floating fasterner formula

[dnobie]

Can someone explain how the floating fastener formula works when you are fastening 3 parts in the assembly with different clearance hole sizes. I have a 1/4 " bolt passing through a clr hole @ .263 MMC, a bushing @ .254 MMC (pressed into another part), then another clr hole @ .263 MMC. I am struggling to conclude how much tolerance is available within the assembly.

Thanks

 

[drawoh]

dnobie,

I do not particularly like the way ASME shows you how to work out positional tolerances. It is easy to work out from first principals.

You have a 1/4" bolt. Presumably, there is a nut at the opposite end, and we can assume the bolt is located precisely at nominal. It occupies a space Ø.250". Your clearance holes must not encroach on this.

Your hole's smallest acceptable diameter must be located accurately enough that it does not occupy the Ø.250" diameter. If your smallest hole is Ø.280" in diameter, it can safely shift .015" off of nominal position, for a positional tolerance of Ø.030".

POSHOLE = MINHOLE - MAXBOLT

If the bolt is a screw, i.e. it is located in a tapped hole, you must account for the positional tolerance of the tapped hole. A 1/4" screw located inside Ø.015", occupies a space of Ø.265". Your Ø.280" hole must clear this, and can therefore shift a maximum of .0075" for a positional tolerance of Ø.015".

POSHOLE = MINHOLE - MAXSCR - POSSCR

If you are clamping something thick, you should account for the tapped hole being at an angle by enlarging your assumed tapped hole positional tolerance, or by applying a projected tolerance zone to it.

JHG

 

[Belanger]

Each component gets its own calculated position tolerance. It would be the MMC of the hole in question minus the MMC of the bolt's major diameter. That would be a position tolerance of dia. .013, .004, and .013 respectively.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems