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kebwi 07-19-2012 09:01 AM

Nuts & bolts: cheapo zinc, grade-8, stainless steel?
 
When designing various braces and brackets for an outdoor, weather-exposed, long-aged (many years) purpose, what are you thoughts on bolt diameter and material? I'm using typical plated steel for the actual bands that wrap around a stove pipe to support it from the side on arms, and I'm still thinking about what the support arms themselves will be (probably aluminum tube or thin-walled conduit, perhaps with a shaft of rebar down the middle for a little extra strength), but I'm totally unsure about the fasteners.

Nuts & bolts obviously come in a variety of diameters and materials. The cheapest and by far most prevalent (by shelf-space) at the store are the zinc-plated fasteners. They seem to be pretty general purpose and I have no idea what applications require upgrading to the next level, either in demands on strength or weather-resistance. Above that, there are at least two options: the pseudo-copper-brass-colored grade 8 bolts (I guess they're called "yellow zinc"), and the stainless steel bolts. Both are *much* more expensive than the zinc-plated bolts, but the stainless steel is also considerably more expensive than the brassy grade 8 bolts.

First question: For a given diameter, are those yellow zinc grade 8 bolts less strong, more strong, or equivalent to stainless?...or if that's the wrong question, then do they "last longer" or "survive weather" better or worse or equivalent to stainless (I can't believe they are more weather resistant than stainless, but are they functionally equivalent in that regard)?

Second question: When "locking" the ends of a band around the stove pipe together, or when attached the ends of the support arms to the band and to a support pole nearby, which of these three materials (or a fourth material I missed) would you use *and* what diameter would you use?

I usually buy the cheap zinc stuff for my builds because I see no reason not to, but I've been thinking I might use the yellow grade 8s for this job...but I'm not particularly inclined to super-splurge on stainless unless there's a reason to believe it would vastly outperform the grade 8s. What do you think of all this?

On a side note, I'm really curious if there is a tradeoff in diameter vs. material: Is a wider zinc-plated bolt just as strong as a thinner grade 8 or stainless bolt...or is that really not how the issue is approached?

Thanks, just conversing, it seemed like an interesting discussion.

Cheers!

Tscarborough 07-19-2012 09:15 AM

Re: Nuts & bolts: cheapo zinc, grade-8, stainless steel?
 
Grade 8 are high strength and not needed. Stainless steel is what I would use, and be careful of mixing various metals to avoid galvanic corrosion. As to size, use what will fit best since pretty much anything over 1/4" will exceed any loads you will put upon it.

kebwi 07-19-2012 10:11 AM

Re: Nuts & bolts: cheapo zinc, grade-8, stainless steel?
 
Erg, the stainless is expensive. It sure is pretty though. Thanks!

A few years ago my basement flooded any my compartmentalized bin of bolts, screws and assorted small gear was submerged for a few days. Everything, including the zinc-plated stuff, looked really bad afterwards, but the stainless was pristine! That experience definitely sold me on stainless, money not withstanding.

nissanneill 07-19-2012 04:07 PM

Re: Nuts & bolts: cheapo zinc, grade-8, stainless steel?
 
Go for the stainless as it will last indefinitely. Grade 304 is fine (food grade) but 316 is better (marine grade). Try not to buy from the local hardware store as they charge like wounded bulls, rather go to the larger supplier/wholesaler/internet seller as they will be half the price. Galvanised is better weathering than the zinc or cadmium plated materials, with stainless the best, so the choice is up to you.
Don't mix tour metals as they will oxidize/rust/react with one another (as Tscarborough suggested) over time causing you concern.

Neill

kebwi 07-19-2012 04:58 PM

Re: Nuts & bolts: cheapo zinc, grade-8, stainless steel?
 
Hmmm, so the band I made to wrap around the stove-pipe, which is "plated steel" (not rust-prone "welder's steel", or so I trust, and which has that weather-proof galvy look...I think) is not right b/c it'll react with the stainless bolts? I should make that out of stainless steel instead? What is stainless steel sheet metal gonna cost? I didn't even see anything like that at the box store, I'll have to get it special or something.

Gulf 07-19-2012 05:29 PM

Re: Nuts & bolts: cheapo zinc, grade-8, stainless steel?
 
Quote:

Originally Posted by kebwi (Post 135256)
Hmmm, so the band I made to wrap around the stove-pipe, which is "plated steel" (not rust-prione "welder's steel", or so I trust, and which has that weather-proof galvy look...I think) is not right b/c it'll react with the stainless bolts? I should make that out of stainless steel instead? What is stainless steel sheet metal gonna cost? I didn't even see anything like that at the box store, I'll have to get it special or something.

I like stainless steel. 316 is what I am most familiar with at work. The band you have (galvanized or maybe gavalume) will probably most likely last many years. You might want to buy a small can of "never seez" (or equivalant). You can coat the inside of your band with it where it touches your SS Chimney. It would be a good idea to apply some to your nuts and bolts also. If you ever have to take it apart the never seez will help the SS from galling.

kebwi 07-19-2012 08:03 PM

Re: Nuts & bolts: cheapo zinc, grade-8, stainless steel?
 
Thanks for all the advice. Stainless 316 seems to be more than twice the cost of stainless 18-8 (which is also labeled 304 as mentioned earlier in this discussion). Golly. I'm not planning on putting my pizza oven at the bottom of a salt-water bay or in a chemical factory. If 18-8 isn't good enough to swing free in the open breeze, then what in the world is it good enough for? 316 is also harder to find. More online sources I've found have 18-8 than 316.

I'll figure something out. Do you think 18-8 would be a catastrophe?

Tscarborough 07-19-2012 08:07 PM

Re: Nuts & bolts: cheapo zinc, grade-8, stainless steel?
 
You will probably be all right using crappy zinc coated fasteners. When you ask for advice, you should always expect that you will be given the best advice, not the most practical.

kebwi 07-19-2012 08:11 PM

Re: Nuts & bolts: cheapo zinc, grade-8, stainless steel?
 
Fair point. Thanks.

RTflorida 07-19-2012 09:56 PM

Re: Nuts & bolts: cheapo zinc, grade-8, stainless steel?
 
OK, now to throw you for a complete loop. You initially mentioned strength concerns along with corrosion resistance. Now you are talking the 400 series stainless. The best commercially available bolts in the US would probably be 431. They give up a bit in cororsion resistance when compared to 316 but have the tensile strength on grade 8 carbon steel. The common misconception is that stainless bolts are stronger than plain steel or zinc coated, NOT true, a 316 bolt has the same strength as grade 2 (the typical HD or Lowes plain steel is a grad 3). Its not until you get into the higher chromium alloys (400 series) that you get strength. If you are interested, I have a supplier in CA that can supply just about anything in the 431 range, don't have the info at hand but I will tomorrow at work.
If you have a friend in the UK, the absolute best would be BUMAX fasteners, Harder thatn grade 8.8 and equal to 10.9 carbon steel. There literature does not give the specific alloy content, but it is also a martensitic alloy similar to the 431. .ultra high strength and very good corrosion resistance. I have not found their US distributor, yet. but have been told that I need to. Price on the 431 bolts will be triple the cost of a 304 and double the price of 316. I just paid over $4 each for 4 dozen #8 x 32 cap screws that were 70cents each in 304. For all practical purposes, get 316 bolts and you will not have any worries,it is a much finer alloy for wet applications.

RT


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