Homemade stuff

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Homemade stuff

Postby GroundquestMSA » May 26, 2014 5:24 pm

A friend of mine is a machinist, and he built me a set of brake bars. I bought a long frame for my micro-rack and wanted to play with the old frame. They are possibly the heaviest set of bars in history, being made from deep-well impact sockets, molybdenum steel, I think. The friction is a little greater, but I don't know if this is because the bars are softer than stainless or because they're a tiny fraction larger. I didn't offset the bars like BMS does.

These are just a novelty (I'm thinking about welding on a ratchet hyperbar) and there's an obvious weight issue, but I'm wondering if there is any reason big fat sockets will make an unsafe descender?

I also built a set of wooden brake bars today, which was interesting. I don't think I would ever use them in a cave or on a cliff, but they worked well, especially after I work-hardened them. Wood being an insulator, I wonder how fast they would reach harmful temperatures? I wonder if I could wad some tinder between the top two bars and start a fire...

I really like seeing and hearing about homemade gear. What have you made, did it work, and what brilliant ideas have you been unable to fabricate due to a lack of resources?
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Re: Homemade stuff

Postby PatB » May 26, 2014 8:00 pm

I'm not a metallurgist, but the things to be concerned about are heat dissipation while on rope and a practical weight/strength ratio you need. I doubt impact socket steel will ever fail you, strength wise, unless you weight as much as a school bus, but you might start to melt nylon if you have to stop half way down a long repel where the steel has retained the heat. The thicker impact socket material will act more as an insulator.
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Re: Homemade stuff

Postby GroundquestMSA » May 26, 2014 8:10 pm

I agree. I'm not worried about strength. I used thicker impact sockets because I thought they would be easier to cut than chrome vanadium; and because I found a bunch of 11mm deep-wells in the barn. I don't know if tactile observation could detect a heat difference between these and normal bars on a 70' rappel, but I'll compare them on the cliff soon.
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Re: Homemade stuff

Postby caver.adam » May 26, 2014 10:30 pm

Heat dissipation is related to the conductivity of the metal, the surface area exposed to a cooler substance (air), and the temperature differential between the metal and the air.

Heat retention is proportional to the amount of metal/volume. This energy can build up

The heat energy comes from friction and if you end the rappel at the same speed you will have the same amount of energy converted to heat from friction regardless of bar type (assuming we can ignore energy converted into sound and vibration). Assuming you stop at the bottom of the pit and then put your feet down, the total energy dissipated to stop you is equal to your potential energy at the top of the rappel. Not stopping before you touch the ground will decrease the heat transferred into your rack and potentially increase the flatness of your skull.

But I have no idea whether one metal will be more abrasive on the rope. Nor their static or dynamic strengths. Nor their proclivity to develop cracks.
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Re: Homemade stuff

Postby GroundquestMSA » May 27, 2014 1:37 am

It looks like molebdenum steel is somewhere between Al and Stainless in terms of thermal conductivity, so a thick-walled but hollow bar shouldn't be dangerously different from typical bars should it? Heat produced by rope friction on a wooden block will go where? Not into the block; into the rope? The air?

I'm guessing that the static or dynamic strengths of brake bars are relatively unimportant.
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Re: Homemade stuff

Postby Scott McCrea » May 27, 2014 7:21 am

You must start a fire with those wooden bars. With video!
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Re: Homemade stuff

Postby GroundquestMSA » May 31, 2014 7:15 am

I zipped down a few times but only managed to badly glaze the rope. I used a junky 9mm static rope that I've had lying around, now the fuzz is gone and it's very hard. I loosed the rope and felt the blocks as fast as I could, but they weren't hot, I could comfortably hold them to my face. The tinder wasn't scorched. As an insulator, the wood "only" heats up at the point of friction, and cools rapidly. The shocking conclusion: wooden brake bars aren't useful, even as an emergency fire starter.
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Re: Homemade stuff

Postby Scott McCrea » May 31, 2014 7:45 am

Can you post a picture of your wooden bars, please? I'm pretty certain that Gary Storrick might want them for his vertical devices museum.
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Re: Homemade stuff

Postby GroundquestMSA » May 31, 2014 11:39 am

:laughing: Sure. Will do later this evening when I can get a proper internet connection.
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Re: Homemade stuff

Postby GroundquestMSA » Jun 9, 2014 10:25 pm

Here you are. Retired tobacco sticks.

Image

Image
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Re: Homemade stuff

Postby gdstorrick » Jun 11, 2014 4:03 pm

Scott McCrea wrote:Can you post a picture of your wooden bars, please? I'm pretty certain that Gary Storrick might want them for his vertical devices museum.


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Re: Homemade stuff

Postby tncaver » Jun 11, 2014 6:25 pm

Maybe it's just me but seems as if wooden brake bars would not have the strength to last very many rappels before total failure. Every rappel would weaken the bars
by reducing thickness.
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Re: Homemade stuff

Postby trogman » Jun 12, 2014 12:34 pm

Definitely a poor mans' rack. I like it, although I wonder how it would work with round bars (dowels).

Thanks for sharing, Jonah.

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Re: Homemade stuff

Postby tamarmole » Jun 12, 2014 4:55 pm

A thing of infinite joy and wonder.
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Re: Homemade stuff

Postby GroundquestMSA » Jun 14, 2014 9:30 am

tncaver wrote:Maybe it's just me but seems as if wooden brake bars would not have the strength to last very many rappels before total failure. Every rappel would weaken the bars
by reducing thickness.


These weren't meant for serious use.
The areas I was most concerned about were around the holes/slots. This is one reason I used square blocks with a groove instead of dowels, to keep more material around the holes. Large dowels would be just as good, but I wouldn't have room for them on an 8" frame.

I am interested to know how much force is acting on a set of brake bars during use. How flimsy can they be before breaking? I'll have to play around some more.
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