Fact or fiction?

Jim Titt wrote: We´ve seen problems with glue-in bolts which move very slightly (we call them "clickies"). Really anoying when you check them on a hot afternoon and they move slightly but when you go back in the morning to replace them they are bomb solid.

Cracks probably absorb thermal expansion / contraction stress from the whole rock, much the same way an expansion joint allows the whole bridge to expand and contract. More, thermal expansion probably is the reason for many splitters existance. A tiny bolt hole is different, with expansion at the order of magnitudes referenced for a single nut. The clicky phenomenon is probably more due to softening of the glue used, not thermal expansion.

Coefficient of thermal expansion is a pretty basic concept, with differing coefficients depending on the material. Also some materials do not have uniform expansion (e.g. crystalline materials).

So you have thermal expansion, and in two materials at once, which will have different coefficients. And the differences between volumetric and linear expansion is significant, but the concept is the same, differing coefficients resulting in differing expansion.

Linear coefficients:
Brass ~ 19
Steel ~ 12
Aluminum ~23
Granite ~8
Ssandstone ~11
Quartz ~ 0.33
Fused quartz ~ 0.6

So you can see that the metals expand significantly more than the rock due to a given rise in temp.

Brassies can be hard to clean in the best case with no temp change and no weighting the piece, and if you whipped on it good luck.

Will S wrote:Coefficient of thermal expansion is a pretty basic concept, with differing coefficients depending on the material. Also some materials do not have uniform expansion (e.g. crystalline materials). So you have thermal expansion, and in two materials at once, which will have different coefficients. And the differences between volumetric and linear expansion is significant, but the concept is the same, differing coefficients resulting in differing expansion. Linear coefficients: Brass ~ 19 Steel ~ 12 Aluminum ~23 Granite ~8 Ssandstone ~11 Quartz ~ 0.33 Fused quartz ~ 0.6 So you can see that the metals expand significantly more than the rock due to a given rise in temp. Brassies can be hard to clean in the best case with no temp change and no weighting the piece, and if you whipped on it good luck.

Also, if I am thinking about this correctly (I don't pretend to totally understand the physics involved here)the materials would expand in opposition to each other, compounding the problem, right? Meaning, thermal volumetric expansion of the rock makes the crack smaller/tighter (surrounding rock larger) and thermal volumetric expansion of the metal makes the gear larger. Both of which serve to make the gear "tighter".

And those coefficients are linear right? So the volumes expand even more if you factor in the 3D shapes than the linear calculations, right?

csproul wrote: Also, if I am thinking about this correctly (I don't pretend to totally understand the physics involved here)the materials would expand in opposition to each other, compounding the problem, right? Meaning, thermal volumetric expansion of the rock makes the crack smaller/tighter (surrounding rock larger) and thermal volumetric expansion of the metal makes the gear larger. Both of which serve to make the gear "tighter". .. right?

No, not correct.
As I mention above, if the whole rock mass warms uniformly, the crack expands along with the rock. But the temperature change is not uniform in the rock, so things are not so simple and the shape of the rock and crack may influence whether the crack widens or contracts. Also, as someone else mentioned, internal stresses in the rock may influence things too.

If your chock had the same thermal expansion coefficient as the rock, and everything was warmed uniformly, then the chock would come out as easily as it went it -- no tightening or loosening whatsoever.

Jon Nelson wrote: No, not correct. As I mention above, if the whole rock mass warms uniformly, the crack expands along with the rock. But the temperature change is not uniform in the rock, so things are not so simple and the shape of the rock and crack may influence whether the crack widens or contracts. Also, as someone else mentioned, internal stresses in the rock may influence things too. If your chock had the same thermal expansion coefficient as the rock, and everything was warmed uniformly, then the chock would come out as easily as it went it -- no tightening or loosening whatsoever.

Sorry, still don't get it. If rock is a volume, and a crack is essentially a space between two volumes, and that material undergoes thermal expansion, ie, the rock volume gets larger with higher temps, why wouldn't the space between two volumes (ie a crack) become smaller as the adjacent volumes become larger?

Oh god, another engineer thread.

