block and strop

[Paul Smith]

Paul,

Your calcs seem to tie in with my previous thoughts on load factors but I can't recall where I first heard it from. Do you have a ref or citation? Cheers.

 

Jamie, It was from an article in essentialARB called taking the guesswork out of rigging. Richard Olley of Kingswood Training wrote it!

 

Cant see a date on the article, Richard gave it to me on a rigging course.

[Jamie M]

Cheers Paul, much appreciated!

[Craig Johnson]

to be correct its Kn not Kg as the log its self weights the same its just got more force, where force=massxacceleration, where A is 9.8m/s/s, so the longer/further the log falls the more it acceleration it has which will generate your peak force when arrested/motion changed. for easy maths every one uese 10m/s/s

so for the first meter/second of fall 80kg x 5m/s/s = 400Kn

because the log is accelerating from 0 to 10m/s/s the mean value for the fist second is 5 but the second second wil 10 to 20 with a mean vaule of 15

so for a two second fall 80Kg x 15m/s/s = 1200Kn and so on.

this would be the froce generated when the the log is stubed off to a dead stop, letting things run will greatley reduce these figures.

craig

[browncow]

The force generated also depends on the amount of rope in the system, the dynamics of that rope and how much your groundy likes you.

[Paul Smith]

The force generated also depends on the amount of rope in the system, the dynamics of that rope and how much your groundy likes you.

 

The 3+1 is based on a free fall but yes to the above.

[kingswood]

Hi

 

I have tried to attach the article Paul is referring to although I admit to knowing much less about computers than rigging. If it isn't attached I could email it to someone else to attach if it would help.

 

This thread is covering a hugely complex subject, but here are some very basic rules:

 

1. Know the Minimum Breaking Load (MBL) of your rope, and divide it by 10 to get the Safe Working Load (SWL).

2. All other parts of the system must have a Higher SWL, in the configuration that they are used.

3. This means that top anchor points must have TWICE the SWL of the rope, even when there is no shockload.

4. Use log mass charts and species conversion factors to get an accurate weight for the log.

5. Current wisdom suggests that you should allow for 30% inaccuracy in this figure.

6. If you can't pre-tension the rope to prevent a shockload, you must let the load run on the capstan.

7. Letting the load run is essential when topping down a stem, because experiment has shown that the load imposed on the top anchor point can be 11 TIMES the weight of the log!

8. Any shockload in the system will massively reduce the weight of the log that can be safely removed.

9. More rope in the sytem is better.

 

It is essential to understand whether the figures on your kit are kg or kn, and whether they are MBLs or SWLs. The ratios between the two (known as the safety factor) are 10:1 for ropes, 7:1 for slings and 5:1 for metal equipemnt.

 

I don't know of any rope system in current use that could safely lower a 1 ton log, even without a shockload. This is not intended as a lesson in rigging - just a pointer to the way rigging should be approached. There is no subject in Tree Surgery where decent training is more crucial - this is not an ad - do it anywhere with a good reputation - just do it! Hope this helps.

 

Kingswood

Rigging for richard.pdf

[BenR]

We lowered some 1ton logs off a big sugar gum over in Oz last year. They were top roped, pre-tensioned and peeled off slowly using a tagline and inch thick lowering line on a huge rigging block. Having said that I wouldnt have been comfortable being in the tree for it as the rigging system wasnt matched properly. Its very easy to say that you can do something no problem and I was pretty confident that it would be fine. But the overloading of the strop for the rigging block is bound to be reducing the cycles to failure significantly. Great article posted by Richard there. Good reading for the go big and go home boys

[BenR]

http://vtio.org.au/Content/wp-content/uploads/2010/07/Working-the-Angles-i.pdf some more good reading on angles and vector forces for those who have not seen it before.

[kingswood]

The VTIO article Ben refers to above gives some really useful, in-depth information for those who want to really understand the forces invloved in various rigging scenarios.

 

Kingswood.