Why are cam lobes not covered with a high friction coating?

As we all know, the expansion range you can get when designing a cam is dependent on the coefficient of friction between the cam lobe and the rock. This is why alien cams are made with a softer, 'stickier' metal. Does anyone know the reason cams are not designed with a very high friction layer along the edge.

I would assume because you could have a fall that ripped the coating from the aluminum base, resulting in catastrophic failure. It seems like this possibility would be worse than the friction that is consistent from aircraft grade aluminum.

^^^This. Coating peels off, cam peels out, zip!

Are cams not strong enough?

Maybe not a coating, but it's worth asking whether the metal that has mechanical properties that we like (aluminum) is also the best metal for its friction coefficient against rock. It might not be. If not, is there a way to bond the second metal (electroplate?) to aluminum?

I think the equally relevant question is why isnt slick rock covered with a high friction coating? Huh?

Darren, it is not about the cams being strong enough. It is about the amount of expansion range you can get while keeping the same strength. Stickier lobes would mean more expansion range without giving up strength.

Its complicated, because the "friction" that makes cams and climbing shoes stick isn't your grandmother's friction, the type you learned about in high school, the "friction," due primarily to forces at the molecular level, governed by Amonton's Laws. The friction of climbing shoes and cams depends to some extent on the interaction of macroscopic irregularities of the surfaces in contact rather than just the molecular forces. The CCH Aliens use of a softer aluminum in order to increase the cam's "bite" is an example of approaches that attempt to enhance non-classical "friction."

One consequence non-classical friction is that Amonton's second law goes out the window, with the area of contact, rather than just the normal force, having some effect on "friction".

The teeth on cams are there to enhance the possibility of the cam "hanging up" on some crack-wall irregularity, and so enhance "friction." DMM's new Dragon Cams claim to increase "friction" by widening the cam surface (which wouldn't make an difference with classical friction) and grooving it, e.g.

Totem takes another approach by knurling the cam surfaces:


But both approaches share a focus on roughing up the cam surface as a way of promoting non-classical "friction."

As for coatings, they just might have some effect. Totem discovered that anodizing their cam surfaces reduced "friction" and alerted their users to this possibility and stopped doing it, so perhaps some coating might increase friction. The problem, already mentioned, would be in keeping the coating on. (The anodizing on the Totem cams wore off fairly quickly and so didn't matter except when the cams were new.)

Using softer metals as CCH did involve a holding-power tradeoff that they didn't mention. A cam can pop out because of a failure of "friction," but if "friction" doesn't fail, then the failure mechanism is a deformation of the cam lobe called shear yield failure. The softer material had a lower shear yield failure threshold, and so might have traded "sticking" under relatively low loads for a lower ultimate extraction value.

I might add that there seems to me to be a certain amount of doublethink involved in practical cam design. On the one hand, groovings and knurlings are added to increase "friction," but on the other hand the fundamental design assumptions leading to the logarithmic spiral shape of the cam lobes are based on Amonton's laws of friction.

rgold wrote:The softer material had a lower shear yield failure threshold, and so might have traded

This was proven in some models by Aric Datesman. Some models of aliens did fail under shear yield. The cams failed below rated strength and smear out leaving a trail aluminium behind.

The QC processess were so bad at CCH that the hardness of the lobes varied considerably between various Alien cams. Apparently the metal feedstock underwent in house heat treatment to soften the lobes up.

I can't imagine that a practically effective high-friction coating would be terribly durable, since durability is an expression of self-bonding strength, and friction is an expression of the bond to external things. The two are not independent of each other.
I'll bet that spraying on some kind of sticky polymer would only work for a pitch or two at best before needing a new application,

The shear yield failure is between the cam and the rock right? You are not talking about the lobe itself shearing in half. If so it sounds like even soft aluminum edging on hard aluminum lobes would have the exact same problem. Anything with more "friction" than soft aluminum would likely be even more prone to this failure mode. Would the shear yield failure threshold be affected by pressure? Thus being somewhat alleviated by wider lobes.

