Physics of Ascenders Slipping

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Physics of Ascenders Slipping

Postby PeterFJohnson » Jul 22, 2012 12:51 pm

Two quick questions based on a conversation in another thread:

1)Has anyone had experience with an ascender slipping after being weight. Meaning you are hanging on it, it is already supporting all your weight in a static situation, and then it suddenly gives. As opposed to the more common situation where you slid your ascender up and when you place weight on it, or during the process of placing weight on it slips or fails to grab.

2)It was my uneducated understanding that the teeth on an ascender function primarily to engage the cam device. Once that happens some combination of the teeth and friction keep the cam engaged. Anyone care to expand upon or correct this model? I am thinking specifically of what Gary Storrick calls eccentric ascenders(load-rope-pivot) but would gladly take a physics primer on the differences between (l-r-p) and (r-p-l) ascenders.
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Re: Physics of Ascenders Slipping

Postby Scott McCrea » Jul 22, 2012 1:00 pm

I can tell you that a buddy got creative one day and filed all the teeth off of his Croll. He was hoping that it would slide up swollen, wet, muddy ropes better. Well, it did go up easily. But, it also slide right back down, weighted or not. He could get it to hold momentarily if he really pushed hard on the cam. Let go and the Croll would slide down. This was on clean rope, too. So, I say yes. The teeth are necessary.
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Re: Physics of Ascenders Slipping

Postby PeterFJohnson » Jul 23, 2012 1:50 pm

Ha, you may have just saved me an old croll. I was thinking about filing off the teeth just to answer question #2. So it seems pretty clear that the teeth are necessary to keep the cam engaged.
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Re: Physics of Ascenders Slipping

Postby Anonymous_Coward » Jul 23, 2012 4:13 pm

Hey Pete I have an ascender that will slip while loaded. All of the teeth at the top of the cam are worn down. The ascender still grabs with the lower teeth, but while weighted you can rotate it forward and cause the cam to become disengaged. It can then be slid down the rope. As soon as you rotate it back towards yourself, the lower teeth will grab and hold body weight again. Kinda freaky, especially when you don't expect it.
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Re: Physics of Ascenders Slipping

Postby Stridergdm » Jul 23, 2012 4:41 pm

Anonymous_Coward wrote:Hey Pete I have an ascender that will slip while loaded. All of the teeth at the top of the cam are worn down. The ascender still grabs with the lower teeth, but while weighted you can rotate it forward and cause the cam to become disengaged. It can then be slid down the rope. As soon as you rotate it back towards yourself, the lower teeth will grab and hold body weight again. Kinda freaky, especially when you don't expect it.


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Re: Physics of Ascenders Slipping

Postby jharman2 » Jul 23, 2012 8:39 pm

I have an ABC handle ascender that slips while weighted. The top teeth are worn out and it intermittently slips even when you attempt to manually thumb the cam closed.

What I found very interesting is that it ONLY slips on 11mm rope. It grabs 10mm and 9mm rope perfectly. The reason is that smaller diameter ropes allow the cam to travel deeper into the shell engaging the lower (good) teeth. So, yes teeth are critical to keeping the device engaged on the rope.
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Re: Physics of Ascenders Slipping

Postby PeterFJohnson » Jul 23, 2012 11:47 pm

I found a couple interesting articles if anyone cares to edumacate demselves. Here, Here, and Here. The first two deal with climbing cams but the mechanics are similar. The last one is off Gary Storrick's site.

All worth a read if you are curious. Once again borrowing Gary Storrick's terminology, as I understand it, there are two broad classes of ascenders:

1)"Non Eccentric": Where the load connects and acts directly on the cam(Petzl Shunt, Black Diamond nForce, Gibbs)
2)"Eccentric":Where the load connects to the ascender shell and only indirectly acts on the cam(most ascenders).

In the 2nd class you need a certain amount of friction to engage the cam to begin with. Specifically the friction between the rope and the cam has to be greater than the rope and the shell(I think...). At this point the cam engages. The spring and the teeth increase this friction. Since the amount of friction required to engage the device should theoretically be less than the friction required to hold a load it should take a reduction of friction for the device to fail after being weighted. A reduction such as rolling the ascender forward a la Andy, or releasing a force-providing-thumb a la Scott's example. Or even something less explainable like John's example. It seems theoretically it should hold once engaged, but in reality it is far too easy to vary the friction to trust your life to an ascender with teeth that worn.

Contrasting that is the 1st class, where since the load directly affects the cam teeth are far less important for engagement.

jharman2 wrote:What I found very interesting is that it ONLY slips on 11mm rope. It grabs 10mm and 9mm rope perfectly. The reason is that smaller diameter ropes allow the cam to travel deeper into the shell engaging the lower (good) teeth. So, yes teeth are critical to keeping the device engaged on the rope.


Seems this is an effect of the equiangular/logarithmic spiral design of cams. It also probably means you climb more 11mm rope than 10mm or 9mm. Not surprising given all the fixed ropes in GV. The fixed ropes also probably tend to be muddier and gritter than push ropes, which compounds the effect.

