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View Full Version : Snatch Blocks (interesting reading)



04OFF
2nd January 2012, 06:49 PM
I came across this while researching Snatch Blocks, i found it very interesting at just how easy you could unkowingly exceed a Snatch Blocks "safe working load", all because you used a Snatch Block in the first place !

Snatch Blocks (http://www.dodgepowerwagon.com/glovebox/snatch.html)


It also appears (from other research) that the larger pulley size Snatch Blocks are better for both Cable and Rope type winches (well, for the rope/cable not really the winch)

One thing i have discovered, is most Snatch Blocks available at common auto stores, have quite small pulley wheels ,and also quite a large gap at one end between the wheel and the housing when its all closed (as you would use it), i can see this gap becoming a potential issue if you use rope instead of cable, as a winch rope under tension could squeeze between the gap (steel cable would not be such a issue as it would not compress)

I ended up with a 8 tonne Maxi Trac 125mm Snatch Block, it has a smaller gap and a larger pulley than all the others i could find in the flesh.

Silver
2nd January 2012, 11:38 PM
Thanks for this interesting info Steve!

I don't understand the example the linked site gives for #2, where the line seems to go from the winch, to a block attached to a tree, and then back to a tree or other fixed anchor next to the car. Apart from being pointless, because it gives no mechanical advantage, it would not double the load on the block.

Maybe I miss read - it has been know to happen!

There are a couple of other points. I suspect that the winch rating is the load at which the winch will stall. The fittings usually show a safe working load which has a safety factor built in depending on application, and is usually considerably less than the load at which the fitting is designed to fail. Now, I am not saying that one should use up the safety factor when doing these calcs, but it is something to keep in mind.

Doubtless a knowlegeable rigger will be along soon to show where I went wrong. :-)

I do get a bit nervous looking at 'Flying Bomb Brand' recovery gear. I saw a 'rated' shackle in a major automotive chain store up here where the thread was so poor on the pin that it would not engage the thread in the shackle - absolutely 2ero quality control. Some things are worth saving $$$ on, some not!

On a related issue, Big Rig is still trying to get blood out of Tank's upholstery that came from the head of a bloke who used a dead tree as an anchor point, with a hand winch - which if it is like mine, would only pull 1.6tonne single line.

Last point, for those who use eye bolts bolted through bull bars etc in their recoveries, can I suggest you do some research on the load ratings for same. Most that I see on bull and tow bars, do not even get to the tonne.

Thanks again for posting the interesting info!

04OFF
3rd January 2012, 11:29 AM
As i understand it Silver (and i could be all wrong)


Fig 2 pictures the winch cable from the car ,through the snatch block (attached to tree), and back to the car (the snatch block is supporting the full load, regardless of mechanical advantage at the winch)

The rest of the description/s has the winch hook attached to somthing other than the car (a rock or tree) in this situation the angle of the rope etc may pay a part in changing the forces the "winch" see's (or needs to pull) but the snatch block still see's the full load.



Basicly it is saying, is that regardless of what the end of the winch rope is attached to (car/ tree/rock), the snatch block is always supporting the "full" load, while its still acknowledgeing that the winches mechanical advantage "will" change with different angles, or change when attaching the hook to the car or to a tree, but regardless of the mechanical advantage making it easy for the winch, the snatch block still supports the full load.




Think if you have a cable crane lifting a load, say the MAX load is 3 tonne, regardless of what load the crane engine (winch) is lifting (with the help of pulleys etc) the lifting hook attached to the load has to lift the 3 tonne, the hook always has to lift whatever the total load is , regardless of all the other mechanical advantage.

The snatch block is like your crane "hook" and will always be supporting the full load, the problem "is", introducing/using the snatch block can now "double" the effective lifting power of the crane engine, so now you can lift 6 tonne before your crane engine starts to struggle, remember ,your hook is only rated to lift 3 tonnes but now with the help of your snatch block, everything else can lift 6 tonnes, you can see how overloading the hook (or snatch block) could be a potential problem.

