huangyhg

floating fastener formula
folks-
there is something that has bothered me for a long time and i hope that maybe you could help me understand it better.
at our company, with many of our purchased rf electrical components we have no control over tolerances and sizes of the component mounting holes. instead this data is determined by the vendor who typically has little or no knowledge of gd&t and the fixed and floating fastener fit formulas. thus, we often have to accommodate odd (often unfriendly!) hole sizes and tolerances.
imagine a component being a simple rectangular plate with, say, four clearance holes passing through it. if you imagine also that this component is mounted to a flat sheet metal surface with four clearance holes (of different diameter and in-pattern tolerance than the component) and you have the picture. the holes in the sheetmetal are positioned and spaced to line up with the mounting holes in the component.
many sources give the following floating fastener formula:
h1+h2=2f+t1+t2
where
h1=the clearance hole diameter in part 1 at mmc,
h2=the clearance hole diameter in part 2 at mmc,
f=the fastener major diameter at mmc,
t1=the feature-relating positional tolerance of the holes in part 1 at mmc, and
t2=the feature-relating positional tolerance of the holes in part 2 at mmc.
let the component be part 1 and the sheet metal mounting surface be part 2.
the component (part 1) has a clearance hole such that h1=.104. this is too small for a no. 4 screw, so we choose a no. 2 with f=.086. the vendor holds the positions of the holes to +/-.005 relative to each other. therefore t1=.014. plugging this all into the formula we get the following formula relating h2 to t2:
h2=.082+t2.
what does this say? does this mean that, say, if t2=.002 then h2=.084? but .084<.086 meaning the screw won't clear the hole!
obviously i'm missing something. also i can't find a reference that can explain this. what additional conditions (besides h1>f and h2>f) or forumlas apply in this case to explain the above formula?
thanks in advance!
tunalover
eng-tips forums is member supported.
if the fastener maj dia is .086, where did .082 come from?
tunalover:
h=f+t or t=h-f
t is positional tolerance diameter
h is taken at mmc (small hole)
f is taken at mmc (large fastener)
what type/brand of fasteners are you using.....milspec fasteners are held to a tighter tolerance than hardware made offshore.
according to the spec, i think you stated the fixed fastener formula.
best regards,
heckler
sr. mechanical engineer
sw2005 sp 2.0 & pro/e 2001
dell precision 370
p4 3.6 ghz, 1gb ram
xp pro sp2.0
nivida quadro fx 1400
o
_`\(,_
(_)/ (_)
do you trust your intuition or go with the flow?
isn't this listed in the machinery handbook?
chris
sr. mechanical designer, cad
solidworks 05 sp2.0 / pdmworks 05
tunalover,
your equation h1+h2=2f+t1+t2 is for floating fasteners. i punched this quickly into a spreadsheet, and actually, it makes sense, as much as math ever makes sense.
the assumption behind the floating fastener theory is that the screw (bolt?) sits at the exact nominal position, and that the two or more holes must clear this at mmc.
for your first hole, you have a bolt .086" diameter, and a positional tolerance of .014". you require a clearance hole of .100" diameter. your clearance hole is larger than that dictated by theory.
you can solve your equation two ways, set t2 and solve for h2, or you can set h2 and solve for t2.
set t2 = 0.
h2 = t2 + t1 + 2f - h1 = .082in !!!
set h2 = .086" (for zero clearance).
t2 = h1 + h2 - 2f - t1 = .004in.
your zero clearance hole can be .002" out of position, and the thing can still be assembled.

jhg
thanks drawoh! your response seems to make sense. i'm one of those guys who needs to spend a day or so thinking about things like this (i need time to think more about your response). then it will make more sense.
i have marked you with a star. thanks!
tunalover
heckler-
what do fastener tolerances have to do with anything? a no 2 screw, for example, is .086 major diameter at mmc regardless of who makes it!
tunalover
tuna,
i should have explained myself more affectively regarding fastener tolerances verses manufacture quality. in the greater sense it does matter if you are in the type of business that dictates using a ms or nas type fastener. but if you are manufacturing wiget fixtures. it has nothing to do with the immediate calculation unless you're buying hardware off-shore then you might want to validate it against commerical or mil specs. but that's getting off the topic here....which i tend to do, a lot!
best regards,
heckler
sr. mechanical engineer
sw2005 sp 2.0 & pro/e 2001
dell precision 370
p4 3.6 ghz, 1gb ram
xp pro sp2.0
nivida quadro fx 1400
o
_`\(,_
(_)/ (_)
"there is no trouble so great or grave that cannot be much diminished by a nice cup of tea" bernard-paul heroux

thanks heckler. where i work, whenever i specify an ms or nas fastener, our production people inevitably earmark commercial parts as alternates. consequently, the stock bins get a mixture of both. i work at one of those places where "quality" is just a feel-good buzzword thrown around by management. when it comes down to it, quality takes a second seat to cutting corners on material costs, shipping product, and getting paid by the customers
sounds like bernard-paul heroux was an emerging hedonist!
tunalover
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