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Cams retracted from ?% to ?%
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verticon


Nov 8, 2008, 12:49 PM
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Cams retracted from ?% to ?%
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I own a full set of DMM 4CU and a set of BD Camalots (up to #3). While trying to figure out how to place my cams better I found different recommendations from different manufacturers:

Black Diamond: "Ideally, each of the cams should contact the rock at lower to mid expansion range (50% to 90% retracted)".
This totally makes sense because the more a cam is retracted, the more the spring will push it against the rock, increasing the friction that holds it in place, thus it's stability.

Wild Country:"Always ensure that all the cams make contact with the sides of the crack, preferably in the middle 1/2 of their expansion range (i.e. the cams should be between 1/4 to 3/4 open)".
This makes sense only from the UIAA point of view (UIAA125-EN12276 Frictional Anchors): "Position 1: s = bmin+[(bmax bmin)/4]; Position 2: s = bmin+[(bmax bmin) 3/4 ], where bmin is the minimum adjustable width, bmax is the maximum adjustable width". This only refers to the positions in which SLCDs will be tested and has nothing to do with the "ideal real life placement".

DMM: "Always ensure that all the cams make contact with the sides of the crack preferably in the middle 1/3rd. of their expansion range (i.e. the cams should be 1/3rd. to 2/3rd. open).
This seems to be just a more conservative approach based on the Wild Country recommendations, althought the Technical Friends and the DMM 4CU have exactly the same cams and the similar sized units have the same expansion range.

And here are my questions:
1. Of course, a SLCD shouldn't be placed fully retracted (or it becomes a bootie) but what do you think it's the reason for the difference between 90% retracted (Black Diamond) and 66.7% (DMM)?
I mean why should I loose 23.3% of the 4CU's working range ?

2. A cam's a cam and a spring acts the same when compressed. Why would a 4CU 66.7% opened still be stable - or a Technical Friend 75% opened, but that's a different company :o) - when a BD Camalot at more than 50 % is not ? So is it really safe to place a 2/3rd. open 4CU ?

3. I try not to get my cams stucked or to open them too much, so I usually place them 40% to 90% retracted, but this is based on intuition.
So, what is the REAL working expansion range on your opinion ?


suilenroc


Nov 8, 2008, 2:01 PM
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Not a technical answer....

If it looks good it probably is - Use the Force you must, Obe Wan Knobe. Cool

ADD:
1. Different lobe shapes.

2. Not all Cams are created equal.

3. The Star Wars reference.


(This post was edited by suilenroc on Nov 8, 2008, 2:17 PM)


yodadave


Nov 8, 2008, 3:01 PM
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Re: [suilenroc] Cams retracted from ?% to ?% [In reply to]
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things that vary with manufacturer,
cam angle
lobe size
single axle or double

these probably all factor in. End of the day stay as close to the best margin of safety as you can and make the best placements you can.


billl7


Nov 8, 2008, 3:02 PM
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verticon wrote:
This totally makes sense because the more a cam is retracted, the more the spring will push it against the rock, increasing the friction that holds it in place, thus it's stability.

I doubt the issue is increase spring-back forces.

If these are all constant-angle cams (constant working-geometry over the range) then then there really isn't any difference from 1% to 100% retracted in an ideal world. But cams get stuck and then won't be available on the next pitch (so go less than 100%). And cams can shift/walk a bit and in doing so become more open (so go for much more than 1%). And cam materials do flex some (again, go for much more than 1%).

For micro cams I think most folks tend proportionately closer to 100% than for bigger cams since total range is quite small (doesn't take much slippage/movement to wider crack to run out of cam range on a micro cam).

Also, different companies might adjust the recommended range due to different materials (harder/softer metal) and/or different angles for the constant-angel part.

Don't know if there are any variable-angle cams - seems doubtful but I don't know.

Search around a bit and you'll find some pretty good discussions about the geometry of typical cams - I think you would enjoy reading.