As already discussed above with a few extra points as I looked into this a few years back for a couple of people.
Cams grip the rock because the lobes deform, if they don´t then the point of contact is infinitely high as the lobes are a curve. If they are ultra-hard then they don´t grip at all or the rock is destroyed. The alternative to making them softer is some way of mechanically engaging with the rock and I worked with a stone-mason with hardened steel lobes with micro-grooving which stuck well onto the rock, the more practical alternative is bonded diamond coating. BUT the downside is placements will become destroyed far quicker than they already are and for that reason alone any attempt to increase holding power by hard coatings of this kind are a terrible idea.
Softer coatings rip off unless the lobes are huge.

Alex R wrote:The shear yield failure is between the cam and the rock right? You are not talking about the lobe itself shearing in half.

No. That would be sliding due to insufficient friction.

Shear failure of the lobe has been observed, though it not a normal failure mode. Imagine a skidding car leaving black tire tracks. The same was observed with some alien cams. The cam skidded out leaving a patch of aluminium. That said it might be debated that the lobes compressed causing the contact angle to change which then caused the skidding. Either way the cause was soft lobes and surface shearing was observed.

Most cam lobes are strong enough to avoid before another part of the cam fails (usually the stem).

patto wrote: This was proven in some models by Aric Datesman. Some models of aliens did fail under shear yield. The cams failed below rated strength and smear out leaving a trail aluminium behind.

I was actually present for some of those tests and could see the sheer yield phenomenon up close and personal. Prior to the shear yield failure, the cam surface flattens under load, as Jim already noted. The flattened material at the edge of the cam is subjected to high tangential loads and deforms tangentially, i.e. "flows" in a direction opposite to the load direction. With the Aliens, this happened before the any other part of the piece broke, and indeed they left aluminum skid marks on the placement jig.

Resistance to shear yield failure depends to some extent on the amount of material in the cam lobe, so small cams will experience this type of failure at lower loads than big cams, which is probably one of several reasons that small cams are less reliable.

An account of some of these issues by David Custer can be found at web.mit.edu/custer/www/rock…

Eric Moss needs to weigh in on this debate. His "just asking questions" ignorance would be really obfuscating.

Jim Titt wrote:bonded diamond coating.

That was the first thing that came to my mind.. cam lobe surfaces similar to a diamond file

Ray Jardine- "the teeth on cams is purely cosmetic, it doesn't enhance holding power"

Forgive the dumb question, but why was anodizing used on the cam teeth in the first place? Purely cosmetic reasons, perhaps?

I think so....I don't buy the "just look at the color for size" thing

rgold wrote: I was actually present for some of those tests and could see the sheer yield phenomenon up close and personal.

Nice. Now I feel dumb for pointing that out. ;-)

rgold wrote:which is probably one of several reasons that small cams are less reliable.

As I understand it most small cams are made of harder aluminium to prevent this as a failure mode. (Expected failure mode in hard rock is still dictated by stem failure.) But then we are back to less, flattening, less surface area for the forces, so certainly softer rock is less likely to hold small cams.

rgold wrote: I might add that there seems to me to be a certain amount of doublethink involved in practical cam design. On the one hand, groovings and knurlings are added to increase "friction," but on the other hand the fundamental design assumptions leading to the logarithmic spiral shape of the cam lobes are based on Amonton's laws of friction.

Can you explain to me like I'm 5 what you mean here? I thought the assumption for the logarithmic spiral was a purely geometrical one, constant cam angle? What I'm seeing is that if we are going off purely Amontons laws (2nd one?), an increase or decrease in area makes no difference one way or the other so at a minimum the grooves can't possibly hurt. And since those laws are not the only thing determining friction in this case, they would probably help?

The extent of my exposure to engineering things is an intro course many years ago, so use small words when you tell me where the doublethink is occuring, thanks

Edit: Rgolds second post clears this up for me somewhat