Snatch Block Pull Weight

[UpperCrust]

So im building a 15' camper on the back of my MB 917 truck. Im thinking the wet weight when all said and done will be about 9,000lbs. I have 2 16.5Klbs winches (front and rear). But I know I need some snatch blocks for when I get stuck in sticky mud or whatever.

But I have a question about what weight rating to get. If I use one snatch block to increase my pull power to 33k lbs, do I need a snatch block with that weight rating? if I use 2 snatch blocks to increase my pull power would I need to have 2 33k lbs blocks or could I use 2 18k lbs?

Can someone help me out with this physics problem?

 

[LocoCoyote]

You might be overthinking this.... Your rigging ( snatch blocks included) need to be rated to the highest load you might pull. It is that simple.

 

[Sabre]

Assuming a straight-line pulling force of 10k lbs, running the winch line through a single pulley will indeed exert 20k lbs of force on the pulley.

The addition of a second pulley, while increasing the winch's pulling force to 30k lbs, will not change the load placed on any single pulley. This load is never more than the sum of the two "falls" of line that run over the pulley wheel. This only applies to two simple pulleys, not to a double block or single pulley with becket.

 

[aardvarcus]

Sabre is right, just remember when multi-rigged you may want to attach the additional pulleys to different anchors or recovery points to spread the load out.

 

[UpperCrust]

OK. So, I just need to divide the total weight pulled by the amount of anchor points (1, 2, or 3) and make sure my blocks are rated for the weight. Makes sense!

 

[NYresQ]

If your winch can pull 16.5K, then the most load a single pulley can have placed on it would be 33K. This would be if you had the winch line run out, through the pulley and then back connected to your truck near the winch and then pulling to complete stall on the winch. Thats either really, really stuck, or you have a winch very undersized for your truck!! Adding more pullies will increase the total pulling force, but as long as they are connected to multiple anchor points or multiple straps, then everything would still be at or below 33K.

But if you connect two pullies to your bumper and two to a tree, you could be pulling as much as 60,000 lbs (or more) against the bolts holding your bumber to the frame, or at the other end you could have 30 tons pulling against that tree in the wet sandy soil... choose your anchor points wisely if you need that much force. I have seen a couple bumpers pulled off of trucks that were used as anchors for other stuck rigs with winches!!!

A pulley used for a directional change would handle less than 33K depending on the angle of the direction change. A 180 degree change (out and straight back to very near the winch itself) would be 33K, and decrease as the angle opened up.

So, yes, you should buy a pulley rated for 33K or a little better for a safety margin... I have a 15K winch and all my hardware and straps have a working load of 35k or better, and my pulleys are both 40k.

 

[UpperCrust]

NYresQ, What pulleys do you have?

 

[NYresQ]

The two I have are heavy monsters with 6" sheaves I got from a rigging company years ago. They are made for up to 1-1/8" cable and are marked 20T. They dont have a brand name on them, but say "Made in USA". They are over kill for what I have, but when I got them from a company that was going out of business for about $150 each, I couldn't pass them up. They were barely used, the sheaves didn't have a mark on them and with some spray paint look new.

They each weigh about 50lbs so I dont keep them in my truck unless I know I am going out in the woods.

I normally keep a Rugged Ridge block (30K) in my bed box along with three Crosby 3/4" links and a Rusty's Offroad tree saver strap which is rated at 60K. Those all fit in a small dewalt tool bag I had laying around and makes it easy to grap and go.

If I know I am going out to play, I grab my high lift, two more extra Large links, the two monster pulleys and a 4"x30' 60K strap as well. Thats my standard winch accessories lot if I plan to use it. One pulley and the rest live in a large canvas tool bag, the other pulley hangs on the wall in the garage.

If you look at industrial pulleys, they normally have a 4:1 safety factor (industry guidlines etc), where most of the stuff made for winching is a 2:1 or 3:1 factor (its completely unregulated), so even getting a lower rated pulley from a rigging source will be much stronger than anything you can buy for "off roading".

But with that safety factor comes a much higher cost. A Rugged Ridge pulley rated for 30K can be had for $70, an industrial rated pulley rated for 15T will be closer to $1,000.

It all comes down to realistic need. If I didn't have those pulleys I got for a steal, I would never buy anything rated at 20T with a 4:1 safety factor built in for just winching...

 

[Brewtus]

I'm a bit late to the discussion here, but the logging pulleys/blocks I have are rated based on cable size not tonnage. i.e. A 3/8" block is capable of breaking a 3/8" cable. So unless your block is too small, you cannot overload the block because the cable will break first.

 

[Kerensky97]

I'm a bit late to the discussion here, but the logging pulleys/blocks I have are rated based on cable size not tonnage. i.e. A 3/8" block is capable of breaking a 3/8" cable. So unless your block is too small, you cannot overload the block because the cable will break first.

Makes sense. Assuming you're not buying off brand consumer pulleys it's the cable that is the weak point of the equation. Most industrial blocks are amazingly stout.
Plus in professional use people are much slower and more deliberate rigging gear than the "Lets just yank him out!" 4WD crowd who has never been through formalized training or certification for heavy duty rigging.