So, the above is mostly notional. I haven't looked at this for a long time. Perhaps someone will improve on this.

Bill L


patto


Nov 8, 2008, 4:36 PM
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Re: [billl7] Cams retracted from ?% to ?% [In reply to]
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If you followed the advice of DMM or BD then there would be gaps in their range between cam sizes.

The numbers are arbitary guides and are more about keeping things simple and safe for those using the cams rather than any ideal range.

Use your own judgement. As others have said its about the balance between getting gear stuck and placing marginal gear. The majority of the range risks neither.


coolcat83


Nov 8, 2008, 4:44 PM
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billl7 wrote:



Don't know if there are any variable-angle cams - seems doubtful but I don't know.

Bill L

slightly off topic but tricams are in a way, not sure what their reccomended placement range is in camming mode.


(This post was edited by coolcat83 on Nov 8, 2008, 4:44 PM)


billl7


Nov 8, 2008, 4:51 PM
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coolcat83 wrote:
billl7 wrote:
Don't know if there are any variable-angle cams - seems doubtful but I don't know.

slightly off topic but tricams are in a way, not sure what their reccomended placement range is in camming mode.

Hah! I think you are right.


hafilax


Nov 8, 2008, 6:05 PM
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AFAIK over-camming is only a problem if it gets stuck.

I don't think that the spring force pushing out on the lobes is a huge issue. I don't know what the spring design goals are but it doesn't take a whole lot more force to pull the trigger at the end of the travel versus the beginning so I would guess that the spring tension is approximately constant. Apparently it's a good idea to set cams anyway at least according to rgold.

As for the wider end of placements there are a few issues that I can think of:
-If it walks to a wider part of the crack it could fall out
-If it's behind an expanding flake or in soft rock it could exert enough force to pull out.
-In severe enough falls some of the lobe material can sheer off and if they're near the end of the range they could pull out.

BD and Metolius seem to agree roughly on the optimal range. Maybe they like the thought of people fixing their cams Wink

In the field you pick the cam that roughly works although I definitely have the tendency of choosing the largest cam that will fit.


hafilax


Nov 8, 2008, 7:26 PM
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Re: [hafilax] Cams retracted from ?% to ?% [In reply to]
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I just thought of another thing. At the outer limit of the cam extension there is more torque on the pins especially for larger cams and if the cam shifts position as it's loaded.


verticon


Nov 9, 2008, 12:58 PM
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C'mon guys, this is the Lab, not the beginner's forum. I wasn't looking for advice on placing cams, but for some meaningful explanations for the "useful range".

I made a graph with working ranges of my rack and according to different manufacturers' specs, it changes shape from "best rack in the universe" to "drop everything and get a real rack" (yeah, I know the one with send it to me for proper disposal...)
The useful ranges in the most plausible graph are varying from 10-25 % to 60-75 % opened, based mainly on intuition and experience with each type of SLCD, but this is way different from the manufacturers' specs and this intrigued me.

Now, you all the engineers from the Lab can you bring some light on this ?


Edited to add this:

Today I received an answer to my questions from a DMM engineer and I think it might be of some interest to you too:

"Firstly I do find it slightly odd that we talk about 1/3 and 2/3 optimum range. We would, as you suggest be better to align ourselves with Wild Country on this, not only because we use the same cam angle but also the EN Standard requires we test our cams at 1/4 open and 3/4 open, the strength rating we give is based on the test in these two positions. We will look at changing this when we reprint the instructions.

The real expansion range of the cams is from fully retracted to fully open. However there are practical problems that the less experienced user may not have appreciated. As you know if a cam is fully retracted in a crack then it is well nigh impossible to remove. This is because you need to retract the cams from the contact surface to remove them. Also cams have a tendency to walk into cracks. Obviously the experienced user can place a cam in a narrower crack and still get it out.

