Screamers and Ice Screws?

[tvashtarkatena]

While we're talking screw strength, here's another fun fact:

 

In good ice, a well placed screw is actually stronger than a V thread, according to recent failure tests performed by Petzl.

[jmace]

using a longer screw to penetrate bad ice and 'get to the good ice' is not a good practice; the increased screw length creates a longer cantilever force on the threads (the bad ice has little compression strength to prevent this, so it's as if the end of the screw is cantilevered out into the air)

 

strength of the screw depends on the angle of the placement and the quality of the ice.

[tvashtarkatena]

I'm not sure of the point of this last post other than a misunderstanding of my post, but some knowledge of how screws typically fail is probably in order. Again, there are some great videos on Petzl's website that show screws failing that are worth watching.

 

Yes, ice quality and placement angle are key; that should be obvious to anyone. What I was addressing was placements in not so ideal ice. The maximum downward angle for proper screw placement is only between 0 and 10 degrees, so there is still a huge cantilever force on the still nearly perpendicular shaft during a fall. Watch Petzl's videos and see for yourself.

 

In any ice, the unthreaded shaft of the screw deforms downward towards the direction of the load. The screw begins to pull out as this is happening, accelerating this process. Placing a longer screw through an outer layer of bad ice without proper excavation will really accelerate this process. This bending transfers the initial cantilevered force on the screw to a longitudinal (pulling) force on the threads. When this force is great enough, the cone of ice holding the threads blows out.

Edited February 29, 2008 by tvashtarkatena

[G-spotter]

Hey Bob have you ever used the "Figure of 1" hooks?

what? what are you trying to say?

 

I read in the AAJ about these big ice fifi hook things used by Russian, Polish, Ukranian climbers for ice and frozen turf. I wondered if you'd ever used them and if so how they compare to Spectres etc?

[G-spotter]

Or were you too busy gobbling your boyfriend's shellac down your throat?

[glassgowkiss]

Hey Bob have you ever used the "Figure of 1" hooks?

what? what are you trying to say?

 

I read in the AAJ about these big ice fifi hook things used by Russian, Polish, Ukranian climbers for ice and frozen turf. I wondered if you'd ever used them and if so how they compare to Spectres etc?

 

no, it's rather ancient method. There was a vast use of "jedynka", which is a rock piton, similar in character and use to birdbeak. I don't know why russians are in the same bag as poles? anyway- i think as far as gear it's bit of misinformation.

[jmace]

Screws should be placed at greater than 10 and less than 30 degrees and yes you should clear bad ice..whatever you cant clear should be strong enough since ice is strongest with downward force

 

I geuss if you fall and more than half your screw is left in the air it may pull out but thats not a cantilever thats just a failure..its almost like for a screw to be cantilevered out you would have good ice up front forming the fulcrum and bad ice in the rear so the back of the screw is ripped out

 

 

[tvashtarkatena]

Well, I'm not going to quibble about the definition of 'cantilever'. Most of the readers here understand the term, and you can't bend anything without a cantilever, or moment force, and screws definitely fail by first bending, then pulling out. Again, I invite anyone whose interested to watch screws fail on Petzl's strength test videos to gain a better understanding of how failure occurs.

 

All modern screw manufacturers recommend that you place the screw perpendicular to the ice surface, not between 10 and 30 degrees as you've recommended. The hangers are not designed for that angle, and, from a practical standpoint, it's difficult enough to clear the ice while rotating the hanger at zero degrees, nevermind some other angle. Again, interested readers should check out the manufacturers websites rather than take the advice of an avatar on a climbing forum.

[jmace]

Short("stubbie")screws when placed in good ice provide a significant amount of protection that was quite unexpected and equivalent of rock gear when placed >10* but <30*. By showing this, we have validated the concept of what angle ice screws should be placed in vertical waterfall ice as previoulsy studied by Leubben and Harmston.

pg13

 

Tvash just having a discussion right not a argument or spray, I am not posting to hear myself just what ive read on the subject..ill give you the cantilever I over simplified it.

 

I get what your saying on clearing ice to place them at 30* but according to the research below maybe you should try in thin ice

 

have a good weeekend iam going home

 

 

Screw Placement

[tvashtarkatena]

With all due respect to whatever tests you've done, I'll go with the manufacturer's recommendations. I believe their testing is probably more comprehensive and rigorous, particularly given their liability exposure. Plus, as I mentioned previously, they design their hangers for a 90 degree pull.

