Help with snatch block calculations

mikey · Jan 27, 2015

I have a trailer with a 12k electric winch. I want to use it to winch up my 5ton M51A2 in the spring, which weighs about 23k.

In a perfect world I could use 2 or more snatch blocks and call it a day. The problem is that I only have 62' of winch cable.

If I had 100' I estimated using 2 blocks and three lines of equal length.

Snatch 1 (a) would need to be better than 24k, snatch 2 (b) better than 16k and the line pull would be 8k, good enough for the winch and the battery.

Since I only have 62' of line and the trailer length is 29' from winch to the ramp ends, I was wondering if I could do the following, please see my poor attempt at CAD.

If I put snatch 2 (b) in the middle of the trailer, I'll have enough line to get back to the bumper (c).

My question is, how does cutting the line lengthy in half, A to B and B to C, affect calculations? Or does it? I can't find any formulas.

I don't want to use a single block because it would require a 48k snatch block and it would put the line pull at 12k maxing the winch and probably the battery (300amp draw @ 12k).

Thanks!

Mikey

NDT · Jan 27, 2015

To do the calculation we will need to know the angle of your ramps. Assuming 30 degrees, the pull needed will be nowhere near 12,000 lbs for a rolling load. A single block halve the line speed and will give your better delicate control.

mikey · Jan 27, 2015

Ramp angle is a great point, thanks. I never figured that into the calculations. I'm not experienced with rigging, which is why I'm turning to the experts.

The Eager Beaver 20 ton tag trailer has a 10 degree angle for 9' (5' dovetail plus 4' ramps).

Mikey

Artisan · Jan 27, 2015

Each roll of wire on the spool changes everything.

Reduce friction, grease? or add wheels or pipes as rollers (the Eqyptian style) or

Chock and block the trailer and use the truck to pull the load onto the trailer.

Give me a big enough fulcrum and lever and I will move the world.

Artisan · Jan 27, 2015

Wait, if it is on wheels you probably have enough to just do it normally...

Put it in neutral and pull as far as you can, THEN use pulleys and other methods to finish.

Tornadogt · Jan 27, 2015

You are not lifting 24000 lbs you are rolling it. A single snatch block is plenty with power to spare. Take your time and be safe.

dawico · Jan 28, 2015

Tornadogt said:
You are not lifting 24000 lbs you are rolling it. A single snatch block is plenty with power to spare. Take your time and be safe.

This.

As stated, it is a rolling load. One snatch block will work but probably isn't even necessary.

A snatch block mounted on a fixed location doesn't add any leverage. It just helps the cable turn a corner.

Depending on your ramp and trailer lowering the hitch will help. Once the first tires clear the ramp they will be rolling down hill helping the pull. Just don't let it get away from you and roll off the front.

I have choked trailers and used my truck to pull stuff up before. It works well if you can keep the trailer stationary and the hitch from rising.

Be careful.

zebedee · Jan 28, 2015

I think you have all the info that you need, as in, you will be fine with one snatch block on the front of the 5 ton at 'A', returning to the winch '12000'.

mikey said:
My question is, how does cutting the line lengthy in half, A to B and B to C, affect calculations? Or does it?

View attachment 539409

But... to tie up the loose end of your post one question... No - the pulley at 'C' (instead of at the winch "12000") doesn't change the calculation re pull factor, it operates the same as if it were at the winch. It naturally changes the over all length of rope reqd, which means you will have to stop winching when the load gets to 'C', requiring chocking or tethering, in order to relocate and continue.

Even though the snatch block at 'C' is "fixed" (post #7), it does allow you to redirect the line back to the load, which moves, therefore adds to the reduction factor.

If you had have needed the additional third line from 'B' to 'C' due to excessive load or resistance (locked wheels, steep gradient etc.) without having enough line to get there, you could use a bridle off of the front of the truck, divorcing the snatch block and final connection 'C' with some heavy chain or similar. Then when the bridle meets the winch, chock/tether the truck, remove bridle, run out line to bumper and start over.

I hope this makes it all as clear as the mud (mire) that you are not stuck in.

swbradley1 · Jan 28, 2015

Have you looked at the manual?

http://www.jatonkam35s.com/DeuceTechnicalManuals/fm21-305ch22vehiclerecovery.pdf

pg 22-5

Imagine your trailer is the winch truck on the left. I'd try it with a straight pull or figure 22-6.

mikey · Jan 28, 2015

Thanks everyone for your responses. I really appreciate it. I'd rather be safe than sorry.

The truck is currently in a field and has been sitting for some time. I planned to pull it a few feet with my tractor and a chain to get the wheels free. Although it does roll freely, I still figured that the soft terrain would add friction. Then we do have the 10 degree ramp for 9'. I could drag it to the road maybe 1/4 mile, but I'm confident after reading this that I should be OK with a single snatch block from right where it sits. Even if it could be done with a straight pull, at least one snatch block will help uncoil the line to get me to the first layer (12k) and cut the line pull in half to help the battery (191amp vs 300amp draw).

