working load limit on fiddle blocks?

That's a damn good question... I have no idea.

I've generally assumed about 1 ton at the shackles in the 5:1 configuration, 400 pounds on the in-haul. I haven't read that anywhere, just made it up for my own use.

I'm pretty sure mine never sees more than 800 or 900 pounds.
 
Deconstruct your setup and find the weakest component.

Each leg of the system supports only it's fraction of the load. In a 3:1 each leg only needs to have a breaking strength of 1/3 the ultimate load [plus a safety factor].

The FB setups that I see in arbo supply catalogs seem to use half inch rope with breaking strengths in excess of 5k#. This is WAY overkill and makes the whole setup bulkier and heavier than really needed.

I read an article about how much pulling power a rope haul team could put into a rescue lift. They found that on a horizontal rope with good traction a person can add about 60% of their body weight to a haul line. So, do the calculations and add more bodies up to the point of breaking the lift system.

If the FB/MA is vertical then the load can get to body weight plus about 60% to account for shock loading if you want to maximize your lift.

After learning this I realized that the input on a FB/MA wouldn't be that high. That meant that I could use smaller blocks and rope. My setup uses 7/16 KMIII.
 
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what is the working load limit on a set of fiddle blocks?

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I believe it's the same as any hardware we use in the field. I use a 5:1 ratio to determine a working load limit for my fiddle blocks.

Joe
 
Hey do the newer fiddle blocks take on dirt and crud any easier with what look like those exposed bearings? Not to hi-jack the thread... I really want to upgrade from a 3:1 to 5:1 but don't know whether to buy FB or double pulleys. HELP?
 
I Made my own system from those 3'' dia 5/8 double sheave pulleys from wesspur and 50 ft of 1/2 tenex and 3/8 for the prusicks. Way cheaper ($80+/-) than fiddle blocks and pretty fast to deploy. Just pull blocks together and wrap a couple times in the middle then dasy chain. Remind me and I'll show it to you Kristian next time I'm up.
 
mark is that breaking strength? cause the hardware on the ones that sherrill sells looks kinda small for my application
 
Rescue guys use the 'rule of 12' on 7/16" rope and the 'rule of 18' on 1/2" rope.

Divide 12 or 18 by the first number of your mechanical advantage system (4, in the case of 4:1 fiddle blocks), and that gives you the number of people you can use to tension the system. Using the figure from above that a person can put force equivalent to 60% of their body weight into tensioning a line, and figuring that your average tree guy is 200 lbs or so, that gives us:

For 7/16" rope: 12/4 = 3

3 x (200 x .60) = 360lbs.

360 x 4 (as in 4:1) = 1440 lbs. WLL for the system

For 1/2" rope: 18/4 = 4.5

4.5 x (200 x .60) = 540

540 x 4 (as in 4:1) = 2160 lbs. WLL for the system

These numbers will vary, based on the size of your people, the rating of your pulleys, and the size of your ropes (both in the block system and your main haul line).
 
I suppose the limit could be set by the force at which the prusik slips on the haul line. That's probably the weakest link, in my opinion.
 
Now that you mention that that is true. I was at a North American training solutions class on rigging. Odis was saying that this is the advantage to using a VT knot to attach your fiddle block to the line was it will slip if subjected to excessive force. I use a 4-2 and works great.
 
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there is no prussic involved. so wll on those fiddle blocks?

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Is it not involved due to you just dont use it or because you have a diffrent block setup that has its on arrester built in? I belive a 8mm six fingure prussik is rated for 4500 lbs and I think even that was tested on a slow pull for slipage.
 
why does everyone want to talk about a prussic. i asked about the fiddle blocks in the link.
 
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