 

I can see where adding a downward (I assume you meant downward)angle theoretically would be a good idea to reduce the cantilever force (oh, wait, there is none, right?), but in practice, a screw placed at such a high angle would thin out the upper part of the cone of ice around the threads that actually holds the screw in too much, thus reducing the overall strength of the screw placement. Perpendicularly placed screws produce the greatest volume of ice in that cone, and thus the strongest screw placement.

 

I'm not what stubbies have to do with this particular issue. Optimal screw placement guidelines apply to all types of screws, regardless of length.

Edited March 1, 2008 by tvashtarkatena

[jmace]

read the article it aint mine man..its sponsored by all the ice screw manufacturers you talk about

 

I just copied the quote it does apply to all ice screws

[tvashtarkatena]

linky? Or is it a google project?

 

Oh, you provided it. Thanks.

 

No argument. Just a discussion.

Edited March 1, 2008 by tvashtarkatena

[tvashtarkatena]

The study posted only tested screws tilted downward, and did not test screws placed perpendicular to the ice, so no comparison was made between the two orientations. The study relied on a previous study's conclusion that -30 degree angle placement can take higher loads than 0 degrees, but without looking at that original study, it's hard to judge how conclusive it was.

 

When I did a search on Harmston, the author of one of those previous studies, I came up with this quote from him:

 

"In my mind, the best placement is always perpendicular to the ice if possible."

 

link

 

It's important to keep in mind that the results of these studies are often ambiguous, involve low sample sizes, and are difficult to draw hard conclusions from, by the authors' own admissions, due to understandably wide variations in ice quality.

 

In the final analysis, manufacturers unanimously agree with Harmston by recommending that screws be placed perpendicular to the ice surface. This practice also saves climbers some unnecessary excavation. Having said that, it's probably no big deal to tilt a screw downward a bit, as long as you don't tilt it upward at all.

Edited March 1, 2008 by tvashtarkatena

[Franko]

the tests done by BD some time ago showed that strength of the screw depends on the angle of the placement and the quality of the ice. also turns out that 13 cm screw held the same as 17cm. one might argue that the energy absorption by the screamer is achieved by lengthening of the fall, hence not real energy absorption is happening.

 

Screamers spread the force out over time, thus reducing the peak momentary force applied to the screw. It's the same way that you spread the force out over time by partially applying the brakes on your car, vs standing on the pedal.

[catbirdseat]

The amount of energy absorbed by the screamer more than makes up for the extra distance you fall. The test results speak for themselves. Peak impact forces are reduced. Whether you want to call it energy absorption or not, they do what they are supposed to do.

[tvashtarkatena]

You can also think of it this way. Falls that generate greater than around 9 KN result in a significantly higher number of screw failures (the uncertainty of these figures from tests is high, so this is by no means an exact limit). A screamer absorbs about 3 KN, and so increases the peak loading a well placed screw can take by about a third.

Edited March 13, 2008 by tvashtarkatena

[selkirk]

One way to think about it is that

 

Your Falling Energy = Potential (PE: how high you are) + Kinetic ( KE: how fast your moving and how fat you are).

 

Your total energy = constant while your falling. As you fall PE goes down and KE goes up.

 

Energy = Force * Distance

 

Once the screamers activates it keeps force constant at 500lbs (? or whatever) over the length of the extension.

So the Energy Absorbed = 500 lbs * Extension (say 18 inches?)

 

Your rope acts in a similar fashion by absorbing energy as a spring

 

Energy = force * distance

 

So long as things stay dynamic and the rope/screamer are extending while providing resistance the force transmitted to the gear isn't so bad (500lbs).

 

Once you run out of extension, and your screamer goes static, and your stretch is eaten up, then you start to get big force as there is still energy to absorb, but it has to be absorbed by a very small extension.

 

Screamers help for the same reason that static runner/static ropes generate high forces.

(Static = little stretch = very little distance, so for a fixed amount of energy, this drives the force component up to disipate the same amount of energy.) In the limit of a pure static (no stretch at all) fall you'd theoretically generate infinite forces.

 

 

Can anybody guess my profession now :p

[Weekend_Climberz]

I thought you were into neuro science.

[tvashtarkatena]

Your rope acts in a similar fashion by absorbing energy as a spring

 

One big difference between scramers and the rope: you don't have to buy your partner a new rope after taking a whipper.

[pdk]

My main thought after reading this thread: there sure are a lot of you who've taken whippers leading ice.