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Thermal Expansion

Original Post
Ryan Arment · · Boulder, CO · Joined May 2010 · Points: 71

In Eldo last week I daintily placed 2 stoppers in cold shady conditions. maybe 30-40min later(I had two followers) the temperature rose what felt like 15degrees F. My partners could not get the nuts out and did very little to try, pulled up once or twice - I watched. On rapel I tried to clean them and they were WELDED like I’ve never seen a nut weld. It looked like the stoppers were actually deforming under pressure.
Has anyone seen this or know something about how much a crack can expand or contract in such a short period of time?

Tom Nyce · · Flagstaff, AZ · Joined Nov 2010 · Points: 45

The expansion can be a serious problem to deal with. I think that sandstone does it the most. I've encountered the problem in Zion, on Spaceshot, which has some long pitches of aid on stoppers. If you lead in the cool shade, and then the sun hits before the follower cleans, the nuts can be impossible to clean.
I was warned about the problem ahead of time, but was pretty skeptical about it (until I experienced it myself).

Highlander · · Ouray, CO · Joined Apr 2008 · Points: 256

I actually experienced the same thing on Space Shot, I led the pitch in the shade, cool fall temps, when my partner followed the pitch they were in the sun and she could not clean 3 of the nuts. I could not believe it as I thought some of them were kind of marginal pieces and thought they would easily come out. I rapped down to give it a try myself, and after 10-15 minutes of banging on them, I gave up, they were welded to the rock. Never experienced anything like that anywhere else.

Rubber Tramp · · Unknown Hometown · Joined Mar 2013 · Points: 5

Expansion will happen, but that sounds a bit extreme. Different rocks types will definitely expand differently. Maybe if you really set them hard under the cooler conditions this resulted in their stuck position after warming?

Max Forbes · · Colorado · Joined Jan 2014 · Points: 108

A quick calculation of thermal expansion:

Change in length = initial length * thermal coefficient of linear expansion of aluminum * change in temperature

This result yields an extremely nominally small answer, (like 10 micrometers of expansion) even using the thickness of the biggest BD stopper, with the assumption that its a solid object and a temperature change of well over 10 degrees C. With that being said I can't think of any better reason as to why this occurred. Curious if anyone else with an engineering background has any additional thoughts.

Andrew Morrison · · Unknown Hometown · Joined Oct 2014 · Points: 30

This is a great question. Firstly, yes, there can be issues with thermal expansion within cracks, though it is typically negligible (compared to that of the metal gear). Metals, however, expand and contact much more, as a function of temperature, which would definitely explain the inability to remove them.
This is described through the equations of thermal expansion, which relate (alpha) and (delta)T to the elongation. Typical values for (alpha) for aluminum are 12-13 *10^-6 in/inF, with steel being about half that. In comparison, quartz (a primary constituent of many types of granite) has a coefficient of thermal expansion of about 0.4.
I don't have the ability to write out the equations from my phone, but they're easy to find, and pretty straightforward.

TL;DR- Place a well-seated piece and temperature increases, it can get stuck.

sanz · · Pisgah Forest, NC · Joined Nov 2011 · Points: 210

I have heard of this phenomenon but never experienced it. However, I was told that the cause is expansion of the nut, not contraction of the crack.

According to the guy who told me about this, aluminum expands somewhat when it warms up, which can happen quickly when it goes from shadow to sunlight and make a tight placement impossible to clean. He swore by it.

This seems more plausible to me than the rock moving that much. It seems like that much movement every time a body of rock warms and cools would lead to a lot more erosion and cracking than we observe.

EDIT - what Andrew said.

Tom Nyce · · Flagstaff, AZ · Joined Nov 2010 · Points: 45

The expansion of the nut isn't the problem. The nut is usually still in the shade of the crack anyway. You have to take into account the size of the thing that is expanding. The nut is less than an inch wide/thick. The rock surface is 20 -40 ft (or more) wide, until the point where it reaches the next crack (which relieves the stress).
My contention is that the nut stays about the same size, while the crack itself gets smaller as the surface heats up. I'd also guess that the effect would be stronger when the cracks are far apart from each other.
I've seen parties that couldn't clean a whole pitch of nuts, and had to bail due to the problem. The nuts practically fell out at the lightest touch for the next party (mine), when they were in the cool of the shade.
Note: Where there aren't a lot of actual cracks existing by faulting or other geologic processes, there really are a lot of shallow, bottoming, cracks. Especially in sandstone. I believe that, in part, these type of cracks form because the surface is expanding and contracting, while the underneath rock is staying at a steady temperature. The cracks form at fairly regular distance intervals. Of course, once they get started, other types of erosion start to add to the process.

JJ Schlick · · Flagstaff, AZ · Joined May 2006 · Points: 11,803

Awesome topic. I've have noted over the years that the cracks on our local columnar basalt can change quite a bit during the course of a year. I notice it most on thin jams I know well, and also small wire placements that at times will be completely bomber, and other times the same piece won't even fit.

BrianWS · · Unknown Hometown · Joined Apr 2010 · Points: 790

I've had this happen numerous times on south-facing cracks at the NRG. Not sure if it's due to the aluminum or the stone itself expanding, but the phenomenon is real (anecdotally, at least...).

A solution that others have sworn by is to pour cool water, if available, onto the nut prior to working it out of the crack. Or, simply wait until the crack goes into the shade and hope that some other party doesn't get there first.