PeterFJohnson wrote:...what Gary Storrick calls eccentric ascenders(load-rope-pivot)...


The original poster clearly doesn't understand what makes eccentric ascenders unique as compared to load-rope-pivot ascenders like the shunt. Idiot...

Thanks everyone for the info and sorry for having to think out load on cavechat. Hopefully someone can correct what I have wrong.

And thanks especially to Gary Storrick, who obviously already discusses all this on his site. A site which seems even more impressive now(Cam angles?! How long did it take to measure all of those?)
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Re: Physics of Ascenders Slipping

Postby Bob Thrun » Jul 24, 2012 12:58 am

I considered ascender slipping and the need for teeth on the cam in my book, Prusiking. Here is the illustration and analysis from the book.
Image
Most eccentric cam ascenders use an equiangular (logarithmic) spiral. On some ascenders, it is easy to replace the cam. It would be easy to lay out the spiral shape and make a series of toothless cams with different cam angles. Very common machine tools would be needed. The smaller the cam angle, the tighter the rope would be squeezed. If the angle is small enough, the ascender will not slip downward. My suspicion is that the ascender will not be easy to slide up the rope either.
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Re: Physics of Ascenders Slipping

Postby NSS8921 » Jul 24, 2012 11:16 am

Peter, back to your original question, yes. This from a previous post:

'by NSS8921 » Mar 14, 2011 6:55 am
The Kong Futura Body ascender is a huge disappointment! I began frogging on 9mm EZ Bend and in mid-stroke, the ascender completely disengaged itself. I re-attached it and tried repeatedly - it consistently 'popped' off. I note that the gap between the cam and the body is much larger than any of my other ascenders - ISC, old and 'new' Jumars and two generations of CMI's.

Next I tried it on 11mm Isostatic - it stayed on the rope but very reluctantly engaged after sliding down a foot. This behavior was also consistent.

Just to be clear, this is the 'body' ascender and not the 'chest' ascender that looks like a croll.'

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Re: Physics of Ascenders Slipping

Postby PeterFJohnson » Jul 26, 2012 7:33 pm

Bob Thrun wrote:I considered ascender slipping and the need for teeth on the cam in my book, Prusiking.


Interesting info. I don't suppose you have a copy of your book you would be willing to part with? I am guessing the answer is no based on an old cavechat thread but figure it can't hurt to ask.
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Re: Physics of Ascenders Slipping

Postby jcg5230 » Dec 17, 2012 8:10 pm

I've noticed that some ascenders have teeth that point at different angles with respect to the rope. My CMI Ultrascender's teeth are more or less perpendicular to the rope, but I've seen that the teeth on some of my friends' Petzl ascenders angle downwards a little. From trying both of them out on muddy ropes, it seems that the angle of the teeth seems to make a noticable difference with regard to slipping (the Petzl's do a little better). Has anyone else noticed this? I'm considering trying a different brand for when my current one wears out - does anyone know of/have a list of which ascenders are of which type?
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Re: Physics of Ascenders Slipping

Postby PeterFJohnson » Dec 18, 2012 7:25 pm

Gary Storrick's site lists "Slopping teeth" or "Straight teeth" in his description for many ascenders:

http://www.storrick.cnc.net/VerticalDev ... ices.shtml

This might be the information you are looking for. However teeth are only one factor when it comes to whether or not an ascender slips. As a counter example, Alpine Caving Techniques recommends a toothless device - the petzl shunt - under extremely icy or muddy conditions. There are other devices that have teeth perpendicular to the rope that also perform well under muddy conditions. Just food for though.
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Re: Physics of Ascenders Slipping

Postby Bob Thrun » Dec 19, 2012 6:31 am

PeterFJohnson wrote:This might be the information you are looking for. However teeth are only one factor when it comes to whether or not an ascender slips. As a counter example, Alpine Caving Techniques recommends a toothless device - the petzl shunt - under extremely icy or muddy conditions.

But the Shunt is not an eccentric cam device. It is a lever device. A lever device could be made to grip on anything if the lever is long enough
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Re: Physics of Ascenders Slipping

Postby PeterFJohnson » Dec 19, 2012 12:38 pm

Bob Thrun wrote:
PeterFJohnson wrote:This might be the information you are looking for. However teeth are only one factor when it comes to whether or not an ascender slips. As a counter example, Alpine Caving Techniques recommends a toothless device - the petzl shunt - under extremely icy or muddy conditions.

But the Shunt is not an eccentric cam device. It is a lever device. A lever device could be made to grip on anything if the lever is long enough


True. I was only trying to point out that teeth are not the only factor that determines whether or not an ascender slips. As you point out, what type of ascender you are dealing with is also a factor.
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Re: Physics of Ascenders Slipping

Postby jcg5230 » Dec 23, 2012 10:11 pm

Thanks Peter, that's exactly the kind of list I was looking for! I had no idea there were so many different ascenders out there...
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