Thanks again!

Mikey

mikey · Jan 28, 2015

zebedee said:
I think you have all the info that you need, as in, you will be fine with one snatch block on the front of the 5 ton at 'A', returning to the winch '12000'.

But... to tie up the loose end of your post one question... No - the pulley at 'C' (instead of at the winch "12000") doesn't change the calculation re pull factor, it operates the same as if it were at the winch. It naturally changes the over all length of rope reqd, which means you will have to stop winching when the load gets to 'C', requiring chocking or tethering, in order to relocate and continue.

Even though the snatch block at 'C' is "fixed" (post #7), it does allow you to redirect the line back to the load, which moves, therefore adds to the reduction factor.

If you had have needed the additional third line from 'B' to 'C' due to excessive load or resistance (locked wheels, steep gradient etc.) without having enough line to get there, you could use a bridle off of the front of the truck, divorcing the snatch block and final connection 'C' with some heavy chain or similar. Then when the bridle meets the winch, chock/tether the truck, remove bridle, run out line to bumper and start over.

I hope this makes it all as clear as the mud (mire) that you are not stuck in.

Thanks specifically for answering that question! Even knowing that I can get this done with one snatch block, I was still very curious to know the answer to that question. I did plan on winching the truck to B, blocking the wheels, then removing B and moving C back to the winch to finish the pull.

Mikey

Whiterabbit · Feb 23, 2015

Don't forget to check out FM 20-22 up in the TM section. It'll answer alot of recovery/winch questions.

mikey · Feb 23, 2015

WOW, thank you! Over 4 years on this site and I've never even seen that FM referenced. That looks like a GREAT resource and read.

Mikey

Csm Davis · Feb 23, 2015

mikey said:
I have a trailer with a 12k electric winch. I want to use it to winch up my 5ton M51A2 in the spring, which weighs about 23k.
I don't want to use a single block because it would require a 48k snatch block and it would put the line pull at 12k maxing the winch and probably the battery (300amp draw @ 12k).
Thanks!
Mikey
View attachment 539409

Okay so quick follow up question I know the block would not need to be 48k but would it need to be 24k or 12k? And what about the 2nd block?

Floridianson · Feb 23, 2015

Artisan Give me a big enough fulcrum and lever and I will move the world.[/QUOTE said:
Give me a place whereon to stand and I will move the Earth. Archimedes

cpf240 · Feb 23, 2015

Wow, not what I was expecting... thought this would be a thread about a woman interfering with another womans attempt to hook up...

Another Ahab · Feb 23, 2015

Artisan said:
Give me a big enough fulcrum and lever and I will move the world.

Floridianson said:
Give me a place whereon to stand and I will move the Earth. Archimedes

Not everybody's going to translate the Greek the same way, but it's the same quote (+/-).

Archimedes, and his 5th Century buddies; amazing bunch, a lot of them were on the ball. Helped give us our Republic today; not too shabby that crowd. Guess it was the olive oil...

mikey · Feb 23, 2015

Csm Davis said:
Okay so quick follow up question I know the block would not need to be 48k but would it need to be 24k or 12k? And what about the 2nd block?

There may be better resources, but at the end of this post I'm listing two websites that helped me determine my calculations.

The first site helped me to determine the resistance of my 23k lb truck being winched onto my trailer with a 10% grade from grass mire. Based on the calculations on that site, my formula looks like this:

(23000 * .15) + (23000 * .20) = 8050 lbs

Once you have the approximate resistance figured out, the 2nd link is an awesome resource to calculate your snatch block totals using multiple blocks and lines. With one block and 2 parts of line, my winch will be pulling 4025lbs and my snatch block will need to be rated for at least 8050. My winch on the first roll is 12k and my snatch block is 20k, so I'll be fine.

Hope this helps you and/or others.

Mikey

http://www.pangaea-expeditions.com/resources/winchworksheet/index.html

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

Whiterabbit · Feb 24, 2015

Don't forget your tackle resistance which is 10% of your load resistance (veh. weight +cargo weight X mire factor - reduction factor) X # number of sheaves (1 less than # of mechanical advantage) which will finally give you your total load resistance.
I used to do alot of 1-1 pulls when I used my 140,000lb winch but with smaller truck winches I had to do the math. (I suck at math) I had a cheat sheet and a calculator. LOL!
The tackle blocks can be smaller as you increase your mechanical advantage. 2-1,3-1,4-1,etc,etc... because the TLR is what the tackle "sees".
Yep, it gets confusing! That's why the Army has a week long course just on wheeled veh recovery H8, and only then can you use a wrecker.
I get a good laugh when Bubba burns up a winch trying to drag a full size truck out of waist deep mud doing a 1-1 pull. You don't need a big winch ($$$), ya' need mechanical advantage!

Whiterabbit · Feb 24, 2015

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.

Help with snatch block calculations

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