04OFF
3rd January 2012, 01:18 PM
There are a couple of other points. I suspect that the winch rating is the load at which the winch will stall. The fittings usually show a safe working load which has a safety factor built in depending on application, and is usually considerably less than the load at which the fitting is designed to fail. Now, I am not saying that one should use up the safety factor when doing these calcs, but it is something to keep in mind.



Exellent point Rick, yes it seems everyone knows to look and check for a WLL number (working load limit) or SWL (safe working load) and as far as i can understand , these safe working loads are worked out by taking 1/5th of the breaking limit. (so the break strain is 5 times stronger than the user end rating)

So thats all great, but your winch cable only comes with a max breaking strain, NOT a safe working load (SWL or WLL), but if we take a 12000lb winch, it may have a cable with a breaking strain limit of about 16000lbs, if we reverse calculate what the normal standard safe working limit would be on something with the same amount of breaking strain, so on the winch rope alone (16000 devide by 5 = 3200) you get 3200lbs or about 1452kg WLL/SWL.




So the safe working load for the average 12000lb winch cable would be less than 1.5 tonnes ,(id guess the average Patrol weighs between 2-3 tonnes GVM) , and thats all when the cable is brand new, after the cable has deteriorated over time ,its breaking strain could be even less.




Now my Snatch Block rating is rated at being "8 tonne max pulling capacity", so "if", that is in fact the safe working load,(not 100% sure its the same thing) will the breaking strain be 5 times this ? (5x8 tonne= 40 tonnes)

If my rope winch is capable of pulling 12000lb (5400kg) and this doubles with the aid of a snatch block to 10,800kg ,my 40 tonne "breaking strain" snatch block should still hold after the winch has stopped pulling, and the winch rope will surley have broken at around 15 tonnes anyway ?

Silver
3rd January 2012, 02:27 PM
thanks Steve!

I think that cleared up the second example a bit for me.

I agree 100% that the snatch block, and whatever it is attached to , shackle, sling or chain, and anchor point is subject to the full load

Silver
3rd January 2012, 02:29 PM
Exellent point Rick, yes it seems everyone knows to look and check for a WLL number (working load limit) or SWL (safe working load) and as far as i can understand , these safe working loads are worked out by taking 1/5th of the breaking limit. (so the break strain is 5 times stronger than the user end rating)

So thats all great, but your winch cable only comes with a max breaking strain, NOT a safe working load (SWL or WLL), but if we take a 12000lb winch, it may have a cable with a breaking strain limit of about 16000lbs, if we reverse calculate what the normal standard safe working limit would be on something with the same amount of breaking strain, so on the winch rope alone (16000 devide by 5 = 3200) you get 3200lbs or about 1452kg WLL/SWL.




So the safe working load for the average 12000lb winch cable would be less than 1.5 tonnes ,(id guess the average Patrol weighs between 2-3 tonnes GVM) , and thats all when the cable is brand new, after the cable has deteriorated over time ,its breaking strain could be even less.




Now my Snatch Block rating is rated at being "8 tonne max pulling capacity", so "if", that is in fact the safe working load,(not 100% sure its the same thing) will the breaking strain be 5 times this ? (5x8 tonne= 40 tonnes)

If my rope winch is capable of pulling 12000lb (5400kg) and this doubles with the aid of a snatch block to 10,800kg ,my 40 tonne "breaking strain" snatch block should still hold after the winch has stopped pulling, and the winch rope will surley have broken at around 15 tonnes anyway ?

too complex for the little knowledge I have Steve :-) One of the more learned rigger types may have a view.

04OFF
3rd January 2012, 03:24 PM
thanks Steve!

I think that cleared up the second example a bit for me.

I agree 100% that the snatch block, and whatever it is attached to , shackle, sling or chain, and anchor point is subject to the full load

Yeh cool , no worries Rick.

Another ^ good point Rick, if you attach a snatch block to a stuck car for recovery, the "anchor point" on the stuck car you attach the snatch block to, will also see the "full load" !