Similarly as you get towards the fully open position there is a judgment call to make with the rock type that the cam is placed in. If it is very soft rock then as the cam engages the rock it can open significantly further, ultimately to a point where the cam would fail. Again experience is probably the best teacher to the user.

This explains why we are fairly conservative when we talk about 'preferable expansion ranges'. Other manufacturers such as Metolius go a stage further by colour dot marking the sides of the cams to indicate the preferred/optimum expansion ranges.

Other issues that are relevant are the aspect ratios of cams. The aspect ratio is the relationship between axle length and the cam 'height'. On a small camming unit the axle length is far greater than the cam height which makes the cam very stable. On a large camming unit the reverse is true and the cam is less stable. Ideally the optimum scenario is when the axle length is equal to the cam height. Also the aspect ratio changes within the expansion range of a camming unit. It the retracted position its aspect ratio is more stable than when the cam is fully open. When the cam is less stable then there is more of an issue of the cam moving or walking in the crack. This could in certain situations affect the holding power of the cam.

Finally, to add to equation is the cam angle. The greater this is then again you tend to lose stability. DMM use 13.75 degree cam angle. BD use a slightly larger one which helps, along with the twin axle design to increase their expansion range. This instability is more noticeable on larger cams. For the cam angle to work well the spring pressure should be even throughout the opening range as this will give the best fit for cam angle and rock. As I mentioned before rock type is very important. Rough rock engages the cams well, where as smooth rock may not even engage the cams on parallel or marginal placements and the cam may just pull straight out. This is especially problematical on polished limestone and slate.

As you know cams are complex bits of kit and their operation is a skill that is acquired through experience. In our instructions we have to educate the user into the correct operation of the devices in a clear and concise manner and if we err on the side of caution you can perhaps see why. Our instructions are guidelines for use. As it says in the instructions, there is no substitute for instruction by a trained and competent person."

The letter has also a footer:"Any views or opinions are solely those of the author and do not necessarily represent those of DMM Engineering / International Ltd."


(This post was edited by verticon on Nov 15, 2008, 11:42 AM)


verticon


Nov 11, 2008, 2:44 AM
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Bump (I added some info in the above post)


(This post was edited by verticon on Nov 11, 2008, 2:45 AM)


hafilax


Nov 11, 2008, 11:13 AM
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verticon wrote:
C'mon guys, this is the Lab, not the beginner's forum. I wasn't looking for advice on placing cams, but for some meaningful explanations for the "useful range".

I made a graph with working ranges of my rack and according to different manufacturers' specs, it changes shape from "best rack in the universe" to "drop everything and get a real rack" (yeah, I know the one with send it to me for proper disposal...)
The useful ranges in the most plausible graph (attached) are varying from 10-25 % to 60-75 % opened, based mainly on intuition and experience with each type of SLCD, but this is way different from the manufacturers' specs and this intrigued me.

Now, you all the engineers from the Lab can you bring some light on this ?


Edited to add this:

Today I received an answer to my questions from a DMM engineer and I think it might be of some interest to you too:

"Firstly I do find it slightly odd that we talk about 1/3 and 2/3 optimum range. We would, as you suggest be better to align ourselves with Wild Country on this, not only because we use the same cam angle but also the EN Standard requires we test our cams at 1/4 open and 3/4 open, the strength rating we give is based on the test in these two positions. We will look at changing this when we reprint the instructions.

The real expansion range of the cams is from fully retracted to fully open. However there are practical problems that the less experienced user may not have appreciated. As you know if a cam is fully retracted in a crack then it is well nigh impossible to remove. This is because you need to retract the cams from the contact surface to remove them. Also cams have a tendency to walk into cracks. Obviously the experienced user can place a cam in a narrower crack and still get it out.

Similarly as you get towards the fully open position there is a judgment call to make with the rock type that the cam is placed in. If it is very soft rock then as the cam engages the rock it can open significantly further, ultimately to a point where the cam would fail. Again experience is probably the best teacher to the user.

