Pre-tensioning rigging lines

[Clicky]

What's everyone using to pre-tension there rigging lines? I've seen stuff like the stein pretension pulley and also the ropejack, and I know about winching bollards but there out of the price range. Would just like everyones thoughts!

[Steve Bullman]

The stein is probably the cheapest option. Personally I very rarely find the need to pretension more than what 2 men can do manually

 

 

Sent from my iPhone using Arbtalk

[TIMON]

We find pulleys, prussiks and caribiners work well in 3:1 (or more) pre-tensioned systems. [emoji106]

 

 

Timon.

[Luckyeleven]

fiddle blocks

[stihlmadasever]

Same as Timon 3:1 with a pulley prussik

[Darrin Turnbull]

[ame]

[/ame]

5 to 1 pull

[David Dobedoe]

Im interested in the practice of pre tensioning lowering ropes / rigging lines.  I think / assume that a snug line reduces slack in the system and and therefore means that when catching falling loads the distance fallen is minimised.

 

However when the rope is tensioned with a pulley system is it not the case that we are impacting on the elasticity of the rope which is designed to absorb the energy?

 

If we are pretensioning a system, if there is a significant advantage, how do we know what level of tension is optimum? Does it change as the scale of equipment changes or the distance between the lowering device and pulley changes?

 

I think that most  of the energy absorption is the friction of the rope around the lowering device. This is also complicated by the choice of how much friction to use (wraps) I would guess that our lowering lines are designed to significantly absorb energy so what proportion of the energy absorption is in the stretch of the rope?

 

What work has been done in measuring how pretension helps or maximises the effectiveness of lowering systems? Any references would be appreciated. 

 

What might I be missing?

 

TIA

David

 

[TIMON]

Try these....

http://www.hse.gov.uk/research/rrhtm/rr668.htm

http://www.treesource.co.uk/the-art-and-science-of-practical-rigging-workbook~4466

https://www.amazon.com/Rope-American-Vertical-Techniques-Rappellers/dp/1879961059

Hope this helps.
Also PM sent.

[emoji106]

[Loler UK]

On 15/09/2017 at 13:56, David Dobedoe said:

I I would guess that our lowering lines are designed to significantly absorb energy so what proportion of the energy absorption is in the stretch of the rope?

You'd guess wrong.

 

The lines used for rigging in arb are 100% polyester and are not really designed with dynamic loading in mind, Polydyne is the most absorbing with it's nylon core but nylon looses strength when wet and the stretch leads to so much sag during normal rigging operations that it can graze/ take out fences.

 

As far as pre tensioning lines that will be shock loaded this is a big NO NO, the split second for which they are unloaded does not give them time to recover, a minimum of 3 mins and preferably longer should be left between subjecting lines to high dynamic loads like when snatching etc. a 'hand tight' vector pull is all that's needed: pull your wraps hand tinght, then 'strum' the line pulling it out 90 degrees from it's direction of travel and work your new tension around the bollard as you pay it back in, this method can achieve a 3:1 MA when done effectively, in theory it could achieve far more but the reality doesn't let us see the true physics at work.

 

A Manufacturer that shall remain nameless once ruched a set of tests for BSEN1891 not giving the rope the full 3 mins recovery time that it was aloud for the dynamic performance test, confident it wouldn't require the full recovery time only to find the cell read over 6KN, oop's (the first drop on virgin line will have only generated 2-3KN, the forces really do increase rapidly when not enough time is given to dynamic recovery)

On 15/09/2017 at 13:56, David Dobedoe said:

 

 

[TIMON]

Here's an example of a pre tensioned system we set up to dismantle a dead/dying sycamore leaning over buildings with asbestos roofs. The tensioned line doubled as a guy line and floating anchor point, tied into the top of the sycamore, through the fork of a neighbouring tree and anchored to the base of yet another tree nearby. We used a 3:1 mechanical advantage system with pulleys and prussic fail to safes to maintain constant tension on the line. We then tied a prussic and pulley to the tensioned line half way between the sycamore and pine to give us a floating anchor point for a separate lowering line to the only viable drop zone.
Took a bit of head scratching to start with but once it was up and running the job went really smoothly.
Very tempting to just say "just cut and chuck", in these situations but just one mishap could have destroyed the roofs below costing £££s (busy garden centre with public nearby too)
We were careful to calculate a very tolerant SWL onto the system.
(Sorry for the long winded explanation)