Author Topic: BMS Micro Rack question  (Read 2027 times)

Offline AlexR

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Re: BMS Micro Rack question
« Reply #25 on: August 10, 2021, 09:48:34 am »
If you don't like the Stop then the answer is simply to make your own.

That looks like the ticket! I assume as the groove doesn't see much load on it abrasion there is negligible? Do you see any accelerated aluminium corrosion due to the Titanium? I suppose there is little difference in the electrode potential between Al-Ti and Al- stainless.
Did you ever consider making Titanium bars? Titanium tubing is commercially available, though the required wall thickness might be an issue - and I wouldn't bloody like to bore round stock on the lathe, I hear it work hardens like an absolute bugger.

Coincidentally I did think about machining a top part with some sort of groove like yours under the incorrect assumption that I wouldn't get enough breaking power off the BMS rack on thin ropes. After my tests I'm confident you could safely descend the sketchiest of ropes, so not needed.

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Re: BMS Micro Rack question
« Reply #25 on: August 10, 2021, 09:48:34 am »
Warmbac

Online PeteHall

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Re: BMS Micro Rack question
« Reply #26 on: August 10, 2021, 11:12:51 am »
I wouldn't bloody like to bore round stock on the lathe, I hear it work hardens like an absolute bugger.

I believe you are correct. We once did a job at a jet engine factory (on the water treatment plant) and had to walk through the machine halls to get to where we were working. The engineers were very happy to show us what they were doing and explained how much of an absolute twat titanium is to work with. You have to get it just right, first time, or it has to be scrapped and re-processed from scratch.

Offline Stuart France

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Re: BMS Micro Rack question
« Reply #27 on: August 10, 2021, 09:15:08 pm »
There are no stainless steel or other ferrous parts in my rack - it's all made of Al / Ti.  My previous descenders were all racks based on stainless steel U pieces with a pigtail on one end for the krab.  The rack bars became unobtainable, and it was quite heavy, but otherwise no complaints.

The main problem for hobby constructors is getting hold of the metal stock.  I got the aerospace grade Al angle from the USA.  A friend over there bought me a couple of hobby lengths of 12" in different sized materials and sent them over here.  The long Ti bars which were already threaded at both ends and the titanium nuts came from a specialist motorbike outfit in the UK.  The ready-to-go rack bars made by Petzl came from Inglesport and I felt these were really well designed and likely to be available as spare parts for some years to come, although I have stocked up on them.  It can all be disassembled easily with one spanner and any worn part then replaced.

The advantage of having the V-shaped brake slot dead centre is that the rack is not pulled to one side when the brake is used and there is none of this left-handed-right-handed nonsense.  I'm a left hander in a mainly right handed world.  The wear on the V slot is minimal as the brake works mainly by the 'wrapped up' shape that the rope adopts when the V is used.


Offline traff

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Re: BMS Micro Rack question
« Reply #28 on: August 11, 2021, 07:57:24 am »
It always makes me laugh when I see 'aerospace grade'. No such thing. A grade of aluminium used in the aerospace industry would perhaps be a better description. I'm no expert but having built my own aircraft I'm very familiar with some of the grades used, some of which would be totally in appropiate in this application. Eitherway it's readilly available in the UK, you just need to know what you are asking for.

Offline AlexR

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Re: BMS Micro Rack question
« Reply #29 on: August 11, 2021, 08:56:28 am »
Well, "aerospace grade aluminium" just sounds better than "Look, the guy in storage mixed up a load of 2011, 6061 and 6082, would take too long to find out what's what, can't we just sell it all under one label?"

Joking aside, given the considerable differences in wear between e.g. the Kong and Petzl racks if I were a betting man I'd say Kong used 60xx (because it's cheap) and Petzl 70xx (because it's hard).

@Stuart:
I understand there are no ferrous metals, but in terms of their position in the galvanic series titanium is comparable to stainless, and galvanic corrosion between aluminium and titanium does definitely occur - ask people with posh titanium fixtures holding down aluminium masts. Though I suspect it won't be an issue under "standard" caving environments.

Offline Stuart France

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Re: BMS Micro Rack question
« Reply #30 on: August 16, 2021, 12:55:57 pm »
Well, I thought 'aerospace' was clearer in this context than '6061-T6'.  It all arrived adorned with bar coded sticky labels in 2ft lengths.  Sorry to disappoint, but there were no mixs-ups in the warehouse.