Nonsense...brass or aluminum have nowhere near the same thermal expansion of rock

csproul wrote: Sorry, still don't get it. If rock is a volume, and a crack is essentially a space between two voluemes, and that material undergoes thermal expansion, ie, the rock volume gets larger with higher temps, why wouldn't the space between two volumes (ie a crack) become smaller as the adjacent volumes become larger?

I cant think of a reason why it wouldn't. Jon is thinking about an unlikely scenario where the whole rock warms uniformly, deep parts warm as fast as the surface. Yes, in that case thermal expansion will simply make everything larger by a factor depending on the material. In the real world, as supported by anecdotal evidence in this thread, rock and cracks follow the premises and conclusions you lay out.

HarryN wrote: I cant think of a reason why it wouldn't. Jon is thinking about an unlikely scenario where the whole rock warms uniformly, deep parts warm as fast as the surface. Yes, in that case thermal expansion will simply make everything larger by a factor depending on the material. In the real world, as supported by anecdotal evidence in this thread, rock and cracks follow the premises and conclusions you lay out.

True, that scenario is unlikely. On the other hand, in the only measurements on real rock, in the articles cited above, the crack actually expanded upon heating.

So, the anecdotal evidence may or may not reflect an accurate understanding of the cause of the nuts coming loose.

The picture csproul has is that of two blocks, separated by a crack, sitting freely on a substrate that does not expand or contract. In this case heating the blocks would make the crack contract as the blocks expanded in 3D. This scenario also seems unlikely. My point is that the actual situation is more complex than either scenario.

a friend of mine told me a story about something that happened to him when he was soloing a wall. he had finished leading a pitch, it was in the sun and really hot. when he rapped the pitch he literally couldn't get any of the nuts out. he got back to the belay and decided to call it a day. next morning he gets up, jugs the pitch, and the nuts came out no problem.

i am surprised that the actual changes in dimensions would be large enough to cause a problem. a) because the stoppers are small and the corresponding delta L would be small. b) because the entire mass of the rock isn't going to change temperature - only the surface to some depth, but not very deep. so again, the delta L will be small. but on the flip side, if you run the pickle jar cap under some hot water for a few seconds you can actually get the jar open. it doesn't take much of an interference fit to really get stuck.

i agree w/ csproul and aleks (myah!), in general the crack would decrease in size as the side surfaces expand towards each other. in the case of the article, i can see a case where the crack expanding would happen. looking at the flake, it does appear to be curved outward. heating it would cause it to expand outwards (similar to pre-heating a bearing prior to a press fit installation increases the inner diameter). very interesting.

slim wrote:a friend of mine told me a story about something that happened to him when he was soloing a wall. he had finished leading a pitch, it was in the sun and really hot. when he rapped the pitch he literally couldn't get any of the nuts out. he got back to the belay and decided to call it a day. next morning he gets up, jugs the pitch, and the nuts came out no problem. i am surprised that the actual changes in dimensions would be large enough to cause a problem. a) because the stoppers are small and the corresponding delta L would be small. b) because the entire mass of the rock isn't going to change temperature - only the surface to some depth, but not very deep. so again, the delta L will be small. but on the flip side, if you run the pickle jar cap under some hot water for a few seconds you can actually get the jar open. it doesn't take much of an interference fit to really get stuck. i agree w/ csproul and aleks (myah!), in general the crack would decrease in size as the side surfaces expand towards each other. in the case of the article, i can see a case where the crack expanding would happen. looking at the flake, it does appear to be curved outward. heating it would cause it to expand outwards (similar to pre-heating a bearing prior to a press fit installation increases the inner diameter). very interesting.

climbing friend,

hmmmmmyaaaaaghhhhhhhhhh!!!! yessssssssssssssssmypreciousssssssss sss ss s ssss

morning dew(condensation) can also help lubricate your nuts...

Please check the math below.

