Floating vs fixed fastner tolerance

[sendithard]

I'm relatively new so my apologies for the basic questions. I'm confused on what the real difference is b/w fixed vs floating fasteners is and then why there is a different equation for the clearance hole. So if there are two .5" plates being secured together you can either thread one side or bolt and nut them. Dealers choice. Why does one situation only require taking the major diameter and adding both tolerances together to get your min size clearance hole. Then the other has a multiplier on it which would make the clearance hole larger as seen here in this video as the equation (fastner diam + tolerance1 + tolerance2*(multiplier)).

https://youtu.be/bThrWWcMTvo?t=231

I don't understand why a fixed vs floating situation you would have differing clearance hole sizes basically. Thanks.

 

[jake.]

Hi sendithard,

I don't understand why a fixed vs floating situation you would have differing clearance hole sizes basically.

*Full disclosure, I did not watch the video that you linked. Here goes:

It is the designer's job to determine what the size of any feature should be to ensure proper functionality of the part. The tolerancing of said features should similarly reflect the functional requirements of the part, while (ideally) being loose enough to allow for economical manufacture. This general concept is what I think you should concern yourself with when designing a part.

To learn more about the fastener formulas, I suggest you review NONMANDATORY APPENDIX B (p. 191) of Y14.5-2009 if you haven't already.

Jake

 

[3DDave]

Is it necessary to describe how addition works?

 

[chez311]

I don't understand why a fixed vs floating situation you would have differing clearance hole sizes basically.

Its right there in the name. In the fixed case the fastener is constrained (fixed) in relation to the threaded hole once torque is applied - ie: if the tapped hole has orientation or position error this will be imparted to the fastener once torque is applied. Not so in the floating case as is it desired to have the fastener constrained to neither hole* (ie: allowed to float). Wouldn't it make sense to consider each case differently?

*This is a simplification for explanation purposes. The hole(s) could be toleranced in such a way that due to some combination of size/position/orientation a zero clearance or interference condition could exist which would apply some degree of constraint. Obviously the desire would be to avoid this with proper tolerancing - hence the floating fastener formula.

 

[drawoh]

sendithard,

I worked this out...

Calculating Locational Tolerances [—] Modelling

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JHG