Petzl Basic Manual- Example Techniques Struggling to understand the benefit of a certain example
Posted 21 February 2012 - 01:59 AM
The first diagram is completely logical and simple: load, pulley fixed to anchor, rope grab, pulling force = grab engages when the force in no longer applied and keeps the load in place.
My issue is with the second diagram on the right:
This shows, one load, anchor point and force, but two rope grabs and two pulleys.
From what I can conclude, the rope grabs are operation in the same direction, preventing the load falling once the force is released. But I can't understand the mechanical benefit, or any benefit at all from using two pulleys and grabs.
My first thought was it was a modified double purchase system, but as the bottom pulley is not secured to the load it would not work to give the same mechanical advantage as a double purchase system? All I can see is that when the force is applied away from the load- pulling the load up, you pull the closest pulley towards yourself, and gain that distance in the height of the load (assuming the second, lower rope grab releases the rope efficiently). Ultimately resulting in very hard, slow progress.
Any clarification in the workings of this system would be greatly appreciated!
Thanks in advance, whilst very confused.
Posted 21 February 2012 - 08:41 AM
You should be aware that releasing the rope-grab can be awkward, especially for inexperienced users (your question hints that you may not have much experience). You need to lift the load slightly whilst disengaging the cam. It may be that the risk of losing control of the load or damaging the rope sheath while doing so could actually outweigh the risk of randomly dropping the load mid-haul.
The diagram on the right does show a system with some mechanical advantage, commonly known as a "Z-hoist". It could be used for hauling a bag up a big-wall rock climb, or for crevasse rescue. Theoretically it might give you a 3:1 mechanical advantage, but in practice the friction in the pulleys reduces that somewhat and you end up lifting around 50% ish the weight of the load. If you can't get your head around it from the drawing, grab some string and have a play - all will become clear.
This old thread might be worth a read (No doubt there have also been others.))
Posted 21 February 2012 - 02:17 PM
To expand on my intended use for the rope grabs. The rig is for three separate, but identical 'Silk Rises', 18m in height and 1.5m wide. The rig is practically identical to a guided silk drop with SWR running on either side of the material, which is eyeleted and snap hooked onto these steels. As the desired effect is to see the rise rather than the drop, the silks will be hauled from the top. The rig is on the front of a multi-story car park of which we have access to the top level. The rope grabs are there to ensure that if the crew let go for whatever reason, we don't suddenly get a drop where it should still be going up, and the rising action can continued as soon as appropriate. The actual load being hauled is minimal- a bit of 2"x1" as a top baton to the silk to ensure a nice crisp edge, the weight of the ripstop material and some friction of all the snap hooks on the SWR- not difficult to haul with one hand and release the grab with the other when it comes to derigging.
I understand your suggestion for the Traxion, however I believe that the Basic will offer me more flexibility to change the rig if the director wants to change the effect during rehearsals. Additionally, 3x Traxions is significantly more costly than 3x Basics. I will think and experiment some more with this.
Thanks again for your input, greatly appreciated.
Posted 21 February 2012 - 04:10 PM
Regarding the relative cost of Basic vs Pro-Traxion, you're right of course although three Pro-traxions are likely to be much more useful things to have after the event than three Basics. There is some middle ground - a Mini Traxion is still more expensive than a Basic, but cheaper than a Pro Traxion. Handled ascenders (Ascension) will also do just about everything that a Basic will, at more or less the same cost and probably more likely to be useful for something else after the event. I'm not sure I agree that the ascenders would be more versatile than than the traxions, but in any case you'd probably need more experience than you have currently to improvise with them to best effect. Do you have plenty of time to play/experiment before the gig?
Besides adapting climbing/caving gear, you might also consider yachting gear. Something like cam cleats perhaps - do you have a friendly marine chandlers anywhere close?
Another thought, given that hemp flymen have been managing not to drop drapes willy nilly for a couple of centuries - if you can't rely on your crew not to let go, maybe you need more reliable crew. Ha ha.
One more slightly off the wall idea: do you have the top floor of the multi-storey to yourself? If there's sufficient space, maybe you could arrange an 18m 'walk' instead of pulling 18m of rope hand over hand - a human 'mule' walks at whatever pace the art dictates, arrives at the out dead and clips a loop to an appropriate anchor. (Kentledge block/vehicle towbar/sling round a pillar/whatever.)
Posted 21 February 2012 - 07:29 PM
Jumars are awkward to unload, needing a slight releasing of the tension and then slightly disengaging the cam and teeth by hand - not a full release that you'd use to unrope them. They basically aren't designed to release "nicely". This is fine for down climbing a single rope if you need to, but a bit of a faff for anything other than a long gear haul that you benefit from an arm break between pulls. Also, when you unload the whole system, the jumar and krab arrangement can slacken, parts invert and generally tangle or then reload at awkward angles (the handled ones sometimes seem worse).
It's deffo something to experiment with first. When I was into all that in my early 20s, I'd spend time harnessed up around the place, practicing dealing with the kit and working out the little vagaries of using it in practice.
This post has been edited by indyld: 21 February 2012 - 07:31 PM