The flake studied is described as being about 60 feet long and bound at both ends. Assume that the crack is two inches wide at the midpoint of the flake. This can be approximated as a right triangle with the rock face as one leg that is 360" long and the 2" crack as the other leg, and thus a hypotneuse that is 360.0056" long. For the crack to increase in width by 1/2" at the midpoint, the hypotneuse only has to increase to 360.0087", which is only 0.0031" longer at the midpoint (and 0.0062 longer for the whole flake). The linear coefficient of expansion of granite is 0.000044 inch per degree F. So, there only has to be a temperature differential of 0.2 degree F between the flake and the rest of the rock for the 60 foot flake to lengthen by 0.0062". That seems very possible, given that the flake has air (which is a poor conductor of heat compared to granite) behind it.

All I know is that cams and nuts that wouldn't budge during the summer have come out with remarkable ease in December. I've done it more often than I can count.

If a crack represents the space between two more or less independent pieces of rock, then it's easy to understand that when those rocks expand, the space between them gets tighter.

Josh Janes wrote:Fact. ... being that a crack or flake is a natural stress line in the rock - probably created by expansion and contraction to begin with, I speculate this is pretty different than a small bolt hole, which is surrounded by one piece of rock on all sides, changing size.

I think Josh expresses here the most likely factors that may lead to some of the observations of released nuts after cooling.

I've never studied geology, but if one considers that cracks initially form in the rock surface due to various stresses as the rock is cooling, then it stands to reason that further cooling would enlarge the cracks further.

This makes sense. Even though the potential expansion is pretty damn small, such small changes in size seem to be capable of causing the nut to hang up on something. Small nuts can be hard to clean regardless, though, especially brassies with brazed cables, even when they haven't been weighted. Intuition says that the whip would probably weld the nut a lot more than heat would.

So to answer the OP, that was a lousy attempt at an excuse by your partner. On the other hand, he is certainly not to blame for the welding of the nut and shouldn't have needed to come up with an excuse in the first place. The nut did it's job and held your fall. For such a tiny piece, I'd just be happy it held and get a new one. If replacing RPs becomes a serious concern for my wallet, then maybe I should take a break from placing myself in a situation where I am liable to whip on them.

I know of a specific cam placement on a south facing route that I have climbed many times- reapeatedly a .4 C4 fits when its cool or cloudy but gotta bump down to a .3 C4 when it's baking in the sun. Is it the cam or is it the rock?

Stephen Felker wrote:I know of a specific cam placement on a south facing route that I have climbed many times- reapeatedly a .4 C4 fits when its cool or cloudy but gotta bump down to a .3 C4 when it's baking in the sun. Is it the cam or is it the rock?

I'd say it's certainly the rock. Think about it - the difference between a cool day and a hot day might produce a dimensional change in the materials in the hundredths of a percent. More or less, I dunno; I'll leave the numbers to the geeks. A change of 0.01% in the size of a cam would scarcely be measurable. But 0.01% change in a 50 ft slab of rock? That's significant.

Everybody - go back and look at the photo of the crushed nut I posted. It didn't get that way because the aluminum got really, really cold!

it's the rock...

from the aforementioned studies

"From May 2010 to October 2013, the scientists identified and measured, every 5 minutes, the deformations present in the near vertical 20 metric ton (293.49 cubic feet) layer of granite slab, which is about 10 centimeters (0.3 inch) thick.

The scientists also monitored the slab movements along the directions it was splitting. Weather conditions including sunlight intensity and air conditions on site were also taken into consideration, even getting the air temperature and humidity from the slab surface and slab gaps that are only about 4.7 inches wide. To gain an independent measurement of the rock motions in an 18-hour period, the experts scanned the slabs from 30 meters away (98 feet) using a laser device.

The study found that in an average day, the slab would have periodic bulging and shrinking of about 8 millimeters (0.31 inch), which is largely due to the temperature variations."

8mm is pretty good sized deflection/expansion

PRRose wrote:Please check the math below.

The value I've seen quoted for the linear expansion coefficient of granite is 4.4e-6 instead of 4.4e-5. My reading of those two articles is that the crack at its widest is 4.7". All in all, I get a temperature change of about 4 F. Finally, modeling the slab as an arc whose chord and sagitta are given makes little difference w.r.t. the triangular model.