Cavechat.org

Hi chaps

I have just mashed the aluminium bars on my micro rack on a fairly heavily contaminated (grit) in situ rope. I am thinking of making some stainless bars (can't buy then in the UK). Do I use 304 or 316?

Whichever is cheaper.

316 specs:
http://www.azom.com/article.aspx?ArticleID=863

304 specs:
http://www.azom.com/article.aspx?ArticleID=965

The mechanical properties are virtually identical. 316 is more resistant to corrosion, but I doubt you'll notice much of a difference as a caver.

Many thanks - confirms my thinking. I was planning to use 316 because of its better corrosion resistance. I am not a caver as such but a mine explorer - some of the mines around this neck of the woods are very acidic so the extra corrosion resistance sounds like a bonus.

If you plan on machining these bars yourself, just take care not to work harden your material too much, which can increase brittleness and induce internal residual stresses. You definitely don't want cracks developing.

Also, if you are planning to use stainless steel tubing, make sure you don't go too thin. I would go at least as thick as the stainless steel bars found on most other racks.

Here are some good tips on stainless steel machining: http://www.azom.com/article.aspx?ArticleID=1178

rlboyce wrote:If you plan on machining these bars yourself, just take care not to work harden your material too much, which can increase brittleness and induce internal residual stresses. You definitely don't want cracks developing.

Also, if you are planning to use stainless steel tubing, make sure you don't go too thin. I would go at least as thick as the stainless steel bars found on most other racks.

Here are some good tips on stainless steel machining: http://www.azom.com/article.aspx?ArticleID=1178

I was planning on using 22mm tube with a 2mm wall thickness

Those are probably appropriate dimensions. The stainless tubes on my rack have an approximate wall of 3/32" (2.4mm), and an OD of 13/16" (20.6mm).

Ideally, after you put your new bars on, you should setup a test rig to verify that the bars can handle the load. I would also advise using a comfortable safety factor (i.e. apply more weight than necessary).

I am by no means an expert on this, but keep in mind that heat may be another big factor to consider. Essentially, a rappel rack is a friction brake, and when a friction brake system heats up it's not as effective. In other words, the less metal, the quicker it will heat up, and the less friction the rack will provide. If you are a larger guy, you may want to consider going with a little thicker tubing. Alternatively, you could make the bars a little longer than what they need to be. As an added bonus, thicker bars will last longer and are capable of taking a greater load (but they will also make the rack a tad heavier). If you don't do a lot of rappelling at any one time or in quick succession, then this is probably a non-issue.

Good luck. Feel free to post some pics of your project!

Thanks for your input - very useful.

It might not be cheap, but I don't see why you couldn't just buy them from one of the American cave outfitters. It's not a prohibited export item, so they should have no problem shipping to you. Of course, if you have the ability and means and desire to make your own, then by all means! I can relate.

graveleye wrote:It might not be cheap, but I don't see why you couldn't just buy them from one of the American cave outfitters. It's not a prohibited export item, so they should have no problem shipping to you. Of course, if you have the ability and means and desire to make your own, then by all means! I can relate.

The rack I am using is a Kong Rakong which is the only micro rack available in the UK (from the ever helpful Starless River). The Rakong is a narrow beast - non of the available stainless bars would fit - hence the need to fabricate. Fortunately I am a fairly practical chap - I made my own chest roller box for my ropewalking set up so brake bars should not be a huge issue.

I have considered buying kit from the US however expense is a significant consideration - On top of purchase costs I will also get hit with shipping costs and import taxes. If this weren't an issue I would have a bright shiney new BMS long micro rack in my grubby hands. Here in the UK Petzl is really the only game in town. I look longingly at the IMO and OR1 websites.

UK vertical culture is pretty conservative. Most people I know consider me to be a dangerous heretic because I use a micro rack rather than the Petzl go and have dared to experiment with ropewalking. Thou wilt do alpine rope work is the almost universal mantra. This conservatism is reflected in what our retailers stock. Thus if you want to make up your own mind rather than following the masses you either need deep pockets or the ability to make your own kit.

by tamarmole » Sun Mar 09, 2014 3:55 pm
I was planning on using 22mm tube with a 2mm wall thickness

For what it's worth, I machine my own replacement microrack bars from the same material as the original manufacturer: 1/2" Schedule 40 304 stainless steel welded pipe (21.34 mm OD x 2.77 mm wall thickness). It is my opinion that Schedule 10 (2.11 mm wall) is too thin for a rack bar. A gritty rope will still wear through stainless steel, but definitely at a slower rate than aluminum.

Although 316SS has better corrosion resistance than 304SS, this higher cost is not justified. If you can survive trips into these acidic mines, so can 304SS. Just rinse your gear off when done. I'd be more concerned with degradation of your nylon harness and rope. For your peace of mind, you may want to buy some full range litmus paper to test the pH of the water in these mines to see if this is an issue. These are widely available from many sources and are cheap.

Plain bars are a breeze to make. Milling the open side for the swinging bar is a bit trickier, although quite doable. Especially if you are, as you say, a "practical chap." If I still had access to machine shop equipment, there's no way I'd pay full price when I could easily make my own. I made several bars a few years ago, and my only regret is that I didn't make one with a hyperbar.

The main bar where you really need to be careful about regarding thickness would be the top bar(s), where you would presumably mill a groove to guide your rope. With these bars, I would suggest starting with a solid bar, and drilling/counterboring a larger hole (thinner wall thickness) near the ends. Then drill through the middle section with ~13 mm drill, leaving more wall thickness to accommodate the groove. I've had SS bars for years, and the amount of wear is very small. It would take a lot to wear one through, and then it it quite easy to detect.

Trogman

I have an old Kong Bonaiti U-rack from a couple of decades ago. I also made up my own stainless bars when the aluminum ones wore out. I used a hacksaw, a file and a drill press. Worked great.

I've never seen a need to mill a rope groove in the top bar, especially with this type of narrow rack. One thing I really like about the old Kong over my usual BMS micro (and presumably the "Rakong" which I am not familiar with) is that it's wide enough to easily descend on a doubled 11mm rope for a pull-down trip or when some extra friction is needed.

Adam Byrd wrote:

by tamarmole » Sun Mar 09, 2014 3:55 pm
I was planning on using 22mm tube with a 2mm wall thickness

For what it's worth, I machine my own replacement microrack bars from the same material as the original manufacturer: 1/2" Schedule 40 304 stainless steel welded pipe (21.34 mm OD x 2.77 mm wall thickness). It is my opinion that Schedule 10 (2.11 mm wall) is too thin for a rack bar. A gritty rope will still wear through stainless steel, but definitely at a slower rate than aluminum.

I have mulled - I think you are right.

My length of 22mm has been relegated to the "it will come in useful sometime" box and I have ordered a length of 1/2" NB schedule 40 316.

Rather than resurrecting the Kong with stainless bars I bit the bullet and bought myself a BMS long micro and am loving it. Shame they are not available in the UK as the shipping and importing costs doubled the price.

Thanks very much for your advice.