Working Load Limit of Pulleys?

[Haironyourchest]

Just occurred to me, does the working load limit of a pulley indicate the total max load of the pulley or just one fall? I have a couple of 4-tonn snatch blocks, but the shackles that came attached to the blocks are WLL 6-tonn. Don't know if that means anything.

 

If I haul an object with my block, with one fall of the rope being pulled by a winch and the other fall attached to an anchor point, and I put 4 tons of force on the rope with the winch, then the total load on the block is 8 tonns.

 

I would have presumed the WWL is that of the mechanical advantage, not the winch pull.

 

Makes me wonder, if I am correct, if people are sometimes overloading their pulleys in the tree. Like a 15kn alloy pulley would have a WWL of 150kg approx - right? This means that if the rope is tied off to the base of the tree, then the WWL should be halved - giving 75kg? Surely that can't be right.

[Aussie treesmith]

It'll be max, 150kg pulley is good for 75kg on each leg, less if you're shock loading it

 

BTW 15kn is 1.5 tonne, 10 newtons to a kilogram, 10,000 newtons to a tonne

 

Bear in mind cycles to failure too, if you load it to the max rated weight, it'll fail sooner

 

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[Haironyourchest]

Nice one, thanks. I just checked my climbing pulley (small one) it actually says on the pulley - 30kn total, 15kn on each leg. Thats a lot for a tiny pulley! I suppose the WWL then would be a fifth of that, so them little pulleys are really only supposed to be used for hoisting 150kg with one leg tied off, which makes sense, given the size of the thing.

[treequip]

Nice one, thanks. I just checked my climbing pulley (small one) it actually says on the pulley - 30kn total, 15kn on each leg. Thats a lot for a tiny pulley! I suppose the WWL then would be a fifth of that, so them little pulleys are really only supposed to be used for hoisting 150kg with one leg tied off, which makes sense, given the size of the thing.

 

The WLL is the max load manufacturers recommend for the item in the recommended configuration and attachment. You can regard the WLL as the Safe working load (SWL)

 

Any load above that and the item should be retired from service.

[Jonny69]

Nice one, thanks. I just checked my climbing pulley (small one) it actually says on the pulley - 30kn total, 15kn on each leg. Thats a lot for a tiny pulley! I suppose the WWL then would be a fifth of that, so them little pulleys are really only supposed to be used for hoisting 150kg with one leg tied off, which makes sense, given the size of the thing.

 

I think you may have your maths out.

 

30kn is 3000kg not 300kg.

 

1500kg each side. Which is 10x your working load of 150kg.

 

Rigging kit is supposed to be 5x working load.

PPE is 10x as I recall.

[Haironyourchest]

I think you may have your maths out.

 

30kn is 3000kg not 300kg.

 

1500kg each side. Which is 10x your working load of 150kg.

 

Rigging kit is supposed to be 5x working load.

PPE is 10x as I recall.

 

Ok so then it would be 300kg WWL on one leg, or 600kg if using it DdRT. Thats a strong wee pulley. If its considered to be a rigging device, that is. I will consider it PPE anyway. Only want to hoist a double glazed unit into place with a pair if them, so they'll be well within their limits. Thanks for your replies

[Jonny69]

Ok so then it would be 300kg WWL on one leg, or 600kg if using it DdRT. Thats a strong wee pulley. If its considered to be a rigging device, that is. I will consider it PPE anyway. Only want to hoist a double glazed unit into place with a pair if them, so they'll be well within their limits. Thanks for your replies

 

I think you'll be fine.

 

Bear in mind the 3000kg is breaking strain. So when new that pulley design would have had 1500kg of pressure applied to each leg and snapped just above that limit.

 

I've never snapped any mechanical equipment used in tree work. Only ever had the ropes break and they were certainly not being used for their intended purpose at the time.

[TIMON]

As others have said the general method for calculating the SWL/WLL

Is 5:1 of the MBS on metal hardware (most

and 10:1 of the MBS of textiles.

Another thing to take into account is that you must measure the peak impact force and not merely the load mass.

There can be a big difference especially in negative rigging situations.