I bought several sizes to experiment with but the stock I eventually used was 1.25x1.25" equal leg, 0.1875 thick, and 0.125 between-leg radius.  The legs with the paired holes for the long titanium bars were shortened to 1.125".  The reason for imperial meaurements is that I bought from USA after failing to find a UK supplier selling in less than industrial quantities.

I've tried various lengths for the long titanium bars and settled on 210mm including 25mm of threads at both ends, initially.  The top angle in the final rack was later fitted with 4mm thick half-nuts underneath (rather than use full nuts which are 10mm thick) and the resultant 6mm excess thread then poking out of the topmost nuts was removed.  I then notched the top fixed Petzl rack bar to accomodate the half nuts so that it sits flush with the top angle to avoid leaving a gap.

The only noticeable wear so far is on the Petzl rack bars, and that is consistent with level of use.  I thought the angle pieces might need periodic replacement (i.e. the V-brake at the top and karabiner hole at the bottom) but they show no significant wear.   There is no corrosion and it is of course stored in a dry place.  Incidentally, I'm not aware of any commercial rack designs that use only a single metal.  The M10 nuts and long bars are the same titanium material.  The 6061 angles are clamped by paired Ti nuts on both sides.  So the 10mm holes through the angle pieces for the M10 bars are not threaded - this is deliberate.

Used it this weekend in some mines and I'm very happy with the smooth action, the speed range from fairly fast to dead slow and precise locking off to do rigging.  What else is not to like?


Offline AlexR

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Re: BMS Micro Rack question
« Reply #31 on: August 17, 2021, 10:22:29 pm »
Well, I thought 'aerospace' was clearer in this context than '6061-T6'.  It all arrived adorned with bar coded sticky labels in 2ft lengths.  Sorry to disappoint, but there were no mixs-ups in the warehouse.

Sorry Stuart - my comments on aerospace grade Al weren't meant to be a swipe at your choice of metal or identification, just a reflection of my dislike of "aerospace grade aluminium" by people selling a product, as it's inherently meaningless.
But as someone who has had trouble sourcing 7075 T6 in smaller quantities I sure understand the struggle in getting your mitts on certain materials.

Incidentally, I'm not aware of any commercial rack designs that use only a single metal.

The BMS rack I asked about in this thread and have now bought and used is all stainless steel. The manufacturer BMS does not disclose which type(s) of stainless, and if you wanted to be pedantic it could be pointed out that the components are probably made of different stainless steels.


BMS micro rack preliminary wear and handling

I've now used the BMS for 360m of descent (completely on rope). 180m on 9mm, 180m on 10.5mm. It handles both diameters really well, on the 10.5 you're starting to run out of space for locking off, but it's just about enough to pass a loop between the bars.
The wear on the bars is already visible, this is for pretty clean but not pristine ropes. Spent a thrilling half hour or so taking some measurements, mean wear is 0.135mm.* The wall thickness varies between 2.80 - 2.88mm. The bars can be flipped, but you also don't want to wear all the way through a bar on either side, so for argument's sake let's say the maximum wear before retirement is 2.80mm, meaning you'll get 7.5km of descent on clean-ish ropes. More if you predominantly cave in nice clean washed Yorkshire potholes using freshly washed ropes.

Althought I keep a log of all my caving, I didn't write down when exactly I replaced parts on the Petzl Stop. As an extremely rough ballpark under comparable conditions the top aluminium bobbin was good for maybe 1.5km, and the stainless steel handle one for 3-4km.


Info of use to absolutely no one:
The bars on the BMS can visually be identified as straight seam welded pipe. There is no benefit to using seamless here, so good price choice. Deviation from roundness 0.12mm +/- 0.02mm.

*mean wear over all bars compared to averaged measurements of ODs. Don't ask me about the uncertainty, there is enough overanalysis here already.

Offline tamarmole

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Re: BMS Micro Rack question
« Reply #32 on: August 18, 2021, 07:00:44 am »


Incidentally, I'm not aware of any commercial rack designs that use only a single metal.

The BMS rack I asked about in this thread and have now bought and used is all stainless steel. The manufacturer BMS does not disclose which type(s) of stainless, and if you wanted to be pedantic it could be pointed out that the components are probably made of different stainless steels.

[/quote]

I designed and a mate built me a BMS clone; we used 316 stainless throughout.  The bars were Schedule 40 316 stainless tube which has a good wall thickness.

 

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