This explains why we are fairly conservative when we talk about 'preferable expansion ranges'. Other manufacturers such as Metolius go a stage further by colour dot marking the sides of the cams to indicate the preferred/optimum expansion ranges.

Other issues that are relevant are the aspect ratios of cams. The aspect ratio is the relationship between axle length and the cam 'height'. On a small camming unit the axle length is far greater than the cam height which makes the cam very stable. On a large camming unit the reverse is true and the cam is less stable. Ideally the optimum scenario is when the axle length is equal to the cam height. Also the aspect ratio changes within the expansion range of a camming unit. It the retracted position its aspect ratio is more stable than when the cam is fully open. When the cam is less stable then there is more of an issue of the cam moving or walking in the crack. This could in certain situations affect the holding power of the cam.

Finally, to add to equation is the cam angle. The greater this is then again you tend to lose stability. DMM use 13.75 degree cam angle. BD use a slightly larger one which helps, along with the twin axle design to increase their expansion range. This instability is more noticeable on larger cams. For the cam angle to work well the spring pressure should be even throughout the opening range as this will give the best fit for cam angle and rock. As I mentioned before rock type is very important. Rough rock engages the cams well, where as smooth rock may not even engage the cams on parallel or marginal placements and the cam may just pull straight out. This is especially problematical on polished limestone and slate.

As you know cams are complex bits of kit and their operation is a skill that is acquired through experience. In our instructions we have to educate the user into the correct operation of the devices in a clear and concise manner and if we err on the side of caution you can perhaps see why. Our instructions are guidelines for use. As it says in the instructions, there is no substitute for instruction by a trained and competent person."

The letter has also a footer:"Any views or opinions are solely those of the author and do not necessarily represent those of DMM Engineering / International Ltd."
^^^ I think I basically covered the same things. ^^^


petit_suisse


Nov 12, 2008, 11:05 PM
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Re: [verticon] Cams retracted from ?% to ?% [In reply to]
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Hi Verticon,

You could find some interesting information here:
http://www.mit.edu/...cking/cams/cams.html

petit_suisse


patto


Nov 13, 2008, 7:42 AM
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verticon wrote:
C'mon guys, this is the Lab, not the beginner's forum. I wasn't looking for advice on placing cams, but for some meaningful explanations for the "useful range".

And that was given. Seriously I don't know what your complaining about.

Your question was answered by forum members. The DMM engineer barely said anything that wasn't said before. Though he did express himself quite well.

As said before cams can be effective over their ENTIRE expansion range otherwise it wouldn't be part of their range. As has also been said as you near the extremes you could face risks of instability or of getting the cam stuck.

Its kinda obvious isn't it.


verticon


Nov 15, 2008, 2:27 AM
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Re: [patto] Cams retracted from ?% to ?% [In reply to]
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I didn't say the DMM guy brought something new, I just quoted him FYI.

Let me make clear the point of all this: I wanted to get some meaningfull data for the range analysis of my rack (the attached table - pdf)

Some obvious conclusions:

1. If one uses the extra expansion range, beyond the one recommended by the manufacturers, one would be at risk (instability, cam slippage, etc.).
2. The marketed range of SLCDs is way different from the real life "safe" range. (the graph at the bottom of the attached PDF)
3. Rangewise, there's no much difference between a BD Camalot and a DMM 4CU for the ideal placement. The only advantage of the Camalot over 4CU is that it can be used in a wider range of "less than ideal placements", in an unsafe overcammed position.

4. It's up to the climber to determine how much unsafe is still safe enough, based on own experience, intuition, technical level, etc.

5. The manufacturers should quit marketing the misleading "expansion range", replacing it with the actual expansion range (crack widths for ideal placements).

What do you think of this ?


(This post was edited by verticon on Nov 15, 2008, 11:45 AM)
Attachments: rack-web.pdf (141 KB)


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