Gunkiemike · · Unknown Hometown · Joined Jul 2009 · Points: 3,492
Max Forbes wrote:A quick calculation of thermal expansion: Change in length = initial length * thermal coefficient of linear expansion of aluminum * change in temperature This result yields an extremely nominally small answer, (like 10 micrometers of expansion) even using the thickness of the biggest BD stopper, with the assumption that its a solid object and a temperature change of well over 10 degrees C. With that being said I can't think of any better reason as to why this occurred. Curious if anyone else with an engineering background has any additional thoughts.
It's not the stopper that's expanding, it's the rock on either (or both) sides of the crack. Yes, it's a tiny, tiny fraction, but calculate 50 ft or more of rock expanding and it's a real issue. Has happened to me in the span of an hour - a lightly placed stopper was super tight and a cam placed mid-range was fully compressed. I've also witnessed flat "pancakes" of rock cup downwards as the sun warms the top surface.
Josh Olson · · Durango, CO · Joined Mar 2010 · Points: 255

More than once I've gone to Devils Lake early on a monday to get the stuck nuts off the top of classic routes that get a lot of toprope action. I don't doubt for a second a porous rock like sandstone would expand and contract enough to death grip some nuts, even over the course of a lengthy lead. Great way to stock up on bail pieces.

sanz · · Pisgah Forest, NC · Joined Nov 2011 · Points: 210

This is an awesome topic, but now I am really curious as to which object expands. You guys made some pretty good arguments for it being the rock.

An idea - can somebody with good calipers measure a few nuts at varying temperatures to see if they change in size?

Ryan Arment · · Boulder, CO · Joined May 2010 · Points: 71

If it’s the rock expanding Im going to have to start paying attention to temperature more for hard projects. Cold for tips hot for OW

Bill Kirby · · Keene New York · Joined Jul 2012 · Points: 480

Is it booty if thermal expansion expands my nuts or the crack I stuck my nut in?

Ryan Arment · · Boulder, CO · Joined May 2010 · Points: 71
Bill Kirby wrote:Is it booty if thermal expansion expands my nuts or the crack I stuck my nut in?
If you walk away from the wall for more than an hour its up for grabs. although this brings up another interesting topic...
sanz · · Pisgah Forest, NC · Joined Nov 2011 · Points: 210
Gunks Jesse wrote: I do, but it's definitely the rock. Max did the calcs and showed (to my satisfaction) that a nut will only expand 10 micrometers with a 10C temp rise. That is 0.0004" for a 50F temp rise. That's beyond the manufacturing tolerance for a nut and most people don't even have tools to measure that. I could run an FEA over lunch but it won't show anything you can't calculate by hand more quickly. The rock is expanding for sure, causing crack contraction. The cracks act as expension joints to relieve stress and allow the rock to move. Most likely there are perpendicular crack systems deeper (5'?) into the rock. At about that point the mass of rock doesn't change temp much.
Fair enough. Pretty wild that rock is moving that much every day!
mark felber · · Wheat Ridge, CO · Joined Jul 2005 · Points: 41
sanz wrote: Fair enough. Pretty wild that rock is moving that much every day!
I remember seeing some pretty scary rockfall on Maroon Peak that started as a trickle of dust when the sun warmed up the rock, and expanded the rocks just enough to start shifting them.

I'm sure we've all seen fixed cams that have been sucked back deep into a crack where noone could reach them, because every time the rock went into and out of the sun it expanded and contracted, loosening and tightening its grip on the cam.
Michael Brady · · Wenatchee, WA · Joined Jul 2014 · Points: 1,316
sanz wrote:This is an awesome topic, but now I am really curious as to which object expands. You guys made some pretty good arguments for it being the rock. An idea - can somebody with good calipers measure a few nuts at varying temperatures to see if they change in size?
You need to have at least a micrometer with a vernier scale to have any hopes of measuring thermal expansion. A 10 degree difference will result in .0002"(about 15 times smaller than a strand of hair or a sheet of paper) of expansion. It is kind of a pain to measure precisely in a non-lab setting as temp. control is critical and the object will either heat or cool rapidly. The easiest way to see large results is to submerge a 6061-T6 (what most climbing gear is made of) rod into a container of water and dry ice then measure and then put the same rod into boiling water and measure again. Its just kinda hard to measure will it is sitting in the two known temperatures. Boiling water usually make it hard to read a vernier scale :)

Would be fun to find a super clean parallel crack and measure it with an inside mic on a really cold and hot day.
Jon Nelson · · Redmond, WA · Joined Sep 2011 · Points: 8,196

Interesting. I would instead guess that, unless the crack completely split the rock in half, the crack would widen with a warming temperature, contract with a cooling temperature.

(As a thought experiment, consider what happens to a hole in a brick of rock when the rock expands - the hole will expand as well, as you can see if you imagine filling the hole back in with rock.)

The issue would then be how much more does the Al expand than the crack. If the nut it already tight, the slightest bit may weld it.

mark felber · · Wheat Ridge, CO · Joined Jul 2005 · Points: 41
Jon Nelson wrote:Interesting. I would instead guess that, unless the crack completely split the rock in half, the crack would widen with a warming temperature, contract with a cooling temperature.
Usually the crack does split the rock completely in half, or the "crack" really consists of two adjacent blocks of rock. If the crack doesn't split the rock completely at first, then enough years/millenia/eons of frost wedging, freeze thaw cycles, etc. should see to it that eventually the crack splits the rock completely in half.
Guideline #1: Don't be a jerk.

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