Floating fastener formula 1

[Tunalover]

Guys,
The floating fastener formula from Appendix B of ASME Y14.5-2009 gives only:

H=F+T​

This is the simplest case where one part has the same hole size and feature-relating positional tolerance as the other. But what is the general case where the hole sizes and feature-relating positional tolerances are NOT the same?




H. Bruce Jackson
ElectroMechanical Product Development
UMD 1984
UCF 1993

 

[3DDave]

Worse - it only applies when the datum reference frames of the two parts are perfectly aligned, which is the actual general case.

I suppose it's H1= F + T1 and H2 = F + T2, but then that assumes the fastener is the same diameter for each portion instead of having a step, so H1 = F1 + T1 and H2 = F2 + T2.

Other than that there's an infinite range of ways to shift tolerance for location from one hole to the other with the projected fixed fastener formula being the limiting case.

 

[greenimi]

I don't even think those formulas needs to be remembered. As long as the VC virtual conditions are the same everything can be shifted accordingly/per the design intent.

 

[Tunalover]

3DDave:
If H1=F+T1 and H2=F+T2 then does it follow that:

​

H1-T1=H2-T2?

H. Bruce Jackson
ElectroMechanical Product Development
UMD 1984
UCF 1993

 

[3DDave]

Yes - so there's one equation with up to four unknowns. Not exactly useful and only applicable for a fastener with a constant diameter for parts where the datum reference frames are perfectly aligned. For some reason the Y14.5 Committee overlooks assembly tolerance.

 

[drawoh]

tunalover,

I do not bother with the formulae. This can easily be modelled from first principals.

In the floating fastener case, you have a fastener with a maximum diameter, positioned exactly at nominal. Your holes must not intrude into that diameter. There are multiple solutions to this problem.

In the fixed fastener case. your keep-out is the maximum diameter of your fastener, plus its positional tolerance. Again, there are multiple ways to not intrude into the space.

--
JHG

 

[jasoncwells]

Keep reading that appendix. There are more general cases with differing tolerances further into it.

Regards,
Jason C. Wells