huangyhg

fixed and floating fastener fit formulas
folks-
this might stir the pot, but here goes. i use the "fit formulas" for robust design.
for fixed fastener cases:
h=f+t1+t2 where h=clearance hole diameter @mmc, f=fastener major diameter @mmc, t1=positional tolerance of, say, clearance hole @mmc and t2=positional tolerance of threaded hole (or captive stud) pitch diameter @mmc.
for floating fastener cases, my references give:
h=f+t where h=clearance hole diameter @mmc, f=fastener major diameter @mmc and t=positional tolerance of both holes @mmc.
the above is just fine and dandy if both holes are of the same diameter and positional tolerance. but often this can't be!
what about the most general floating fastener case where the holes in both parts may have different diameters and positional tolerances? is it true that:
h1+h2=2f+t1+t2? if so, under what conditions is this true? why don't the textbooks address this more general (and practical) scenario? if this formula is true, why don't the books publish it?
tia
tunalover
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asme y14.5m-1994, appendix b mentions the use of different diameter holes with a floating fastener application as follows:
"b3 floating fastener case
where two or more parts are assembled with fasteners, such as bolts and nuts, and all parts have clearance holes for the bolts, it is termed the floating fastener case. see fig. b-l. where the fasteners are of the same diameter, and it is desired to use the same clearance hole diameters and the same positional tolerances for the parts to be assembled, the following formula applies:
h = f + t
or
t = h - f
example: given that the fasteners in fig. b-1 are 3.5 diameter maximum and the clearance holes are 3.94 diameter minimum, find the required positional tolerance:
t = 3.94 - 3.5
= 0.44 diameter for each part
any number of parts with different hole sizes and positional tolerances may be mated, provided the formula h = f + t or t = h - f is applied to each part individually."
the floating fastener formula is simply the same as saying "clearance equals tolerance". since each hole individually provides its own clearance for locational tolerance @mmc, the positional tolerance can be calculated independently for each hole size in the assembly stack.
gdt_guy
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