snatch blocks

[Cillyman]

Hello all I was wondering how you rate a snatch block for tree work ? What I mean is if you have a block that is rated at say 2 ton working capacity. How do you rate the block when you will be allowing the load to fall a short distance before the block catches it. I guess it would then be a dynamic load as apposed to a static load. I am not sure if my terms are right. Thanks for any education you can give me.

Scott opcorn:

 

[kennertree]

Most arborist equipment has a tensile strength which is where it breaks, due to probability of dynamic loading loading it will have a safe working load which is a ratio of 10 to 1. For example, a rope that has a tenesile strength of 10,000 lbs, the safe working load is 1,000 lbs.

 

[BostonBull]

CillyMan

By the wording you used you must be a lineman, or have a lineman background?

Could be way off here too though.

 

[Ekka]

You have to anticipate the shock load or dynamic load and ensure it stays below the blocks WLL or SWL.

So if the block is two ton you better cut pieces a lot smaller.

Also where the load will be braked on a friction device the load at the block doubles. So 1 ton hanging there doing nothing puts 2 ton on the block.

A piece of timber weighing 100lb falling 4' becomes approx a 400lb dynamic load. If the piece is cut 4' above the block then it will have to fall 8' before the block starts to catch it. So then you'll see around 1600lb of dynamic force.... all for a 100lb piece.

My advice, tone it down, pieces no more than 10% of blocks SWL or WLL.

Doesn't ISA teach this in their CA?

 

[Cillyman]

Ok so I see a 10 to 1 ratio for dynamic loading. So a 2 ton block would be good for about 200 lbs of dynamic load ? Also I see that the dynamic loads increase with the distance dropped and that the load on the block doubles when braked to a stop with a friction device. 100lb becomes 200 when hanging from the block. Ekka I would assume the the letters SWL mean safe working load ? What do the letters WLL stand for. Thank you for your replies. Scott :newbie:

Oh Boston Bull not a lineman, actually I am a Sheriffs Deputy

 

[SRT-Tech]

W.L.L. = Working Load Limit

 

[Ekka]

My advice, tone it down, pieces no more than 10% of blocks SWL or WLL.

Just for clarity and to re-affirm what Treeco wrote.

The SWL or WLL of the block has already been adjusted in.

But where there is no mention or stamping be careful.... like ropes are usually rated breaking strength.

So a 2t block can take 2 ton, no worries, and yes that's only 1 ton a side... however, if the piece you cut is going to dynamic load the block that's where things go way off the scale.

In a vid I'm about to put up you'll see the the block is well below the head I cut out, the forces on the block in that video would have easily gone 7X the weight of the piece, maybe even 10x.

I estimated the piece at being 100kg, a 4' fall to the block, 4' past the block, then braking, I reckon if you dyno'd that load at the block it would have peaked around 1000kg.

But hey, the rope is only experiencing about 1/2 of that, so 500kg on the rope which is rated 6000kg breaking so I'm OK there. The block can take 2000kg all day every day as that's its SWL so 1000kg is OK there also.

But what this all means is KNOW YOUR ****, know your drops, calculate your margins, and dont have idiots on lowering ropes being BANK ROBBERS!

And you being a deputy sherrif know that BANK ROBBERS hold everything up!

I have seen 50kn steel biners straightened out, blocks go flying out of trees, lowering devices race up trees and lowering ropes breaking ... all due to overloading. Mind you, not on my jobs.

You need to match your gear, if the block is 2ton then match it with slings and rope accordingly.

Unlike USA we use a 1/6 SWL for ropes. My lowering rope is 6000kg breaking so 1000kg SWL. The rope's SWL should be around half the blocks SWL. (remember the block experiences twice the load of the rope). What attaches the block to the tree has to be equal to the block in SWL. I attach mine with doubled lowering roped so it's pretty close unless you want to argue about loss of strength due to a cow hitch.

Cheers.

Link to video

http://www.arboristsite.com/showthread.php?t=47587

 

[Cillyman]

Ok block ratings have the 10 to 1 or possibly 5 to 1 safety factored in already. A two ton block will take a 2 ton dynamic load. I have to figure my length of drop and wieght of the load for a total dynamic load figure. Just curious why the rope has half of the dynamic load ? Is that because it is doubled across the block pulley ? Thank you for your input everyone, you are great teachers Scott

 

[ropensaddle]

Also a ground man that knows how and when to let
a piece run a little will help a great deal! If he can let it run
and slow it down it greatly reduces shock load! If he holds tight
when rope loads and stops too fast well you probably know this.
I know I probably everyone has pushed the envelope while blocking
down and I have a chain made out of high test for serious loads
heavy but can count on not breaking! do not use while climbing
unless I am tied in a different tree as don't like to kiss trunk lol.