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impulse force on cams

changie

14-Sep-2005
1:32:26 PM

My physics is also pretty rusty. If between 20 years since uni and 10 since I've had to do this sort of calc so here goes...

Assuming the belayer is as close as practical to vertically below the cam. If the climber weighs 80kg then the section of rope between the climber and the karabiner carries 80kg. The rope then curls around the karabiner back down to the belayer. Assuming no losses, doesn't the rope between the karabiner and the belayer also need to support 80kg (or the rope starts to accelerate)? So, if I am not mistaken (big if) you have the same rope with forces in it pointing in the same direction (downwards) leading to a static load in the karabiner (and hence the support - whatever it is) of 0.16kN (160kg).

The same logic, if correct, would apply to dynamic/impact loads. Haven't investigated the referenced equations but one shouldn't be so brazen as to assume they are correct...

Changie

Changie

14-Sep-2005
4:36:45 PM

Looks great! But why are you doing this? Is it because you think there's a problem with cams? or just for fun? How did you model the material, and the interface between the cam and the rock?


cheesehead

14-Sep-2005
10:07:28 PM

I think constrained (no rotation) makes sense - you're testing for loadings through the lobes in a fall, rather than holding force.

Though I guess having a 'non welded axle' allows the elastic properties of the lobes to generate stresses as the lobes compress and rotate ever-so-slightly.

Maybe run simulations for different loading directions...

changie

14-Sep-2005
10:20:53 PM

Great! you picked a very interesting subject. You'll find the stresses will change as you vary the restraint conditions. In reality the rock has a Young's modulus "E" which is sometimes modeled as a bunch of spring constants - your program may let you do that. I'd avoid the fixing rotation (can't see that happening - ever).

Note to that when the load comes on the cam, apart from "camming", will slip, unless it's jammed up against a surface. This will be difficult to model

Another couple of aspects which you should be able to model are
. a "high" spot (eg a Buffalo quartz crystal). FEA should give you stress intensities - localized yielding of the metal.
. lateral load, say at the tip of the one of the cams. Beware the type of finite element you use - some assume a zero out-of-plane stress.

When cams are tested to destruction, do you know what the failure mode is?

Changie

changie

14-Sep-2005
10:31:52 PM

Mmm, in my view the cam can rotate where it interacts with the rock. If you imagine you're at the rock/metal interface - the cam itself can rotate but not displace. We're talking about a support condition and not the element condition.


cheesehead

15-Sep-2005
7:56:36 AM

Yeah, Cosmos is very "cartoon FEA". So easy, but limited. We used ABAQUS a lot, which was uber-powerful, but we only had the text input for model creation*. The subject turned into a ghost-chasing coding exercise and soon had nothing to do with FEA. For now, stick with Cosmos, and forget about making your life harder with "better" software :)

*ie, you can buy a graphical package like Solidworks or similar to generate models. We had to plot nodes in a text file :(

Changie

15-Sep-2005
10:27:37 AM

I'd stick with what you've got already. FEA is time consuming stuff and if you don't have a graphical interface you'll be chasing your tail finding errors.

Given that I don't do this stuff anymore, my knowledge of "modern" FEA programs is limited - when I started engineering we were still developing interesting new (3D) elements and coding them up! The basics were there though some programs that come to mind are SAP (not the accounting program), LUSAS, E-TABS etc. Most of them spawned from Unis.

Don't know much about failure mode in cams. Would anyone else know?


shmalec

15-Sep-2005
1:12:27 PM

There is a fair bit of voodoo involved in getting these things right. keep experimenting but careful what you do with the results.

I'd try and remove any fixed connections to the rock. You may have to reduce the interface to a line to do it.

Failure will not occur with a peak stress at yield. You can safely let it go up to the material ultimate stress.

Have you actually modelled the pin or have you just got 4 independant analyses of the lobes?

The more powerful the package you use, the harder it is to learn it. So stick with something simple. The combined CAD/FEA you have is the way to go for a project like this. Have fun and careful with your units.

PS. FEA and manufacturability are completely unrelated. :)

Changie

15-Sep-2005
1:54:57 PM

Wow!

I agree with the yield bit - that's why I asked the question about failure modes.

In reality you probably need only model one lobe - this is what we would have done in the good ol' days with limited computational power. [That is we'd overnight the run or even sometimes "overweekend" the run - nasty if there are errors!]

mockmockmock

15-Sep-2005
3:05:09 PM

Dear Geekasaurus, that's the whole forum not one person in particular.

If the model is on a BD C4. what kind of placement is in the instance. I haven't tried it yet but because the C4 has dual axles it can be placed behind a hole and used like a chock. No camming action used, just a passive piece of pro. Would this change the force ratings?


shmalec

15-Sep-2005
5:18:37 PM

On 15/09/2005 mockmockmock wrote:
>Dear Geekasaurus, that's the whole forum not one person in particular.
>

Geekasaurus sounds really cool!! I can just imagine this really huge T-rex with coke bottle glasses and a cord jacket....

Changie

15-Sep-2005
8:46:27 PM

Bart Simpson once used "dorkus merlorkus"


IdratherbeclimbingM9

21-Sep-2005
3:22:33 PM

Good enlightening pic ti.
It shows (as expected?) the axle also takes quite a bit of any significant loading.
Interesting that there is not more yellow/orange colouring. If the red is from the constraint, then it seems to indicate that the device is well within its capabilities with a 12kN loading??
Given the axle loading stress in the head of the stem; it is little wonder that nmonteith and others have managed to rip the stems from (flexible) cams placed in such a way that the 'stem to cam junction' is bent 90 degrees (at that point) in real life when fallen upon.

>(as I am very weary of them myself).
End of term fatigue setting in ... ?
Weary or wary !


nmonteith

21-Sep-2005
4:49:50 PM

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neats

21-Sep-2005
4:58:33 PM

Ha! Sorry, I thought that was funny!!

Changie

22-Sep-2005
8:57:09 PM

Looks great. I assume red is a compressive stress. In your analysis does any part of the cam go into tension? Can't tell from the pictures. Looking at the output, you can reduce the calculation time by modeling half the cam. Good idea for the disclaimer given the sort of techo discussion preceeding. Onto the next project!

There are 36 messages in this topic.