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atlnq9
Apr 2, 2009, 12:54 AM
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Can the same concept of the logarithmic spiral be applied to the cam of an ascender? After reading and rereading the articles, http://www.rockclimbing.com/...post=2032811#2032811 , on cam design it makes since to me that they can. Does anybody have a different thought? I don't have an easy way to measure any of my ascenders.
(This post was edited by atlnq9 on Apr 2, 2009, 12:55 AM)
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philbox
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Apr 2, 2009, 11:13 AM
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Ascenders by and large are designed for a particular purpose i/e ascending up a particular size of rope (obviously). Thus the cam design is fit for the purpose it was designed to serve.
That being said not all ascenders are created equal. Some ascenders will actually chop yur rope given enough shock load on them.
I can vouch for SRTE ascenders. They are designed so that if a shock load greater than (from memory) 700kgs occurs then the ascender will strip the sheath but under no circumstances will it ever chop through the core. Yes you'll enjoy a short ride whilst the shock load is dissapated but you will survive.
I would think that if a logarithmic spiral was designed into a cam on an ascender such that a greater and greater compressive force was to be applied to the rope then under certain extrem e circumstances the rope would be chopped. Just my conjecture is all.
There have been a huge amountof differing types of ascenders around for ages, almost bewildering actually. Some very creative designs too. Might be a great contest to consider for the future.
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misanthropic_nihilist
Apr 2, 2009, 11:42 AM
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If the ascender's cam is logarithmic, it will always make the same angle between down and where the cam contacts the rope. Here's an easy way to figure out if the cam is logarithmic.
Materials:
Ascender
Ropes of Varying Diameter
Paper
Straightedge
Tape
Marker
1. Tape the ascender to the piece of paper so it stays in place.
2. Insert a rope into the ascender.
3. Place the straightedge on top of the ascender so it runs between the axis of the cam and where the cam contacts the rope tangentially.
4. To the side of the ascender, use the marker to mark the line where the straightedge lies.
5. Repeat steps 2-4 with varying rope sizes.
If all of the lines you draw are on top of each other, the cam is logarithmic. If not, it's not.
*Note: you must use rope sizes that contact the cam tangentially. If the rope is too big/small and contacts the cam at the edge, your results will be incorrect.
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patto
Apr 2, 2009, 11:52 AM
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I suspect that it would be that hard to design a cam that doesn't chop ropes.
For starters the cam should not touch the metal wall when there is no rope in the device. If the cam does touch the wall you have designed a device that will chop.
I don't have ascenders but I would be interested to see how close the cam gets the the wall in various designs. The closer it gets then the more likely it is to chop.
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JAB
Apr 2, 2009, 12:01 PM
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patto wrote: I don't have ascenders but I would be interested to see how close the cam gets the the wall in various designs. The closer it gets then the more likely it is to chop.
It's not that straightforward. Some ascenders will start to slip at a certain amount of force, instead of cutting through the rope.
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bigjonnyc
Apr 2, 2009, 12:14 PM
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patto wrote: I suspect that it would be that hard to design a cam that doesn't chop ropes.
It'd actually be pretty easy to create a design that incorporates a mechanical stop that keeps the cam from compressing past a certain point.
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atlnq9
Apr 3, 2009, 5:11 AM
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bigjonnyc wrote: It'd actually be pretty easy to create a design that incorporates a mechanical stop that keeps the cam from compressing past a certain point.
My personal thought then would be that the teeth would be more prone to ripping the rope sheath since compressive force would no longer be applied as more downward force was applied.
philbox wrote: I would think that if a logarithmic spiral was designed into a cam on an ascender such that a greater and greater compressive force was to be applied to the rope then under certain extrem e circumstances the rope would be chopped. Just my conjecture is all.
Maybe I am misunderstanding what you are saying but that is exactly how the cam on an ascender works. The harder you pull on it the more force it exerts on the rope to create a greater frictional force counteracting the downward force. I would be very hard pressed to ever say that an ascender can be deigned which cannot cut a rope, especially toothed one which cannot slip.
misanthropic_nihilist wrote: Here's an easy way to figure out if the cam is logarithmic.
patto wrote: For starters the cam should not touch the metal wall when there is no rope in the device. If the cam does touch the wall you have designed a device that will chop.
But in theory it seems similar: you are designing something to fit a certain size range to accommodate different sized ropes and rope compression which provides more force against it as more downward force is applied.
Edit: Edited for grammer
(This post was edited by atlnq9 on Apr 3, 2009, 5:15 AM)
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moof
Apr 5, 2009, 6:09 AM
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Per the original question:
Yes, most Jumar style ascenders use a logarithmic spiral section for their cam. It predates the usage in Friends. I don't know the full range, but one I looked up the other day (google "storrick ascender" for a great site) that was about 25 degrees in cam angle (much steeper than cams).
Ascenders have a very rough set of design goals:
1. Grab onto 8-12 mm ropes (some even larger for rescue purposes)
2. Work on muddy ropes in caves.
3. Work on icy ropes.
4. Slide up easily on lightly loaded lines, and still clamp down on a loaded line.
5. Not break when loaded over an edge (common when turning the lip in caving).
6. Be cheap enough to make and sell while still turning a profit.
7. Be quick and easy to get on and off the rope when intended, and damn hard to get off the rope when not intended.
Given all that, the current offerings work pretty well.
Beyond the Jumar style, several ascenders use a lever in conjunction with a log spiral, i.e. the load levers the cam into the rope in addition to the camming action, as opposed to a simple cam on an axle. Most notably good old Gibbs are the most prolific, but the new BD ascenders do the same.
The better question to be asking is why do ascender cut ropes? With a 25 degree cam angle a 1000 lb load puts over 2000 lbs pressing the cam into the rope, and only one spot in the rope (factoring in the friction of the rope against the cam body is beyond me).
If the goal is not cut the rope very high loads, the most obvious solution is to increase the contact area. Prussik's for example clamp onto a large area of the rope, over a long length as well as the entire circumference. So if you don't mind a 4" radius cams on your ascenders you could double the contact area and greatly increase the peak load to sever the rope. Nobody would buy them I'm guessing.
(This post was edited by moof on Apr 5, 2009, 3:41 PM)
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atlnq9
Apr 10, 2009, 2:50 AM
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Thanks, I thought it sounded right, now I start designing my next project! (It is a secret though)
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