blocks in all different sizes not just 3/4 and don't trust the military gear unquestionably.

Your different sized snatch blocks should be used with the corresponding wire rope size that way you won't overload a snatch block.. or are you saying you have different rated snatch blocks with similar shieve throats (need a better word for the groove for the cable!)?
The rear winch double snatch blocks are obviously intended for double the load, but have you noticed that they are of very similar design and construction to the single ones... therefore the single ones are way over sized - I think it's easier for fabrication to have similar parts etc... Your caution of "military gear" is commendable but I don't think I'd worry too much, so long as you check for damage and don't use the 1" manila rope snatch blocks for steel rope (A.L.L. page C-2 items 3940-00-926-3710 and 3940-00-926-3719)!

Ohhhhhh yeah you know calling a redneck backwards is fighting words!

... well I'm glad I removed the @%&@€$$ infront of "backwards" before I posted it then! and remember - even though I am a "ferrr-ner", I am still a Southerner!

 

[TruAg]

Couldn't believe it took 7 or 8 replies before someone through out the "have you looked at the TM?" Lol

Thanks for posting this. Lots of good advice and now I know where to look in the TM.

 

[73m819]

What I meant was WHAT EVER the total weight of the load that needs to be moved, WILL fall on the snatch block and the attachment to the load, this weight (load) can add up QUICK, so the load needs to be figured out, rolling resentences in dirt, load weight, dragging brakes, ramps, ect.. Once this is figured out then add at least 25% as a safety factor, this IS the size of the snatch block and attachment that is needed. If the winch can not handle 1/2 of the snatch block load, go to plan B because plan A is not workable.

This is all VERY SIMPLE, it does not need rocket sciences to figure this stuff out. You ought to move a rigged up oil rig 200' around another rig if you want a REAL thrill in figuring out what rigging, multiple pulling points, loads on these points, multiple pulling sources, the pivot, ect., and everything has to work the first time or the rig could collapse (would not be a good thing)

 

[mikey]

I just want to say thank you to Ron (73m819). He pm'd me with his # and offered me his time and experience to ensure I had a safe and successful recovery this spring.

It was an excellent conversation and I truly appreciate someone with the experience he has reaching out to a less experienced member to offer such advice. TM's are great resources, but nothing compares to the experience of others who have been doing this for most of their lives.

Thanks Ron!

Mikey

 

[73m819]

Thank You, Mikey was on the right track, though I think he was surprised HOW FAST the load weight that needed to get moving added up.